Document Type

Thesis

Date of Degree Completion

Spring 2023

Degree Name

Master of Science (MS)

Department

Geological Sciences

Committee Chair

Carey Gazis

Second Committee Member

Lisa Ely

Third Committee Member

Walter Szeliga

Fourth Committee Member

Tom Ring

Abstract

Delaying groundwater discharge into rivers until it is critically needed during baseflow conditions provides promise for lowering elevated stream temperatures and improving habitat for aquatic species. Increasing groundwater storage may accomplish this in locations where excess spring runoff can be captured and allowed to infiltrate into the subsurface for later beneficial use, a process known as Managed Aquifer Recharge (MAR). Here, MAR via an infiltration basin is considered at a site along the Teanaway River in central Washington State. The effects of simulated ephemeral ponds of sizes varying from 554 m3 to 2430 m3 (0.449 acre-feet to 1.97 acre-feet) on the existing groundwater flow regime are investigated using a transient MODFLOW groundwater flow model.

The groundwater flow model is calibrated against 217 groundwater head observations at the site over a span of 2 years. Secondary calibration is performed by comparing the MODFLOW model to an analytic water balance developed by idealizing the water table with a least-squares plane of best-fit to determine the change in groundwater storage from water table fluctuations. Both the transient groundwater flow model and the analytic water balance model employ specific yield in the calculation of groundwater storage. Coupling of the two models produces a weighted average of specific yield at the site for the shallow alluvial aquifer and upper 10 m of sandstone bedrock of 0.12.

Comparison of head elevations and mass balances between the calibrated groundwater flow model and the different ephemeral pond scenarios indicates that increased infiltration contributes to an increased overall volume of the system in the short term, with the additional water returning to baseline levels by September following the drying up of the pond at the end of April. The maximum increase in groundwater elevation is 0.76 m. The total increase in volume of the system is offset by increased discharge across the downgradient boundary of the site and to the river, with a modest increase in evapotranspiration. For the storage and hydraulic conductivity conditions of the shallow alluvial aquifer at the site, MAR in the spring would not have lasting effects into summer when increased baseflow is desired.

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