ASSESSING GROUNDWATER AND NUTRIENT FLUX TO DEER LAKE NEAR GRAND RAPIDS MINNESOTA USING A MULTI-SCALE HYDROGEOLOGICAL APPROACH

Lakeshore residents in Itasca and surrounding counties in northern Minnesota are concerned about degradation of their lake water quality. However, there are few data at present to suggest the trophic status and the sources of water and nutrients to these remote lakes. Through a Minnesota Clean Water Partnership Grant, a study was initiated in 2010 on Deer Lake, a 36-m-deep, 1600-ha lake near Grand Rapids, MN with the objective to determine the flux of water and nutrients to (and from) the lake. A multi-scale approach that includes minipiezometers, seepage meters, nutrient samples from private wells, stable isotopes of water (δ¹⁸O and δ²H) from all waters, groundwater dating, and a 2-D analytic element model was implemented in summer 2011. The lake has few surface inlets and appears to be dominated by groundwater inflow and outflow, potentially from a shallow unconfined aquifer and a deeper confined aquifer at about 90 ft depth. Preliminary water-table mapping suggests that the lake is a flow-through lake, with most water flowing out the south side of the lake. Seepage meters (42) show a range of flux values from 0.005 to .23 μm/s and were consistent with hydraulic head gradients from minizpiezometers (0.02 to 0.08) and the conceptual model of flow. Stable isotope samples taken from a private well on the south side of the lake and lake water in June 2010 and summer 2011 show that private well water is preferentially enriched with δ¹⁸O over δ²H (-9.62‰, -73.63‰ respectively) relative to the local meteoric water line, suggesting that lake water enriched in δ¹⁸O over δ²H (-5.00‰, -47.07‰ respectively) is flowing into the aquifer. Isotopic analysis of precipitation samples between 5/22/11 and 6/15/11 along with previously unpublished data yeilded a local meteoric water line of δ²H = 8.52 δ¹⁸O+ 14.84 (R² = 0.9764). Shoreline minipiezometers and lake water were sampled in the summer of 2011 for Total N, NH₄, SRP, TDP, TP, alkalinity, pH, and Cl. Results show alkalinity values ranging from 140 to 495 mg/L (as CaCO₃). Total dissolved P concentrations up to 353 ug/L occur in some minipiezometers but most lie in the 6 to 8 ug/L range. Isotope and nutrient values from the minipiezometers will be compared to those from 25 private wells to track sources of water and nutrients from the aquifer and into and out of the lake.