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National Water-Quality Assessment Program
National Synthesis on Volatile Organic Compounds

Approach to an Assessment of Volatile Organic Compounds in the Nation’s Ground Water and Drinking-Water Supply Wells

By Michael J. Moran, John S. Zogorski, and Barbara L. Rowe

U.S. GEOLOGICAL SURVEY
Open-File Report 2005-1452

Abstract

The National Water-Quality Assessment (NAWQA) Program of the U.S. Geological Survey (USGS) developed an approach for a national assessment of the occurrence, status, and distribution of volatile organic compounds (VOCs) in samples of ground water from aquifer studies and in samples from drinking-water supply wells, specifically domestic and public wells. Data on VOCs in samples from aquifers and from domestic and public wells had three sources: (1) NAWQA sampling of ground water in aquifer studies and of shallow ground water in areas of agricultural or urban land use from 1993–2002, (2) retrospective data from other Federal, State, and local agencies that sampled ground water in aquifer studies from 1985–1997, and (3) sampling of ground water from public wells used as a source of drinking water from 1999–2000 (source-water survey).

Data for assessing VOCs in ground water came from the NAWQA sampling of aquifers and from retrospective data (1 and 2). Domestic wells were the most commonly sampled well type in these aquifer studies. Data for assessing VOCs in domestic well samples came from the NAWQA sampling of aquifers and shallow ground water and from retrospective data (1 and 2). Data for assessing VOCs in public well samples came from NAWQA sampling of aquifers and shallow ground water and from the source-water survey (1, 2, and 3).

Fifty-five VOCs were included in the national assessment. All ground-water samples were collected using consistent, prescribed field protocols and were analyzed using approved analytical methods. All ground-water collection activities and laboratory procedures included the collection of various quality-control samples in order to ensure the quality of the data.

Comparisons of detection frequencies and detected concentrations among individual VOCs, groups of VOCs, aquifer studies, and data sets were performed using two assessment levels: (1) an assessment level of 0.2 microgram per liter (µg/L), and (2) an assessment level of 0.02 µg/L. Selection of the 0.2-µg/L assessment level was based on historical laboratory reporting levels of VOCs used by the USGS National Water-Quality Laboratory (NWQL) prior to April 1996, and selection of the 0.02-µg/L assessment level was based on a new, low-level analytical method developed by the NWQL and used since April 1996.

Relational analyses using multivariate logistic regression were performed on VOC data from aquifers, domestic wells, and public wells to better understand the natural and anthropogenic factors that control or influence the occurrence of VOCs. Ancillary data used in the relational analyses represent a variety of anthropogenic and hydrogeologic controls on the occurrence of VOCs. For aquifers, relational analyses were performed only on NAWQA data analyzed using the new low-level analytical method for 10 frequently detected VOCs. For domestic wells, relational analyses were performed at assessment levels of 0.2 µg/L and 0.02 µg/L for 6 and 10 frequently detected VOCs at these levels. For public wells, relational analyses were performed at an assessment level of 0.2 µg/L for nine frequently detected VOCs.

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Contents

Abstract

Introduction

Purpose and Scope of Report

General Information on VOCs

Overview of Approach

VOCs Selected

Ground-Water-Quality Data

NAWQA Data

Retrospective Data

Source-Water Data

Field Sampling and Laboratory Analytical Methods

Field Sampling and Handling

Laboratory Analytical Methods

VOC Analyses Prior to April 1996

VOC Analyses after April 1996

Field and Laboratory Quality Assurance and Quality Control

Field Quality-Control Samples

Laboratory Quality-Control Samples

Organic Blind Sample Project

Laboratory Certification

Computational and Statistical Methods

Detection Frequencies and Concentrations

Assessment Levels

Statistical Tests

Comparisons of VOC Concentrations to Drinking-Water Standards and Health-Based Screening Levels

VOC and Ancillary Data Sets

VOC Data Used in Assessment of Ground Water

VOC Data Used in Assessment of Drinking-Water Supply Wells

VOC Data Used for Comparison to Drinking Water

VOC Data Used in Relational Analyses

Ancillary Data Used in Relational Analyses

Hydrogeologic Variables

Anthropogenic Variables

Other Ancillary Data

Summary

References

Figures

1–9. Maps showing:

1. Study Units where the National Water-Quality Assessment Program has completed an occurrence survey of volatile organic compounds in aquifers.

2. Median long-term method detection level for each volatile organic compound and the estimated rate of false negatives when using an assessment level of 0.02 microgram per liter.

3. Flow chart showing decisions for placing each volatile organic compound into a group reflecting confidence of detection frequencies when using an assessment level of 0.02 microgram per liter.

4. Locations of wells sampled in aquifer studies for a national assessment of volatile organic compounds.

5. Locations of centroids of wells in aquifer studies included in a national assessment of volatile organic compounds.

6. Locations of domestic wells sampled for a national assessment of volatile organic compounds (VOCs) and included in the relational analyses of VOCs in domestic wells at an assessment level of 0.2 microgram per liter.

7. Locations of public wells sampled for a national assessment of volatile organic compounds (VOCs) and included in the relational analyses of VOCs in public wells at an assessment level of 0.2 microgram per liter.

8. Locations of wells used in the relational analyses of volatile organic compounds in aquifers.

9. Locations of domestic wells used in the relational analyses of volatile organic compounds at an assessment level of 0.02 microgram per liter.

Tables

1. Predominant uses for the 55 volatile organic compounds (VOCs) included in a national assessment and for other VOCs analyzed in some samples.

2. Number of wells available for analysis in a national assessment of volatile organic compounds by data source and water use.

3. Median long-term method detection level, and percent of detections less than or equal to 0.05 microgram per liter for volatile organic compounds in ground water, data on spiked concentrations and recoveries, and confidence of detection frequency computations.

4. Maximum Contaminant Levels and Health-Based Screening Levels for the 55 volatile organic compounds included in the national assessment.

5. Characterization of data for assessment of volatile organic compounds in ground water by water use and by source of data.

6. Number of domestic and public wells used for assessment of volatile organic compounds in drinking-water supply wells by source of data.

7. Distribution of wells used in the explanatory analysis of volatile organic compounds in aquifers by water use.

8. Hydrogeologic and anthropogenic variables that were used in the logistic regression analyses.


Send questions or comments about this report to M.J. Moran (605) 394-3244.

For more information about USGS activities in South Dakota, visit the USGS South Dakota Water Science Center home page.

For more information about USGS National Water-Quality Assessment Program, visit the NAWQA Program home page or for more information about the USGS National Water-Quality Assessment Program Volatile Organic Compound National Synthesis, visit the VOC National Synthesis home page.



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