Assessing background levels of specific conductivity using weight of evidence.

There are many ways to estimate background levels, and many types of evidence may contribute to determining whether a water, air, or soil is at background. As a result, it is important to define background in each case and to weigh the available evidence to determine the best estimate of background. A weight-of-evidence approach is demonstrated that assesses whether the background SC is sufficiently similar in streams of Ecoregion 70 in West Virginia and Ohio. During planning, five relevant considerations were identified to assess background SC: physical properties, measured SC, spatial distribution of low SC sites, biological properties, and data relevance and reliability. For each consideration, diverse types of evidence were generated, evaluated, and synthesized using weight of evidence. In the example, evidence was weighed for the hypothesis that background SC is similar in two areas in Ecoregion 70, the Western Allegheny Plateau in the eastern United States. Where, as in this case, background is not well characterized by measurements, because data sets are small or sampling designs or anthropogenic inputs may influence estimates of background, it is suggested that information about regional properties, related to and affected by SC, may be used to determine whether SC in the less characterized area is sufficiently similar to a well characterized area.