Date of Final Oral Examination (Defense)

12-2011

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Hydrologic Sciences

Department

Geosciences

Major Advisor

Shawn Benner

Abstract

This study was initiated to evaluate potential source(s) of elevated uranium in ground and surface waters of the Treasure Valley in southwest Idaho. Groundwater in the area exhibits widespread but complexly distributed uranium concentrations up to 110 mg L-1, well in excess of the U.S. EPA drinking water standard of 30 mg L-1. Data from field sampling (surface water, groundwater, and solid sediments), laboratory experiments, and geochemical and isotopic analysis constrain the source of the elevated uranium. Results from surface water sampling show significant downstream increases in uranium concentrations. With irrigation return waters and shallow groundwater returns indicated as the primary contributors toward elevated uranium concentrations, evidence suggests that a near-surface uranium source exists within the valley. When evaluated for isotopic composition, these surface waters consistently evolved toward a common nexus of 234U/238U and 87Sr/86Sr isotopic composition that is also shared by the estimated mean groundwater composition and several of the most elevated groundwater samples. Analysis of a wide variety of geologic materials representing aquifer sediments did not uncover materials containing particularly high bulk uranium contents (avg. of 3.5 ppm). Furthermore, isotopic analysis of nearly all the solids produced low 234U/238U ratios that are incompatible with the source material. In addition, isotopic results definitively indicate that the analyzed fertilizers cannot be the source of the uranium. Only two shallow geologic samples collected from terrace and floodplain sediment yielded high enough234U/238U ratios to match the projected source signature. Isotopic and elemental differences between three selective leaching treatments applied to each solid show that, on average, the most uranium and highest 234U/238U ratios were associated with the carbonate extraction. The two high 234U/238U solids did not contain particularly high carbonate contents, and it appears that the carbonate leaching solution acts to assist in releasing the source uranium from sorption and exchange sites in some shallow, fine-grained, clastic sediments of the Gowen Terrace and modern floodplain formations.

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