Title

A Geochemical Investigation of Groundwater Sources, Blackfoot River, Idaho

Publication Date

5-2009

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Hydrologic Sciences

Department

Geosciences

Major Advisor

James McNamara, Ph.D.

Abstract

An isotopic study of groundwater/surface water mixing in the agricultural Snake River Floodplain (Blackfoot, Idaho) was performed using river, canal and irrigation well samples. Strontium (87Sr/86Sr) and uranium (234U/238U) isotopes were used to spatially constrain mixing ratios of Snake and Blackfoot River water, show alternative geochemical influences on the system such as evaporation and host-rock mineralogy, and delineate a groundwater divide between two rivers hydraulically tied to a network of irrigation canals.

Construction and calibration of a river/subsurface mixing model (Faure, 1986), based on Sr isotopic ratios required analysis of the following: 1. Effect of water mass mixing on well isotopic composition. 2. Effect of evaporation on well Sr, Cl concentrations, and model calibration. 3. U isotope ratios and major ion concentrations were used to add additional constraints to the mixing model.

The mixing model indicates that Snake River water predominantly influences the shallow aquifer in the valley. However, there is a clear plume of Blackfoot River water superimposed on the predominant Snake River water. Two end-member water (Snake and Blackfoot Rivers) and evaporative mixing were found to be the main controls on shallow groundwater composition throughout the study site. Two anomalous groundwater regions were observed along the margins of the study area. A low U, high Sr isotopic zone (LUHSZ) and a high U, high Sr (HUSZ) isotopic zone are likely due to Snake, Blackfoot River mixing, host-rock mineralogy from mountain blocks or a unique tributary source. During the irrigation season, a seasonally high water table induces water to the shallow aquifer observed in the similar Sr isotopic compositions in canals and wells. This shallow aquifer is a major influence on groundwater isotopic compositions and is the result of Snake and Blackfoot River water recharge. Groundwater contours and a general end-member divide have been mapped based on proportions to the two rivers.

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