Watersheds disturbances

The primary pathway of Hg to aquatic systems is considered to be atmospheric transport and subsequent deposition, which has caused an accumulation of Hg in watershed soils (Mason, et al., 1994). Recent work has shown that watershed disturbance such as clear cutting results in the increased export of Hg from the watershed to aquatic systems (Porvari, et al., 2003) and others have found agricultural and urbanized land use to be important factors influencing Hg export (Fitzgibbon, et al., 2008 Mason Sullivan, 1998). Thus the recovery of aquatic systems from anthropogenic Hg may depend on watershed characteristics which will differ among watersheds. Therefore we compared time to recovery to watershed attributes (e.g., %urban, susceptibility to erosion, watershed to lake area ratio, etc...) to test the hypothesis that the rate of recovery from Hg enrichment is influenced by watershed controlled pathways and stressors. 

Meyer, J. L. and Tate, C. M. (1983). The effects of watershed disturbance on dissolved organic carbon dynamics of a stream. Ecology 64, 33-44. 

Miller, J.H. and M. Newton (1983). Nutrient loss from disturbed forest watersheds in Oregon s Coast Range. Agric. Ecosyst. Environ., 8 153-167. 

In a chemically disturbed landscape unit, exposure concentrations, as well as population and community responses, may show clear spatial patterns. Assessing risks at a landscape scale (e.g., watershed), however, requires the development of a... 

Suspended solids have the potential to silt out stream channels, rivers, lakes, and reservoirs they inhibit aquatic life and are expensive to remove from water. In some industries (e.g., mining) suspended solids, along with various other pollutants, are regulated by law, which requires that sediment ponds at the base of disturbed watersheds be built with sufficient detention time so that the water released meets certain sediment and water chemistry criteria (Tables 9.1 and 9.2). 

Kao, S. J., and Liu, K. K. (2000). Stable carbon and nitrogen isotope systematics in a human-disturbed watershed (Lanyang-Hsi) in Taiwan and the estimation of biogenic particulate organic carbon and nitrogen fluxes. Global Biogeochem. Cycles. 14(1), 189—198. 

Lewis, W. M. (2002). Yield of nitrogen from minimally disturbed watersheds of the United States. 

In natural lacustrine and slowly-accumulating reservoir sediments, core dating with the isotope °Pb has been used extensively (Schell and Earner, 1986). Appleby and Oldfield (1983) found that the constant rate of °Pb supply model (CRS) provides a reasonably accurate sedimentation chronology. The basic assumption of the CRS model is that the rate of supply of excess °Pb to the lake is constant. This model, thus, assumes that the erosive processes in the catchment are steady and give rise to a constant rate of sediment accumulation (MAR) (Appleby and Oldfield, 1983). In practice, for reservoirs, this assumption is rarely met because, for example, an increase in the MAR caused by land disturbances, such as those associated with the urban development, transports additional surficial soils and sediments to the lake. This additional erosion increases the MAR and also increases the rate of supply of °Pb to the lake. In general, because excess °Pb is an atmospheric fallout radionuclide, the model works better in low sedimentation rate, atmospherically dominated lakes with undisturbed watersheds, than in high sedimentation rate, fluvially dominated urban lakes and reservoirs. 

The gradient approach requires a great deal to be known about the landscape of the watershed, airshed, or other division of the area under consideration. Ideally, it is nice to have data on land use, geology, the hydrology, soil types, sediment composition, types of contaminants, the history of disturbance, and other information available when deciding a sampling plan. These data may not be available, and this uncertainty should be reported. 

This estimate is compared to the specific activity of likely sources (upland and marine) in Table XIII. It appears that neither material scoured from the mud-water interface of Long Island Sound nor material eroded from the upland surface is an allowable source, since only material from beneath these surfaces is of sufficiently low activity. It is relatively difficult to effect deep (18-20 cm) erosion of submarine sediment in Long Island Sound, but common for upland soil to erode deeply, especially when land is disturbed by agriculture or construction. This indicates that the source of low-specific-activity inorganic matter is the eroding subsurface material of the watershed. 

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