Groundwater’s dynamic role in regulating acidity and chemistry in a precipitation-dominated lake
The role of groundwater in regulating the hydrologic and chemical regime ofan “acid-susceptible lake situated in a sandy silicate aquifer in northern Wisconsin was determined by direct groundwater measurements using a network of 68 piezometers. Groundwater inputs were compared with precipitation and dryfall components of the hydrologic and chemical budgets for the lake. Groundwater contributed only 10\% of the water to Crystal Lake in 1982 but was the major source of alkalinity, calcium, magnesium, sodium, potassium, silicon, and chloride. Precipitation contributed 90\% of the water and was the major source of hydrogen and sulfate ions. Continuing analyses suggest that groundwater consistently supplies the bulk of the inoorganic constituents to the lake on annual basis. The flux of groundwater and dissolved solids to the lake is highly seasonal, with large pulses of groundwater inflow occuring after spring snowmelt and in the fall. For certain nonconservative elements such as silicon, the periodic influx of chemically concentrated groundwater can trigger significant changes in the chemical and biological balance in the lake. Despite the fact that the aquifer contains no carbonate minerals and the area experiences acid precipitation (pH = 4.6), the alkalinity of the small amount of groundwater entering Crystal Lake is more than enough to neutralize all of the acidity contributed by precipitation. In-lake sulfate reduction may generate an equal amount of alkalinity. This buffering of pH helps to regulate aluminum concentrations at low levels, thus minimizing potential aluminum toxicity in the lake. Most lakes in the region probably receive even larger amounts of alkalinity-rich groundwater than Crystal Lake, because most lie further downgradient in the regional flow system. This may help to explain why most lakes in this “acid-susceptible area are no more acidic today than they were 50 years ago.