Considering Variable Hydrogeology in an Assessment of Regional Shallow Ground-Water Quality in the Mid-Atlantic Coastal Plain, USA
Abstract
The variable surficial hydrogeology of the Mid-Atlantic Coastal Plain affects the nature and extent of natural and human influences on regional shallow ground-water quality. Water-quality data collected from 533 shallow wells (median depth of 11 meters) in unconfined aquifers of the Coastal Plain (New Jersey through North Carolina) were compiled from various sources for regional synthesis. These data were compared to a surficial hydrogeologic framework that was developed to provide a template for understanding the major physical processes affecting chemical transport and transformations in the shallow hydrologic system. Seven hydrogeologic subregions of the framework were defined to represent areas of similar geology (primarily siliciclastic sediments) along a continuum of sediment textures and physiography. Although the usefulness of data compiled from multiple sources for regional assessments is limited, broad patterns of similar water chemistry are apparent when the compiled data are analyzed within the context of the hydrogeologic framework. The quality of shallow unconfined ground water in the Mid-Atlantic Coastal Plain is related to aquifer redox properties and the distribution of soluble minerals, as well as land use. In agricultural areas, for example, nitrate concentrations (as nitrogen) rarely exceed 3 mg/L (milligrams per liter) where dissolved oxygen concentration is low (less than 1.4 mg/L), but commonly exceed 10 mg/L where dissolved oxygen concentration is higher (greater than 7.7 mg/L). Conversely, concentrations of phosphorus (which can be mobilized in ground water under reducing conditions) among all land uses decrease with increasing dissolved oxygen concentration (rho = -0.438, p < 0.0001). Regionally, specific conductance, pH, and concentrations of organic carbon, iron, and most nutrients and major ions are higher in the Coastal Lowlands subregion (a poorly drained area of abundant organic matter and little dissolved oxygen) than in a subregion of the Middle Coastal Plain with similar land use but coarser, more weathered sediments (mostly quartz) and better drainage. Nitrate and dissolved oxygen concentrations are lower in the Coastal Lowlands subregion. These differences likely are related to the greater availability of soluble aquifer materials in the Coastal Lowlands, as well as the variable redox and drainage conditions. Among other more hydrogeologically variable subregions, ground-water quality typically is indistinguishable and intermediate between these two subregions.
- Publication:
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AGU Spring Meeting Abstracts
- Pub Date:
- May 2001
- Bibcode:
- 2001AGUSM...H31E04A
- Keywords:
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- 1831 Groundwater quality