Document Type

Thesis

Date of Degree Completion

Fall 2012

Degree Name

Master of Science (MS)

Department

Geological Sciences

Committee Chair

Carey Gazis

Second Committee Member

Audrey Huerta

Third Committee Member

Dimitri Vlassopoulos

Abstract

Groundwater, the primary source for potable water in the Columbia Plateau, has experienced significant declines over the past 25 years. Groundwater geochemistry and multivariable statistics were employed to identify and characterize the hydrofacies present, constrain recharge sources, and assess mixing of groundwater in the Royal Slope area of the Columbia Plateau. Geochemical analyses for major ions, stable isotopes, and radiogenic isotopes (14C and tritium) for age dating were performed on groundwater samples collected in 2010 from 24 wells in the study area. The results confirmed that the groundwater evolution in the study area follows the typical Columbia River Basalt Group (CRBG) trend of increasing Na and K with residence time. Four distinct hydrofacies groups were identified by major ion chemistry. Group 1 groundwater samples are from the overburden aquifers. Groundwater samples from these aquifers are isotopically similar to local surface water and precipitation, suggesting a significant component of modem recharge. Groundwater samples in Groups 2, 3, and 4 are from the CRBG aquifers, with Group 4 representing the most evolved end-member. Group 2 is characterized by increased surface water contribution mixing with relatively less evolved CRBG groundwater, as indicated by younger apparent 14C ages (usually< 6000 years), higher concentrations of Ca, Mg, NO3-N, and tritium, and relatively heavy stable isotopic signatures. The geochemistry of Group 3 falls in an intermediate range and generally on a mixing line between Groups 2 and 4, indicating groundwater mixing. Group 4 is characterized by more evolved CRBG groundwater, with higher concentrations of Na, K, and F, and lower concentrations of Ca, Mg, NO3-N, and tritium. Group 4 samples are the most isotopically depleted with respect to heavy isotopes; this likely represents recharge during ancient times and is supported by apparent 14C ages of 12,000 to 22,000 years old. The geochemistry, stable isotope, and groundwater age data suggest that modern groundwater recharge depends on a number of factors, e.g., well depth and type, location of well, and aquifer unit and characteristics. Generally, the wells open to the shallower aquifers, e.g., all wells in Groups 1 and 2, receive more modern recharge than those open to the deeper aquifers, e.g., most wells in Group 4. Ancient groundwater recharge in the deeper aquifers, with little indication of modern recharge, confirms that groundwater mining is occurring.

Language

English

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