Nonpoint Source Assessment & Prioritization

Every two years since the 1980s, water quality degradation caused by nonpoint sources (NPS) of pollution has been assessed on a per hydrologic unit basis along with some indicators of where such degradation might have its greatest negative impact. Results are in the Draft NPS chapter of the Virginia Water Quality Assessment (305b) Report, published by DEQ. Assessment results are reported to the U.S. Environmental Protection Agency, Congress and the Virginia General Assembly.

Origins of the 2016 Nonpoint Source Pollutant Loads

For the 2016 assessment, estimations of the nonpoint source pollutant loads of nitrogen (N), phosphorus (P) and sediment (S) by 6th-order hydrologic units of the National Watershed Boundary Dataset have been calculated using a load simulation model and data developed by the Department of Conservation and Recreation and the Virginia Tech Department of Biological Systems Engineering.

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
  • 21 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 2016 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, evapotranspiration, dissolved pollutant factors) in the aforementioned NPS load computer model. The model replicated many of the NPS pollutant-producing processes and created additive estimated nonpoint source 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 2016 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 2016 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

Aside from the ranking of the UALs above, other NPS pollution measures can be useful for prioritizing NPS reduction activities. In the 2016 study, two biological assessments were also reported – an evaluation of public surface water protection needs and a modified index of biological integrity (mIBI). Combined with other relevant NPS measures, evaluations of NPS conditions can help planners and decision-makers target which hydrologic units should be given primary consideration for the implementation of NPS pollution control measures. Attention should be directed to those units with higher rankings in the prioritization categories as well as those units downstream of polluting conditions.

The mIBI was 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.

To evaluate on a hydrologic unit basis the effect to the human population that depends on surface water for drinking that NPS pollution can have, sources (intakes) of public surface water supplies and the population they each serve were 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. The process for ranking the various protection zones or overlapped portions of zones scores can also be found in the NPS Chapter of the 305b Report.

Prioritization component values and rankings can be obtained from DCR’s NPS databases.

How 2016 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 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.

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Virginia Department of
Environmental Quality
P.O. Box 1105
Richmond, VA 23218
(804) 698-4000


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