Spatial-Temporal Aspects of Water Quality

Environmental agencies, federal, state and tribal, must evaluate water quality using chemical, biological, thermal and usage criteria over vast spatial and long-term temporal domains. The Environmental Protection Agency (EPA) is publishing a report on the possible effects of the Clean Air Act Amendments of 1990 on the acid status of surface waters in several regions of the US. The first part of this talk will identify statistically important features of the data underlying this report, and limitations of statistical techniques available to design such studies and analyze the resulting data.

The second part of the talk will describe a specific estimation problem for acid status of surface waters in the Northeastern United States. One of the tools used to evaluate characteristics of acidity is the cumulative distribution function of slope trends (acid concentration/year). An understanding of the distribution of these slopes helps evaluate the impact of the Clean Air Act Amendments of 1990. For example, the proportion of lakes whose acidic concentration has been decreasing can be estimated. A hierarchical model is constructed to describe these slopes as functions of available auxiliary information, and constrained Bayes techniques are used to estimate the ensemble of slope values.