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Groundwater nitrate contamination is a well-documented issue in the Southern
Willamette Valley (SWV) of Oregon, as a Groundwater Management Area (GWMA) has
recently been declared. As a GWMA, groundwater nitrate monitoring must occur until
regional concentrations are below 7 mg/L NO3-N. However, the presence of temporal
variability can make it difficult to determine if contamination exceeds a threshold and if
contamination is increasing or decreasing over time. To examine the potential impact of
temporal variability on groundwater nitrate monitoring in the SWV, a well network was
created and sampled monthly for 15 months. Results indicate that substantial intra-well
temporal variability is present, and that spatial variability of groundwater nitrate is
greater than temporal variability. Generally, temporal variability was associated with
recharge events, which flushed higher concentration soil-water into the aquifer. Though
individual wells showed seasonality, network-wide seasonal trends were not statistically
significant (which is believed to be caused by a dampening effect due to local
heterogeneities). From a monitoring perspective, this implies that less frequent
groundwater nitrate sampling (such as quarterly) can capture network-wide seasonal
response to the same degree as monthly sampling.
To determine how long-term land management practices are likely to impact
regional nitrate leaching and future monitoring trends, a nitrogen loading model was
created for the SWV. Present-day data were used to calibrate and validate the Soil and
Water Assessment Tool (SWAT) model, with 3 alternative future scenarios then being
evaluated. The effects of agrarian Groundwater Best Management Practices (GW-BMPs)
were examined with respect to nitrate leaching in present and future scenarios. Modeled
values indicate that agrarian GW-BMP implementation is a more effective agent for
reduced nitrate leaching than land use change alone. Together, land use change and the
adoption of GW-BMPs were found to decrease nitrate leaching values by 32 to 46% of
their present-day rates. These predicted results do not include the impact of
denitrification or changes in septic leaching, and therefore should be regarded with
caution as they do not completely represent future conditions. Considering this, a
conservative conclusion which can be drawn is that GW-BMP implementation is a safer
alternative than reliance on projected land use/crop change alone for lessening
groundwater nitrate concentrations in the GWMA. This is the first study to successfully
apply SWAT as a tool to examine the spatial and temporal variability of nitrate leaching. |
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