The U.S. Environmental Protection Agency (EPA) has completed its five-year National Survey of Pesticides in Drinking Water Wells (NPS). Between 1988 and 1990, EPA sampled approximately 1300 community water systems (CWS) wells and rural domestic wells (RDW) for the presence of 101 pesticides, 25 pesticide degradates, and nitrate. The survey results statistically represent approximately 94,600 drinking water wells at community water systems and over 10.5 million rural domestic wells throughout the United States. Samples were collected from Bonner and Latah counties in Idaho for this survey.

EPA designed the survey with two principal objectives: (1) to determine the frequency and concentration of the presence of pesticides and nitrate in drinking water wells nationally; and (2) to improve EPA's understanding of how the presence of pesticides and nitrate in drinking water wells is associated with patterns of pesticide use and the vulnerability of groundwater contamination. The survey focused on the quality of drinking water in wells. The survey was designed to yield valuable information on both the frequency and levels of pesticides, pesticide degradates, and nitrate present in rural domestic (private) wells and community (public) drinking water wells on a nationwide basis. The survey, however, was not designed to provide an assessment of pesticide contamination in drinking water wells at the local, county, or state levels.

The NPS was designed to provide statistically valid estimates of the presence of pesticides and nitrate in the nation's well water supply. EPA used survey research methods which represent the total population of community water system (CWS) wells and rural domestic wells (RDW) in the U.S. The survey design ensured that water samples would be taken from wells located in areas with a wide range of levels of pesticide use and groundwater vulnerability. The survey design also ensured that a known proportion of the wells selected were from areas where pesticides were more heavily used and also from areas that were the most vulnerable to groundwater contamination. This scientific sampling process was designed to achieve a high level of precision. The survey had two distinct statistical design components: one for sampling CWS wells and another for sampling rural domestic wells.

Community Water System (CWS) Wells
CWS wells were defined for the NPS as wells in systmes of piped drinking water that either have at least 15 connections or serve at least 25 permanent residents. To be eligible, a system must have had at least one operating well (at the time of sampling) that was being used to supply drinking water. EPA targeted two categories of CWS wells in the Survey:

1. Community water system wells nationally, which included all CWS wells in the continental United States plus Alaska and Hawaii; and
2. Community water system wells in counties with high groundwater vulnerability, which made up about 22 percent of all CWS wells.

1. Rural domestic wells nationally, which includes all rural domestic wells in the continental United States plus Alaska and Hawaii;
2. Rural domestic wells in counties with high pesticide use;
3. Rural domestic wells in counties with high groundwater vulnerability;
4. Rural domestic wells in the "cropped and vulnerable" areas of counties; and
5. Rural domestic wells in counties with high pesticide use and high groundwater vulnerability.

Pesticide Results
EPA estimates that 9,850 (10.4 percent) CWS wells and 446,000 (4.2 percnet) rural domestic wells in the United States contain pesticides or pesticide degradates at or above the minimum reporting limits used in the Survey. The two pesticides analytes most frequently detected were DCPA acid metabolites and atrazine.

DCPA acid metabolites, a degradate of DCPA (dimethyl tetrachloroterephthalate), is the most commonly detected pesticide or pesticide degradate in the survey. EPA estimates that 6,010 (6.4 percent) of CWS wells and 264,000 (2.5 percent) rural domestic wells contain at least 0.10 micrograms/L of DCPA acid metabolites.

Atrazine is the second most commonly detected pesticide. EPA estimates that atrazine is present, at or above 0.12 micrograms/L, in about 1,570 (1.7 percent) CWS wells and 70,800 (0.7 percent) rural domestic wells nationwide.

Atrazine is known by the common names AAtrex, Atratol, Gesaprim, and Zeaphos, and is used to control many annual broadleaf weeds and certain grasses on cropped land. It is also used for general weed control on non-cropped industrial land, selective weed control in conifer restoration and Christmas tree plantations, and non-selective control of vegetation on fallow land.

In addition to DCPA acid metabolites and atrazine, ten other pesticides were detected above Survey reporting limits. The following list identifies all pesticides or pesticide degradates that were detected above minimum reporting limits.

*Community Water System (CWS) Wells: atrazine, DCPA acid metabolites, dibromochloropropane, dinoseb, hexachlorobenzene, prometon, and simazine.

Table 1. Estimated Number and Percent of Community Water System Wells Containing NPS Analytes.
Analyte	Estimated Number	Estimated Percent
Nitrate	49,300	52.1
DCPA acid metabolites	6,010	6.4
Atrazine	1,570	1.7
Simazine	1,080	1.1
Prometon	520	0.5
Hexachlorobenzene	470	0.5
Dibromochloropropane (DBCP)	370	0.4
Dinoseb	25	<0.1

*Rural Domestic Wells: alachlor, atrazine, bentazon, DCPA acid metabolites, dibromochloropropane, ethylene dibromide, ethylene thiourea, gamma-HCH (lindane), prometon, and simazine.

Table 2. Estimated Number and Percent of Rural Domestic Wells Containing NPS Analytes.
Analyte	Estimated Number	Estimated Percent
Nitrate	5,990,000	57.0
DCPA acid metabolites	264,000	2.5
Atrazine	70,800	0.7
Dibromochloropropane (DBCP)	38,400	0.4
Prometon	25,600	0.2
Simazine	25,100	0.2
Ethylene dibromide	19,200	0.2
Gamma-HCH (Lindane)	13,100	0.1
Ethylene thiourea	8,470	0.1
Bentazon	7,160	0.1
Alachlor	3,140	<0.1

For analytes with established or proposed EPA Lifetime Health Advisory Levels (HALs) or Maximum Contaminant Levels (MCLs), most observed detections of nitrate, pesticides, and pesticide degradates were at levels well below the associated MCLs and HALs. An MCL is an enforceable standard defining the maximum permissible level of a contaminant in water that is delivered to any user of a public water system. MCLs set achievable levels of drinking water quality to protect human health. A lifetime HAL represents the concentration of a contaminant in water that may be consumed over an average human lifetime without causing adverse health effects.

Based on the results of the Survey, EPA estimates that 1,130 (1.2 percent) CWS wells and 254,000 (2.4 percent) rural domestic wells nationwide contain nitrate exceeding EPA's HALs and MCLs of 10 milligrams per liter (mg/L).

Based on those pesticides and pesticide degradates with HALs or MCLs, EPA estimates that at most 750 (0.8 percent) CWS wells nationally have at least one pesticide detection above the respective HALs and MCLs. For rural domestic wells, about 19,400 (0.2 percent) contain at least one pesticide detection above the HALs and 60,900 (0.6 percent) contain at least one pesticide above the MCL.

This article was adapted from a summary of the pesticide survey published by EPA in December, 1990.

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