Submitted to: Catena
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/11/2009
Publication Date: 10/15/2009
Citation: Rhoton, F.E., Bennett, S.J. 2009. Soil and Sediment Properties Affecting the Transport and Accumulation of Mercury in a Flood Control Reservoir. Catena. 79: 39-48.
Interpretive Summary: Grenada Lake is a relatively large flood control reservoir in north central Mississippi that captures runoff for a 78,000 hectare watershed that has a long history of erosion and channel instability. Mercury found in some fish species in this water body is of some concern since it exceeds Food and Drug Administration standards for human consumption. Since the primary source of mercury in this reservoir is the sediment transported from the upland soils and stream channels in the watershed, we characterized watershed soils, stream sediment, and reservoir sediment for several physical and chemical properties that influence the transport of chemical pollutants with sediment. Our data showed that the mercury is most concentrated in the surface of the watershed soils where it is more susceptible to runoff and erosion processes, and that is not closely associated with soil clay and organic matter components responsible for most chemical transport. However, reservoir and stream sediment data showed that mercury was picked up by the clay portion of sediment. These results indicate that during the erosion and sediment processes dissolved mercury is picked up by suspended sediment clay and organic matter and is carried into Grenada Lake primarily on fine clay particles. This information can be used to select best management practices to reduce the content of particulates in runoff from upland soils and mercury concentrations in sediment for improved water quality.
Technical Abstract: Mercury accumulations in some fish species from Grenada Lake in north Mississippi exceed the Food and Drug Administration standards for human consumption. This large flood control reservoir serves as a sink for the Skuna and Yalobusha River watersheds whose highly erodible soils contribute to excessively high sediment yields and impaired water quality. This study was conducted to characterize the distribution of total Hg in watershed soils and determine the relationship between the easily transportable clay and organic C (OC) fractions and the movement of Hg from upland sources to reservoir sinks. Cores were collected from soils, of different land-use, representative of the three soil orders (Alfisols, Entisols, Vertisols) found in the watersheds. Sediment cores were collected from the Yalobusha River and Grenada Lake. All cores were segmented and dried in the laboratory. The fine earth fractions (< 2 mm) were characterized for total Hg, particle size distribution, organic C, and pH. Mercury concentrations ranged from 10 to 112 µg kg-1 in the soil profiles, with average regression coefficient (r2) values of 0.104 for Hg versus clay, and 0.362 for Hg versus OC. River sediment cores had Hg concentrations ranging from 0 to 38 µg kg-1, and significant (1 % level) r2 values of 0.611 versus clay and 0.447 versus OC. Mercury in the lake sediment ranged from 0 to 125 µg kg-1. The r2 values for Hg versus clay were 0.813, and 0.499 for Hg versus OC, both significant at the 1 % level. These results indicate that total Hg is only poorly correlated with clay and OC in these soil profiles because is most concentrated in the near surface horizons, due to atmospheric infall, that are generally low in these two soil components. The statistically significant relationships obtained for Hg versus clay and OC in the sediment cores are the result of particulate clay and OC interacting with dissolved Hg in the runoff during the sediment transport process. The higher r2 values for the lake sediment are due to the preferential transport and deposition of fine clay fraction which substantially enhances the exchange capacity of the total clay fraction.