Geochemical Analysis of Groundwater in Eastern Ada County

Science Center Objects

Groundwater is the primary source of water supply to Ada County’s growing population. Because of a proposed development near Mayfield, the Idaho Department of Water Resources (IDWR) initiated the East Ada County Hydrologic Project to improve the scientific understanding of the groundwater resources in the area.

The IDWR asked us to try to answer the following questions:

What direction is the groundwater flowing?

How old is the groundwater?

How is the aquifer recharged?

The IDWR's East Ada County Hydrologic Project is part of its Comprehensive Aquifer Management Program that “integrates hydrologic realities with social needs.”

The IDWR asked the USGS to apply its expertise in geochemical analysis to help describe the groundwater resources of eastern Ada County. We collected and analyzed water samples from Indian Creek and a small number of wells near Mayfield. We interpreted the data and published the results in a peer-reviewed report.

Groundwater flow direction

Our study confirmed the findings of previous studies that groundwater generally flows to the southwest. However, it is unknown how faults and fractures affect groundwater flow.

Age of water

Higher nutrient concentrations and younger carbon-14 dates indicated that water in the perched aquifer is younger than water in the deep aquifer.

Recharge sources

The perched aquifer is likely recharged from winter precipitation and surface water infiltration. The deep aquifer is likely recharged from high-elevation precipitation, surface water infiltration, and upwelling geothermal water.