Ambient water quality in aquifers used for drinking-water supplies, Gem County, southwestern Idaho, 2015

Scientific Investigations Report 2016-5170
Prepared in cooperation with Gem County, Idaho, and the Idaho Department of Environmental Quality
By:  and 



In recent years, the rapid population growth in Gem County, Idaho, has been similar to other counties in southwestern Idaho, increasing about 54 percent from 1990 to 2015. Because the entire population of the study area depends on groundwater for drinking water supply (either from self-supplied domestic, community, or municipal-supply wells), this population growth, along with changes in land use (including potential petroleum exploration and development), indicated to the public and local officials the need to assess the quality of groundwater used for human consumption. To this end, the U.S. Geological Survey, in cooperation with Gem County and the Idaho Department of Environmental Quality, assessed the quality of groundwater from freshwater aquifers used for domestic supply in Gem County. A total of 47 domestic or municipal wells, 1 spring, and 2 surface-water sites on the Payette River were sampled during September 8–November 19, 2015. The sampled water was analyzed for a variety of constituents, including major ions, trace elements, nutrients, bacteria, radionuclides, dissolved gasses, stable isotopes of water and methane, and either volatile organic compounds (VOCs) or pesticides.

To better understand analytical results, a conceptual hydrogeologic framework was developed in which three hydrogeologic units were described: Quaternary-Tertiary deposits (QTd), Tertiary Idaho Group rocks (Tig), and Tertiary-Cretaceous igneous rocks (TKi). Water levels were measured in 30 wells during sampling, and a groundwater-level altitude map was constructed for the QTd and Tig units showing groundwater flow toward the Emmett Valley and Payette River.

Analytical results indicate that groundwater in Gem County is generally of good quality. Samples collected from two wells contained water with fluoride concentrations greater than the U.S. Environmental Protection Agency (EPA) Maximum Contaminant Level (MCL) of 4 milligrams per liter (mg/L), six wells contained arsenic at concentrations greater than the EPA MCL of 10 micrograms per liter, and a sample from one well exceeded the MCL of 15 picocuries per liter for alpha particles. Although previous samples collected from some wells in Gem County contained nitrate concentrations greater than the MCL of 10 mg/L, the largest concentration detected in the current study was 5.2 mg/L. Total coliform bacteria was detected in four groundwater samples.

Three volatile organic compounds (VOCs) were detected in samples collected from five wells, and five compounds of the triazine class of herbicides were detected in samples from five wells; no concentrations were greater than applicable EPA MCLs. Methane was detected in samples from 36 wells, with the concentration in 1 well large enough to be considered an explosion hazard by U.S. Office of Surface Mining guidelines. Stable isotope signatures of methane in six samples suggest that naturally occurring methane in Gem County is probably of both thermogenic and biogenic origin.

Suggested Citation

Bartolino, J.R., and Hopkins, C.B., 2016, Ambient water quality in aquifers used for drinking-water supplies, Gem County, southwestern Idaho, 2015: U.S. Geological Survey Scientific Investigations Report 2016–5170, 33 p.,

ISSN: 2328-0328 (online)

Study Area

Table of Contents

  • Abstract
  • Introduction
  • Description of Study Area
  • Previous Work
  • Study Methods
  • Hydrogeology
  • Ambient Water Quality
  • Additional Needs for Groundwater-Quality Monitoring
  • Summary and Conclusions
  • Acknowledgments
  • References Cited
  • Appendix A. Water-Quality Data
Publication type Report
Publication Subtype USGS Numbered Series
Title Ambient water quality in aquifers used for drinking-water supplies, Gem County, southwestern Idaho, 2015
Series title Scientific Investigations Report
Series number 2016-5170
DOI 10.3133/sir20165170
Year Published 2016
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Idaho Water Science Center
Description Report: v, 33 p.; Appendix A
Country United States
State Idaho
County Gem County
Online Only (Y/N) Y
Additional Online Files (Y/N) Y
Google Analytic Metrics Metrics page
Additional publication details