MERCURY DEPOSITION AND SOURCES FOR THE UPPER GREAT-LAKES REGION

Mercury concentrations and depositions for northeastern Minnesota were measured in precipitation to investigate depositional trends, relationships with major cations and anions, and possible source emission regions. Results for 1987-1990 showed that environmentally significant amounts of Hg are present in precipitation and air and are subsequently deposited to remote lake watersheds. Volume-weighted concentrations of total Hg in precipitation averaged about 18 ng Hg L-1 with calculated annual depositions near 15-mu-g Hg m-2. Mercury concentrations in precipitation are positively correlated with the major ions, conductivity, and pH, and are negatively correlated with precipitation volume. The best predictor equation from stepwise regression has an r2 of 0.65 with Mg and chloride concentrations as predictor variables. From measurements of Hg in rain concentrations as a function of time within events, scavenging ratios for "washable" Hg were calculated to be 140 +/- 80 (mass based at a 1 mm hr-1 precipitation rate). Up to about 10% of the total Hg in air is subject to washout by precipitation for a given event. Air parcel back-trajectories indicate that possible source regions within 72-hr travel time were located mostly to the south, southeast, and southwest, up to 2500 km distance away but local sources may also be important.