Use of a Hydrogeologic Framework in
Relating Surficial
Hydrogeology to Shallow Ground-Water Quality in the
Mid-Atlantic Coastal Plain
Hancock, T.C., USGS, Richmond, VA; Ator, S.W., USGS, Baltimore, MD; and Denver, J.M., USGS, Dover, DE
Local topography and the texture and chemical
composition of surficial and near-surface geologic materials affect the movement
of water and the transport and reactivity of chemical constituents, which in
turn affect the quality of ground-water resources. In order to evaluate the
effects of variable surficial hydrogeology on shallow ground-water quality, the
USGS analyzed water-quality data collected from unconfined aquifers of the
Mid-Atlantic Coastal Plain (New Jersey through North Carolina) in the context of
a newly developed surficial hydrogeologic framework.
The results illustrate the importance of considering surficial hydrogeology in
the design of water-monitoring programs and the interpretation of results.
A surficial hydrogeologic framework was developed based on the variable topography, texture, and depositional history of surficial sediments as delineated by newly available regionally consistent maps of the Mid-Atlantic Coastal Plain. The framework was designed to define subregions of the Coastal Plain within which the natural physical factors controlling the occurrence and movement of chemicals into shallow ground water and small streams are relatively consistent. Seven hydrogeologic subregions were distinguished by physiography and the bulk texture of surficial sediments.
All seven subregions represent areas of primarily
unconsolidated siliclastic sediments along a continuum of texture and drainage
characteristics, from sand and gravel that provide well-oxygenated conditions to
fine-grained sediments providing poor drainage and reducing conditions. The
Coastal
Lowlands (subregion 1) is primarily flat, low-lying, and poorly drained with
numerous wetlands. Sediments are primarily fine-grained estuarine or near-shore
deposits, which typically contain organic-rich matter. The Middle Coastal Plain
(MCP) was subdivided into four subregions which reflect
differences in sediment texture and stream dissection. The MCP-Mixed (subregion
2) is moderately dissected with laterally and vertically variable sediments,
including coarse sands associated with shorelines and estuarine and lagoonal
silts and clays. The MCP-Fine (subregion 3) is more heavily
dissected with predominantly fine-grained sediments at the surface. The MCP-Sands
with Overlying Gravels (subregions 4 and 5) contain predominantly well-drained,
permeable, weathered, coarse-grained deposits, but subregion 4 is moderately
dissected and subregion 5 is completely incised in most places. The Inner
Coastal Plain (subregion 6) includes an outcrop belt of deeply weathered
sediments. The Alluvial and Estuarine Valleys (subregion 7) contains a mixed
sequence, with coarse alluvial deposits at depth and finer, estuarine sediments
near the surface.
We compiled data on major ions, nutrients, and pesticides in water samples collected from 1987 through 1997 by the U.S. Geological Survey and other Federal and State agencies from 533 wells in unconfined aquifers in the Mid-Atlantic Coastal Plain. The data were screened to eliminate bias toward sites at the same location or sites sampled multiple times. The data were grouped by subregion and evaluated to distinguish differences in the quality of water between subregions. Geographical data for the Mid-Atlantic Coastal Plain were compiled for comparison to water quality, both within each hydrogeologic subregion and independent of subregion. For this study, Multi-Resolution Landscape Characterization (MRLC) land covers were combined to five major classes: agriculture, forest, urban land, wetland, and barren land. Data analyses were designed to evaluate the relation of both hydrogeologic subregion and land use to measured ground-water chemistry.
Concentrations of most major ions and nutrients
are similar among many of these subregions, reflecting similar geology. However,
water-quality differences were observed in subregions with distinctly different
sediment texture, topography, geochemistry, and land use. For instance,
concentrations of most major ions and reduced nutrients are higher in subregion
1, which is a poorly-drained area of abundant organic matter and little
dissolved oxygen. Dissolved oxygen and nitrate concentrations are the highest in
subregion 4, which has similar land use to subregion
1 but coarser, more weathered sediments and better drainage.
Pesticide concentrations are related to both hydrogeology and land use. Because pesticides are applied to the land surface, land use within individual subregions is a source of bias not eliminated by the initial screening of data. Detection frequencies of commonly used agricultural pesticides are greater in subregion 6 than in subregion 1. In subregion 1, fine grained, organic-rich soils most likely retard the transport of pesticides from the land surface to ground water. Subcropping geologic units in subregion 6 are deeply weathered where exposed, and can be quite permeable, promoting the transport of pesticides from the land surface to the ground water. The soil and hydrogeologic conditions in subregion 4 are also particularly favorable for the transport of pesticides. However, detection frequencies were typically lower in subregion 4 than subregion 6, probably because a larger proportion of data from subregion 6 was collected in agricultural areas, where there is a greater likelihood that pesticides are applied.
Hancock, T.C., S.W. Ator, and J.M. Denver, 2000, Use of a Hydrogeologic Framework in Relating Surficial Hydrogeology to Shallow Ground-Water Quality in the Mid-Atlantic Coastal Plain [abs.], in Younos, T. and Poss, J., eds., 2000 Virginia Water Research Symposium, Advances in Land and Water Monitoring Technologies and Research for Monitoring of Water Resources, Special Report SR 19 2000, Virginia Water Resources Research Center, Blacksburg, Va., 2001.
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