Extending the isotope based (d 18O) mass budget technique for lakes and comparison with solute based lake budgets
Groundwater inflow and outflow rates were determined for nineteen lakes with no associated streams (seepage lakes) in Vilas County, Wisconsin using the oxygen isotopes (1BQf16Q) and steady state mass-balance techniques using solutes. Estimates from the index-lake technique, normalized to lake surface area, range from 0.02 to 0.28 m/yr for inflow and 0.22 to 0.48 m/yr from outflow. This is equivalent to 2\% to 26\% of all of the annual inflows (groundwater input divided by all hydrologic inputs). Lakes receiving relatively large amounts of groundwater receive groundwater that is more evolved (more dissolved solids) than lakes receiving minor amounts. This has important implications for assessing acid sensitivity of lake systems. Three factors influence the hydrologic budgets of lakes with no associated streams: position in the regional groundwater flow system, lake morphometry, and geology of the contiguous aquifer. Lakes located near groundwater divides receive the least amount of groundwater and can be viewed as recharge lakes. Lakes that are intermediate and low in the flow system act as flow-through lakes (after Born et al., 1979). In general, large lakes receive more groundwater normalized to surface area) than smaller lakes. Lakes located in outwash receive less groundwater than a nearby lake in till. Groundwater o1BQ is uniform at the scale of Vilas County. Twenty-seven samples show the groundwater o18Q to be at -11.2 o/oo ± 0.7 o/oo (one standard deviation). A slight trend of depletion of groundwater toward Lake Superior was found. This trend can be accounted for by higher winter precipitation to the northwest and suggests higher recharge rates in the northwest portion of the county. Lakes with mean depths greater than five meters were isotopically invariant through the summer of 1991. For lakes with mean depths less than five meters, seasonal isotopic variation (from May to October) follows a linear relation of increasing variability with decreasing mean depth. Most of the study lakes showing seasonal variation display a steady evolution from isotopically depleted in the Spring to isotopically enriched in the Fall. The solute mass balance technique of lake budget calculation is limited due to heterogeneity of groundwater chemistry. Results using calcium and magnesium as tracers compare favorably with isotope estimates. The use of potassium as an indicator of groundwater fluxes is confounded by contribution of potassium to lakes by leaf litter. Biological cycling and removal limit the use of silica as a solute tracer. The solute mass balance is not amenable to the use of. chloride, because concentrations in precipitation are similar to groundwater and some portions of the aquifer are affected by roadsalt contamination. The inverse use of the solute mass balance equation with estimate of groundwater discharge and recharge rates from isotope data provides a powerful tool for determining the average, volume weighted chemistry of discharging groundwater. This technique may be used to assess the biological removal of solutes (eg. the incorporation of silica in diatom frustules) and aerosol inputs (eg. potassium contributions to lakes by leaf litter).