Human activities in urban and rural areas can have a significant impact upon the quality of surface water. Impacts upon water quality can originate from both point and non-point sources. Runoff from non-point sources carries sediment with contaminants, such as fertilizers, pesticides, hydrocarbons, and metals. Ultimately this sediment is deposited in lakes and rivers where it can be used to monitor water quality and environmental change.
O'Shaughnessy and Griggs Reservoirs lie within the Scioto River drainage in Delaware and Franklin Counties of central Ohio, and provide long-term records of sediment deposition in the vicinity of the Columbus metropolitan area. Griggs Reservoir presently sits within the Columbus metropolitan area. Analysis of aerial photographs of this area indicates that its surroundings are now 70% developed (primarily suburban and urban land uses) whereas 50 years ago they were 6% developed with land use predominantly agricultural. O'Shaughnessy Reservoir lies to the northwest of Columbus proper and its surroundings are now 23% developed whereas 50 years ago they were only 5% developed. The objective of this study is to use the sediment record from O'Shaughnessy and Griggs Reservoirs to determine the effects of these land-use changes on sediment loading and contaminant supply within the Scioto River watershed. Sedimentation rates in the two reservoirs have been determined from bathymetric profiles taken every 10-30 years. These sedimentation rates have generally decreased through time, although the upstream half of Griggs Reservoir has changed from an older zone of slight net erosion, caused by the construction of O'Shaughnessy Dam, to a younger zone of slight new accumulation. This general decrease in sedimentation rates is connected to land use and reflects the combination of improved agricultural practices and the decrease in sediment yield that accompanies the predominance of urban/suburban land use.
Surface sediment of both reservoirs contains no detectable amounts of pesticides or polycyclic aromatic hydrocarbons. Bioavailable phosphorus concentrations in O'Shaughnessy Reservoir are higher that those in Griggs Reservoir, as a result of the greater agricultural land use in the O'Shaughnessy drainage. Metal (Cu, Cr, Pb, Mn, Ni, Zn) concentrations are closely linked to clay abundance, and have significant anthropogenic sources. Phosphorus and metal mass accumulation rates parallel sediment mass accumulation rates; thus, these contaminants are primarily from non-point sources. Metal sources include industrial and residential waste, runoff from roads, the burning of fossil fuels, and fertilizers. Concentrations of metals in the reservoir sediment fall between guidelines used by the EPA for Lowest and Severe Effect Levels. The majority of each metal leaves the reservoir sorbed to suspended sediment, very little leaves the reservoir as dissolved load, and approximately 5% of each remains in reservoir sediment. With a greater influx of metals, the potential exists for concentrations to reach the Severe Effect Level.