Using geographic information systems and regression analysis to evaluate relationships between land use and fecal coliform bacterial pollution

Geographic Information Systems (GIS) and regression modeling techniques were used to evaluate relationships between land use and fecal pollution in Murrells Inlet, a small, urbanized, high-salinity estuary located between Myrtle Beach and Georgetown, SC. GIS techniques were used to identify and calculate land use and spatial variables to be used in a regression model. The regression analysis was performed to identify specific land-use characteristics that may influence fecal coliform densities in the estuary. The regression modeling used land-use parameters to explain the variability of fecal coliform densities measured monthly at 21 locations in the estuary over the 10-year period from 1989 to 1998. Individual regression models were generated for each season, and for a combined data set. The results of the regression analyses indicate that proximity to areas with septic tanks, and rainfall runoff from urbanized areas are important predictors of fecal coliform densities in the estuary. Sampling sites closer to areas with high densities of active septic tanks or more urbanized land uses tended to have higher fecal coliform densities. Although these results may suggest that septic tanks are a substantial human source of fecal pollution, previous research has indicated that the fecal pollution in those areas is probably not from human sources. The areas of Murrells Inlet with higher septic tank densities also are located in areas of high housing density, are in close proximity to the land-water interface, and are in the extreme upper reaches of the estuary, where flushing and dilution effects may be reduced. The higher fecal coliform densities observed at these locations may be a coincidental result of these factors and fecal deposition from pets, and not the direct result of fecal pollution input from the septic tanks.The results also show seasonal differences in the dynamics of fecal coliform bacterial pollution in the estuary. The winter model included a rainfall interaction term, which indicates that ground saturation effects may be an important part of fecal deposition in winter months. The water quality management implications of this research include identification of strategies to reduce or intercept urban stormwater runoff, reduction of waste dumping from boats, and reduction of pet waste.