Metal Release from Acid Snowmelt

Abstract

An investigation of seven Wisconsin lakes, five in the northern part of the state, (Bass, Buteau, Otter, Turtle, and Clara) and two in the central part, (Jacqueline, and University) was conducted for trace metal mobility in response to acid snowmelt. The five northern lakes are small oligotrophic, low alkaline, bog influenced lakes. This area was chosen due to it’s combination of acid loading and geological sensitivity. The central Wisconsin site contains two lakes; a shallow soft water, lake, and a hard water, oligotrophic lake. This area has received more acidic precipitation than the northern lakes, but is more geologically resistant to inputs of acidic material. Lake water samples were collected during ice cover, ice out (spring thaw), and late summer. Parameters measured included trace metals (Al, Cd, Cu, Fe, Ni, Zn), anions (S04, N02/N03, Cl), alkalinity, hardness, and pH. Snow samples were also collected around each lake. In addition to the above mentioned parameters, acidity measurements were conducted on the snow samples. Sediment samples from each lake were analyzed for trace metal concentration. Trace metals were analyzed using a Varian atomic absorption unit, using both the units flame and furnace modes depending on the sample. Anions (SO4 and Cl) were analyzed on a Dionex ion chromatograph. NO2/NO3 analyses were conducted using a Technicon Auto Analyzer II. A laboratory sediment leachate study was also conducted using H2S04, HN03, HCL, H2C03. In this study the pH of lake water in combination with sediments was decreased to 4.00 (equlibrium pH with H2CO3 was 4.60). The trace metal concentration following this treatment was then measured and compared to values before acidification. Trace metals increased following snow melt in all seven study lakes. There appears to be two major sources of these increases: the displacement of base metal ions from the soil/sediment by hydrogen ions contained within the percolating meltwater, and the snowpack itself. Whether the increases are the result of cation exchange or directly from trace metals contained within the snow, there is an increase of trace metals, approaching toxic levels, occurring during the spring thaw in these Wisconsin lakes. Considering the number of Wisconsin lakes with similar geology and acid loadings, the potential ecological damage is great. Due to the similar nature of Jacqueline lake and the northern lakes, they responded similarly to inputs of acid snowmelt. Increases in alkalinity, hardness, pH, and trace metals appear to be caused by cation exchange reactions occurring between the meltwater which is high in hydrogen ion concentration and the soils and sediments of the watershed. As this meltwater percolates through the soil it displaces cations and base metal ions carrying them into the lake, thus increasing the alkalinity, hardness, and pH. This may be indicative of a loss of this area's limited buffering capacity through leaching processes. University lake was the only hard water lake included in this study and the only lake which showed a decrease in pH, alkalinity, and hardness values. This may be the result of meltwater entering the lake directly, due to the frost depth or nature of the surrounding soils, thus allowing the meltwater to express itself strongly within the water column. This would account for the decreases in pH, alkalinity, and hardness. In addition the relatively high concentration of alkalinity, and hardness found in University lake would require large amounts of base ions flushed into the system to show a significant increase. Nitrate/nitrite concentrations decreased for all lakes following snowmelt, probably due to biological utilization as the meltwater percolated through the surrounding soil, along with early phytoplankton growth within the water column. Trace metal concentrations decreased later in the summer, likely due to adsorption and precipitation to the sediments.