MODULE-3 Susceptibility and Vulnerability

Assessing Contaminant Leaching Rates of Your Drinking Water System

Since there are many threats to drinking water resources, it is important to clearly identify vulnerabilities and provide proper protection. The 1996 amendments to the federal Safe Drinking Water Act (SDWA) require states to implement Source Water Assessment Programs (SWAP). The Washington State Department of Health's SWAP was developed with public participation to improve protection of public water supply sources and further protect the health of Washington's citizens. Washington's source water assessments must:

  1. Identify the drinking water source area;
  2. Identify potential contamination sources;
  3. Assess how susceptible the drinking water source is to contamination; and
  4. Ensure assessments are made available to the public.

As indicated above, the first step to protecting the drinking water supply is to map the area of contribution to the well, or the wellhead protection area (WHPA). A discussion of how to delineate a wellhead protection area is provided in Module 1.

Then, an inventory of all potential sources of groundwater contamination in and around the WHPA should be conducted. An inventory is a required component of a local wellhead protection program. To help local communities with this task, DOH has created a technical assistance document, "Inventory of Potential Sources of Contamination within Washington's Wellhead Protection Areas."

The next step is to evaluate the vulnerability of the water system. This includes a full inventory of potential contamination sources, including septic systems, agriculture, landfills, junkyards, and underground fuel storage tanks. Assessing how vulnerable the drinking water source is to contamination should be done quantitatively. There are several ways in which local communities can quantify their risk; one of which is to conduct a nitrogen loading analysis. This analysis is especially applicable in Washington where nitrate-nitrogen is a priority pollutant of concern. Nitrate is also an indicator of overall water quality.

Nitrogen Loading Assessment

A nitrogen loading study is a very effective tool for assessing water quality impacts in rural areas. These sources typically include wastewater, residential and agricultural fertilizers, road run-off and precipitation. A nitrogen loading analysis can be conducted for both existing and build-out (future development) conditions. A build-out analysis can help to predict future development conditions. This should include a review of the community's zoning code, and determination of land uses for each developable parcel. Once the saturation/build-out analysis has been completed, all potential sources of nitrogen can be assessed and tabulated.

An analytical model has been developed to predict the nitrogen loading from land uses within WHPAs. This model allows for cumulative impact assessment, meaning that it provides a comparison of the impacts of the proposed project with other development which might also be affecting a resource area and predicts the additive effects of all development within a particular area. Use the Hydrologic/Nitrogen Budget Calculator (located at the bottom of the left-hand sidebar) to find out what your community's nitrogen loading budget is.

Using vulnerability assessments to protect drinking water supplies

A quantified assessment of drinking water supply vulnerability can be used to achieve a variety of management goals:

Targeted management options. Once the specific vulnerabilities of a water system are determined, management options can be prioritized, and resources can be allocated accordingly. For example, if nitrate levels are high because of increased residential development and thus, more individual on-site sewage systems, then resources should be focused on wastewater management. Assessment results could be provided to residents and maps that display the susceptibility could be used for education.

Growth management. Those areas that are most susceptible to groundwater contamination will be identified through the vulnerability analysis. These areas should be protected using smart growth techniques, such as Low Impact Development (LID), Transfer of Development Rights (TDR), and Overlay Zoning, which are further discussed in Module 5.

Targeted groundwater monitoring. Inspections, data collection, and other monitoring efforts can be targeted to vulnerable areas where land uses that are known sources of contaminants already exist. For example, public water systems are required to sample wells periodically for contaminants. Targeting sampling to those areas identified as more vulnerable to the contaminant of interest would result in more effective and less costly monitoring programs.

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