Mercury geochemistry of the Scioto River, Ohio: Impact of agriculture and urbanization

The Scioto River is a major tributary to the Ohio River, which runs from its headwaters in row crop dominated agricultural lands in central Ohio, south through the city of Columbus, the 15th largest city in the USA. During a high flow event in April 2004, water was collected from seven locations along the river, and analyzed for a number of chemical constituents including total and dissolved Hg. Total Hg concentration increased continually downstream with the highest concentrations found at Commercial Point, just below two large sewage treatment plants that serve metropolitan Columbus. The highest Cl− concentration was also found there. The highest NO3- concentrations were found in the agriculturally dominated portion of the river. The highest dissolved Hg concentration occurred in downtown Columbus. Using flow data from the day when the samples were collected, Hg yields were calculated at three locations within the basin: at Prospect in the northern, agriculture-dominated part of the basin; at Bellepoint located upstream of two reservoirs, just north of Columbus proper; and at downtown Columbus. The dissolved Hg yields in ng km−2 s−1 increased by a factor of 2 from Prospect to Bellepoint and then another 50% at Columbus. The particulate Hg yields increased only 10% from Prospect to Bellepoint, but 30% from Bellepoint to Columbus, with the particulate Hg yields about ∼2.5 to 4.5 times greater than the dissolved ones. These data suggest that yields of particulate Hg are affected more by urbanization than agricultural activities. The cause of the increasing yields of dissolved Hg as the river proceeded downstream is not clear at this time. It is assumed that the last increase is due to input from the urbanized portion of the watershed. As demonstrated previously, it appears that urbanized regions may retain a lower percentage of atmospherically deposited Hg than other landscape types.