Protecting Public Health and the Environment.

Rathdrum Prairie Aquifer Boundary

In 1977, a petition request was submitted to the EPA to designate the RPA as a sole source aquifer. On February 9, 1978, EPA granted the request (Federal Register Vol. 43, No. 28, Thursday, February, 9, 1978). EPA defines a sole-source aquifer as one that 1) supplies at least 50% of the drinking water in the area overlying the aquifer, and 2) has no alternative drinking water source(s) which could physically, legally, and economically supply all those who depend on the aquifer for drinking water.

The map below shows three separate boundaries. The dark blue line shows the EPA aquifer boundary, the red line shows the US Geological Survey (USGS) aquifer boundary, and the yellow line shows the sole source aquifer boundary.

(Click on map to enlarge.)

Boundary - SmallAs seen on the map, the EPA aquifer boundary and the sole-source aquifer boundary are not always the same. Within the sole-source aquifer boundary, water on the surface of the land infiltrates, or seeps downward, to the ground water below, recharging the aquifer. The sole-source aquifer boundary excludes areas where recharge comes from sources other than the land surface. One such area is near the southern shore of Lake Pend Oreille, where most of the recharge to the RPA is from the lake and not from infiltration from the land surface.

The red line on the map shows where the aquifer boundary was recently defined by the USGS as part of a hydrogeological study of the aquifer in Idaho and Washington. The USGS boundary, which is based on geologic mapping conducted by the Idaho Geological Survey and the USGS, is located largely where the glacial flood deposits are adjacent to surrounding bedrock.

DEQ has classified the RPA as a sensitive resource aquifer (IDAPA, following EPA’s aquifer boundary (dark blue line) seen in the map above. Because of this classification, all activities that could impact the water quality of the RPA must be carried out so they maintain or improve existing quality of the ground water (IDAPA

The Panhandle Health District also recognizes the EPA aquifer boundary and has adopted rules to allow only one septic tank and drainfield for every 5 acres over the aquifer to avoid impacting water quality. A map of the EPA aquifer boundary used by both DEQ and the Panhandle Health District can be found on the RPA Reports/Publications web page.


In January 2007, Kootenai County established the RPA Protection District. The aquifer protection district boundaries correspond to the areas over the RPA and adjacent upland areas that contribute water to the RPA. The upland areas are also known as Critical Aquifer Recharge Areas. Creation of the aquifer protection district allows Kootenai County to charge an annual fee to residences and businesses with the boundary. The money is used to fund aquifer protection programs and activities.