Evaluating differing trends across aquifer layers in a heterogeneous aquifer system

Abstract:

The Washington State section of the Columbia Plateau Regional Aquifer System (CPRAS) is well recognized as an agricultural industry, generating billions of dollars annually. Groundwater sourced from the CPRAS is an important resource that is in high demand for agriculture. Groundwater level declines have been documented in this region.

In this study, we examined publicly available groundwater level data from wells throughout the CPRAS in Washington State, along with a 3-dimensional stratigraphic model (CPRAS model) to evaluate the risk to groundwater availability by aquifer, as well as investigate potential inter-connectivity between aquifers.

Groundwater level measurements from production wells were collected from the database of Environmental Information Management (EIM) of the Washington State Department of Ecology. A total of 217 wells were selected to meet a criterion of 20 minimum observations representing a minimum of 20 years of spring annual data. Long-term trend analysis was computed for 1975-2019. The hydrogeologic sections of the CPRAS model were used together with the well locations, completion depths, and land surface elevation to identify the hydrogeologic unit each well was completed.

A total of 110 wells were completed in the Grande Ronde, 62 in the Wanapum, 33 in Saddle Mountains, and 12 in the Overburden. The mean of the trends in the Grande Ronde was a decline of 2.7 ft/yr with 90% of the wells declining while that of the Wanapum was a decline of 2.5 ft/yr with 89% of the wells declining. Within the Saddle Mountain, the mean of the trends and percentage of declining wells were 0.6 ft/yr decline and 73% respectively while that of the Overburden was 0.1 ft/yr and 50% respectively.

The well hydrographs were used to examine whether the layers in the heterogeneous aquifer system were connected. Similar hydrographs and the presence or absence of faults or folds were used to identify and better understand the interconnectivity of the aquifer layers.

A risk map was produced from the long-term groundwater level trends to help ascertain the risks involved in intense groundwater withdrawals. The results of the trend analyses by aquifer layer and the risk map are essential for the successful implementation of effective groundwater resource management strategies including groundwater rights allocations.