A Reference Stream for the Karst Area of Shenandoah Valley
by Wayne Webb
In Virginia, a stream is listed as impaired if it fails to meet the State water quality standards that support the designated uses of all of Virginia’s rivers and streams. If a stream fails to meet a designated use and its associated water quality standard, then the State develops a total maximum daily load (TMDL) for the pollutant that will bring the stream into compliance with water quality standards. One of these designated uses for all of Virginia’s streams is the support of a healthy and diverse population of aquatic life. When streams do not support this designation, a stressor analysis is performed during the TMDL development process. Once the primary stressor(s) of the biological community is identified, a TMDL is developed for that particular pollutant. In cases where a numeric water quality criterion does not exist, the stream of interest is compared to a “healthy” reference stream to determine a tolerable pollutant load that will not compromise the biological community.
Other considerations are defined by the use of the water body and can include numeric State-wide water quality standards. For fresh water contact recreation (wading, swimming, fishing), for example, E.coli bacteria densities are to be less than a prescribed concentration for a defined frequency.
Unfortunately, there are no streams in the karst area of the Shenandoah River valley that DEQ has used as a reference stream for various stressors of the biological community identified in streams throughout the region. And there is a need for at least one or more reference streams because of the large number of streams in the karst area of the Valley that are listed as impaired.
Karst geology imparts a unique hydrology that has a major impact on stream flow and water quality. In the Shenandoah Valley most people live in karst areas and most farming occurs in the karst areas. Consequently the karst streams receive a strong environmental imprint caused by land use and land cover that is substantially different from mountain streams with low human populations and little or no farming. However, in the Shenandoah Valley mountain streams are being used as reference streams for karst streams. Comparing karst streams to non karst streams is like comparing apples and oranges. A reference stream should exhibit the qualities that one would expect to find in a stream in a similar environment that is not polluted.
Defining what the standards should be for a reference stream is a challenge. For many of the karst streams in the Valley the impairments are E. coli bacteria
and sediment. Sediment is usually listed as the cause of the lack of benthic diversity. E coli exceedances are typically caused by cattle waste deposited directly in the stream, direct discharge of untreated sewage or wild animal waste, and runoff of manure from pasture and cropland.2014 Monitoring Calendar
The karst streams of the Valley are in sediment that in Clarke County is called Weaver soil. Weaver soil is the result of deposition by precipitation of calcium carbonate dissolved from limestone bedrock and formed when ground water is warmed as it flows from springs into the stream and oxygenated as it flows over riffles and waterfalls. Ground water comprises more than 90% of all the water that is contributed directly to karst streams because of the permeability of fractured limestone. Thus a stream from which cattle are excluded and which does not receive untreated sewage should represent an example of a stream that is not polluted and could be considered for “reference” stream designation for benthic invertebrates.
Beginning in 1997 the Clarke County Planning Department began a concerted effort to have livestock excluded from the entire 8.87 mile length of Page Brook because it makes up a substantial part of the recharge zone for Prospect Hill Spring. (This spring is the potable water supply source for Millwood, Boyce, Waterloo and White Post ) That effort continues today and is nearing completion. Recent efforts supported by the Lord Fairfax Soil and Water Conservation District, NRCS and local land owners include installing off-stream watering facilities for cattle and stream exclusion. Approximately 90% of the length of Page Brook is now protected from cattle access. There are only about 20 residences within 1000 feet of Page Brook and there are no known direct sewage discharges. . Based on existing conditions in the Page Brook watershed and recent improvements made to the stream including establishment of a considerable amount of livestock exclusion fencing, it is expected that Page Brook should soon be meeting the bacteria water quality standard under most conditions, ultimately leading to removal of its bacteria impairment in the next several years.
Although streams usually need time to recover after management practices are put into place, the benthic invertebrate communities in Page Brook could, in the future, represent what will be the indigenous benthic communities of a stream in the karst agricultural area of the Valley that has received the best available BMP technology.. Thus the invertebrate community of Page Brook could eventually be considered a reference community for the karst agricultural streams of the Valley.
There have been and continue to be evaluations of Page Brook that can contribute to a bench mark to which other streams can be compared.
During a period from 1995 to 1998 LeAnne E. Astin conducted an evaluation of the benthic invertebrates in the upstream portion of Page Brook west of VA Rt. 617. That work was reported in Bioassessment of BMP Effectiveness in an Agriculturally Impacted Watershed in Clarke County, VA a Master of Science thesis submitted to George Mason University in 2000.
The Astin report contained three salient points. First, there are no reference streams in the karst area of the Valley. Second, exclusion of cattle from the stream is necessary for E. Coli reduction. And third, exclusion is needed to improve benthic diversity. Benthic diversity was noted to recover in a few 1000 feet down stream of the last cattle access.
Recently, as part of the Spout Run TMDL Clean-up Plan and improvement effort, the Piedmont Environmental Council is collecting benthic macro invertebrate samples at the mouth of Page Brook. The Friends of the Shenandoah River (FOSR) has collected and analyzed water samples for nutrients from Page Brook since 1997 at Route 617. More recently, also as part of the Spout Run improvement efforts, FOSR is collecting water samples for nutrient and bacteriological analysis at the mouth of Page Brook.
The FOSR data show that the nitrate concentrations in Page Brook are less than 2 mg/l whereas most of the karst streams in the Valley have higher nitrate concentrations. Thus a repeat of the Astin study for the entire length of Page Brook and with the FOSR data there would be documentation of a karst stream to which other karst streams could be compared. Such a study could result in a dramatically improved basis for determining whether or not many streams rising in a karst geologic environment are in fact impaired and if so, what Best Management
Practices are needed to correct such impairments.
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