Hydrogeologic field investigation and groundwater flow model of the southern Willamette Valley, Oregon Public Deposited



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  • 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 to 7.22 x 10-3 m/s, vertical hydraulic conductivities (Kv) from 3.48 x 10-6 m/s to 3.84 x 10-6 m/s, and storage coefficients from 0.05 to 0.15. Groundwater age ranged from 13 years to >50 years, with the greatest ages resulting from wells that penetrated the semi-confining Willamette Silt. Groundwater ages were compared to model particle travel times using MODPATH and used as calibration targets. Groundwater ages along with nitrate, chloride, sulfate, and dissolved oxygen concentrations were used to reconstruct past contaminant loading and observe data trends. Spatial distributions of hydraulic conductivity were estimated using wells with specific capacity data and an empirical relationship (T = 158.48sc, where T = transmissivity (ft2/d) and sc = (gal/min/ft); R2 = 0.61) between wells in the study area that contained both specific capacity and aquifer test data. 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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