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Comparison of Random and Systematic Site Selection for Assessing Attainment of Aquatic Life Uses in Segments of the Ohio River

EPA 600/R-06/089
September 2006

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Abstract

The Clean Water Act (CWA) section 305(b) requires that states report biennially on the water quality standards (WQS) attainment status of all waters. The United States Environmental Protection Agency (USEPA) has recognized that biological assessment data are very valuable and should serve as a core indicator in determining aquatic life use attainment status (USEPA 2002). However, great (or large-floodplain) rivers, such as the Ohio River, are one type of aquatic resource for which biological assessment data typically have been deficient, primarily due to difficulty in sampling (Emery et al. 2003). The Ohio River Valley Water Sanitation Commission (ORSANCO), a compact of eight states and the federal government is responsible for assessment of the Ohio River. Recognizing the abundance of stresses on this system and the lack of biological data for the river, ORSANCO developed a bioassessment program several years ago. In 1990, ORSANCO began a Long-Term Intensive Survey (LTIS) of the Ohio River to provide high sample density (one sample every 3.2-6.4 km) from selected reach segments of the river. Beginning with this effort, a fish-based index of biotic integrity (IBI) for the Ohio River was developed (Emery et al. 2003). Although the LTIS approach provided valuable data for the development of a fish IBI and biocriteria, this approach was very resource-intensive and time-consuming and was not feasible for routine sampling of the entire river. To support the development of a cost-effective program for routine monitoring, the minimal amount of effort required to produce an adequate assessment needed to be determined.

A probability-based sampling design, in which sites are randomly selected from the population of possible sites, is an effective way to reduce effort and still collect data representative of an entire resource. By simulating a probability design using the intensive survey data already collected, the number of sites required to adequately represent the condition of a pool was determined. Two approaches were used to analyze the simulation data, and both assume that the intensive survey data provide the most accurate representation of condition available. The first approach was to determine the minimum number of sites that provided a similar condition estimate to that of the full set of sites, indicating some stability in the estimate. This was achieved by calculating variables for various subsets of sites and identifying the number of sites at which distributions of variables no longer differed from those of the full set of sites. Simultaneously, the number of sites required to obtain an estimate of condition with a specific level of precision was determined. A second and more commonly used approach to determining the adequacy of sampling was based on the relative proportion of species collected. The number of 500 m reaches required to collect 80-90% of the observed taxa in the pool was calculated as a way to determine the number of sites at which a sufficient level of effort has been expended to estimate biological condition. By examining the results of both approaches for this study, a probability sampling design was developed that will still be rigorous and will provide known confidence around any estimate of condition for Ohio River navigational pools while

This research indicated that 15 sites (compared to 20-32 sites sampled in the LTIS) may be adequate to draw conclusions about the overall condition of a navigational pool. However, in some cases, additional sites may need to be sampled to achieve the desired level of precision around the condition assessment. Because there are a large number of pools in the Ohio River, an approach that allows ORSANCO to sample and assess more pools each year will result in a more robust assessment of the river for the 305(b) report. A suggested approach is to sample all of the navigational pools of the Ohio River over a 5-year period. In each pool, an initial sampling of 15 sites is carried out, and additional sampling is completed only if required to make a definitive assessment of the pool. This approach would limit the resources required to assess an individual pool, such that additional effort would only be required in those pools that are of more marginal condition. In addition, this approach will help ORSANCO to identify those pools in which biological condition is most impacted and to prioritize any mitigation or restoration efforts. Additional sampling of individual sites may be required to determine causes of impairment within pools but may be guided by the data acquired through the random sampling design.

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