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Table of Contents  Previous Chapter
The occurrence of volatile organic compounds (VOCs) in water is of
national concern because of their relatively high aqueous solubility,
mobility, and persistence, because many are known or suspected carcinogens,
because of their widespread use, and because they have been found in drinking
-water supplies. Because of this national concern, VOCs were selected for
National investigation (hereafter termed "National Synthesis") by the U.S.
Geological Survey's National Water-Quality Assessment (NAWQA) Program in
1994.
The broad goals of this National Synthesis are to: (1) describe current
water-quality conditions with respect to VOCs; (2) define trends, or lack of
trends, in VOCs in surface and ground water; and (3) identify, describe, and
explain causal relations among the occurrence and distribution of VOCs in
surface water and ground water, and natural and human factors.
The National Synthesis of VOCs in ground water has three objectives:
(1) to describe their occurrence, status, and distribution; (2) to determine
relations among VOCs in shallow ground water and natural and human factors;
and (3) to determine, compare, and contrast the occurrence, transformation,
transport, and fate of selected VOCs in the hydrologic cycle for several
regionally or nationally important aquifer systems.
The description of VOC occurrence, status, and distribution in ground
water focuses on major aquifers of the United States. Occurrence describes
the presence or absence of VOCs, their frequency of occurrence, and their
ranges of concentrations. Status compares the concentrations of VOCs
detected in relation to water-quality regulations or advisories, such as
Maximum Contaminant Levels, Proposed Maximum Contaminant Levels, Maximum
Contaminant Level Goals, and Health Advisories. Distribution describes
the variability of VOCs in ground water, areally and by depth. This report
describes the study design for conducting such an assessment.
The assessment focuses on aquifers, or parts of aquifers, that are
currently used or have the potential to be used as sources of water supplies,
using data collected as part of local, State, and Federal ground-water
monitoring programs since 1985. Assessment by aquifer and comparison of
results among aquifers will be completed for those aquifers for which
adequate spatial or depth-related data are available. Assessment of VOCs
in aquifers also will be completed at regional and national scales.
A set of criteria for well-network design, well construction, sample-
collection methods, and methods of laboratory analysis must be met before
VOC data are used for assessment. An appropriate well-network design will
provide a generally unbiased, random, equal-area distribution of sampling
sites throughout the aquifer, or part of the aquifer, of interest. Well-
construction information must be sufficient to ensure that the hydrogeologic
unit (or units) represented by the water level measured and the hydrologic
unit (or units) contributing water to the well are known. In addition, the
well construction and pumping equipment in the well need to be of a type
that are not likely to affect concentrations of VOCs in the water sample.
VOC data will be considered suitable for use in the occurrence assessment
if nationally accepted methods for collection and analysis were used and
if the quantitation level for VOC analytes was less than about 5 micrograms
per liter; laboratory analysis was done by a laboratory certified by the U.S.
Environmental Protection Agency; and the sample was collected from untreated
(raw) water at or near the well head before being held in a pressure tank or
holding tank.
In 1991, the U.S. Geological Survey's (USGS) National Water-Quality Assessment
(NAWQA) Program began the transition from a pilot program to full-scale
implementation. The long-term goals of the NAWQA Program are to describe
the status and trends in the quality of a large, representative part of the
Nation's surface-water and ground-water resources and to provide an improved
understanding of the primary natural and human factors that affect the quality
of these resources. The concepts, implementation, and design of the NAWQA
Program are described in Hirsch and others (1988), Leahy and others (1990);
Leahy and Thompson (1994), and Gilliom and others (1995).
The NAWQA Program has two major components: (1) hydrologic investigations
of large river basins and aquifer systems, referred to as Study-Unit
Investigations (fig. 1), and (2) a National Synthesis that is organized to
provide information about water-quality topics of regional and national
concern.
 Figure 1. Location of National Water-Quality
Assessment Program Study Units and their proposed implementation
dates (from Gilliom and others, 1995).
Study-Unit Investigations and their scheduling are described in Leahy and
others (1990). As of 1995, four water-quality topics of regional and
national concern have been selected for NAWQA's National Synthesis.
Studies of pesticides and nutrients began in 1991, and studies of volatile
organic compounds (VOCs) and of aquatic ecosystems began in 1994.
The purpose of this report is to describe the plan for national assessment
of the occurrence, status, and distribution of VOCs in ground water of the United States, with a focus on major aquifers. The assessment relies largely on data collected by NAWQA Study Units (fig. 1). Seven pilot NAWQA Study-Unit Investigations began in 1986. Three of these pilot investigations collected ground-water-quality data. Twenty additional Study-Unit Investigations began in 1991 and 15 began in 1994. Additional investigations are scheduled to begin in 1997, some of which might be continuations of the pilot investigations that began in 1986.
During 1995, a major effort of the investigations that began in 1994 is analysis of existing water-quality data. Results of this Retrospective Analysis will help guide the Study-Unit ground-water design by developing an understanding of the occurrence, status, and distribution of VOC target analytes and other chemical constituents in ground water in the Study Unit. These Retrospective Analyses also are anticipated to be a primary source of data for the planned national assessment described in this report. Because of the importance of these Retrospective Analyses to this assessment, the plan described herein is presented as guidance to the 1994 Study Units for compilation of VOC data during their Retrospective Analyses. However, the guidance also generally applies for all other VOC-data compilations that will be used by the VOC National Synthesis for the assessment described in this report.
The authors would like to thank James Stark of the NAWQA Upper Mississippi River Basin Study Unit and Anthony J. Tesoriero of the NAWQA Puget Sound Study Unit for their technical reviews of this plan. The authors also thank Lanna Combs of the USGS in Lawrence, Kansas for providing an editorial review of the report and Ella Decker of the USGS in Huron, South Dakota, for preparing it for publication.
VOCs were selected for National Synthesis because of the occurrence of this constituent group in many of the Nation's water supplies (Tennant and others, 1992; Pankow and Cherry, 1996). The broad goals of the VOC National Synthesis reflect those of the entire NAWQA Program: (1) to describe current water-quality conditions; (2) to define trends, or lack of trends; and (3) to identify, describe, and explain causal relations among the occurrence and distribution of VOCs in surface water and ground water and natural and human factors. Fifty-five VOCs, including of halogenated alkanes, halogenated alkenes, alkyl benzenes, aromatic hydrocarbons, halogenated aromatics, ethers, and others (table 1), have been identified for study (John Zogorski, written commun., 1995). The VOCs in table 1 are collectively called the NAWQA "VOC target analytes" in this report.
VOCs are of concern in ground water because of their relatively high aqueous solubility, mobility, and persistence, because many VOCs are known or suspected carcinogens, and because of their widespread use. The National Synthesis of VOCs in ground water has three objectives: (1) describe their occurrence, status, and distribution (VOC occurrence assessment); (2) determine relations between VOC target analytes in shallow ground water and natural and human factors; and (3) determine, compare, and contrast the occurrence, transformation, transport, and fate of selected VOC target analytes in ground water for several regionally or nationally important aquifer systems.
The objective of the assessment of VOCs in ground water discussed in this report is to describe the occurrence, status, and distribution of VOCs in major aquifers of the United States, both for individual aquifers and collectively. Occurrence describes the presence or absence of VOCs, their frequency of occurrence, and their ranges of concentrations. Status compares the concentrations of VOCs detected in relation to water-quality regulations or advisories, such as Maximum Contaminant Levels, Proposed Maximum Contaminant Levels, Maximum Contaminant Level Goals, and Health Advisories. Distribution describes the variability of VOCs in ground water, areally and by depth.
Table 1. Volatile organic compound target analytes1 in the National Water-Quality
Assessment Program
[CAS, Chemical Abstract Services; U.S. EPA, U.S. Environmental Protection Agency;
USGS, U.S. Geological Survey; VOC, volatile organic compound; N/A, there is no
parameter code for this compound; IUPAC, International Union of Pure and Applied
Chemistry]
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VOC compound, by compound class2 CAS number U.S. EPA and USGS
parameter code
-------------------------------------------------------------------------------------------
Halogenated Alkanes
Tetrachloromethane (Carbon Tetrachloride) 56-23-5 32102
Trichloromethane (Chloroform) 67-66-3 32106
Hexachloroethane 67-72-1 34396
1,1,1-Trichloroethane 71-55-6 34506
Bromomethane 74-83-9 34413
Chloromethane 74-87-3 34418
Chloroethane 75-00-3 34311
Dichloromethane (Methylene Chloride) 75-09-2 34423
Tribromomethane (Bromoform) 75-25-2 32104
Bromodichloromethane 75-27-4 32101
1,1-Dichloroethane (1,1-DCA) 75-34-3 34496
Trichlorofluoromethane (CFC 11) 75-69-4 34488
Dichlorodifluoromethane (CFC 12) 75-71-8 34668
1,1,2-Trichloro-1,2,2-trifluoroethane (Freon 113, CFC 113) 76-13-1 77652
1,2-Dichloropropane 78-87-5 34541
1,1,2-Trichloroethane (1,1,2-TCA) 79-00-5 34511
1,2-Dibromo-3-chloropropane (DBCP) 96-12-8 82625
1,2,3-Trichloropropane 96-18-4 77443
1,2-Dibromoethane (EDB) 106-93-4 77651
1,2-Dichloroethane 107-06-2 32103
Chlorodibromomethane 124-48-1 32105
Halogenated Alkenes
Chloroethene (Vinyl Chloride) 75-01-4 39175
1,1-Dichloroethene 75-35-4 34501
Trichloroethene (TCE) 79-01-6 39180
1,1,2,3,4,4-Hexachloro-1,3-butadiene (Hexachlorobutadiene) 87-68-3 39702
Tetrachloroethene (PCE) 127-18-4 34475
cis-1,2-Dichloroethene 156-59-2 77093
trans-1,2-Dichloroethene 156-60-5 34546
Bromoethene 593-60-2 50002
cis-1,3-Dichloropropene 10061-01-5 34704
trans-1,3-Dichloropropene 10061-02-6 34699
Aromatic Hydrocarbons
Benzene 71-43-2 34030
Naphthalene 91-20-3 34696
Styrene (Vinyl Benzene) 100-42-5 77128
Alkyl Benzenes
1,2-Dimethylbenzene (o-Xylene) 95-47-6 77135
1,2,4-Trimethylbenzene (Pseudocumene) 95-63-6 77222
Isopropylbenzene (Cumene) 98-82-8 77223
Ethylbenzene 100-41-4 34371
n-Propylbenzene 103-65-1 77224
n-Butylbenzene 104-51-8 77342
1,4-Dimethylbenzene (p-Xylene) 106-42-3 385795
1,3-Dimethylbenzene (m-Xylene) 108-38-3 385795
Methylbenzene (Toluene) 108-88-3 34010
Halogenated Aromatics
1,2,3-Trichlorobenzene 87-61-6 77613
1,2-Dichlorobenzene (o-Dichlorobenzene) 95-50-1 34536
1,4-Dichlorobenzene (p-Dichlorobenzene) 106-46-7 34571
Chlorobenzene 108-90-7 34301
1,2,4-Trichlorobenzene 120-82-1 34551
1,3-Dichlorobenzene (m-Dichlorobenzene) 541-73-1 34566
Ethers and Other Oxygenated Compounds
Diisopropyl ether (DIPE) 108-20-3 81577
Ethyl tert-butyl ether (ETBE) 637-92-3 50004
tert-Amyl methyl ether (TAME) 994-05-8 50005
Methyl tert-butyl ether (MTBE) 1634-04-4 78032
Others (Aldehydes and Nitriles)
2-Propanal (Acrolein) 107-02-8 34210
2-Propenenitrile (Acrylonitrile) 107-13-1 34215
Other Halogenated Alkenes1
1,2-Dichloroethene (mixed isomers) 540-59-0 N/A
Dichloroethene (all isomers) 25323-30-2 N/A
1,3-Dichloropropene (mixed isomers) 542-75-6 N/A
Other Alkyl Benzenes1
Xylene (mixed isomers) 1330-20-7 81551
Other Halogenated Aromatics1
Dichlorobenzene (mixed isomers) 25321-22-6 N/A
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1Mixed isomers are not target analytes. Volatile organic compound mixed isomers of the
target analytes in the U.S. Geological Survey National Water-Quality Assessment Program
are included in this table for reference because they are commonly measured or reported.
2IUPAC nomenclature (Synonym).
3This is a combination of p- and m-Xylene. These compounds do not elute separately with
the current analytical procedure.
Analysis of temporal trends in VOCs is not a primary objective of the assessment described in this report because identifying temporal trends, or lack of trends, and interpreting those trends requires historic VOC and ancillary data that most likely are not widely available. However, VOC data sets suitable for trend assessment might be identified during the conduct of the assessment described herein.
The assessment will be nationwide for aquifers, or those parts of aquifers that are currently used or have the potential to be used as sources of water supplies. Parts of aquifers at or immediately downgradient of known sources of contamination generally are unsuitable for use as a water supply, and assessment of these parts of aquifers is beyond the scope of this assessment. Assessment of the quality of these parts of aquifers is being done in other programs, such as the U.S. Environmental Protection Agency (USEPA) RCRA (Resource Conservation and Recovery Act) and CERCLA (Comprehensive Environmental Response, Compensation, and Liability Act) Programs.
Interpretations of the reasons for observed occurrence and distribution of VOCs, such as the relation of the occurrence and distribution of VOCs to natural factors and human activities, probably will be minimal. Interpretation will be difficult because of a number of complicating factors, including the lack of consistency in design and conduct of the studies from which the VOC data are derived and because of the lack of detailed, ancillary information associated with the VOC data that would be required for this type of interpretation. Data deficiencies will be identified as part of the assessment. When feasible, enhancements to the data sets, such as subsequent compilation of important ancillary data, will be completed.
This assessment will be done by collective analysis of data from monitoring programs across the Nation. Data from these monitoring programs will be compiled into a national data base for this analysis. Criteria for monitoring-program design, sample-collection methods, and methods of laboratory analysis will be met before data are entered into this data base. The assessment will be based on data collected since 1985, for as many of the 55 VOCs currently (1996) on the list of NAWQA VOC target analytes (table 1) as possible. Assessment by aquifer and comparisons of results among aquifers also will be completed for those aquifers that have adequate data, spatially or by depth.
The proposed timeline for data collection, compilation and analysis, and reporting of results of this assessment is shown in table 2. A relatively limited assessment of VOC occurrence, status, and distribution will be reported in 1997, using: (1) data compiled by 1994 NAWQA Study Units during their year of Retrospective Analysis in 1995; (2) data from Study-Unit Surveys completed by 1991 NAWQA investigations; (3) data compiled by 1991 NAWQA investigations during Retrospective Analysis in 1992, to the extent that data were compiled; and (4) data compiled by NAWQA VOC National Synthesis from the pilot NAWQA investigations and selected retrospective data from State, multi-State, and Federal programs not covered by items (1) to (3). A subsequent assessment of VOC occurrence, status, and distribution will be reported in 2001. This last assessment will use all of the data from items (1) through (4) plus data compiled from States not covered in item (4), as well as (5) retrospective data compiled by NAWQA Study Units in States covered by 1997 NAWQA investigations, if not already completed by the VOC National Synthesis, (6) data from Study-Unit Surveys completed by 1994 NAWQA investigations, and (7) data from Study-Unit Surveys by the 1997 NAWQA investigations that have been collected by the year 2000 (table 2).
Table 2. Timeline for data collection, compilation, and reporting of results for the
assessment of occurrence, status, and distribution of volatile organic
compounds in ground water
[VOCs, volatile organic compounds; NAWQA, National Water-Quality Assessment]
--------------------------------------------------------------------------------------------------------------------------
Data analysis
Data compilation by VOC
into national VOC National
Data source for assessment of occurrence, status, Data collection occurrence data Synthesis
and distribution of VOCs in ground water completed base completed completed
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(1) Retrospective data compiled by 1994 NAWQA Study Units. 1995 1996 1997
(2) Data from Study-Unit Surveys by 1991 NAWQA Study Units. 1995 1996 1997
(3) Retrospective data compiled by 1991 NAWQA Study Units1. 1992 1995 or later 1997 and 2000
(4) Retrospective data compiled by NAWQA VOC National Synthesis Ongoing until Some compilation in 1997 and 2000
from the pilot NAWQA studies and selected State, multi-State, and 2000. 1995 and 1996.
Federal programs not covered by 1991 or 1994 NAWQA Study Total compilation
Units. in 2000.
(5) Retrospective data compiled by NAWQA Study Units in States 1998 1999 2000
covered by 1997 NAWQA Study Units, if not already completed by
VOC National Synthesis
(6) Data from Study-Unit Surveys by 1994 NAWQA Study Units 1998 1999 2000
(7) Data from Study-Unit Surveys by 1997 NAWQA Study Units2. 2001 2001 22001
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1Compilation of retrospective VOC data by the 1991 NAWQA Study Units into a national
occurrence data base was not part of their plans for Retrospective Analysis. However,
compilations that were completed and that meet criteria described in this report will
be used in the assessment of occurrence, status, and distribution of VOCs in ground
water. Also, some additional compilation might be completed by the 1991 Study Units as
part of their low-level assessment activities from 1997 to 2000.
2The intensive data-collection period for the 1997 NAWQA Study Units is from 1999
through 2001. Therefore, data collection for some Study-Unit Surveys will be completed
during 1999 and 2000. These data will be available for analysis in the year 2001.
The plan for the assessment of occurrence, status, and distribution of VOCs in ground water consists of selecting VOC and related data from wells, submitting those data to a national data base, and analyzing and summarizing those data. Data selection consists of an inventory of local, State, and Federal monitoring programs followed by an evaluation of the suitability of data collected in those monitoring programs or well networks for the assessment. VOC and ancillary data subsequently are selected from suitable programs or well networks. Submittal of the data into a national VOC data base is completed in a prescribed format. Once all data are in the national VOC data base, the occurrence, status, and distribution of VOCs will be summarized and presented at aquifer, regional, and national scales.
The purpose of the inventory of local, State, and Federal monitoring programs (see "Supplemental Information" section) is to identify programs in which VOC and associated ancillary data, such as well location, well construction, and hydrogeologic information, are being collected that are suitable for describing the occurrence, status, and distribution of VOCs in major aquifers. The inventory also might identify programs in which VOC and associated ancillary data are being collected for (1) analyses of relations among VOCs in shallow ground water and natural and human factors and (2) analysis of temporal trends of VOCs in ground water.
Concentration data for all VOC analytes available from those on the target list (table 1) need to be compiled. In addition, ancillary data are needed for the National Synthesis to describe and interpret the occurrence, status, and distribution of VOCs in major aquifers across the Nation. Compilation of well location, well construction, hydrogeologic, and other data (table 3) is needed for this analysis to the extent that these ancillary data are available. Data in table 3 were requested in 1995 for compilations being done by the 1994 Study Units during their year of Retrospective Analysis. These data will be entered into a national VOC occurrence data base. Additional information about the data elements listed in table 3 and probable formats for data submittal are given in table 4.
Table 3. Data requested for submittal to the national volatile organic compound(VOC)
occurrence data base for use in assessment of occurrence, status, and
distribution of VOCs in ground water
[VOC, volatile organic compounds; NWIS, U.S. Geological Survey National Water Information
System; rdb, relational data base; GIS, geographic information system; LSD, land-surface
datum; NGVD, National Geodetic Vertical Datum; C, component in U.S. Geological Ground-Water
Site Inventory; P, U.S. Environmental Protection Agency parameter code; N/A, No parameter
code for this compound]
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Description of component, parameter, or other data NWIS component (C) or parameter (P)
code, or other description
-------------------------------------------------------------------------------------------------------------------------------
Cross reference of well to monitoring program As an rdb table
Monitoring program description from VOC inventory table As an rdb table
Cross reference of well to aquifer in which the monitoring well is screened As an rdb table
Description and ancillary information about the aquifer/hydrogeologic unit for which VOC As an rdb table
data are compiled and submitted to the National data base
Boundary of each aquifer/hydrogeologic unit for which VOC data are compiled and submit GIS coverage
ted to the national data base
Source agency code C004
Site ID (station number) C001
Latitude C009
Longitude C010
Local well number C012
District code C006
State code C007
County code C008
Type of site C002
Record classification C003
Altitude of land surface C016 (same as P72000)
Primary use of site C023
Primary use of water C024
Depth of well C028 (same as P72008)
Agency use of site code C803
Locator sequence number C815
Casing material C080
Depth to top of this open interval C083 (same as P72015)
Depth to bottom of this open interval C084 (same as P72016)
Type of openings in this interval C085
Depth to top of interval C091 (same as P72002)
Depth to bottom of interval C092 (same as P72003)
Aquifer code C093
Lithology code C096
Description of material C097
Contributing unit C304
Aquifer-type code C713
Aquifer code C714
Site ID (only needed if the water-quality data are from a different data base than the STAID (same as C001)
well-construction data)
Date DATES
Time TIMES
Agency collecting sample (code number) P00027
Agency analyzing sample (code number) P00028
Well selection criteria, code P84144
Project component, code P84145
Sampler type, code P84164
Sample purpose code P71999
Elevation of land surface datum (LSD, in feet NGVD) P72000 (same as C016)
Depth of hole, total (in feet) P72001
Depth to top of water-bearing zone sampled (in feet) P72002 (same as C091)
Depth to bottom of water-bearing zone sampled (in feet) P72003 (same as C092)
Pump or flow period prior to sampling (in minutes) P72004
Sample source (codes) P72005
Sampling condition (code) P72006
Depth of well, total (in feet) P72008 (same as C092)
Depth to top of sample interval (in feet below LSD) P72015 (same as C083)
Depth to bottom of sample interval (in feet below LSD) P72016 (same as C084)
Depth below land surface to water level (in feet) P72019
VOC-concentration data for all analytes on the target list, as available See table 1 for P codes
(see table 1 for list of target analytes)
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Table 4. Details of data requested for submittal to the national volatile organic
compound (VOC) occurrence data base for use in assessment of occurrence,
status, and distribution of VOCs in ground water
[See footnote 1 for additional data elements required for creation of a site file in
the U.S. Geological Survey Ground-Water Site Inventory (GWSI) data base; QWDATA, U.S.
Geological Survey Water-Quality Data Base; GIS, geographic information system; LSD,
land-surface datum; DD, National Water-Quality Assessment Program (NAWQA) data
dictionary; NWIS, U.S. Geological Survey National Water Information System]
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Suggested storage
Example of data location of data
Data element or other information Source for definition of data element2 element element3
------------------------------------------------------------------------------------------------------------------------------------------------
Monitoring Program/Network and Aquifer Information
Cross reference of well to monitoring See "Supplemental Information" See "Supplemental netsite.rdb file
program Information"
Monitoring program description from do. do. netdes.rdb file
VOC inventory (form B)
Cross reference of well to aquifer in do. do. aqsite.rdb file
which the monitoring well is
screened
Description and ancillary information do. do. aqdes.rdb file
about the aquifer/hydrogeologic unit
for which VOC data are compiled
and submitted to the national data
base
Boundary of each aquifer/hydrogeo Table 17 in DD. GIS coverages: Hydro Upper glacial aquifer GIS coverage
logic unit for which VOC data are geologic unit boundaries polygons
compiled and submitted to the 1:100,000, UNITNAME 40, 40, C:
national data base Name of aquifer or hydrogeologic unit
Well Site and Construction, and Hydrogeologic Data Associated with the Analyte-
Concentration Data
Site (Station) identification number C001 431209100501701 GWSI
Station name C012 (C900 also is automatically populated 120N69W14ACCD or GWSI
when C012 is populated) Cuny well near
Rapid City, SD.
Latitude C009 431209 GWSI
Longitude C010 1005017 GWSI
State C007 SD GWSI
Monitoring program well-selection P84144 in DD 200 QWDATA
criteria
100 for site selected because near/within
local problem area
200 for site selected without regard to
local problem area
Altitude of land surface C016 1000 GWSI
(if readily available)
Use of site C023 O (for observation) GWSI
Use of water (primary use) C024 P (for public supply) GWSI
Aquifer type (for the aquifer that was C713 C (for confined single GWSI
sampled) aquifer)
Primary aquifer (for the aquifer that C714 100CNZC (for Cenozoic) GWSI
was sampled)
Well depth (in feet below land surface) C028 140.5 GWSI
Water level (in depth below land P72019 (In the event that no water level 75.3 QWDATA
surface) and date of measurement was measured at the time of sampling,
use GWSI water-level components -
C237 with date C235. If there are no data
in C237 and C235, use data in C30 and
C31.)
Collecting agency P00027 1028 QWDATA
Analyzing agency P00028 1028 QWDATA
Casing material C080 S (for steel) GWSI
Depth to top of open interval C083 132 GWSI
(perforated or screened, in feet
below LSD)
Depth to bottom of open interval (per C084 137 GWSI
forated or screened, in feet below
LSD)
Type of opening C085 R (for wire-wound GWSI
screen)
Hydrogeologic Data for the Aquifer that was Sampled
Contributing unit (for the aquifer C304 P (for primary) GWSI
that was sampled)
Depth to top of unit (that was sam C091 80 GWSI
pled, in feet below LSD)
Depth to bottom of unit (that was sam- C092 155 GWSI
pled, in feet below LSD)
Unit identifier C093 100CNZC (for Cenozoic) GWSI
Lithology C096 SDGL (for sand and GWSI
gravel)
About 90 possible terms are listed under
this C code to describe the principal
lithology of the aquifer that was sampled.
To the extent possible, without incor
rectly identifying the lithology, catego
rize lithology by broad categories, such
as igneous, metamorphic, sedimentary,
basalt, granite, sandstone, shale, lime
stone, dolomite, sand and gravel, silt and
clay, till.
Lithologic modifier C097 Brownish red GWSI
Hydrogeologic Data Repeated for Major Hydrogeologic Units Overlying the Sampled Aquifer to
Record Hydrogeologic Information About Those Units That Might Affect or Provide Explanation
for Concentrations of VOCs in the Sampled Aquifer
Contributing unit C304 N (for contributes
GWSI
no water)
Depth to top of unit (in feet below C091 15 GWSI
LSD)
Depth to bottom of unit (in feet C092 80 GWSI
below LSD)
Aquifer code (geologic age and C093 100CNZC (for Cenozoic) GWSI
unit identifier)
Lithology C096 CLSD (for clay, some GWSI
sand)
About 90 possible terms are listed under
this C code to describe the principal
lithology of the major hydrogeologic
units overlying the sampled aquifer. To
the extent possible, without incorrectly
identifying the lithology, categorize
lithology by broad categories, such as
igneous, metamorphic, sedimentary,
basalt, granite, sandstone, shale, lime
stone, dolomite, sand and gravel, silt and
clay, till.
Lithologic modifier C097 Grey, soft GWSI
For Each VOC Analyte
Analyte concentration4 P codes (including Remarks code) and P34235 = 0.1(<:::) or QWDATA
value for each analyte. P34235=<0.1 for
benzene dissolved that
If the analyte was not measured, but cannot was measured and
be easily removed from the list of ana was below the
lytes, indicate this missing value as --. quantitation level of
Do not enter nonmeasured values as = 0. 0.1 µg/L.
Concentrations reported as non-detections
(ND) will be of limited value for this
assessment of occurrence, status, and dis
tribution of VOCs in ground water
because of the wide possible range of
quantitation levels that could be associ
ated with an ND. For this reason, it is
requested that an extra effort be made
to determine the quantitation level. If
the analyte was measured but not
detected (ND) or if a nondetection is indi
cated as a zero value, then the value (0 or
ND) should be modified to indicate the
value is less than the quantitation level
for that analyte. If the quantitation level
absolutely cannot be obtained, indicate as
ND.
Date sample was collected At a minimum, the year that the sample was 930723 (for QWDATA
(YearMonthDay) collected should be listed. Include month YYMMDD)
and day, when that information is avail
able.
------------------------------------------------------------------------------------------------------------------------------------------------
1The following additional GWSI data elements - C codes are required for establishing a
site file for entry of data into GWSI; Station locator sequence number - C815; Agency code - C004;
District - C006; County - C008; Agency use - C803; Station type - C802; Data reliability - C003;
Site type - C002.
2Source : C, component number in GWSI; P, parameter code in QWDATA; DD, see NAWQA data
dictionary.
3Suggested storage location: rdb file, relational data base file for submittal to IDB
(NAWQA Interim Data Base), QWDATA, or GWSI in the U.S. Geological Survey's National Water Information
System (NWIS).
4Do not include analyses of quality-control samples.
After the VOC inventory described in "Supplemental Information" (at back of this report) is completed, the suitability of existing VOC data for use in the assessment of occurrence, status and distribution of VOCs in ground water is evaluated by each Study Unit in consultation with the VOC National Synthesis staff. In general, the criteria for selection of wells for NAWQA Study-Unit Surveys (Lapham and others, 1995) should be followed when selecting wells for the assessment described in this report. VOC data are selected for the assessment on the basis of well-network design, well construction, and sample-collection and laboratory-analysis criteria that need to be met for each well (table 5). Selected VOCs that are measured by several different laboratory schedules or methods are listed in table 6. Not all schedules and methods that include measurements of the indicated compounds are included in table 6. Some or all of the VOC data that meet these criteria will be compiled by the 1994 Study-Unit Staff. The same approach will be used by the VOC National Synthesis to compile data from selected local, State, multi-State, and Federal programs not compiled by Study Units.
A well network is best suited for this national assessment of occurrence, status, and distribution of VOCs in ground water if its design results in a generally unbiased, areally distributed, random selection of sampling sites throughout the aquifer or part of the aquifer of interest (table 5). This network design is based on random selection with equal-area distribution (Alley, 1993). Computerized methods for random site selection are presented in Scott (1990).
It is unlikely that many existing monitoring programs follow the ideal design because of the inability to install wells exactly at randomly located sites. Thus, a common compromise is to install wells as close as possible to the randomly located sites. In many monitoring programs, the cost of drilling precludes well installation. In this case, rather than installing wells, existing wells that are located as near as possible to the randomly located site and that meet certain construction criteria are selected for sampling. This approach is used for NAWQA Study-Unit Surveys (Gilliom and others, 1995; Lapham and others, 1995). For the assessment described in this report, deviation from the ideal network design is complicated further by the fact that VOC data must have been collected. Thus existing wells need to be found that collectively result in a network of wells that closely fits the ideal network design, that have been sampled for VOCs, and that meet the other suitability criteria in table 5.
Table 5. Criteria for selection of volatile organic compound (VOC) data for assessment
of occurrence, status, and distribution of VOCs in ground water
----------------------------------------------------------------------
Well-Network Design
? Wells are suitably located in relation to the desired spatial and
depth design.
? All wells or a subset of wells from one or more well networks
collectively result in a generally unbiased, random, equal-area
distribution of sampling sites throughout the aquifer or part of
the aquifer of interest.
Well Construction1
? The hydrogeologic unit (or units) represented by the water level
measured and the hydrologic unit (or units) contributing water
to the well are known.
? The well construction and pumping equipment in the well are
known to be of a type that are not likely to affect concentrations
of VOCs in the water sample.
Criteria for Selection of VOC Data
VOC data are considered suitable for use in the assessment if:
? National methods for collection and analysis of VOCs were
used (information on analytes measured by different methods is
presented later in this report). The method detection limits or
the reporting levels for VOC analytes should be less than about
5 micrograms per liter.
? Laboratory analysis was done by a laboratory certified by the
U.S. Environmental Protection Agency.
? The location of the well from which the sample was collected is
known by latitude and longitude.
? The sample was collected from untreated (raw) water.
? The water sample was collected at or near the well head before
being held in a pressure tank or holding tank.
? The analyte name is identified by P code (see table 1) and the
analyte concentration is known.
? The date (at a minimum, the year) of sample collection is
known. (In general, the latest measured concentration of the
analyte from a well after January 1, 1985, is requested. How
ever, in some cases this criteria may not apply. See "Selection
of a Volatile Organic Compound Analysis From Several Avail
able for a Well" for further guidance.)
? Quality-control data have been used to evaluate and, if neces
sary, to censor the environmental data prior to its compilation
at a national level to the extent that quality-control data are
available.
----------------------------------------------------------------------
1See Lapham and others (1995) for discussion of possible effects of well construction
on the chemistry of a water-quality sample.
Table 6. Selected volatile organic compounds (VOCs) measured by indicated U.S.
Geological Survey National Water-Quality Laboratory schedules and U.S.
Environmental Protection Agency methods1
[NWQL, U.S. Geological Survey National Water-Quality Laboratory; U.S. EPA, U.S.
Environmental Protection Agency; CAS, Chemical Abstracts Services; RCRA, Resource
Conservation and Recovery Act; x, analyte measured by that schedule or method; VOC,
Volatile Organic Compound; GC/MS Gas Chromatography/Mass Spectrometry]
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
NWQL | ____________________________________________________U.S. EPA method numbers_____________________________________________________________ |
schedule Ground water, soil,
CAS Compound | _________numbers________________ | _______________________________ Drintking water_____________________________________ | ___RCRA___ | __________and so forth_____________ |
number name 2090 1380 9090 1307 1401 502.1 502.2 503.1 504 505 524.1 524.2 525.1 603 611 612 625 1624 1625 8240 8270 8010 8015 8020 8030 8100 8120
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
56-23-5 Tetrachloromethane (carbon x x x x x x x x x x
tetrachloride)
67-66-3 Trichloromethane (chloroform) x x x x x x x x x x
67-72-1 Hexachloroethane x x x x x x
71-55-6 1,1,1-Trichloroethane x x x x x x x x x x
74-83-9 Bromomethane x x x x x x x x x
74-87-3 Chloromethane x x x x x x x x x
75-00-3 Chloroethane x x x x x x x x x
75-09-2 Dichloromethane (methylene x x x x x x x x x x
chloride)
75-25-2 Tribromomethane (bromoform) x x x x x x x x x x
75-27-4 Bromodichloromethane x x x x x x x x x x
75-34-3 1,1-Dichloroethane x x x x x x x x x x
75-69-4 Trichlorofluoromethane (CFC 11) x x x x x x x x x x
75-71-8 Dichlorodifluoromethane (CFC 12) x x x x x x x x x
76-13-1 1,1,2-Trichloro-1,2,2- x x x x
trifluoroethane (CFC 113)
78-87-5 1,2-Dichloropropane x x x x x x x x x x
79-00-5 1,1,2-Trichloroethane x x x x x x x x x
96-12-8 1,2-Dibromo-3-Chloropropane x x x x x x x x
(DBCP)
96-18-4 1,2,3-Trichloropropane x x x x x x x x
106-93-4 1,2-Dibromoethane (EDB) x x x x x x x x x
107-06-2 1,2-Dichloroethane x x x x x x x x x x
124-48-1 Dibromochloromethane x x x x x x x x x
75-01-4 Chloroethene (vinyl chloride) x x x x x x x x x x
75-35-4 1,1-Dichloroethene x x x x x x x x x x
79-01-6 Trichloroethene (TCE) x x x x x x x x x x x
87-68-3 1,1,2,3,4,4-Hexachloro-1,3- x x x x x x x x x
Butadiene
127-18-4 Tetrachloroethene x x x x x x x x x x x
156-59-2 cis-1,2-Dichloroethene x x x x x x x x
156-60-5 trans-1,2-Dichloro-ethene x x x x x x x x x x
593-60-2 Bromoethene x
10061-01-5 cis-1,3-Dichloro-1-propene x x x x x x x x
10061-02-6 trans-1,3-Dichloro-1-propene x x x x x x x x
71-43-2 Benzene x x x x x x x x x x
91-20-3 Naphthalene x x x x x x x x
100-42-5 Styrene (vinyl benzene) x x x x x x x x x
95-47-6 1,2-Dimethylbenzene (o-xylene) x x x x x x x x x
95-63-6 1,2,4-Trimetylbenzene x x x x x x
98-82-8 Cumene (isopropylbenzene) x x x x x x
100-41-4 Ethylbenzene x x x x x x x x x x
103-65-1 n-Propylbenzene x x x x x x
104-51-8 n-Butylbenzene x x x x x x
106-42-3 1,4-Dimethylbenzene (p-xylene) x x x x x x x x x
108-38-3 1,3-Dimethylbenzene (m-xylene) x x x x x x x x x
108-88-3 Methylbenzene (toluene) x x x x x x x x x x
87-61-6 1,2,3-Trichlorobenzene x x x x x x
95-50-1 1,2-Dichlorobenzene x x x x x x x x x x x x x
106-46-7 1,4-Dichlorobenzene x x x x x x x x x x x x x
108-90-7 Chlorobenzene x x x x x x x x x x x x
120-82-1 1,2,4-Trichlorobenzene x x x x x x x x
541-73-1 1,3-Dichlorobenzene x x x x x x x x x x x x x
108-20-3 Diisopropyl ether (DIPE) x
111-44-4 Bis (2-chloroethyl) ether x x x x
123-91-1 1,4-Dioxane x x
637-92-3 Ethyl tert-butyl ether (ETBE) x
762-75-4 tert-Butyl formate (TBF)
994-05-8 tert-Amyl methyl ether (tame), x
(2-Methoxy-2-methyl-butane)
1634-04-4 Methyl tert-butyl ether (MTBE) x x x
107-02-8 2-Propenal (acrolein) x x x x x x x
107-13-1 2-Propenenitrile (acrylonitrile) x x x x x x x
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
1Not all schedules and methods that include measurements of the indicated compounds are included in this table.
NOTES:
The xylene concentrations are reported as totals using U.S. EPA methods.
Method 502.1: Title: Halogenated VOCs in Water. Matrix: Drinking water (finished
or any treatment stage) and raw source water.
Method 502.2: Title: Halogenated VOCs in Water. Matrix: Drinking water (finished
or any treatment stage) and raw source water.
Method 503.1: Title: Aromatic and Unsaturated VOCs in Water. Matrix: Drinking water
(finished or any treatment stage) and raw source water.
Method 504: Title: 1,2-Dibromoethane and Dibromochloropropane in Water by
Microextraction. Matrix: Finished drinking water and unfinished
ground water.
Method 505: Title: Organohalide Pesticides and PCBs.
Method 524.1: Title: VOCs in Water by Purge and Trap GC/MS. Matrix: Drinking water
(finished or any treatment stage) and raw source water.
Method 524.2: Title: VOCs in Water by Purge and Trap GC/MS. Matrix: Drinking water
(finished or any treatment stage) and raw source water.
Method 525.1: Title: General Purpose Organics.
Method 603: Title: Acrolein/Acrylonitrile.
Method 611: Title: Haloethers.
Method 612: Title: Chlorinated Hydrocarbons.
Method 625: Title: Acids (Phenols), Base/Neutral Organochloride Pesticides and PCBs.
Method 1624: Title: VOCs by Isotope Dilution GC/MS.
Method 1625: Title: Semi-Volatile Organic Compounds by Isotope Dilution GC/MS.
Method 8010: Title: Halogenated Volatile Organics. Matrix: Ground water, soils,
sludges (water miscible liquid wastes and nonwater miscible wastes).
Method 8015: Title: Nonhalogenated Volatile Organics. Matrix: Ground water, soils,
sludges (water miscible liquid wastes and nonwater miscible wastes).
Method 8020: Title: Aromatic Volatile Organics. Matrix: Ground water, soils, sludges
(water miscible liquid wastes and nonwater miscible wastes).
Method 8030: Title: Other Nonhalogenated VOCs. Matrix: Ground water, soils, sludges
(water miscible liquid wastes and nonwater miscible wastes).
Method 8100: Title: Polynuclear Aromatic Hydrocarbons. Matrix: Ground water, soils,
sludges (water miscible liquid wastes and nonwater miscible wastes).
Method 8120: Title: Chlorinated Hydrocarbons. Matrix: Ground water, soils, sludges
(water miscible liquid wastes and nonwater miscible wastes).
Method 8310: Title: Polynuclear Aromatic Hydrocarbons. Matrix: Ground water, soils,
sludges (water miscible liquid wastes and nonwater miscible wastes).
Method 8240: Appendix IX Volatiles (RCRA).
Method 8270: Appendix IX Semivolatile (RCRA).
Well networks used in monitoring programs for which objectives are different from the objectives for this assessment might be based on a design other than unbiased, areally distributed, random selection of sampling sites. Nevertheless, monitoring programs designed either for similar assessments of VOCs or for other objectives still might be considered if, by selecting all or a subset of wells from one or more of these programs, the resulting network collectively produces a network design based on random selection with equal-area distribution.
Table 7. Examples of monitoring-program or well-network characteristics,
requested data compilation, and anticipated analysis of the data
[VOCs, volatile organic compounds]
--------------------------------------------------------------------------------------------------------------------------------
Amount of
Distribution of data that
wells across are readily Requested
Total number the hydro- available number of Requested
Example of wells geologic unit/ in an wells for amount of
monitoring in the aquifer/part of electronic which data data1
program network(s) the aquifer data base are compliled compliled Anticipated analysis of data
--------------------------------------------------------------------------------------------------------------------------------
1 Much greater Approximate All. All wells in All. Aquifer- and national-scale analyses of
than 100. equal-area the network. occurrence, status, and distribution of
distribution. VOCs.
2a do. do. Very little. All wells in Data in the Aquifer- and national-scale analyses of
the network. electronic occurrence, status, and distribution of
or data base. VOCs.
2b do. do. do. About 100 As much as Aquifer- and national-scale analyses of
wells possible. occurrence, status, and distribution of
randomly VOCs to the extent supported by the
selected available ancillary data.
with equal-
area
distribution.
3 do. Highly All do. All. Aquifer- and national-scale analyses of
variable. occurrence, status, and distribution of
VOCs.
4 do. do. Very little. do. As much as Aquifer- and national-scale analyses of
possible. occurrence, status, and distribution of
VOCs to the extent supported by the
available ancillary data.
5 Much less Approximate All. As many All. National-scale analyses of occurrence,
than 100. equal-area wells as status, and distribution of VOCs.
distribution. possible.
6 do. Highly variable. Very little. do. As much as National-scale analyses of occurrence,
possible. status, and distribution of VOCs to
the extent supported by the available
ancillary data.
--------------------------------------------------------------------------------------------------------------------------------
1Data for assessment of occurrence, status, and distribution of VOCs in ground water
described in tables 3 and 4.
Different approaches for design of a well network from one or more monitoring programs might be necessary to fit local situations. For purposes of illustration, six hypothetical situations (table 7) are described in the following paragraphs with suggested approaches for selection of wells and data compilation for this assessment. The approaches are intended to result in networks of wells that are based on random selection with equal-area distribution (table 5). The monitoring-program/well-network characteristics described as example monitoring programs 1 and 6 of table 7 cover anticipated end members of types of monitoring programs/well networks for which data might be compiled. It is assumed that the criteria for selection of VOC data in table 5 are met in all examples.
Example 1 in table 7 describes a ground-water-quality monitoring program designed to describe the occurrence and distribution of VOCs. The well network consists of a large number of wells that are distributed uniformly throughout the aquifer of interest. Sampling sites were selected randomly by equal-area distribution with respect to both location and depth in the aquifer, and wells were installed at those sites. A broad suite of VOCs were analyzed in water from all wells. Nondetections are reported as values less than the quantitation level. Extensive ancillary data about each well are in an electronic data base. All wells in the network are selected for compilation of all available VOC and ancillary data requested by the VOC National Synthesis (table 3).
Example 6 in table 7 describes a situation in which one or more well networks are used. Each network consists of small numbers of wells, distributed vertically and horizontally throughout the aquifer of interest. Some networks might be biased toward a specific well type, for example, public-supply, domestic-supply, or wells installed at or near known sources of contamination. The total number of wells available is small, and the distribution of wells across the aquifer is highly variable. Few VOC analyses and (or) little ancillary data are available. The approach is to select as many wells as possible from the combined monitoring programs such that there is a reasonably equally distributed number of randomly selected wells across the aquifer.
Well networks that have the characteristics described in example 6 will be the least useful of the six listed in table 7 for this assessment because of the small number of wells and the small amount of ancillary information available for each well. Data from these programs will be useful in aggregate, but little or no interpretation of the occurrence, status, and distribution of VOCs for an individual aquifer will be possible. Nevertheless, compilation of data from these programs is preferred to no compilation at all.
Well construction, including construction materials, the design of the well, and (or) installation methods can result in a well being unsuitable for sampling of targeted water-quality constituents, such as VOCs (Lapham and others, 1995). For example, glued PVC monitoring wells, to the extent possible, should not be selected as part of a well network for this assessment because organic compounds, such as tetrahydrofuran, methylethylketone, methylisobutylketone, and cyclohexanone, can leach from the glue used to bond unthreaded polyvinylchloride casing. Wells that are screened in several units, that contain multiple screens in different units, or that have long well screens make determination of the source of water to the well difficult. Therefore, wells with these types of design should not be selected, if possible. Oil, grease, and other foreign materials on drilling and associated equipment can be introduced to water-bearing units during drilling, well completion, and well development if not removed from the equipment prior to its use. This potential for contamination needs to be considered when gathering and interpreting information about candidate wells. Low-capacity wells should be selected in preference to high-capacity wells because high-capacity wells can draw water from units other than the unit of interest.
It is important to document information regarding construction of a well when submitting the VOC data to the national VOC occurrence data base (table 3), particularly for those wells where construction likely might affect concentrations of VOCs in the water sample.
An additional complication to the assessment described in this report, which can result from using a network of existing wells, is that well type can bias the results (Alley, 1993; Gilliom and others, 1995; Lapham and others, 1995). For example, although pre-screening of water quality prior to well installation of small public-supply wells probably is not done, pre-screening of water quality from test wells might be performed prior to installing large municipal water-supply wells. A result of pre-screening would be the absence of large municipal supply wells in areas of ground-water contamination. In addition, public-supply wells in areas of contaminated ground water might be removed from service once a contaminant is detected. Consequently, well networks that consist only of large municipal water-supply wells could result in a bias in the assessment toward areas of aquifers that are not contaminated with VOCs. Conversely, well networks for this assessment that consist only of wells installed immediately downgradient of sources of known contamination could result in a biased interpretation that the aquifer is largely contaminated with VOCs.
One of the most common types of wells sampled for VOCs are monitoring wells installed at and near sites of known contamination. Therefore, this well type probably will be one of the most common types from which VOC data are available. Because the focus of this assessment is on aquifers, or those parts of aquifers, that are currently used or have the potential to be used as sources of water supplies, selection of monitoring wells that were installed at or immediately downgradient of sources of known contamination would not be appropriate. However, wells installed upgradient of these sources to determine "background" water quality in the aquifer might be selected.
Wells should be selected to minimize bias attributable to well type. Often one of several alternative wells of different types that have been sampled for VOCs could be selected at or near a randomly located site. If several wells are available near a randomly located site, the established priority for selecting a well (table 8) is applied. In these situations, the well selected should be the one that is most likely to have been installed with the least regard to the presence or absence of VOCs. Thus, monitoring wells installed to meet the objective of the assessment described in this report, or wells installed to meet other objectives, but without regard to the presence or absence of VOCs, have a higher priority for selection than wells installed in areas where it is assumed that VOCs are absent or than wells installed in areas where data indicate that VOCs are present (table 8). For example, in a situation where the alternative is to select either a domestic well or a large municipal-supply well, the domestic well probably is the one most likely to have been installed with the least regard to the presence or absence of VOCs and would be the well selected for this assessment.
In some cases, a well might have been sampled multiple times. In these cases, a single VOC analysis is selected to represent the well. The decision of which VOC analysis to select is based on the objectives and sampling design of the monitoring program in which the well is located. Following are examples of monitoring programs in which wells are sampled multiple times, and the suggested approaches for selecting an analysis to represent each well in that program. The intent of the suggested approaches is to minimize bias in the analyses selected with respect to describing the occurrence and distribution of VOCs in aquifers. Other approaches might need to be developed by Study Units for specific monitoring programs. In these cases, an approach should be developed that minimizes bias in the data with respect to the occurrence-and-distribution objective. The Study Unit needs to document that approach for future reference.
Table 8. Suggested priority for well selection for assessment of occurrence,
status,and distribution of volatile organic compounds (VOCs) to minimize bias
attributableto well type
---------------------------------------------------------------------
Well selection based on type, from highest (1)
to lowest (4) priority
---------------------------------------------------------------------
(1) Wells that were installed without regard to the presence or
absence of VOCs, such as monitoring wells installed based
on random selection of sites with equal-area distribution
throughout an aquifer.
(2) Wells that were installed for objectives other than assessment
of the occurrence and distribution of VOCs, but were
installed without regard to the presence or absence of VOCs,
such as observation wells installed for water-level measure
ments, public-supply wells installed without pre-screening
for VOCs, and some domestic- and industrial-supply wells.
(3) Wells that were installed at locations where it is assumed that
VOCs are absent, such as test wells for locating future water
supplies.
(4) Wells that were installed in areas where data indicate that
VOCs might be present, such as monitoring wells installed
upgradient of sources of known contamination to measure
"background" conditions.
---------------------------------------------------------------------
In some monitoring programs, all wells are sampled at about the same time (synoptically) and at the same frequency. For example, every well in the network is sampled every year in October. In this case, the most recent analysis is selected. However, in all cases, analyses collected prior to January 1985 are not selected. The reason for this is to obtain the most recent data within the last decade describing the occurrence and distribution of VOCs for all aquifers studied. In some monitoring programs, the frequency of sampling might depend on the detection of VOCs in the well. For example, the frequency of sampling might be increased (for example, to quarterly sampling) if a VOC is detected, whereas other wells in the monitoring program would continue to be sampled at a lower, routine frequency (for example, once every year). In this case, it is suggested that the latest measurement after January 1, 1985, at the low, routine frequency be selected. In some monitoring programs, a "stopping rule" is used to determine if sampling of a well can be discontinued. In this example, sampling is continued only as long as VOCs are detected. If the latest analysis is selected, results will be biased toward nondetections. In this case, it is recommended that one analysis from the well be selected randomly from all analyses made after January 1, 1985.
Quality-control (QC) samples might be included as part of some monitoring programs used for National Synthesis of VOCs. QC samples for VOCs include equipment blanks, field blanks, trip blanks, and field-spiked, replicate samples. Bias and precision measurements from these QC samples reflect combined on-site and laboratory errors that occur during data collection. Other QC samples might be collected to estimate errors associated with a specific on-site or laboratory procedure. To the extent that data from QC samples are available, these data need to be used to evaluate and document the quality of the environmental data.
Data from QC samples are used to make general inferences about bias and measurement precision for selected VOC analytes (Koterba and others, 1995). QC data can be used to demonstrate the effectiveness of equipment-decontamination procedures, to measure changes that occur in analyte concentrations from time of collection to time of analysis, and to support the decision to censor environmental data. For example, use of QC data to isolate and eliminate sources of sample contamination or bias that occur as a result of sample collection and processing are described in Koterba and others (1991) and Christenson and Rea (1993). Use of the magnitude of error estimates from QC data to provide an indication of the quality of ground-water data collected is described in Koterba and others (1991; 1993).
The magnitudes of the method detection limits or reporting levels for VOC analytes are criteria for selection of an analysis. The method detection limit or the reporting level for a VOC analyte should not be greater than about 5 µg/L (micrograms per liter) for that data to be selected for use by the National Synthesis (table 5).
Confusion about differences between instrument detection limits, method detection limits, and reporting levels warrants brief repetition here of a discussion in U.S. Geological Survey (1994). An instrument detection limit is an analyte's mass equivalent that would correspond to a signal equal to three times the standard deviation of a series of 10 replicate measurements of a reagent blank signal (Currie, 1988; U.S. Geological Survey, 1994). This detection limit is for a pure analyte and refers only to the sensitivity of the instrument. This is the lowest detection limit that can be achieved and is usually the one reported by instrument manufacturers.
The method detection limit (MDL) is the minimum concentration of a substance that can be measured and reported with 99-percent confidence that the analyte concentration is greater than zero and is determined from analysis of a sample in a matrix containing the analyte (U.S. Environmental Protection Agency, 1992). The 99-percent confidence is defined as three times the standard deviation calculated from seven replicate analyses of a matrix spiked at a concentration of 2.5 to 5 times the instrument signal-to-noise ratio, or 1 to 5 times the estimated detection limit. In this definition of MDL, the analyte has been through all steps of the method (extraction, isolation, analysis). In addition, the MDL is matrix specific. Given these matrix effects and the unavoidable analyte losses throughout the analytical procedure, the MDL can be 2 to 100 times higher than the instrument detection limit.
The reporting level cited by the U.S. Geological Survey's National Water-Quality Laboratory (NWQL) is the concentration that the laboratory feels it can reliably report, regardless of varying sample matrices in a given medium. In some cases, the NWQL reporting levels are lower than their corresponding U.S. EPA MDL's for regulated analytes. The choice of a reporting level reflects many variables, not all of them quantifiable. For some schedules, the NWQL might report reporting levels equal to MDL's. Also, for some new methods, for those analytes for which identity is assured, the NWQL will report all concentrations determined that are greater than the instrument detection limit (uncensored) whether or not they are less than, equal to, or greater than the MDL.
Submittal of the VOC and ancillary data to the national VOC occurrence data base is accomplished through a formal request. This request was made in the summer of 1995 for data compiled by the 1994 Study Units from their Retrospective Analysis. The request was similar to the environmental and quality-control data requests made to the 1991 Study Units by National Synthesis in 1994. Detailed instructions describing the submittal procedure and format were provided at the time of the data request.
VOC concentration and ancillary data at each well that are requested are listed in table 3. Additional information about the VOC concentration and ancillary data requested is provided in table 4. In addition, general information about the monitoring programs/networks from which data were compiled and about the aquifers/hydrogeologic units investigated are requested. This general information is requested as rdb files similar to those in tables 9-12. Table 9 is a cross reference between wells for which VOC data are submitted and monitoring program(s) from which these wells were sampled; the information in table 10 describes each inventoried monitoring program in table 9; table 11 is a cross reference between wells for which VOC data are submitted and the aquifer in which each well is screened; and table 12 describes information about each aquifer in table 11. The information in table 12 will be used for reporting on general characteristics of the aquifers studied and for grouping of aquifers with common characteristics during analysis.
The amount of VOC and ancillary data available will determine the extent of the description and interpretation of the occurrence, status, and distribution of VOCs in major aquifers across the Nation (table 7). Example programs that have the characteristics described in example 1 of table 7 will be the most useful types of programs for analysis by the VOC National Synthesis because of the large number of wells sampled and the large amount of ancillary data known about each well. A large number of analyses of VOCs from an aquifer is desirable because the percentage of an aquifer contaminated by VOCs is likely to be small. The large amount of ancillary data is desirable because more data will enable more extensive analysis at the aquifer scale---for example, analysis of occurrence, status, and distribution of VOCs in relation to factors such as depth to the water table, well depth, and well type. The situation described in example 6 of table 7 is probably a common situation that might be encountered. However, programs that have the characteristics described in example 6 will be the least useful for the assessment of those listed in table 7 because of the small number of VOC analyses and the small amount of ancillary information available about each well. Data from these programs will be useful in a national aggregation, but little or no interpretation of occurrence of VOCs for an individual aquifer will be possible. Nevertheless, compilation of data from these programs is preferred to no compilation at all.
Table 9. Example of a relational data-base file (called netsite.rdb) that identifies each well, for which volatile
organic compound (VOC) analyte-concentration data are submitted, in relation to the monitoring
program/network from the VOC data-inventory tables (see "Supplemental Information"section)
[Information in this table is for illustrations purposes only; GWSI, U.S. Geological Survey Ground-Water Site
Inventory]
----------------------------------------------------------------------------
# netsite.rdb
# Tab-delimited RDB table for each well submitted to the VOC occurrence data base.
# Gives STAID for each well for which VOC data are submitted and NETCODE value
# identifying monitoring program(s) in which the well was sampled from Study Unit
# VOC data-inventory table (See "Supplemental Information" section).
#
# STAID = Station identification number of well in the network (GWSI C001)
# NETCODE = Network code value or prefix that identifies the monitoring-program number
# recorded in the Study Unit VOC Inventory Table (see "Supplemental Information" section).
# This number is the four-letter Study-Unit identification followed by an arbitrary sequence
# number assigned by the Study Unit; for example, LINJ03.
#
#
STAID NETCODE
15s 8s
450715074230101 LINJ01
450715074230101 LINJ03
450715074230101 LINJ05
452043074445601 LINJ02
450715072301102 LINJ02
450715072301102 LINJ03
450715072301102 LINJ05
451134074012301 LINJ02
441220074221102 LINJ02
450715074230101 LINJ03
451043074445701 LINJ03
450725072301202 LINJ03
451434074013301 LINJ03
451434074013301 LINJ05
441220074221102 LINJ05
450715074230101 LINJ05
461043074446701 LINJ05
460725072301202 LINJ05
451734074012901 LINJ05
. .
. .
. .
----------------------------------------------------------------------------
Table 10. Example of a relational data-base file (called netdes.rdb) that describes
general information about each monitoring program in the volatile organic
compound (VOC) data-inventory tables for which VOC analyte-concentration data
are submitted
[Information in this table is for illustrations purposes only; RCRA, Resource Conservation
and Recovery Act; GWSI, U.S. Geological Survey Ground-Water Site Inventory]
---------------------------------------------------------------------------------------------------------------
# netdes.rdb
# Tab-delimited rdb table of descriptions of MONITORING PROGRAMS/WELL NETWORKS
# from which VOC and associated ancillary data were compiled for the national
# Retrospective Analysis of occurrence, status, and distribution of VOCs in
# ground water. This information is from the VOC data-inventory tables (see
# "Supplemental Information" section). Additional information is compiled about
# these programs/networks in the VOC data-inventory tables.
#
#
# NETCODE = Network code value or prefix that identifies the monitoring-program number
# recorded in the Study Unit VOC data-inventory tables. This number is the four-
# letter Study-Unit identification followed by an arbitrary sequence number assigned
# by the Study Unit; for example, LINJ03.
# NETDESC = Descriptive text describing monitoring program.
# STATE = State(s) covered by the monitoring program (use two-letter postal State
# abbreviations, in caps; if program covers more than one State, separate States by
# commas, for example, NY,NJ).
# DEPT = Department conducting the program.
# EXTENT = Areal extent of program (if statewide, give State name; if county-wide,
# give county name; if aquifer-wide, give aquifer name; RCRA-site name; and
# so forth).
# SQMILES = Area covered by the program, in square miles.
# BEGINYR = Year monitoring program began.
# ENDYR = Year monitoring program ended or `- year'(for example `-1995') if program
# is ongoing (use four digits for year; do not enter future dates).
# USE = Primary use(s) of water from the wells in the network (use GWSI C24 codes)
# as entered in the VOC data-inventory tables, in order of largest to smallest use
# (separate uses by commas, for example, H, I, N, P).
# NUMSITE = Number of sites (wells) sampled in the network (VOC data are submitted
# for all or for a subset of these total number of sites).
#
NETCODE NETDESC STATE DEPT EXTENT SQMILES BEGINYR ENDYR USE NUMSITE
8s 132s 8s 12s 12s 8n 15s 10s 12s 8n
LINJ02 Aquifer monitoring program since early 1980's in Nassau Cty using monitoring wells NY,NJ Nassau Cty Nassau Cty
200 1981 -1995 P 450
LINJ04 Monitoring of domestic supply wells since 1986 NJ Ocean Cty Health Dept 500 1986 1986 H 10000
---------------------------------------------------------------------------------------------------------------------------------------
Table 11. Example of a relational data-base file (called aqsite.rdb) that
identifies each well, for which volatile organic compound (VOC) analyte-
concentration data are submitted, in relation to the aquifer in which the
monitoring well is screened
[Information in this table is for illustrations purposes only; GWSI, U.S. Geological Survey
Ground-Water Site Inventory]
--------------------------------------------------------------------------------------------------------
# aqsite.rdb
# Tab-delimited rdb table for each well submitted to the VOC occurrence data base.
# Gives HYDROGEO-UNIT NAME identifying aquifer associated with each well for
# which VOC data are submitted to the national data base.
#
# STAID = Station identification number of the well (GWSI C001)
# HYDROGEO-UNIT NAME = Short name assigned by the Study Unit that identifies the
# aquifer (or hydrogeologic unit or subunit) in which each well is screened
# and for which VOC data are submitted.
#
# To the extent possible, the National Synthesis objective is to describe the
# occurrence, status, and distribution of VOCs by aquifer and to summarize these
# data nationally. In some cases, the amount of existing VOC data might not be
# sufficient to warrant description at the aquifer scale (for example, analyses
# from a minimum of about 20 to 30 wells distributed throughout the aquifer), but
# data might be sufficient to describe VOC occurrence at a more regional scale; for
# example, by a hydrogeologic unit that consists of a group of aquifers, or by a
# subunit defined by the Study Unit on the basis of stratification by physiographic
# division and geology.
#
#
STAID HYDROGEO-UNIT NAME
15s 20s
450715074230101
450715074230102
450715074230103
452043074445601
450715072301102
450715072301103
450715072301104
451134074012301
441220074221102
450715074230101
451043074445701
450725072301202
451434074013301
451434074013302
441220074221102
450715074230101
461043074446701
460725072301202
451734074012901
. .
. .
. .
--------------------------------------------------------------------------------------------------------
Table 12. Example of a relational data-base file (called aqdes.rdb) that
describes information about each aquifer/hydrogeologic unit for
which volatile organic compound (VOC) analyte-concentration data
are submitted
[Information in this table is for illustrations purposes only; GWSI, U.S. Geological Survey
Ground-Water Site Inventory]
---------------------------------------------------------------------------------------------------------
# aqdes.rdb
# Tab-delimited rdb table of descriptions of aquifers (hydrogeologic units or
# subunits) for which VOC and associated ancillary data were compiled.
# This table describes properties of each aquifer (or hydrogeologic unit or subunit)
# as a whole, not at individual wells. This information in this table is repeated
# for each aquifer.
# For example, if data are submitted for three aquifers, this table would have three
# rows of
# information, one row for each aquifer.
#
# Following is a list of information for each aquifer/hydrogeologic unit to be included
# in an rdb table. The format is the: VARIABLE NAME (field length and type) =
# Description of requested information [example of how information is coded].
#
# HYDROGEO-UNIT NAME (20s) = Short name assigned by the Study Unit that identifies
# the aquifer (or hydrogeologic unit or subunit) in which each well is screened,
# and for which VOC data are submitted [Canoe River aquifer].
# AQDES (132s) = Descriptive text describing the aquifer (or hydrogeologic unit
# or subunit)
# [Unconfined, stratified-drift aquifer composed of sand and gravel up to 150
# feet thick].
# LITH (4s) = Lithologic description of the aquifer/hydrogeologic unit as a whole
# (use definitions in GWSI C096) [SDGL].
# CONSOL (2s) = Degree of consolidation of the aquifer/hydrogeologic unit (UC,
# unconsolidated; SC, semiconsolidated; CN, consolidated) [UC].
# ATC (1s) = Aquifer type code for aquifer/hydrogeologic unit as a whole using the
# definitions in GWSI (Ground-Water Site Inventory - C713: U, Unconfined
# single aquifer; N, unconfined multiple aquifers; C, confined single aquifer;
# M, confined multiple aquifers; X, Mixed (confined and unconfined) multiple
# aquifers) [U].
# DPTHTOP (10s) = Approximate range in depth to top of (saturated part of) aquifer,
# in feet below land surface [0-20].
# SATTHK (10s) = Approximate range in saturated thickness, in feet [0-120].
# K (10s) = Approximate range in hydraulic conductivity, in feet per day [50-350].
# T (10s) = Approximate range in transmissivity, in thousands of feet squared per
# day [0-300].
# RECH (5n) = Approximate average (annual) recharge to the aquifer, in inches per
# year [22].
# AQAREA (6n) = Area of aquifer/hydrogeologic unit, in thousands of square
# miles [2000].
# PUMPT (5n) = Total pumping from the aquifer in 1990, in millions of gallons per
# day [12].
# PPWS (5n) = Percentage of PUMPT used for public drinking-water supply [75].
# PDWS (5n) = Percentage of PUMPT used for domestic drinking-water supply [20].
# PIC (5n) = Percentage of PUMPT used for industrial and commercial water supply [5].
# PIR (5n) = Percentage of PUMPT used for irrigation [0].
# POPT (5n) = Total population served by pumping from the aquifer in 1990, in
# thousands [3200].
# PPOPP (5n) = Percentage of POPT served by public water supply [50].
# PPOPD (5n) = Percentage of POPT served by domestic water supply [50].
# AQSTATE (12s) = State(s) that the aquifer covers, use two-letter postal abbreviations,
# in caps) [NY,NJ].
# PRECIP (6n)= Average annual precipitation over the aquifer, in inches per year [45].
# LANDUSES (25s) = Predominate land uses overlying the aquifer (use Level II-
# classifications in Anderson and others, 1976, table 2). For example, for cropland
# and pasture, use code 221) [111-117, 221, 223].
# POPUL (6n) = Population overlying the aquifer, in thousands [5000].
---------------------------------------------------------------------------------------------------------
Volatile organic compounds (VOCs) were selected for National Synthesis by NAWQA because of the widespread occurrence of this constituent group in many of the Nation's water supplies. The broad goals of the VOC National Synthesis are (1) to describe current water-quality conditions; (2) to define trends, or lack of trends; and (3) to identify, describe, and explain causal relations among the occurrence and distribution of VOCs in surface water and ground water and natural and human factors. Fifty-five VOCs, including of halogenated alkanes, halogenated alkenes, aromatic hydrocarbons, alkyl benzenes, halogenated aromatics, ethers, and others, have been identified for study.
VOCs are of concern in ground water because of their relatively high aqueous solubility, mobility, and persistence, because many VOCs are known or suspected carcinogens, and because of their widespread use. The National Synthesis of VOCs in ground water has three objectives: (1) describe their occurrence, status, and distribution; (2) determine relations between VOC target analytes in shallow ground water and natural and human factors; and (3) determine and compare the occurrence, transformation, transport, and fate of selected VOC target analytes in ground water for several regionally or nationally important aquifer systems.
The objective of the assessment of VOCs in ground water discussed in this report is to describe the occurrence, status, and distribution of VOCs in major aquifers of the United States, both for individual aquifers and collectively. The assessment will be nationwide for aquifers, or those parts of aquifers, that are currently used or have the potential to be used as sources of water supplies. Occurrence describes the presence or absence of VOCs, their frequency of occurrence, and their ranges of concentrations. Status compares the concentrations of VOCs detected in relation to water-quality regulations or advisories, such as Maximum Contaminant Levels, Proposed Maximum Contaminant Levels, Maximum Contaminant Level Goals, and Health Advisories. Distribution describes the variability of VOCs in ground water, areally and by depth.
A set of criteria for well-network design, well construction, sample-collection methods, and methods of laboratory analysis need to be met for data to be used in this assessment. A well network suitable for this assessment is one in which all wells or a subset of wells from one or more well networks collectively results in a generally unbiased, random, equal-area distribution of sampling sites throughout the aquifer, or part of the aquifer, of interest. Well-construction information must be sufficient to ensure that the hydrogeologic unit (or units) represented by the water level measured and the hydrologic unit (or units) contributing water to the well are known. In addition, the well construction and pumping equipment in the well need to be of a type that are not likely to affect concentrations of VOCs in the water sample. VOC data are considered suitable for use in this assessment if: (1) national methods for collection and analysis of VOCs were used and the method detection limits or the reporting levels for VOC analytes are less than about 5 µg/L; (2) laboratory analysis was done by a laboratory certified by the U.S. EPA; (3) the location of the well from which the sample was collected is known by latitude/longitude; (4) the sample was collected from untreated (raw) water; (5) the water sample was collected at or near the well head before being held in a pressure tank or holding tank; (6) the analyte name is identified by parameter code, and the analyte concentrations are known; (7) the date (at a minimum, the year) of sample collection is known; and (8) quality-control data have been used to evaluate and, if necessary, to censor the environmental data prior to its compilation at a national level to the extent that quality-control data are available
This assessment will be done by collective analysis of data from monitoring programs across the Nation. Data from these monitoring programs will be compiled into a national data base for this analysis. Criteria for monitoring-program design, sample-collection methods, and methods of laboratory analysis will be met for data to be entered into this data base. The assessment will be based on data collected since 1985, for as many of the 55 VOCs currently (1996) on the list of NAWQA VOC target analytes as possible. Assessment by aquifer and comparisons of results among aquifers also will be completed for those aquifers that have adequate data, spatially or by depth.
- Alley, W.M., ed., 1993, Regional ground-water quality: New York, Van Nostrand Reinhold, 634 p.
- Anderson, J.R., Hardy, E.E., Roach, J.T., and Witmer, R.E., 1976, A land use and land cover classification system for use with remote sensor data: U.S. Geological Survey Professional Paper 964, 28 p.
- Christenson, Scott, and Rea, Alan, 1993, Ground-water quality in the Oklahoma City urban area, in Alley, W.M., ed., Regional ground-water quality: New York, Van Nostrand Reinhold, chap. 24, p. 589-611.
- Currie, L.A., 1988, Detection -- Overview of historical, societal, and technical issues, in Currie, L.A., ed., Detection in analytical chemistry--Importance, theory, and practice: New York, American Chemical Society Symposium Series 361, 191st meeting, April 13-18, 1986, p. 1-62.
- Gilliom, R.J., Alley, W.M., and Gurtz, M.E., 1995, Design of the National Water-Quality Assessment Program -- Occurrence and distribution of water-quality conditions: U.S. Geological Survey Circular 1112, 33 p.
- Hirsch, R.M., Alley, W.M., and Wilber, W.G., 1988, Concepts for a National Water-Quality Assessment Program: U.S. Geological Survey Circular 1021, 42 p.
- Koterba, M.T., Banks, W.S.L., and Shedlock, R.R., 1993, Pesticides in shallow ground-water in the Delmarva Peninsula: Journal of Environmental Quality, v. 22, no. 3, p. 500-518.
- Koterba, M.T., Shedlock, R.J., Bachman, L.J., and Phillips, P.J., 1991, Regional and targeted ground-water-quality networks in the Delmarva Peninsula, chapter 6, in Nash, R.G., and Leslie, A.R., eds., Ground-water residue sampling design: Washington, D.C., American Chemical Society Symposium Series 465, p. 111-138.
- Koterba, M.T., Wilde, F.D., and Lapham, W.W., 1995, Ground-water data-collection protocols and procedures for the National Water-Quality Assessment Program -- Collection and documentation of water-quality samples and related data: U.S. Geological Survey Open-File Report 95-399, 113 p.
- Lapham, W.W., Wilde, F.D., and Koterba, M.T., 1995, Ground-water data-collection protocols and procedures for the National Water-Quality Assessment Program--Selection, installation, and documentation of wells, and collection of related data: U.S. Geological Survey Open-File Report 95-398, 69 p.
- Leahy, P.P., Rosenshein, J.S., and Knopman, D.S., 1990, Implementation plan for the National Water-Quality Assessment Program: U.S. Geological Survey Open-File Report 90-174, 10 p.
- Leahy, P.P., and Thompson, T.H., 1994, U.S. Geological Survey National Water-Quality Assessment Program: U.S. Geological Survey Open-File Report 94-70, 4 p.
- Pankow, J.F., and Cherry, J.A., 1996, Dense chlorinated solvents and other DNAPLs in groundwater: Portland, Oreg., Waterloo Press, 522 p.
- Scott, J.C., 1990, Computerized stratified random site-selection approaches for design of a ground-water-quality sampling network: U.S. Geological Survey Water-Resources Investigations Report 90-4101, 109 p.
- Tennant, P.A., Norman, C.G., and Vicory, A.H., Jr., 1992, The Ohio River Valley Water Sanitation Commission's Toxic Substances Control Program for the Ohio River: Water Science and Technology, v. 26, no. 7-8, p. 1779-1788.
- U.S. Environmental Protection Agency, 1992, Protection of environment (Appendix B to Part 136--Definition and procedure for determination of the method detection limit--Revision 1.11): U.S. Code of Federal Regulations, Title 40, parts 100-149, revised as of July 1, 1992, p. 565-567.
- U.S. Geological Survey, 1994, Still more insights into sample handling and analysis at the NWQL:National Water Quality Laboratory Newsletter, v. 2, no. 4, p. 3.
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