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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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