GEOLOGY

South Harrison Township lies in the Atlantic Coastal Plains specifically the inner-coastal plain (see physiographic map on right). Therefore the sediments are comprised of both continental and marine sediments. The Coastal Plain consists of a series of unconsolidated deposits of sand with clay, silt, and gravel formations. From the time before dinosaurs, the Cretaceus Period, through the Tertiary and the Quarternary Periods, sedimentation within the Coastal Plain has been occurring. This constant geological process has resulted in a wide diversity of characteristics in sedimentation ranging from heavy clays to coarse riverbed gravel. Thick beds of horizontal strata at varying depths and thicknesses have been continuously deposited at varying depths and thicknesses over the Coastal Plain. The result is thick beds of sand, clay, and gravel extending hundreds of feet below existing contours.

Each geological formation left its signature in the size and type of sediments it contained. These ranged from glauconitic clays to well sorted quartzes sand grains. Uncemented beds of sands are ideal for groundwater flow and withdrawal. Beds of thick clay are often associated with these thick beds of sand and serve as confining elements to various aquifer systems inhibiting an exchange between the various aquifers. As would be expected, the combination of sandy/gravel deposits serve as an excellent water source. From the perspective of surface geology, the Bridgeton Formation, the Kirkwood Formation, The Navasink Formation, and the Cohansey Formation with a small portion of the Cape May Formation are evident in the Township (see geologic formation map). The Bridgeton Formation has been identified as a geological formation containing radium.


WATER BEARING AND CONFINING UNITS

Sandwiched between layers of impervious materials are the major water bearing formations. The available water bearing capacity and yields from these formations are shown in Table 2.

The Bridgeton Formation is found in the eastern area of the Township. It lies directly over the Kirkwood/Cohansey System. When used as an aquifer, yields between 10-50 gallons per minute can be expected. The dominant material in this formation is sand and gravel. Where this formation lies over the Kirkwood/Cohansey Formation, a layer of clay serves as a confining layer. Recent studies have found radioactive radium deposits in this formation although they are not universally found in the formation. Water percolating thorough the Bridgeton Formation is speculated to be the cause of radium contamination in the underlying Cohansey Formation when it is detected.

The uppermost significant water bearing unit in South Harrison Township is the Kirkwood and Cohansey Formations which contains the Kirwood/Cohansey aquifer.

The Kirkwood-Cohansey aquifer is approximately 100 feet thick and consists of highly permeable sand with some clay. The hydrological importance of this aquifer system is, not only its yields, but also its localized ability to recharge lower water bearing units. This ability to recharge also mirrors its ability to contaminate potential water sources. It is bounded below by poorly communicable clayey sediments. These poorly-permeable clayey sediments include several formations. Some of these formations also contain small-to-moderate water yields such as the Fallsington formation and the Red Bank sand. However, taken as a whole these clayey formations are considered a composite-confining unit and extend approximately to 160 feet below the surface.

At the base of the confining units is the Navasink formation which is noticeable during well-drilling operations because of its green and black color and its clayey consistency. It acts as an upper boundary for the Wenonah-Mount Laurel aquifer. The outcrop of this and the Hornerstown formation is very narrow and is located near the western boundary of the Township. It is a very minor aquifer source throughout its location in Gloucester County. When it is tapped as a water source, isolated yields up to 50 gallons per minute are possible.

The Wenonah-Mount Laurel aquifer is bounded both above and below by confining units. This aquifer lies about 200 feet below ground level. The Wenonah-Mt. Laurel formations outcrop in bands ranging between 0.3-3 miles lying parallel to the Delaware River. It is composed of fine to coarse-grained sand ranging in thickness between 65-95 feet, and, as a source for well water, can provide up to 200 gallons per minute. Recharge of this aquifer comes from seepage from the Navasink/Hornerstown formations and from precipitation where it outcrops. The waters from the Delaware River would serve as a primary recharge area. Fortunately, thick alluvial deposits protect the aquifer by preventing seepage.

The Merchantville formation is situated below the Wenonah-Mount Laurel aquifer with the Woodbury clay and Merchantville formation forming the Merchantville-Woodbury confining unit.

The Potomac Raritan-Magothy (PRM) aquifer system lies below the Merchantville-Woodbury confining unit. It is a major aquifer system. The PRM formations outcrop along the Delaware River in a band several miles wide in places. These formations consist of sand and clay, and range in thickness between 150-500 feet. Where these formations are the thickest, the water bearing capacity is the highest. As much as 1400 gallons per minute have been withdrawn. Recent studies of the PRM aquifer have led to restrictive legislation for major purveyors of water pumping permits. The Division of Water Resource at the Department of Environmental Protection (DEP) has declared the PRM aquifer as part of Critical Area Number 2 (Figure 7) because danger of salt water intrusion from the Delaware River Basin poses a threat to the water quality of this and other aquifer systems that outcrop close to sea level. Counties in Critical Area 2 include Burlington, Camden, and portions of Gloucester.


SALT WATER INTRUSION

Although most regional aquifer systems of the Atlantic Coastal Plain have an abundance of water for both commercial and domestic use, over-pumping or unrestricted withdrawal of water from an aquifer can result in salt water intrusion into the aquifer from an adjacent salt water or brackish water body, in this case the Delaware River and Delaware River Basin. Seasonal variations in local climate in Southern New Jersey can result in too little recharge through rain water into a aquifer such as the Passaic-Raritan-Magothy aquifer system. When pumpage from the aquifer exceeds the rate of recharge by rain water, brackish water containing chloride salt contamination enters the aquifer system. The Division of Water Resources of the DEP has recommended that Gloucester County, as one of the counties in Critical Area Number 2, find regional alternative water sources such as the Mount Laurel-Wenonah aquifer. As part of a yet-to-be completed study of ground water in Southern New Jersey, the United States Geological Survey used a regional aquifer systems analysis computer model in 1988 to find the limits for a critical demand of the Mount Laurel-Wenonah aquifer. This enabled the DEP to recommend that alternate potable water sources be developed and evaluated.

In South Harrison Township there are 16 major water users. A major user is one permitted to withdraw 100,000 gallons per day or more from aquifers. Major users must obtain withdrawal permits, pursuant to the Water Supply Management Act, from the DEP Bureau of Water Allocation.

The Bureau, through implementation of the Act, is attempting to reduce impacts to critical aquifer systems to assure the users have a safe and dependable yield. Most major users in the Township draw from the Raritan-Magothy Aquifer because of the possible yields. Most domestic water withdrawals are from the Cohansey/Kirkwood Aquifer which are shallow wells that would be too difficult to map. They would occur where there was no service through an approved distribution system.