Geologic and Hydrologic Properties

Figure 5. Different units of the aquifer system have varying thicknesses due to tectonic forces that warped the bedrock (Trapp and Horn, 1997).

Figure 6. The impact crater formed by a meteorite during the late Eocene is outlined in purple (Charles, 2016).

Geologic Description

The NACP consists of unconsolidated to semi-consolidated sediments in the shape of a wedge that thickens and dips towards the southeast (Charles, 2016). These sediments range in age from early Cretaceous to Holocene and overlie crystalline bedrock from the Precambrian to Paleozoic time (Masterson, et al., 2016). Varying types of sediments exist within the system including clay, silt, sand, lignite, gravel, and limestone (Trapp and Horn, 1997). The aquifers are constructed mostly of sand, gravel, and limestone while the confining units are made up of silt, clay, and silty or clayey sand.

The deposition of the sediments within the NACP occurred in non-marine, marginal marine, and marine settings (Trapp and Horn, 1997). Streams within alluvial and deltaic environments deposited the lower sediments of the Cretaceous age while marine transgressions are responsible for the sediments deposited during the late Cretaceous, early Tertiary times. A regression followed during late Tertiary and eventually non-marine sediments were deposited during the Miocene and Quaternary periods. Due to the variety of depositional environments that formed the aquifer system, the sediment types and textures can vary greatly within short horizontal and vertical distances (Trapp and Horn, 1997). Local aquifers can be identified within this system, even with complex interbedding of fine and coarse grained sediments, but they are often grouped by similar hydrologic characteristics, resulting from geologic origins to form regional aquifers.

The surficial aquifer is considerably heterogenous due to multiple origins of the unconsolidated sand and gravel sediments (Masterson, et al., 2013). Glacial, marine, beach, dune, and terrace deposits ranging from Miocene to Holocene exists within the surficial aquifer. Values for porosity and permeability for this unconfined aquifer is not readily available, but due to the aquifer being composed of sand and gravel, porosity probably ranges from 25-50% and intrinsic permeability most likely ranges from 10^(-2) to 10^(3) darcys (Fetter, 2001). The Potomac aquifer's unconsolidated sediments were deposited during the Cretaceous by fluvial-deltaic conditions (Masterson, et al., 2013). The aquifer is also highly heterogenous, consisting of medium to fine grained sand with interbedding clay and slit lenses. Porosity and permeability are also not readily available for the Potomac aquifer. Estimates for its porosity within the sandy areas are 20-35% and 33-60% within the clay and silt lenses (Fetter, 2001). The Potomac's intrinsic permeability is likely 10^(-2) to 10^(2) darcys within the sand and 10^(-6) to 10^(-3) darcys within the clay and silt lenses.

Unconformity persists in the system in terms of the thicknesses of the sediments as well. The surface of the crystalline bedrock is irregular due to tectonic forces that warped it to form alternating arches and troughs (Trapp and Horn, 1997). Sediments were deposited onto these arches and troughs, creating varying thicknesses in varying locations (Figure 5). In Virginia, the sediments within the NACP aquifer system were dramatically altered when a meteorite struck the land near the mouth of the Chesapeake Bay during the late Eocene (Figure 6; Masterson, et al., 2013). At the time, the impact occurred at a shallow-shelf environment during a marine transgression and penetrated the sediments all the way down to the bedrock. The crater was filled with slumped blocks of pre-impact sediments along with impact-related sediments.

Hydrologic Properties

The geologic characteristics of the Northern Atlantic Coastal Plain aquifer system has created a series of unconfined as well as confined aquifers (Figure 7). An unconfined aquifer, the surficial aquifer, rests at the top of the system and is underlain by confining units of clay and silt (Trapp and Horn, 1997). Below the uppermost confining unit is a series of confined aquifers as well as more confining units of clay and silt. The Potomac aquifer is the lowermost, overlying basement bedrock, and is therefore confined. Water moves most readily through the aquifers but in areas where the confining units contain sand, leakage can occur (Trapp and Horn, 1997). To a degree, each aquifer in this system is hydraulically connected with one another.

The thickness of the entire system can vary greatly due to the irregularity of the basement rocks caused by tectonic forces. Sediments near the New Jersey coastline can be as thick as 4,000 feet, while those near the Maryland coast are approximately 8,000 feet (Trapp and Horn, 1997). Sediments composing the NACP along the North Carolina coast are the thickest, reaching 10,000 feet. The thickness of each regional aquifer varies considerably as well. The surficial aquifer averages at 50 feet thick while the Potomac aquifer ranges in thickness from a thin edge under the Fall Zone to thousands of feet offshore (Masterson, et al., 2013).

Figure 7. Generalized cross section of the NACP aquifer system starting in the west, at the Fall Zone, extending eastward, to the Atlantic Ocean (Masterson, et al., 2016).

Hydraulic Properties

Although the aquifer system consists of ten regional aquifers spanning over 6 states, hydraulic properties such as hydraulic conductivity, storativity, transmissivity, and specific storage are more readily available for aquifer units that receive the most use (Masterson, et al., 2013). With that said, hundreds of measurements for hydraulic conductivity and transmissivity exists for the Potomac aquifer. Horizontal hydraulic conductivity for the Potomac can range anywhere from 1 ft/d to 500 ft/d, but averages around 30 to 200 ft/d throughout the whole region. Because the Potomac is considerably thick, values as large as 58,000 ft^(2)/d have been recorded for transmissivity (Masterson, et al., 2013). Values of horizontal hydraulic conductivity for the surficial aquifer are similar to that of the Potomac aquifer, ranging from less than 1 ft/d to 500 ft/d. Values of transmissivity and specific yield have been calculated in the surficial aquifer in Maryland and are 510 to 53,500 ft^(2)/d and 0.15, respectively (Andreasen, et al., 2013). Hydraulic properties of the confining units are not well known, but a few studies have found values for vertical hydraulic conductivity ranging from 3.0×10^(-9) to 0.1 ft/d. A storage coefficient (storativity) value of 1.0×10^(-4) has been used for previous regional simulations of the NACP aquifer system, which translates to specific storage of 2.0×10^(-7) to 1.0×10^(-5) ft^(-1). Estimated values for specific storage of the confining units are 5.0×10^(-4) to 1.0×10^(-6) ft^(-1) (Masterson, et al., 2013).

Natural Chemistry of the Groundwater

The natural chemical character of the groundwater within the NACP aquifer system is a reflection of physical, chemical, and biological processes that occur within the region (Knobel, et al., 1998). The physical process of hydrodynamic dispersion affects the concentration gradients between the salty and fresh groundwater bodies, mineral dissolution and precipitation is caused by chemical processes that changes the groundwater along flow paths, and carbon dioxide is produced by biological processes within deeply buried sediments. Throughout the entirety of the aquifer system, areas with high recharge rates have variable compositions where the water is not dominated by any cations or anions. Down-gradient from these areas, the groundwater is usually dominated by calcium and magnesium bicarbonate, slowly grading into sodium bicarbonate. Within the most down-gradient regions of the aquifer system, near the coast, the water is highly concentrated by sodium chloride.

Within the regional aquifers throughout the system, the chemistry of the groundwater is mostly dependent on the depositional environment of the sediments within the aquifer (Knobel, et al., 1998). The surficial aquifer has variable geochemistry throughout the NACP region. Areas overlain by soils rich in organic matter and clays usually contain a large amount of dissolved iron. In contrast, deeper into the aquifer where the sandy materials are not near organic rich soils, the groundwater lacks dissolved iron but has a generous amount of nitrate. Within the deeper, confined aquifers such as the Potomac, cation exchange reactions are important factors in the groundwater chemistry (Knobel, et al., 1998). A predictable pattern occurs by the removal of cations from the solution followed by a replacement with sodium. First, calcium dissolves and is removed from the solution followed by magnesium and potassium to sodium.

Relationship to the Unsaturated Zone

The surficial aquifer is the only aquifer within the system that is in direct contact with the unsaturated zone. Even with its direct contact, the amount of precipitation that percolates down into the unsaturated zone, reaching the surficial aquifer, is limited. Only 31% of the total amount of precipitation that falls within the Northern Atlantic Coastal Plain region enters the underlying aquifers while 69% is lost to evaporation, transpiration, or surface runoff (Masterson, et al., 2013). Because precipitation is the primary source of recharge to the aquifer system, the 31% that does travel through the unsaturated zone is highly important. Many of the soils that exist along the northern East Coast are significantly sandy, so it is interesting that a relatively small portion of the precipitation reaches the aquifer system.

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