Title

Investigating Recharge Routes to the Treasure Valley Aquifer System, Idaho Using Noble Gas Thermometry

Publication Date

12-2008

Type of Culminating Activity

Thesis

Degree Title

Masters of Science in Geosciences

Department

Geosciences

Major Advisor

James P. McNamara

Abstract

Over the past few decades, urban development has been the motivation behind many studies related to water resources, most notably in the western United States where the availability of water varies seasonally as well as year to year. The Treasure Valley in southwest Idaho shares these water resource management concerns. Recent trends in population growth along with changes in land use have raised concerns over water budget issues within the Treasure Valley, with strong emphasis on recharge routes and the effects of irrigation. Previous water balance studies have identified that the major source of recharge to the Treasure Valley Aquifer is seepage from canals and irrigation water, while less than 1% is estimated to originate from mountain block recharge through the adjacent Boise Front. This estimated value of mountain block recharge is poorly constrained and much lower than estimates from other similar mountain block adjacent aquifers. Further study of sources and routes of recharge to the Treasure Valley is important for identifying seasonal variability of recharge as well as future water management issues.

This study demonstrates the effectiveness of using noble gas thermometry methods to identify and discern between mountain block recharge and recharge through irrigation sources. Noble gas thermometry is the use of dissolved gas concentrations within groundwater to identify climatic conditions at the time of recharge. This process makes use of Henry's Law solubilities, which relate gas solubility to, most strongly, temperature and pressure. Concentrations of dissolved gases in groundwater represent climatic conditions (mean annual temperature, atmospheric pressure) of the recharge source. Assigning specific temperatures to individual recharge sources such as mountain block recharge, precipitation, and irrigation enables us to identity recharge sources to the Treasure Valley Aquifer based on noble gas concentrations extracted from groundwater. Mixing of multiple sources is approached through a three component mixing model developed in this study.

Calculated noble gas recharge temperature values from wells and springs within our study area ranged from 5.9 to 18.6̊ C. With the mean annual temperature for the Treasure Valley being roughly 11̊ C, temperatures greater than 11̊ C suggest contribution from a seasonally-dependent source (irrigation water), temperatures less than 11̊ C suggest water recharged at higher elevations than the valley floor (mountain block recharge). Higher recharge temperatures were generally found in groundwater wells located beneath agricultural lands, while lower temperatures were found in wells nearer to the basin margin and beneath urban land. This suggests that where irrigation activity is minimal, near the basin margin, the dominating recharge source may be mountain block recharge. With the data provided in this study, we developed the hypothesis that recent irrigation activity is changing and overwhelming previous natural recharge conditions.

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