Idaho source water assessments

A source water assessment gives information on the potential contaminant threats to a public drinking water source. The Idaho Department of Environmental Quality (DEQ) says that, as required by the Safe Drinking Water Act, Idaho completed and summarized assessments of all recognized public water sources, and that more than 3,500 public water system sources have been assessed. The assessments are searchable online and shown on an interactive map.

In a Phase I ESA

ASTM E1527-21 does not name source water assessments among its standard record sources. They are useful for two questions: whether a property lies in the area that feeds a public well, spring or intake, and what potential contaminant sources DEQ has already inventoried nearby.

• Each report has three parts. DEQ says a report defines the zone of contribution, which is the part of the watershed or subsurface area contributing water to the well, spring or surface water intake. It identifies the significant potential sources of drinking water contamination in that area, which DEQ calls the potential contaminant source inventory. And it determines the likelihood that the water supply will become contaminated, which DEQ calls a susceptibility analysis.
• The inventory lists potential sources. DEQ's word is potential: the inventory identifies significant potential sources of contamination. Read an entry as a prompt to check the property's own records, not as a finding.
• DEQ names the kinds of threat. DEQ says drinking water sources are often vulnerable to contamination from land use practices such as farming and from potential contaminant sources such as gas stations in the vicinity of wells and intakes.
• The assessments cover public systems. DEQ describes the assessments as covering public water sources. It says a public water system well must have at least 15 service connections or regularly serve an average of at least 25 individuals at least 60 days out of the year, and that there are approximately 2,000 public drinking water systems in Idaho. For a private well, see groundwater quality and nitrate priority areas.
• Protection is voluntary. DEQ describes source water protection as a voluntary community effort, which may involve a protection plan and regulatory or non-regulatory management practices. DEQ names land use regulations and permitting among the regulatory approaches a community may use, so check with the city or county whether local rules apply to a protection area.

DEQ says capture zones delineated for source water assessments are an optional layer on its nitrate priority area map.

Where to look a site up

• DEQ's Source Water page links the Source Water Assessment Database. DEQ says it gives the assessed sources, a potential contaminant inventory, a contamination susceptibility summary, maps of the source water delineation and a summary report, and that it can be searched by county, public water system number, water system name and type of source.
• The same page links the Source Water Assessment Map. DEQ's mapping page links a map in the same address folder under the name Source Water Assessment and Protection, and says it shows Idaho's source water protection areas and potential contaminant inventory.
• DEQ's Source Water page also links a Source Water Protection Activity Guide, which it describes as a tool for identifying potential sources and types of contamination that could threaten drinking water sources.

Getting the file

The assessment reports are reached through the database. For records that are not posted, DEQ takes requests under the Idaho Public Records Act through its Public Records Request page, which lists the source water assessment tool among the resources that can often answer a question without a formal request.

Sources

Idaho Department of Environmental Quality pages, read October 1, 2026:

• Source Water, last modified January 15, 2026.
• Wells, last modified November 24, 2025.
• Groundwater Quality, last modified September 23, 2026.
• Interactive Mapping and GIS Data, last modified August 3, 2026.
• Public Records Request, last modified August 18, 2026.