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USGS

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Synthesis of Ground-Water Quality
at the Principal-Aquifer Scale

Why Synthesis at the Principal-Aquifer Scale?

Analysis of ground-water quality in Cycle I of NAWQA (1991-2000) occurred at the study-unit (local) scale by Study Units and at the national scale by National Synthesis. Analysis at the study-unit scale provides assessment of water-quality conditions locally, for example for an aquifer or part of an aquifer, including presence and absence of constituents, constituent concentrations when detected, spatial patterns of occurrence and non occurrence, and determination of factors related to the local occurrence of a contaminant. Analysis at the national scale provides analyses similar to those of local studies but as summaries for the Nation. National occurrence assessment is achieved by compiling data from the local studies into national data sets.

Statistical and other types of models also were used at both the local and national scale for spatial extrapolation or prediction of water-quality conditions at unmonitored locations. For example, both local and national statistical models were used to establish quantitative relations between constituent concentrations measured at sites and a variety of anthropogenic and natural factors that are hypothesized to affect a constituent's concentration in ground water. Example factors included constituent uses and releases to the environment, hydrology, climate, and soil characteristics. For use in these statistical models, all potential explanatory variables are required to have values available for all locations across the modeled area. National models, therefore, require national spatial data of all explanatory factors used in the model. National spatial data usually are generalized and, therefore, do not include smaller-scale variability that might occur locally or regionally. Prediction of concentrations at the local or regional scale from national-scale models, therefore, may or may not accurately predict local or regional conditions.

It is anticipated that analysis at an intermediate, regional scale between local and national scales, i.e. by principal aquifer, will complement and extend the findings of the local studies to fill critical gaps in our understanding of ground-water quality and will help link the findings at the local and national scales. Consequently, synthesis of ground-water quality by principal aquifer was identified as an additional important synthesis activity by NAWQA during Cycle II. About 1/3 of the principal aquifers of the U.S. are part of this regional synthesis by principal aquifer during Cycle II.

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Email: Ken Belitz (kbelitz@usgs.gov)