Surficial Hydrogeologic Framework Hypotheses for Chemical Transport

Surficial hydrogeologic region	Conditions Affecting Ground-Water and Surface-Water	Hypothesized Speciation and Concentration of Nitrogen	Hypothesized Potential for Pesticide Mobility
I	Predominantly reducing conditions in poorly drained, organic sediments; oxidizing conditions in sandy beach ridges. Organic matter affects water chemistry. High water table limits infiltration. Runoff potential is low, except to unbuffered drainage ditches in fields.	Ammonia and organic nitrogen dominant in poorly drained organic sediments; nitrates present in water from predominantly sandy sediments. Water will generally contain relatively low nitrate and relatively high ammonia and organic nitrogen concentrations. Nitrogen may be present from natural and anthropogenic sources.	High potential for pesticides to be bound by fine-grained sediments and organic matter in soils of poorly drained areas. There may be some transport of pesticides through sandy surficial sediments into ground and surface waters in areas where pesticides are applied. Pesticides may runoff into drainage ditches.
II	Wide range of chemical environments associated with variations in sediments from coarse sands to clays, silts, and organic matter. This area has a variable depth to the water table and variable potential for infiltration.	Occurrence and concentrations of nitrogen species will vary widely in association with variations in sediment-type distribution, land use, and oxidation-reduction conditions.	Occurrence and concentrations of pesticides will vary widely in association with variations in sediment-type and land use distribution.
III	Fine-textured surficial sediments will promote over land runoff and limit infiltration. The presence of an unconfined aquifer will be limited. Most ground water will be confined and under reducing conditions.	Concentrations of nitrogen species in shallow ground water will be very low because of confined conditions. Nitrogen species could be present and transported to surface water in runoff; concentrations will depend on land use.	The potential for pesticides to infiltrate into ground water is low because of confined conditions. Pesticides may be transported to surface water in overland runoff from areas where they are applied.
IV	Sandy surficial sediments promote oxidizing conditions and infiltration of water into the surficial aquifer. Overland runoff is limited, but possible, in areas with higher slope. Stream channels are generally sandy, but can contain significant organic matter.	Nitrate dominates nitrogen speciation. Concentrations vary with land use and can be quite high in ground water and surface water of areas with nitrogen application because of the predominance of sandy sediments. Nitrate concentrations in surface water may be affected by denitrification in stream channels with organic sediments.	The potential for pesticides to be transported into ground water is relatively high. Pesticide transport will be affected by variability in land use and soil characteristics. The presence of organic matter in stream beds may limit transport from ground water to surface water in some areas.
V	Sandy surficial sediments promote oxidizing conditions and infiltration of water into the surficial aquifer. Runoff potential is high in areas with steep slopes. Stream incision extends into confining beds and confined aquifers where reducing conditions affect water chemistry.	Nitrate dominates nitrogen speciation. Concentrations will vary with land use. Nitrate concentrations in surface water may be affected by denitrification in areas where streams incise into aquifers and confining beds under reducing conditions. Nitrogen in surface water will be mostly associated with runoff.	The potential for pesticides to be transported into ground water is relatively high. Pesticide transport will be affected by variability in land use, and soil characteristics. Most pesticide occurrence in surface water will be associated with runoff because of stream incision into confined aquifers and confining beds.
VI	Water chemistry is affected by variability in the permeability and lithology of different geologic units that subcrop and outcrop in this area. Oxidizing conditions occur in surface and near surface sand units and in the highly weathered soils. Reducing conditions occur where aquifers are confined and in confining beds.	Nitrate dominates nitrogen speciation in sediments under oxidizing conditions. Concentrations will vary with land use. Nitrate will be denitrified to form nitrogen gas under confined conditions. Runoff will be an important nitrogen transport mechanism in areas with limited infiltration and high slope.	The potential for pesticides to be transported into ground water is moderate in areas with sandy surficial sediments because of loamy soils. Pesticide transport will be affected by variability in land use. Most pesticide occurrence in surface water will be associated with runoff because of stream incision into confined aquifers and confining beds.
VII	The predominantly fine-grained nature, relatively high organic-matter content of shallow sediments, and shallow water table may result in reducing conditions in shallow ground water. Areas with sandy surficial sediments may have oxidizing conditions. Topography is locally relatively flat, promoting infiltration and reducing runoff.	Transport, speciation, and concentrations of nitrogen will depend on local conditions and be variable. In flat areas with fine-grained near-surface sediments, ammonia and organic nitrogen could be the dominant nitrogen species. Overland transport of nitrogen should be limited because of locally flat topography.	The presence of fine-grained sediments, organic matter and shallow water table on valley terraces will limit pesticide mobility. In areas with sandy surficial sediments, pesticides may be present in ground water. Overland transport to surface water will be limited by flat topography.

Go to the Northern Atlantic Coastal Plain Aquifer System Home Page

Go to the NAWQA Ground-Water Synthesis by Principal Aquifer Home Page