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Scientific Investigations Report 2010-5144

Prepared in cooperation with the Idaho Department of Water Resources

Distribution of Isotopic and Environmental Tracers in Groundwater, Northern Ada County, Southwestern Idaho

By Candice B. Adkins and James R. Bartolino

ABSTRACT

Residents of northern Ada County, Idaho, depend on groundwater for domestic and agricultural uses. The population of this area is growing rapidly and groundwater resources must be understood for future water-resource management. The U.S. Geological Survey, in cooperation with the Idaho Department of Water Resources, used a suite of isotopic and environmental tracers to gain a better understanding of groundwater ages, recharge sources, and flowpaths in northern Ada County. Thirteen wells were sampled between September and October 2009 for field parameters, major anions and cations, nutrients, oxygen and hydrogen isotopes, tritium, radiocarbon, chlorofluorocarbons, and dissolved gasses. Well depths ranged from 30 to 580 feet below land surface. Wells were grouped together based on their depth and geographic location into the following four categories: shallow aquifer, intermediate/deep aquifer, Willow Creek aquifer, and Dry Creek aquifer.

Major cations and anions indicated calcium-bicarbonate and sodium-bicarbonate water types in the study area. Oxygen and hydrogen isotopes carried an oxygen-18 excess signature, possibly indicating recharge from evaporated sources or water-rock interactions in the subsurface. Chlorofluorocarbons detected modern (post-1940s) recharge in every well sampled; tritium data indicated modern water (post-1951) in seven, predominantly shallow wells. Nutrient concentrations tended to be greater in wells signaling recent recharge based on groundwater age dating, thus confirming the presence of recent recharge in these wells. Corrected radiocarbon results generated estimated residence times from modern to 5,100 years before present. Residence time tended to increase with depth, as confirmed by all three age-tracers. The disagreement among residence times indicates that samples were well-mixed and that the sampled aquifers contain a mixture of young and old recharge. Due to a lack of data, no conclusions about sources of recharge could be drawn from this study.

First posted July 20, 2010

For additional information contact:
Director, Idaho Water Science Center
U.S. Geological Survey
230 Collins Road
Boise, Idaho 83702
http://id.water.usgs.gov

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Suggested citation:

Adkins, C.B., and Bartolino, J.R., 2010, Distribution of isotopic and environmental tracers in groundwater, northern Ada County, southwestern Idaho: U.S. Geological Survey Scientific Investigations Report 2010–5144, 30 p.



Contents

Abstract

Introduction

Description of Study Area

Methods

Distribution of Isotopic and Environmental Tracers in Groundwater

Application of Tracers in Groundwater

Suggestions for Further Study

Summary and Conclusions

Acknowledgments

References Cited


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