TRANSIENT HYDROGEOLOGICAL CONTROLS ON THE CHEMISTRY OF A SEEPAGE LAKE

A solute mass balance method was used to estimate groundwater inflow a nd outflow rates for Nevins Lake, Michigan, a seepage lake in the uppe r peninsula that historically has shown extremely variable water chemi stry compared with most other seepage lakes. A 4-year Study (1989-1992 ) of the hydrology and geochemistry of Nevins Lake and its contiguous groundwater system revealed that changes in the mass of dissolved solu tes are the result of annual hydraulic gradient reversals. A pronounce d acidification of Nevins Lake from 1986 to 1988 was likely caused by drought-induced diminished groundwater inflow rates. In this study, di ssolved calcium (the major cation in water of Nevins Lake, groundwater , and precipitation) was used for estimating mass flow rates. During t he 1989-1992 period, Nevins Lake showed a reproducible annual cycle in calcium mass. Immediately following spring snowmelt and the resulting hydraulic gradient reversal, the mass of dissolved calcium in the lak e increases rapidly, and then it decreases steadily throughout the sum mer and early fall, at which time the lake becomes hydraulically mound ed and receives no groundwater inflow. Groundwater flow rates estimate d by the solute mass balance method are sensitive to assumed solute co ncentrations in discharging groundwater. Pore water samples from the l ake bed are shown to be more representative of water discharging to th e lake than are samples from piezometers near the lake shore, but spat ial and temporal variability in pore water chemistry must be considere d. Stable isotope analyses (O-18 and H-2) of lake water, groundwater, and pore water samples show that water discharging to Nevins Lake in t he spring is entirely recycled lake water, and no groundwater derived from terrestrial recharge reaches the lake. The conceptual model formu lated during this study linking lake chemistry and the contiguous grou ndwater system and general groundwater flow patterns surrounding highl y transient lake systems are likely transferable to other similar syst ems.