Origins | Ranking using 2010 loads | HU prioritization | Use
Every two years since the 1980s, DCR has evaluated the potential for water quality degradation caused by nonpoint sources (NPS) of pollution on a per hydrologic unit basis along with some indicators of where such degradation might have its greatest negative impact. Results are in the NPS chapter of the Virginia Water Quality Assessment (305b) Report (PDF), published by the Virginia Department of Environmental Quality (DEQ). Assessment results are reported to the U.S. Environmental Protection Agency, Congress and the Virginia General Assembly.
Origins of the 2010 NPS Pollutant Loads
For the 2010 assessment, estimations of the NPS pollutant loads of nitrogen (N), phosphorus (P) and sediment (S) by 6th-order hydrologic units of the National Watershed Boundary Dataset (NWBD) have been calculated using a NPS load simulation model and data developed by DCR and the Virginia Polytechnic Institute and State University (VPI&SU) Department of Biological Systems Engineering (BSE).
The voluminous statewide data requirements of the model include:
- detailed land use from interpreted imagery supplemented with tillage practice data, the 2007 Census of Agriculture, the 2006 Virginia Agriculture Statistics, grazing and manure application practices, and mining activities.
- hydrologic soil groups, average water content and K factors of all soils.
- stream flows from gauge stations
- 19 years of climate records from a multi-state area
- growing seasons
- dominant crop types by hydrologic unit
- Chesapeake Bay Watershed Model output
- animal numbers by type and location
- location and numbers of population using septic systems
- distribution and extent of agricultural conservation practices
- slope
- manure application schedules by manure type
Much of this information was gathered in very small units - in either small cells of less than a quarter acre or as coordinate pairs (x, y) - and then aggregated into hydrologic unit level data. For the 2010 NPS Pollution Assessment and Prioritization study, data were developed and loads calculated for 1,236 of the 1,247 6th-order hydrologic units of the Virginia NWBD (the other 11 units were open water only).
Loadings were developed using the above data as well as many additional data layers and a host of modeling factors (i.e., build-up rates, curve numbers, uptake rates, evapo-transpiration, dissolved pollutant factors) in the aforementioned NPS load computer model. The model replicated many of the NPS pollutant-producing processes and created additive estimated NPS pollutant loads from each. Those processes (load components) are:
- Nitrogen (N) = groundwater_N + septic_N + dissolved_N + washoff_N
+ sediment_attached_N
- Phosphorus (P) = groundwater_P + septic_P + dissolved_P + washoff_P
+ sediment_attached_P
- Sediment (S) = summation of load for each detailed land use (see report for detailed land uses)
Load estimates by detailed hydrologic units as calculated above for N, P and S were separately calculated for agriculture, urban and forest land use classes. A total NPS load per hydrologic unit per pollutant was calculated by summing up the NPS loads of the land use components.
Best management practices (BMPs) from the DCR Agricultural BMP Cost-Share Program, as well as from the USDA Natural Resource Conservation Service (NRCS) and others, were used by the Chesapeake Bay Program Office (CBPO) to produce reduction factors that were then updated by DCR and used to reduce the estimated loads initially calculated by the model per hydrologic unit in Virginia.
The modeling performed in the 2010 NPS pollution assessment is edge of stream. This differs from the CBPO loading estimates, which are to the fall line of each river system, and from the load reductions calculated in the DCR Agricultural BMP Cost-Share program, which are loadings to the edge of field.
Rankings of Hydrologic Units Using the 2010 NPS Pollutant Loads
As part of each NPS pollution assessment, the estimated loads per pollutant per hydrologic unit are divided by the land area of the hydrologic unit to create a unit area load (UAL) per pollutant per hydrologic unit. For the purpose of targeting NPS pollutant reduction activities, hydrologic units are ranked per NPS pollutant (nitrogen, phosphorus, sediment and total) based on the UAL values of each into three categories:
- High - units with the greatest UALs (top 20 percent)
- Medium - units with moderate UALs (middle 30 percent)
- Low - units with the lowest UALs (bottom 50 percent)
Estimated loads, UALs, and rankings can be obtained from DCR’s NPS databases. Results are reported and mapped in the 305(b) water quality report.
Hydrologic Unit Prioritization
The categories of the prioritization component of the NPS pollution assessment and prioritization process are intended to help planners and decision-makers target the hydrologic units that should be given primary consideration for the installation of NPS pollution control measures. Attention should be directed to those units with higher rankings in the prioritization categories and with high NPS pollutant loads from the assessment component, either directly or upstream. Those prioritization categories are:
- 303d listed NPS impaired waters
- rivers
- lakes
- Biological health issues
- public source water protection
- aquatic species measures
The extent of NPS impaired waters by water feature type by hydrologic unit is one set of measures. Impaired waters were determined by DEQ and reported in the 2008 303d report. DCR performed a hydrologic unit level analysis of the final 2008 303d database to produce the NPS impaired waters measures found in the 2010 305b report. The report also includes a total maximum daily load (TMDL) status of the impairments. This is the status at the time of the report as opposed to a current TMDL status.
Another prioritization measure in the reports is a modified index of biological integrity (mIBI) produced by the Virginia Commonwealth University (VCU) Center for Environmental Studies (CES). The mIBI values are derived from more than 162,000 stream-dependent records maintained by DCR, the Virginia Department of Game and Inland Fisheries (DGIF), and VCU. The academically established process for determining IBI scores can be found in the NPS chapter of the 305b report (PDF).
The 2010 prioritization process also includes a measure of the effect to the human population that depends on surface water for drinking. The sources (intakes) of public surface water supplies and the population they serve was obtained from the Virginia Department of Health (DOH). DOH has also established a standard zone of protection from these sites. This is the area where activities likely to cause water quality degradation are most likely to affect the surface water supply. This area most likely differs from the area of the population served.
Prioritization component values and rankings can be obtained from DCR’s NPS databases.
How 2010 Assessment Products Are Used
Various state programs rely on specific products from these assessments in evaluating program conditions and targeting activities and funding. In general, DCR attempts to maximize limited resources and funds by targeting the high-load high-priority ranked units for NPS pollution reduction activities. However, different programs may target using different rankings (i.e., target units with high-ranked agricultural loads of N, high ranked total urban loads or units with a high impaired waters percentage within a TMDL study area). The rankings table (XLS) of the NPS chapter of the 305(b) report contains a small set of flagged conditions. Other customized targeting methods can be developed by combining various ranked components in a way that meets program-specific targeting needs. For example, DCR's Agricultural Cost-Share Program staff uses a customized Agricultural Nonpoint Source Assessment ranking that considers only the agriculture loading ranks in targeting BMP recruitment