Groundwater flow model for the lower Malheur Basin near Ontario, Oregon Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/7p88cj80p

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  • The shallow, poorly-confined, sand and gravel aquifer underlying the lower Malheur River basin is contaminated by nitrate and metabolites of the herbicide dimethyl tetrachloroterephthalate (DCPA), first detected in 1983 and 1985, respectively. The aquifer supplies water for the Ontario municipal water system, domestic and industrial uses, and irrigation. In response to this finding, the Oregon Department of Environmental Quality (DEQ), the Oregon Water Resources Department (WRD), and Oregon State University (OSU) have initiated a study of the groundwater quality and hydrogeology of the aquifer. The study includes analyzing groundwater samples for nitrate and DCPA metabolites (by DEQ); hydrogeological characterization, including measuring water levels and aquifer hydraulic parameters (by WRD); and developing a numerical groundwater flow and solute transport model (by OSU). This paper presents a regional groundwater flow model that will be used to estimate aquifer parameters and groundwater velocities for input into a solute transport model, and to identify additional data needs. A two-dimensional, steady state, finite element model was calibrated to a set of 36 water level measurements taken by WRD personnel in late September 1988. The model incorporates recharge from infiltration of surface irrigation water and leakage from irrigation ditches and withdrawals by production wells. Transmissivity was estimated from five pumping tests and using aquifer thickness data from well logs. The model was calibrated using a trial and error procedure. Surface irrigation recharge rates were estimated by minimizing the mean-squared-error (MSE) between predicted and measured water levels at the 36 wells. The calibrated model produced an MSE of 25.3 ft² and a mean error of 1.0 feet. The estimated recharge from surface irrigation is about eight times that from leakage through unlined ditches; however, many small, unlined ditches were not modeled as separate line sources and were lumped with the irrigation recharge term. Therefore the model suggests both irrigation water infiltration and irrigation ditch leakage contribute significant quantities of recharge. The model suggests that Dork Canal and Arcadia Drain can be approximated as specified head boundaries and that recharge may occur from uplands aquifers. Model predictions would be most improved by better definition of leakage from irrigation supply ditches, groundwater interception by major drainage ditches and shallow drainage systems on individual farm tracts, and the rate of withdrawal by producing wells. For transient flow analyses, better definition of the aquifer storativity is needed.
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