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Setting Benchmarks for Reference Condition

The most common approach for using reference sites to set attainment benchmarks is to select a percentile of the reference site index scores below which is considered degraded. The selection of the percentile should consider the coarseness, or restrictiveness, of the screening criteria. In the Mid-Atlantic Region, West Virginia sets a benchmark at the 5th percentile of the reference sites; Maryland uses the 9th percentile; Virginia uses the 10th percentile; and EPA’s Mid-Atlantic Integrated Assessment (MAIA) uses the 25th percentile. In addition to percentile selection, the West Virginia Department of Environmental Protection (WVDEP), the Maryland Department of the Environment (MDE), and the Virginia Department of Environmental Quality (VDEQ) also include separate measures of index precision surrounding the bench-mark in order to capture uncertainty for regulatory program use. These programs demonstrate innovative approaches for setting attainment benchmarks, as well as recommended practices for defining and implementing reference condition.

West Virginia BenchmarkThe West Virginia Stream Condition Index (WVSCI) has a precision estimate of 7.4 units. Therefore, setting the benchmark at a score of 68 (the 5th percentile) effectively creates a “gray zone” between 68 and 60.6.
West Virginia DEP set a low benchmark (at the 5th percentile) because their screening criteria included a secondary review of the candidate reference sites that passed the initial objective screening criteria. Using BPJ in the secondary review, the number of sites dropped from 349 candidate reference sites to 216 reference sites (Southerland 2006). The BPJ included a review of each candidate site for its proximity to upstream point source discharges and an evaluation of anthropogenic activities and disturbances near the candidate sites. For their stream condition index (SCI), WVDEP (2006) determined a precision estimate of 7.4 out of 100 units. This uncertainty in the index itself sets the 5th percentile benchmark of 68 effectively to 60.6, the range between those values being in a “gray zone”.

The West Virginia Stream Condition Index (WVSCI) has a precision estimate of 7.4 units. Therefore, setting the benchmark at a score of 68 (the 5th percentile) effectively creates a “gray zone” between 68 and 60.6. See figure at right.

Virginia Benchmark
Virginia DEQ (2006) sets the attainment benchmark at the 10th percentile of their reference sites. Much like West Virginia, the precision estimate for Virginia’s stream condition index (VSCI) is ±7.9, which generates a “gray zone” around the benchmark. As a result, the 10th percentile benchmark of 60 effectively becomes a range from 55 to 63, where sites scoring above 63 are viewed as healthy streams and those below 55 are deemed moderately to severely stressed.

Maryland Benchmark
The Maryland Department of Natural Resources did not establish a specific benchmark based on a distribution of their reference sites. They maintain a categorical index score rating similar to the metric scoring procedure of 1, 3 and 5, where any score above 3 is considered acceptable. To facilitate the use of the Maryland IBIs for the regulatory agency, a statistical measure of uncertainty (confidence interval) is used to determine whether the mean of the results from the sites sampled in a watershed is above or below the Index of Biotic Integrity (IBI) value considered indicative of satisfactory water quality (i.e., 3). Where at least ten sites have been sampled in a watershed, watershed-specific confidence intervals are calculated. If the upper bound of the confidence interval is less than 3, that watershed is designated as not meeting water quality criteria (MDE 2004). For comparison with other states’ methods, it was determined that the MDE index benchmark of 3 was equivalent to the 9th percentile of the reference sites (Southerland 2006).

Mid-Atlantic Integrated Assessment Benchmark
For a report card on the state of streams and rivers in the Mid-Atlantic region, data was combined from two sample surveys conducted in the region by MAIA from 1993 to 1998 (Stoddard et al. 2006b). MAIA established two benchmarks based on the distribution of reference sites. The 25th percentile value set the lower limit on “good” condition. The 1st percentile was used as the threshold below which values were deemed “poor.” Values between the 1st and 25th percentiles were designated as “marginal.” These classifications were deliberately used so as not to conflict with regulatory terms used by the States.

The two benchmarks set by MAIA established three categories of condition: poor, marginal and good. (Stoddard et al. 2006b)
Above - The two benchmarks set by MAIA established three categories of condition: poor, marginal and good. (Stoddard et al. 2006b)

 

Material above taken from Best Practices for Identifying Reference Condition in Mid-Atlantic Streams.

Learn more about reference condition by reading:

Citations:

Bryce, S.A., D.P. Larsen, R.M. Hughes, and P.R. Kaufmann. 1999. Assessing relative risks to aquatic ecosystems: A Mid-Appalachian case study. Journal of American Water Resources Association. 35:23-36.

Davis, W.S., and J. Scott. 2000. Mid-Atlantic Highlands Streams Assessment: Technical support document. EPA-903-B-00-004. U.S. Environmental Protection Agency, Region 3, Office of Research and Development, Ft. Meade, MD.

Stoddard, J.L., A.T. Herlihy, B.H. Hill, R.M. Hughes, P.R. Kaufmann, D.J. Klemm, J.M. Lazorchak, F.H. McCormick, D.V. Peck, S.G. Paulsen, A.R. Olsen, D.P. Larsen, J. Van Sickle, and T.R. Whittier. 2006b. Mid-Atlantic Integrated Assessment (MAIA): State of the flowing waters report. EPA-620-R-06-001. U.S. Environmental Protection Agency, Office of Research and Development, Washington, DC. February 2006.

Southerland, M., J.H. Vølstad, L. Erb, E. Weber, and G. Rogers. 2006. Proof of concept for integrating bioassessment results from three state probabilistic monitoring programs. EPA-903-R-05-003. U.S. Environmental Protection Agency, Office of Environmental Information and Mid-Atlantic Integrated Assessment Program, Ft. Meade, MD.

West Virginia Department of Environmental Protection (WVDEP). 2006. West Virginia 2006 Section 303(d) List, updated 3/22/2006. Available at http://www.dep.state.wv.us/show_blob.cfm?ID=10203&Name=303(d)_Listing_Rationale_Only.pdf

 

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