A MULTI-METHOD EVALUATION OF GROUNDWATER TRENDS IN THE COLUMBIA RIVER BASIN

Collins Kissi Asante-Sasu

doi:10.7273/000006472

The Columbia Plateau Regional Aquifer System (CPRAS) is a heavily irrigated agricultural region. In some parts of the region, groundwater is the only source of water for domestic and municipal water supply as well as irrigation. In other areas, groundwater serves as a supplementary source to surface water, especially during droughts. Declining groundwater trends have been reported in the CPRAS, mainly due to long-term pumping for irrigation purposes. Little attention has been given to how the trends vary between the shallower, mostly unconfined, and deeper, mostly confined, portions of the aquifer. This dissertation employs a multidisciplinary approach, integrating hydrogeological analysis, remote sensing, GIS techniques, and statistical methods to examine regional groundwater status, trends, and vulnerabilities in localized subareas in Eastern Washington, historically and during drought periods specifically. Statistical analyses are conducted to quantify trends in groundwater levels and vulnerabilities. The remote sensing approach employs the GRACE satellites to study groundwater storage changes. The findings reveal significant trends in groundwater declines in all aquifer layers of the CPRAS. Localized subareas such as the Odessa subarea, which has groundwater as the main source of water supply for all uses, experience very high significant groundwater declines in the Grande Ronde layer. The degree of groundwater vulnerabilities due to over abstraction varies both vertically and spatially. While a lot of the subareas experience high vulnerabilities, some experience positive trends indicating no vulnerabilities based on groundwater level change. The dissertation also found that the GRACE satellites are not able to capture groundwater dynamics in the confined aquifers of the CPRAS. Groundwater used as a drought mitigation tool shows that the variances of groundwater level differences between post-drought years and non-drought years are significantly different based on a 95% confidence interval, but the true means are not statistically different. The percentage declines in water level between post-drought years and non-drought years also vary based on the years when droughts were declared, and the aquifer layers the wells are screened in. This dissertation contributes to our understanding of groundwater trends and vulnerability due to over abstraction and provides valuable insights for addressing the challenges facing groundwater resources in the context of growing human demands and environmental change.

A MULTI-METHOD EVALUATION OF GROUNDWATER TRENDS IN THE COLUMBIA RIVER BASIN

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