USGS FS-122-96: Pesticides in Public Supply Wells of Washington State
Central Columbia Plateau - Yakima River Basin
NAWQA Study -
Publications
Pesticides in Public Supply Wells of Washington State
USGS Fact Sheet 122-96
The Safe Drinking Water Act requires quarterly pesticide monitoring of Washington State's 5,400 Class A* public water supply wells, beginning in 1995. In a 3-year sampling period, the cost of monitoring is estimated to be about $3,300 per well. For the 40% of Washington's Class A public water systems that have 15 to 100 connections, the cost per household could reach $70 per year for each well in the system.
In 1994 the Washington State Department of Health (WDOH) created a program, approved by the U.S. Environmental Protection Agency (EPA), enabling public water systems to apply for waivers from quarterly monitoring. Wells for which waivers are granted must be at low risk of contamination from 25 pesticides specified by the EPA Phase II-5 Rule.
In cooperation with the WDOH, the U.S. Geological Survey (USGS) began a statewide sampling program in 1994 to assess the vulnerability of Washington State public water systems to pesticides. 1,326 Class A public supply wells were selected for sampling-1,103 using USGS geographic information systems (GIS) data and random selection software, and 223 as a high-risk group based on nitrate concentration and well depth. The WDOH used three contract laboratories to collect and analyze samples from across the state. The USGS National Water Quality Laboratory analyzed 220 duplicate samples-80 from wells in the Puget Sound Basin and 140 in the Central Columbia Plateau. Because the USGS lab can detect pesticides at very low concentrations, USGS data were used to validate contract lab results and to provide additional low-level pesticide data. Study costs ($1.4 million) were recovered by waiver fees.
*Class A: Generally, community water systems with 15 or more connections
Statewide Sampling Results* and Waiver Program
*Contract lab results reported by WDOH, Spring 1995
Sampling results for 1,326 public supply wells
- Pesticides were detected in 6% of 1,103 randomly selected public supply wells sampled across Washington.
- 21 of 27 analyzed pesticides were detected.
Pesticides detected in three or more wells were:
atrazine; simazine; dicamba; 2,4,5-TP;
2,4-DB; picloram; metribuzin
- The concentration of pentachlorophenol exceeded the EPA maximum contaminant level (MCL) in one well. Dieldrin and endrin concentrations exceeded EPA health advisory levels in one well each. However, EPA drinking water standards have not been established for 11% of the pesticides detected by contract labs in this study.
- More than 10% of wells with detections had more than one pesticide detected.
Risk assessment
Factors that correlated with pesticide detection were:
- Land use predominantly agricultural or urban
- Well depth less than 125 feet
- Nitrate concentration greater than 2.7 mg/L
(Steve Swope, Pacific Groundwater Group, written commun., 1994)
Waivers
On the basis of this sampling and risk assessment:
- 74% of Washington public supply wells were designated low risk, so were granted full monitoring waivers.
- 20% of public supply wells were designated moderate risk, so were granted partial monitoring waivers.
- 6% of public supply wells were designated high risk, so were denied monitoring waivers.
Savings
Annual monitoring savings
resulting from waivers : $ 6.0 million
Study cost : $ 1.4 million
1994 Lab Comparisons Contract labs USGS lab
---------------------- ----------------- ---------------
Cost per sample $375-500 $370
Number of 27 48
pesticides
analyzed
Schedules EPA 515.1 USGS 2001
analyzed 525.1
---------------------- ----------------- ---------------
USGS Sampling Results Highlight Other Issues
USGS results for 220 duplicate samples
- Pesticides were detected in 30% of 159 randomly selected duplicate samples analyzed by the USGS in the Puget Sound Basin and the Central Columbia Plateau.
- 13 of 48 analyzed pesticides were detected.
Pesticides detected in three or more wells were:
atrazine; desethyl atrazine; simazine; prometon;
p,p'-DDE; tebuthiuron; metribuzin
- No pesticide exceeded the EPA MCL. The concentration of dieldrin exceeded an EPA health advisory level in one well. However, EPA drinking water standards have not been established for 23% of the pesticides detected by USGS in this study.
- 68% of wells with detections had more than one pesticide detected.
Why does the USGS detect more pesticides?
- Differences in method detection limits (MDLs) account for much of the difference between contract lab and USGS findings.
- The MDL is the smallest concentration of a compound that the laboratory can reliably detect.
- For many of the pesticides analyzed in this study, the USGS MDLs are ten times lower than the MDLs of the contract labs used by WDOH. (For a list of USGS MDLs used in this study, see Wagner and others, 1995.)
- In most cases, this means that if a well had pesticides detected by the USGS but not by a contract lab, the pesticides were detected at very low concentrations, below the contract lab's MDL (shaded area of graph, right).
Why are low detection limits needed?
- Early warning and trend detection
While detection limits near the MCL may be suitable for compliance monitoring, lower detection limits make it possible to detect trends and protect source-water before it becomes significantly contaminated.
- Risk analysis
Risk analysis links pesticide detection to predictive factors. High detection limits decrease the number of pesticide detections on which to base the analysis. Including low-level detections produces a more rigorous risk analysis.
- Comparison and quality control
A comparison of duplicate samples shows close correspondence between concentrations of atrazine reported by the USGS and a contract lab, for concentrations above the contract lab's MDL (unshaded area of graph).
Duplicate samples analyzed for atrazine by both USGS and a contract lab
USGS Fact Sheet FS-122-96 by Sarah J. Ryker and Alex K. Williamson
References:
Washington State Department of Health, Spring 1995, Preliminary Results of the
Areawide Groundwater Monitoring Project: Washington State Department
of Health, 9 p.
Wagner, R.J., Ebbert, J.C., Roberts, L.M., and Ryker, S.J., 1995, Agricultural pesticide applications and observed concentrations in surface waters from four drainage basins in the Central Columbia Plateau, Washington and Idaho, 1993-94: U.S. Geological Survey Water-Resources Investigations Report 95-4285, 50 p.
For health information, contact your water utility or county health agency, or the Washington State Department of Health: 1 (800) 521-0323.
For further technical information contact:
Project Chief
1201 Pacific Avenue - Suite 600
Tacoma, WA 98402
Phone: (206) 593-6530 x2687
Email: rwblack@usgs.gov
May 1996
Suggested citation:
Ryker, S.J., and Williamson, A.K., 1996, Pesticides in public supply wells of Washington State: U.S. Geological Survey Fact Sheet 122-96, on line at <URL: https://wa.water.usgs.gov/ccyk/fs-122-96.html>, last updated March 16, 1998
For a paper copy of this report, email: rwblack@usgs.gov
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Last modified: Mon Mar 16 08:37:22 1998
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