Science Center Objects

Ground water is a significant source of drinking water in Washington State, and keeping it free of contamination is important for public health. Public supply wells are frequently tested for nitrate concentrations, but private wells are tested only when they are drilled. This limits information about the potential exposure to elevated nitrate concentrations in private wells.

To help the Washington State Department of Health estimate nitrate concentrations in drinking water from private wells, the USGS is developing a tool that is based on statistical analyses of previous nitrate measurements and their relation to well characteristics, such as depth, location, and surrounding land use. The USGS is using several data bases in the analyses to determine which well characteristics best predict nitrate concentrations.

9722-CYH - Analysis of Nitrate Concentrations in Aquifers of Washington State - Completed FY2008

Problem - Ground water is a significant source of drinking water in Washington State, and assuring that aquifers remain free from contamination is important for maintaining public health. In parts of the state, nitrate concentrations in ground water are elevated as a result of a variety of land-use practices, including fertilizer application, dairy operations and ranching, and septic-system use. To help assure that nitrate concentrations in drinking water remain below levels that are considered a health hazard, the Washington State Department of Health (WADOH) requires that public-supply water systems regularly measure nitrate concentrations. Such testing is only required of self-supplied systems (private wells) at the time that wells are drilled; and, as a result, citizens and public health officials have only limited information about the potential exposure to elevated nitrate concentrations for people whose primary drinking-water sources are private wells. The WADOH has asked the USGS to develop statewide tools that show the relation between the locations and depths of wells and likely nitrate concentrations. This information could be used to help estimate exposure to nitrate in drinking water in parts of the state without measurements of nitrate concentrations in ground water.

Objectives - The general objective is to estimate nitrate concentrations in ground water throughout most of the State of Washington. Specific objectives are to:

  1. Estimate the likely nitrate concentration for a given location and depth; and
  2. Estimate the likely depths at which selected nitrate concentrations, such as 3 mg/L, are present (indicating anthropogenic impacts).

Relevance and Benefits - This study is consistent with national USGS mission and goals and with water-resource issues identified in the USGS Washington Water Science Center (WAWSC) Science Plan. Specifically, the study addresses drinking-water quality, which is a priority issue for both the Strategic Directions of the USGS Water Resources Discipline (1999-2008) and the WAWSC Science Plan.

Approach - The objectives of this study will be addressed by utilizing a previously compiled database of wells with relatively recent (after 1995) measurements of nitrate concentrations and well information-including well location and depth, and by conducting a logistic regression of nitrate concentrations versus a series of independent variables such as soils, land use, and climatic metrics.