Statewide non-point-source pollution assessment methodology

We present a new methodology for performing Virginia's statewide, watershed-level non-point-source pollution assessment. The problems and inconsistencies inherent in previous methodologies were addressed and improved upon. The revised methodology utilizes geographic information system technology, a database management system, and computer modeling. Statewide land cover, soils, and topographic digital data were used to develop input parameters for a simple nutrient and sediment loading model, resulting in an assessment based on estimated loadings for each of the predefined 493 hydrologic units in the state. The resulting model output includes runoff volume, sediment loading, and dissolved and sediment-attached nitrogen and phosphorus. The estimated loads were used to rank the hydrologic units for potential non-point-source pollution and compared with rankings from a previous assessment. The revised methodology is based on annual pollutant loadings, and no attempts were made to "weight" or subjectively impose importance to sources or constituents as with the previous methodology. The new methodology, resulting watershed priorities, and recommendations for further refinement are presented in this paper.