MODULE-3 Susceptibility and Vulnerability

Studies regarding the Susceptibility and Vulnerability of Washington's Drinking Water Supply

The following section provides studies conducted on the vulnerability of Washington's water supply systems to nitrogen and pesticides.

Vulnerability Assessment Studies, Washington

Nitrogen

Because of agricultural activities and increased rural development, Washington? s drinking water resources are vulnerable to increased nitrate loading. Private wells are most in danger, since they are typically drilled to shallower depths, where nitrate is found in higher concentrations; however, shallow public drinking water wells and public wells (in unconfined aquifers) found in areas of intense development are also at risk.

Assessment of aquifer susceptibility to nitrate

A study was conducted by the United States Geological Survey (USGS) in 1996 to determine the groundwater vulnerability to nitrogen based upon well depth and aquifer material. According to the USGS, in the Puget Sound Basin, shallow wells in aquifers that have coarse-grained glacial deposits at the surface and a high percentage of urban (residential, commercial, and industrial) and (or) agricultural land use in the vicinity are most vulnerable to nitrate contamination.

For example, shallow wells in agricultural areas overlying coarse-grained glacial deposits, like much of the Lower Nooksack Valley, or in urban areas overlying such deposits, like parts of Tacoma and Olympia, have a greater than 50% probability of nitrate concentrations at or above 3 milligrams per liter (mg/L). Nitrate concentrations at or above 3 mg/L are well above naturally occurring levels in the Puget Sound Basin, indicating an anthropogenic (human-related) source of nitrate."

The study used a vulnerability model to relate existing nitrate data to factors describing aquifer susceptibility (the ease with which a contaminant can reach the aquifer, determined by natural factors) and contaminant availability (the availability of sources of nitrate at or near the land surface, determined by anthropogenic factors). Significant factors were well depth, surficial geology, and the percentage of agricultural and urban land use within a 2-mile radius of a well. See the figure below for the study's results.

Pesticides

Starting in 1995, the United States Safe Drinking Water Act has required quarterly pesticide monitoring of Washington State's approximately 5,400 Group A public water supply wells. The cost of this sampling equates to approximately, $1,500 per well per year, and for the 40% of Washington's Group 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 (DOH) 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-V Rule.

In cooperation with the DOH, the USGS conducted a statewide sampling program in 1994 to assess the vulnerability of Washington State public water systems to pesticides. One thousand three hundred and twenty six (1,326) Group 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 following results were obtained from the public well supply study:

  • Pesticides were detected in 6% of (66 of 1,103) randomly selected public supply wells sampled across Washington.
  • Twenty-one of the analyzed pesticides were detected.
  • Pesticides detected in three or more wells included: atrazine, simazine, dicamba, 2,4,5-TP, 2,4-DB, picloram, and 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 laboratories in this study.
  • More than 10% of wells with detections had more than one pesticide detected.

State-wide pesticide sampling results

The study also concluded that the following factors were correlated with pesticide detection:

  • Land use predominantly agricultural or urban
  • Well depth less than 125 feet
  • Nitrate concentration greater than 2.7 mg/L

The study identified wells that were granted waivers, and concluded the following.

  • 74% of Washington public supply wells were designated low risk, and were thus 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.

More recently, the Washington State Department of Agriculture (WSDA) initiated a joint study with the U.S. Geological Survey (USGS) and the Washington State Department of Ecology (DOE) to conduct a statewide aquifer vulnerability study. This project is essential for identifying aquifers vulnerable to agricultural practices throughout the state, and will provide valuable information for pesticide management plans that may be necessary in the future.

The aquifer vulnerability study uses soil data provided by the National Resource Conservation Service (NRCS), rainfall data provided by the U.S. Weather Bureau, pesticide properties provided by the EPA and other available data such as a pesticide usage report conducted in 1998. The data were analyzed by modeling software developed by the EPA called PRZM3. The results will help indicate areas in the State that have the greatest potential for groundwater contamination. The study is complete and a USGS report is pending.

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