Hydrogeologic Field Investigation and Groundwater Flow Model of the Southern Willamette Valley, Oregon |
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Craner Thesis |
Elevated groundwater nitrate (NO3-) concentrations in the Southern
Willamette Valley (SWV) caused the Oregon Department of Environmental Quality (ODEQ) to declare a Groundwater Management Area (GWMA) in Spring, 2004. To better understand direction of groundwater flow, groundwater age, and nitrate transport pathways of the SWV we developed a steady-state numerical groundwater flow model using MODFLOW with MODPATH. Model development was supplemented by field investigations of local outcrops, pump and slug tests, and laboratory analyses to determine groundwater age and groundwater chemistry. Field work included the construction/collection of cross-sections and stratigraphic columns; 12 slug tests and 3 pump tests to determine hydraulic conductivity and storativity; 10 groundwater ages using CFC-11, CFC-12, and CFC-113; 3 wells instrumented to collect long-term continuous water level measurements; 42 wells selected for quarterly manual water level measurements; and 14 groundwater samples to determine pH, dissolved oxygen, specific electrical conductance, chloride, sulfate, and nitrate concentrations. Slug tests determined horizontal hydraulic conductivities (Kx) from
4.19 x 10-8 m/s to 4.62 x 10-4 m/s. Pump tests
determined Kx-values from 3.59 x 10-4 m/s The calibrated groundwater flow model is intended to help make management decisions, establish monitoring programs, and to be used as an outreach education tool. Model simulations were run in key areas to demonstrate model capabilities and create visual aids for outreach education. This study suggests it may take 10’s of years to see measurable declines of groundwater nitrate in some locations. It is our hope that educating stakeholders about local groundwater flow along with stressing the use of Best Management Practices (BMPs) will result in better decision making and lead to a reduction of groundwater nitrate concentration in the SWV. |
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