Statewide GIS/census data assessment of nitrogen loadings from septic systems in Pennsylvania

Despite the role of septic systems in surface and groundwater N contamination, little statewide information has been collected or is available on the amount of N pollution from septic systems. This study reports on the development of a methodology to assess N released from septic systems at the state level and presents results of N loading estimates in Pennsylvania watersheds. The methodology describes steps to compute N loadings per watershed unit area within a Geographic Information System (GIS) framework based on (i) information on septic systems and population from the 1990 Census data, (ii) soil limitations to proper purification of septic system effluent acquired from the State Soil Geographic (STATSGO) data base, and (iii) daily N estimates per capita in septic system effluent from the literature. In this study, watersheds were ranked from 1 to 104 and grouped into three significantly different classes (high, medium, and low) according to their N loadings. The N loading ranges corresponding to these classes were 1.16 to 0.71 kg ha^-1 yr^-1, 0.66 to 0.41 kg ha^-1 yr^-1, and 0.38 to 0.03 kg ha^-1 yr^-1, respectively. High-producing watersheds were generally located in suburbs adjacent to larger metropolitan areas. The watershed ranking, groups, and N loading results provided by this methodology can be combined with similar information from agriculture, atmospheric deposition, etc. in the attempt to account for all sources of nonpoint N pollution in a state or region.