Susceptibility index to surface contamination for the Little Cross Creek watershed, Cumberland County, North Carolina

An index of surface-water contamination potential was constructed for the Little Cross Creek Basin, a 9.7-square-mile, water-supply watershed in Cumberland County, North Carolina. The index was developed because previous water-quality investigations raised concerns regarding inputs of bacteria, suspended sediment, and phosphorus from nonpoint sources in the watershed. A geographic information system was used to build map overlays and to categorize and rate three factors that affect the transport of water and contaminants-land-surface slope, distance to water, and land use/land cover. Each factor was weighted to reflect its potential contribution to surface-water contamination; the factors then were combined to estimate susceptibility values for the entire watershed. The numerical susceptibility values were categorized to indicate lowest to highest potential for surface-water contamination, and a map was produced showing the spatial distribution of these categories within the watershed.

The susceptibility index for about 17 percent of the Little Cross Creek watershed is rated in the high or highest category. These areas have high slopes, short distances to the nearest surface water, impervious land cover, and land uses that generate contaminants. About 38 percent of the watershed area is rated as having low or lowest susceptibility to contamination. These areas contain flat terrain, greater distances to water, land cover that promotes infiltration, and land uses that pose little risk for generating contaminants. Approximately 43 percent of the watershed is in the moderate category of susceptibility. Open water, which is not rated, accounts for the remaining area.

The susceptibility index provides water-resource managers with a tool that can aid in prioritizing areas within the Little Cross Creek Basin for monitoring, protection, and remediation. Previous suspended sediment, total phosphorus, and fecal coliform data collected in the Little Cross Creek watershed support the results of the susceptibility analysis. Although this susceptibility index is specific to the Little Cross Creek Basin, the methods used to develop the index are transferable to other watersheds.