8-13D-9-13: GROUND WATER:

Ground water in the Hidden Springs area is found in permeable sedimentary aquifers and in fractured rock aquifers. The fractured rock aquifers appear to be limited to the slopes on the north side of Dry Creek. Depth to ground water varies, and ranges from a few feet below ground surface along Dry Creek, to more than one hundred feet (100') below ground surface in upland locations. Tables 2 and 3 provide actual soils logs and ground water depth data for a portion of the valley floor in Hidden Springs.

   •   Alluvium: Recent alluvium in the Dry Creek valley consists of unconsolidated sand, gravel, and silt that has been eroded from nearby uplands and deposited by Dry Creek and tributary streams. Thickness of the alluvium is generally less than fifty feet (50'). Ground water from the alluvium has been tapped by shallow wells for irrigation and domestic use.

   •   Terteling Springs Formation: The Terteling Springs formation is composed of a layered sequence of lakebed sediments - primarily sand, sandstone, silt, clay, and mudstone. Saturated sand and sandstone layers within the Terteling Springs formation act as productive aquifers. Domestic and irrigation wells within the project area vicinity tap aquifers within the Terteling Springs formation. Clay, silt, and mudstone layers act as aquitards, restricting the movement of ground water. Clay, silt, and mudstone sediments are believed to be the predominant saturated sediments south of Dry Creek.

   •   Volcanic Rocks: Volcanic rocks within the project area consist primarily of basalt flows. Most of these volcanics have relatively low permeability and are not considered to be significant aquifers, although some ground water production has been reported from basalt aquifers in areas of the foothills.

   •   Granite: Cretaceous-age granitic rocks of the Idaho Batholith are exposed within the Currant Creek drainage, east of Cartwright Road. These rocks generally have very low porosity and permeability and do not serve as productive aquifers.

Ground waters with temperatures in excess of eighty-five degrees Fahrenheit (85°F) (i.e., low-temperature geothermal waters) are generally restricted from development for municipal and irrigation uses by the 1982 Idaho Geothermal Resources Act. In addition, water temperatures in excess of eighty-five degrees Fahrenheit (85°F) are not desirable for domestic use due to aesthetics, chemical composition and increased potential for bacterial contamination. As a result, warm ground waters are probably not available nor desirable for Hidden Springs water supply development.

The depth to ground water data and soils logs presented in Tables 2 and 3 confirm the notion that Dry Creek acts as a drain for upstream events. Clearly the shallow ground water flow direction has component vectors both down the valley parallel to Dry Creek and perpendicular toward Dry Creek. It is not clear if deep ground water flow is parallel to Dry Creek, or south toward the Boise River. Local ground water flow in the Hidden Springs area is probably influenced by geologic units and structure, with preferential movement through higher permeability materials in old streambeds and along fault-related fracture paths.

Ground water discharge currently occurs at domestic and irrigation wells, at springs, and through streambeds within the gaining reaches of Dry Creek and Currant Creek. Surface water and ground water are generally in direct hydraulic connection in the vicinity of springs and in the gaining reaches of Dry Creek and tributary streams.

During periods without direct runoff of precipitation or snowmelt, the flowing reaches of Dry Creek are probably a reflection of the local water table. Many of these reaches dry up during the summertime, suggesting a seasonal lowering of the local water table at that time.

Given the distance and changes in geologic conditions, the aquifer hydraulic characteristics in the Hidden Springs project area cannot be directly inferred from the characteristics in the Lower Dry Creek area.

Ground Water Quality: Analyses of water samples from wells in the vicinity of the project area are reported by Parliman (1983) and Baker (1991). These analyses include data for both warm water and cold-water aquifers in the Dry Creek Valley. Based on these data, ground water from deep cold-water aquifers in the Hidden Springs project area is anticipated to be good quality for domestic use, with moderate levels of hardness and no water quality parameters in excess of EPA maximum contaminant levels (MCLs). Geothermal aquifers are also anticipated to be of good quality, with the exception of fluoride concentrations which may exceed EPA MCLs.

The shallow alluvial aquifer is often in direct hydraulic connection with the surface water flow of Dry Creek and as a result is not sufficiently protected from surface contaminants for domestic consumption. For that reason, the shallow alluvial aquifer is not as desirable for domestic use as deeper, more protected aquifers of the Terteling Springs Formation. Well logs suggest that clay layers should provide protection from surface and surficial activity for wells tapping aquifers below one hundred feet (100').

Reported well yields in the Dry Creek Valley range from over two thousand (2,000) gallons per minute (gpm) to less than five (5) gpm. In general, the higher yield wells are located in the valley floor, west of the project site. The higher yield wells tap sand aquifers within the Terteling Springs Formation and the lower part of the recent alluvium. The low yield wells are typically located in either the granite foothills north and northeast of the site or in the thick blue clay sediments south of the site.

   Figure 2-G

(Ord. 1002, 10-28-2025)