QUANTIFYING SURFACE WATER-GROUND WATER EXCHANGE USING TEMPERATURE PROFILE INVERSE MODELING IN A RIPARIAN WETLAND

Second Creek is wild rice stream located on the Iron Range in northeast Minnesota that has been impacted by mining pollution. In order to understand how mining-derived sulfate affects biogeochemical cycling at Second Creek, surface water-ground water exchange must be quantified, because it controls geochemical gradients in the sediment. We employed inverse modeling of temperature profiles to estimate hyporheic flux at the site. Temperature profile methods have been most widely applied in streambeds with sediments that are sand-size and greater and support relatively high flux magnitudes. In contrast, the Second Creek study site is a riparian wetland where low hyporheic flux is expected due to high organic content in the sediments.

Streambed temperature profiles were measured continuously over the summer of 2016 at three locations across a transect spanning from the main stream channel to the flanking wetland area. The data were collected using low-cost, open-source vertical temperature profilers and “ALog” data loggers. The USGS model 1DTempPro was applied to the temperature data, along with co-located head data, at each location to estimate hydraulic conductivity across the transect. The sediment thermal parameters used in the model were constrained based on the sediment bulk density, which is strongly controlled by organic content. The estimated hydraulic conductivity values were applied to the measured head gradients to generate time series of hyporheic flux time across the transect over the summer. Results showed spatial variability in both hydraulic properties and hyporheic flux. Across the transect, flux was upward toward the surface water for nearly the entire summer, though the magnitude of the flux varied dynamically in response to variable weather conditions and one flux reversal occurred following a strong late-summer storm event.