​Source water assessments

The 1996 amendments to the federal Safe Drinking Water Act created new requirements to protect drinking water at its source, recognizing the need to address risks beyond traditional treatment. The amendments directed every state to complete “source water assessments" for all public water systems with at least 15 service connections or that serve more than 25 people year-round. These assessments delineate each system's source area (for groundwater or surface water) and identify potential sources of contamination.

To meet these requirements, the Oregon Health Authority (then the Department of Human Services) and the Oregon Department of Environmental Quality jointly developed Oregon's Source Water Assessment Plan with input from a citizen advisory committee which included six state and federal agencies, as well as 18 additional representatives of organizations, advocacy groups, and municipalities.  EPA provided general guidance but allowed states to design their own approach. EPA approved Oregon's plan in June 1999.

At that time, Oregon had 2,656 public water systems, including 1,171 that met the federal definition and required full assessments. Smaller systems received streamlined assessments. All required assessments were completed between 1999 and 2005.

Since the first source water assessments were completed in 2005, state agencies have significantly expanded analytical capabilities, including more detailed data for analyzing natural characteristics and potential pollutant sources. The state agencies began completing Updated Source Water Assessments in 2016 to provide the water systems and communities with more detailed and updated information on the watershed or recharge area that supplies their well, spring or intake. Much of the source water assessments information is now also provided in GIS layers that provide a combined drinking water protection database that is available to local governments, planners, state and federal agencies, consultants, communities and the general public.

• Download or request Source Water Assessment Reports and updates.
• Access Source Water Assessment Maps and GIS Data

Communities now have maps of their source areas and identified natural and human-caused risks that may affect drinking water quality. More information on each of these components is listed below. Public water systems and local communities can use the information in the assessments to voluntarily develop place-based plans and implement drinking water protection strategies. 

The source water assessment process begins by “delineating" the areas that supply groundwater wells and surface water intakes. For groundwater systems, this means identifying the land area overlying the part of the aquifer that contributes water to the well, wellfield, or spring. Oregon defines the extent of this capture zone using groundwater time-of-travel (TOT) through the aquifer. For systems with populations of more than 500, the TOT is 10 years. For systems serving less than 500, the TOT is 15 years because the method of delineation is much less precise.

Each delineated area is further divided into two- and five-year TOT subzones to help communities target protection strategies. The two-year zone is generally prioritized because potential pollution sources located there pose higher risk due to their proximity, and because microbial contaminants are most likely to reach the well within this period.

Each water system receives a topographic map showing the delineated time-of-travel zones. All wells are precisely located using Geographic Positioning System. For geologically complex systems, the state consults U.S. Geological Survey experts to develop the conceptual model and identify the parameters used in the delineation. See OHA's Delineation of drinking water protection areas for groundwater sources web page for more information.

For surface water systems, the delineation is based on US Geological Survey Hydrologic Unit Code (HUC) fifth-field watershed boundaries. All intakes are GPS-located, and the delineated area includes the entire watershed upstream of the intake.

After delineating the entire watershed, sensitive areas within the watershed are identified. These include land adjacent to the stream, high-erosional areas, landslide prone areas, and other natural factors that increase the risk of contamination of the surface water. The result is an identification of a subset of the entire watershed. For groundwater systems, sensitivity is evaluated based on depth to water an overburden characteristics, aquifer characteristics, infiltration potential, previous monitoring history, and source construction.  Sensitive areas are those where potential contamination sources or land-use activities, if present, have a greater potential to affect the water supply. This is analogous to the time-of-travel zones for groundwater systems.​

​​Potential contaminant sources are assigned low, moderate, or high risk ratings based on the type of facility and the substances involved. These ratings were developed by the advisory committee that developed the state's Source Water Assessment Plan using conservative, worst-case assumptions, such as the absence of good management practices or pollution prevention measures. For example, worst case scenario could mean the facility or activity is not employing good management practices or using pollution prevention techniques or technologies.

Because the assessments were completed for all public water systems statewide within limited time and resources, the risk ratings reflect general, statewide conditions rather than site-specific circumstances. The committee developed relative risk ratings for more than 100 types of potential contaminant sources. As a result, these ratings represent a high-level summary and do not necessarily indicate actual risk at a specific location, especially where contaminants are properly managed and best practices are in place.

Oregon's Source Water Assessment Methodology - Risk levels for potential contaminant sources

​The susceptibility analysis provides water systems with information on where the greatest risk occurs and where to focus resources for protection. Susceptibility is defined as the potential for contamination in the source area to reach the intake on the surface water body or the well(s) being used by a public water system for drinking water purposes. Whether or not a particular drinking water source becomes contaminated depends on three major factors: 1) occurrence of a facility or land use that releases contamination, 2) location of the release, and 3) hydrologic and/or soil characteristics in the source area that allow transport of the contaminants to the surface water body.

In conducting a susceptibility analysis for each public water system, the state uses information from the delineation (the most sensitive areas) and inventory. Results of the inventory are more meaningful when proximity to the well or intake is considered, along with the associated risk rating of the source, and whether it is located within a sensitive area. In general, land uses closest to the intake/well and those with the highest risk rating pose the greatest threat to a drinking water supply. The presence and locations of potential contamination sources within sensitive areas will determine where the water system has the highest susceptibility to contamination. The susceptibility analysis cannot predict when or if contamination will actually occur but recognizes conditions that are highly favorable for contamination to occur. If a contaminant release to soils or water should occur in a sensitive area, it's very likely that contamination of the water body would occur if remedial actions are not taken.

When several high- or moderate-risk sources are located within sensitive areas, a public water system may also be said to have a high overall susceptibility to contamination. If a public water system's drinking water source is determined to be of high susceptibility, it is recommended that the system identify those condition(s) that lead to the high susceptibility and take steps to protect the resource (such as reducing soil erosion or working directly with facility operators to implement sound management practices). Water systems with a low susceptibility should consider all identified factors that could lead to higher susceptibility in the future and prepare a strategy to protect the resource for the future. The end product of a susceptibility analysis is an overlay of inventory results with a map of sensitive areas.

​Public water systems that want to voluntarily develop local source water protection plans or strategies can take the next step by updating and enhancing their Source Water Assessment. An enhanced assessment looks more closely at what is actually happening on the ground within the drinking water source area, including site-specific land uses and practices.

State-completed source water assessments provide a useful, statewide starting point, but they offer a broad snapshot and may not reflect current conditions. Since many assessments were completed, land uses may have changed due to new development, agricultural practices, wildfire impacts, or industrial activity. Updating the assessment helps ensure the water system's understanding of potential risks is current and accurate.

An enhanced inventory builds on the original assessment by incorporating local knowledge from water system staff, landowners, planners, community members, and partner agencies. Water systems can work with DEQ, OHA, and other state agencies and use available tools such as updated GIS data, contaminant inventory resources, and groundwater or watershed mapping guidance.​

​Drinking water sources, whether from watersheds or groundwater recharge areas, can be affected by both natural conditions and human activities. Identifying potential sources of pollution helps communities understand local risks and take steps to reduce them.

The core purpose of the source water assessment is to identify land uses and activities that could pose risks to public water systems. This information is compiled in an “inventory." In 1998–1999, a statewide advisory committee determined what activities to include and how inventories should be completed. A complete list of inventoried land uses and activities is available on the Potential Contaminants and Risks web page.

The inventory is an important tool because it:

• Identifies where potential contaminant sources are located, particularly those posing the greatest relative risk

• Helps educate the public about possible threats to drinking water

• Increases awareness for landowners and facility operators within the source area

• Provides a foundation for developing a local source water protection plan

Inventories focus primarily on potential sources of contaminants regulated under the federal Safe Drinking Water Act. These include contaminants with established maximum contaminant levels, contaminants addressed under the Surface Water Treatment Rule, and the microorganism Cryptosporidium. DEQ defined a significant source of contamination as any facility or activity that stores, uses, or produces these contaminants of concern and has a sufficient likelihood of releasing those contaminants to the environment at levels that could contribute significantly to the presence of these contaminants in the source waters of the public water supply. 

The inventory identifies several broad categories of potential contaminants, including:

• Microorganisms (e.g., viruses, Giardia, Cryptosporidium, fecal bacteria)

• Inorganic compounds (e.g., nitrates, metals)

• Organic compounds (e.g., solvents, petroleum products, pesticides)

• Turbidity and sediment

Contaminants can enter groundwater, rivers, or lakes from activities occurring on the land surface or below ground, and from both widespread (nonpoint) sources and individual (point) sources. To identify potential sources, DEQ relied on readily available information, including reviews of databases from DEQ, EPA, and other agencies; interviews with public water system operators; and field observations.

The assessments use conservative, “worst-case" assumptions when identifying potential risks. In practice, many facilities and land uses operate under current regulations and use best management practices that reduce or prevent contamination. It's important to note that the assessments list all POTENTIAL risks; many risks listed in the assessment don't present actual risks to the water system. Environmental contamination is unlikely to occur when contaminants are handled and used properly, or when best management practices are employed. Day-to-day operating practices and environmental awareness varies greatly from one facility or land-use activity to another. Due to time constraints, in-depth analysis or research is not completed to assess each specific source's compliance status with local, state and/or federal laws.

The assessment also makes general assumptions about activities associated with certain land uses. For example, rural residences associated with farming operations are assumed to have potential sources such as fuel storage, chemical storage or mixing areas, and equipment repair. These assumptions can be refined as communities move beyond the assessment phase. Before developing a protection plan, DEQ recommends conducting an enhanced, site-specific inventory. During that process, sources found to present little or no risk can be removed from consideration.​