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Comparison of Diffusion- and Pumped-Sampling Methods to Monitor Volatile Organic Compounds in Ground Water, Massachusetts Military Reservation, Cape Cod, Massachusetts, July 1999-December 2002By Stacey A. Archfield and Denis R. LeBlancToxic Substances Hydrology Program
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The citation for this report, in USGS format, is as follows:
Archfield, S.A., and LeBlanc, D.R., 2005, Comparison of diffusion- and pumped-sampling methods to monitor volatile organic compounds in ground water, Massachusetts Military Reservation, Cape Cod, Massachusetts, July 1999- December 2002: U.S. Geological Survey Scientific Investigations Report 2005-5010, 53 p.
For more information about USGS activities in Massachusetts and Rhode Island, visit the USGS MA-RI Water Science Center Home Page.
To evaluate diffusion sampling as an alternative method to monitor volatile organic compound (VOC) concentrations in ground water, concentrations in samples collected by traditional pumped-sampling methods were compared to concentrations in samples collected by diffusion-sampling methods for 89 monitoring wells at or near the Massachusetts Military Reservation, Cape Cod. Samples were analyzed for 36 VOCs. There was no substantial difference between the utility of diffusion and pumped samples to detect the presence or absence of a VOC. In wells where VOCs were detected, diffusion-sample concentrations of tetrachloroethene (PCE) and trichloroethene (TCE) were significantly lower than pumped-sample concentrations. Because PCE and TCE concentrations detected in the wells dominated the calculation of many of the total VOC concentrations, when VOC concentrations were summed and compared by sampling method, visual inspection also showed a downward concentration bias in the diffusion-sample concentration.
The degree to which pumped- and diffusion-sample concentrations agreed was not a result of variability inherent within the sampling methods or the diffusion process itself. A comparison of the degree of agreement in the results from the two methods to 13 quantifiable characteristics external to the sampling methods offered only well-screen length as being related to the degree of agreement between the methods; however, there is also evidence to indicate that the flushing rate of water through the well screen affected the agreement between the sampling methods. Despite poor agreement between the concentrations obtained by the two methods at some wells, the degree to which the concentrations agree at a given well is repeatable. A one-time, well-bywell comparison between diffusion- and pumped-sampling methods could determine which wells are good candidates for the use of diffusion samplers. For wells with good method agreement, the diffusion-sampling method is a time-saving and cost-effective alternative to pumped-sampling methods in a long-term monitoring program, such as at the Massachusetts Military Reservation.
Abstract
Introduction
Purpose and Scope
Description of Study Area
Study Design
Well Selection
Monitoring-Well Construction and Installation
Data Collection
Diffusion-Sampling Method
Pumped-Sampling Method
Collection of Additional Samples
Laboratory Analysis
Data Analysis
Comparison of Diffusion- and Pumped-Sampling Methods
Comparison of Methods by Volatile Organic Compound
Comparison of Methods by Well
Comparison of Additional Samples
Quality-Assurance Samples
Replicate Samples
Repeated Sampling
Comparison of Wells Sampled with Two Pumping Methods
Samples Collected above the Well Screen
Comparison of Sampling Methods to Characteristics External to the Methods
Study Limitations, Methods Agreement, and Long-Term Monitoring
Limitations of the Study Design
Limitations of the Statistical Analyses
Factors Affecting the Agreement between Methods
Long-Term Monitoring with Diffusion Samplers at the Study Area
Summary
Acknowledgments
References Cited
Appendix: Analytical Results for Selected Volatile Organic Compounds by Well for Diffusion- and Pumped-Sampling Methods, Including Wells with Two Sets of Samples Collected or with Replicate Samples Collected, and Samples Collected for Quality Assurance, Massachusetts Military Reservation, Cape Cod, Massachusetts, July 1999–December 2002
1. Map showing location of study area and monitoring well sites where groundwater samples were collected by diffusion- and pumped-sampling methods, Massachusetts Military Reservation, Cape Cod, Massachusetts, July 1999– December 2002
2–4. Graphs showing:
2. Typical results of pumped- and diffusion-sample concentrations for volatile organic compounds from two wells: A, 27MW0026B; and B, 27MW0023A, at the Massachusetts Military Reservation, July 1999–December 2002
3. Comparison of pumped- and diffusion-sample concentrations for volatile organic compounds detected in ground water from A, more than six; and B, six or fewer wells at or near the Massachusetts Military Reservation, July 1999–December 2002
4. Comparison of total volatile organic compound concentrations for samples collected by pumped- and diffusion-sampling methods in wells at or near the Massachusetts Military Reservation, July 1999–December 2002
5. Comparison of A, relative percent difference between first and second sampling; and B, pumped- and diffusion-sample concentrations from repeated sampling of selected wells at or near the Massachusetts Military Reservation, July 1999–December 2002
1. Well construction and sample information for wells with samples collected by diffusion- and pumped-sampling methods, Massachusetts Military Reservation, Cape Cod, Massachusetts, July 1999–December 2002
2. Volatile organic compounds and detections of compounds in samples collected by diffusion- and pumped-sampling methods from 89 wells at or near the Massachusetts Military Reservation, July 1999–December 2002
3. Results of sign tests applied to determine if total volatile organic compound concentrations in samples collected by the diffusion-sampling method were biased relative to concentrations in samples collected by the pumpedsampling method for wells at or near the Massachusetts Military Reservation, July 1999–December 2002
4. Analysis of volatile organic compounds for samples collected from well 03MW0214B, Massachusetts Military Reservation, July 1999
5. List of quantitative characteristics explored to determine if a relation was present between the characteristic and the degree of agreement of the diffusion- and pumped-sample concentrations and the reasons why each characteristic might affect the agreement between the methods
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