- Groundwater Management
- Other Actions
Wellhead Protection Area
Groundwater provides drinking water for about 65 percent of Washington’s residents—in some counties it approaches 100 percent. Because groundwater can be vulnerable to contamination, public water systems must take preventative measures to minimize the possibility that land uses will contaminate the underlying groundwater.
A wellhead protection area (WHPA) is defined as the surface and subsurface area surrounding a well or wellfield that contaminants are likely to pass through and eventually reach the well(s). Most of the land in any WHPA is not owned or managed by the utility so good stewardship and local regulations are important to protect the source and those who drink the water. WHPAs are divided into time periods – 6-month, 1, 5 and 10-year times of travel zones. Each area represents the length of time it would take a particle of water or contaminant to move from a specific point in the aquifer to the source well.
Current WHPAs for sources in the Lower Yakima Valley project area have been established using the calculated fixed radius (CFR) approach, resulting in the standardized “bullseye” mapping surrounding sources. The Department of Health has recently contracted to update WHPAs for 20 sources in the lower Yakima River Valley using a numerical model developed by the USGS.
The WHPAs will include the 6-month and 1-, 5- and 10-year time of travel zones (ToTs). A numerical method uses site-specific data assimilated from a variety of sources, including well logs and hydrogeologic reports, to determine the time of travel areas. This method of WHPA delineation results in a more accurate representation of groundwater flow and better identifies areas to focus outreach and education efforts.
The contractor has started working on updating the WHPAs and preliminary model results are expected in mid-April. Final time of travel maps and a summary report will be completed by July 31, 2023.
Groundwater and Nitrate Concentration Mapping Tool
To aid in the targeting of nonpoint source pollution reduction measures, this project supports the development of a groundwater contour mapping modeling tool in the Lower Yakima Valley, utilizing existing ambient groundwater data collected from July 2021 through June 2022 by the Washington State Dept of Ecology. Groundwater contour mapping increases the visible connection between land use practices on the surface and groundwater recharge, including nitrate contamination, and allows growers to observe the potential radius of their fields’ influence and for stakeholders to identify influences that are up-gradient of the wells. Developing a model for mapping groundwater elevations and nitrate concentrations provides partners with a valuable tool for tracking changes in nitrate concentrations in relation to management measures used.
Read the methodology document summarizes the available data sources and the proposed methodology for developing the groundwater contour mapping tool: