32 Hydrogeology

Added due to amount of discipline abstracts submitted.

Tuesday, 10 March 2026
1:30 PM - 5:30 PM
RCC, 106

Regional Nitrate Monitoring Using Stream/Road Crossings

Paper number: 32-2
Program: Technical Sessions
Time: 2:00 PM – 2:15 PM

Authors

Abstract

Nitrate concentrations greater than the health standard of 10 mg/L are common in surface and groundwater in the Central Sands region of Wisconsin. This region, written about by Aldo Leopold in the Sand County Almanac, is home to agriculture, natural areas including trout streams, and rural homeowners. The many streams in this region are baseflow dominated with minimal run-off input.  As a result, the water quality of a stream reach is closely related to the water quality of the groundwater discharge into that stream reach.

To understand the distribution of nitrate in the groundwater of the region, we measured nitrate concentrations in streams at road crossings using a portable UV nitrate sensor. The UV sensor was checked against laboratory measurements to assess its accuracy. Benefits of this monitoring approach are easy and long-term access to the monitoring locations since the roads are public access to navigable water of the state. In addition, there is no expensive infrastructure to build or maintain. Out of approximately 1400 road and stream crossing locations, we measured nitrate at over 500 sites between May and August 2024.

We used these data in conjunction with particle tracking from a regional groundwater flow model.  These data, combined with the model and land use in the groundwater contributing area for the stream above the point of nitrate measurement, provide a measure of correlation between land use and the nitrate concentrations in the streams.  We found that agriculture, more specifically potato/vegetable agricultural land use, was correlated (R2=0.47) with nitrate when including all the groundwater contributing areas for the stream reaches.  When the groundwater contributing areas were limited to only those greater than 10,000 acres, the correlation increased to R2=0.86.  This result suggests that caution should be applied when attempting to state correlation between land use and stream water quality when the groundwater contributing areas are below a size threshold.  This also supports the intuitive conclusion that large land use changes will be more likely than small land use changes to affect stream water quality.

Geological Society of America Abstracts with Programs. Vol. 58, No. 1

doi: 10.1130/abs/2026SE-411645

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