Before she came to work as my graduate student, Amy Haase wrote a master's thesis on water quality in Pennsylvania watersheds. Using data from about 40 watersheds, she and her masters advisor found empirical relationships between two land-use parameters: wooded area and impervious surface area (ISA), and three water quality parameters: concentrations of nitrogen, phosphorus and sediments. These concentrations were then compared with results from an earlier survey of ecosystem quality, to determine whether the land use in a given watershed was likely to lead to significant ecosystem impacts. They presented their results in the form of a GIS-driven web interface, that allowed stakeholders to predict water quality in their favorite watershed by tracing its boundaries on a map.
My contribution to published paper for this project was to plot pollutant concentrations against land use parameters using the empirical relationships. Each pollutant has a cutoff level above which the ecosystem is expected to be impacted (below). Note that nitrogen concentrations depend only on wooded area, and not on ISA. By contrast, sediments are weakly dependent on ISA and phosphorus concentrations are strongly dependent on ISA. A cautious watershed manager should thus use predicted phosphorus concentrations to determine safe land use. For these Pennsylvania watersheds, areas with at least 75% woodland and less than 5% ISA may be considered pristine.
Figure Caption: Impairment thresholds for three pollutants as a function of wooded area (%wood) and
impervious surface area (%ISA). The diagonal line cutting off the upper right hand corner corresponds to
%wood + %ISA = 100.