Confined Aquifer System Information

Geology

The Coastal Plain is generally divided into two regions known as (1) the Tidewater region, and (2) the Inner Coastal Plain region (Winner and Coble, 1996). The study area is located mostly within the Inner Coastal Plain region (Figure 2).

The Inner Coastal Plain is characterized by land surface altitudes that range from about 50 feet in the east to over 700 feet above sea level along the western boundary (Winner and Coble, 1996). The northwestern portion of the study area contains a unique subdivision of the Inner Coastal Plain, known as the Sand Hills (Stuckey, 1965). Scotland, Hoke, Cumberland and a small section of Bladen counties contain some area of the Sand Hills subdivision. Figure 2 shows the distribution of the Inner Coastal Plain and the Sand Hills subdivision.

Figure 2: Subdivisions of the North Carolina Coastal Plain (adapted after Stuckey, 1965 and Heath, 1980).

Sediments that comprise the Coastal Plain are primarily stratified sequences of clays, sand, gravel, fossil shell layers and limestone, some of which form hard beds. The sediments that dip and thicken to the eastsoutheast, were deposited on crystalline bedrock in a marine environment (near and offshore) during transgressions and regressions of sea level (Figure 3). The section shown in figure 3 is what we would expect to find in the subsurface from west to east.

Hydrogeology

Within the Coastal Plain, groundwater is stored in unconfined and confined to semiconfined aquifers. An aquifer is a rock (consolidated or unconsolidated sediments) unit that will yield an economical quantity of water to wells and springs. The unconfined aquifer is the first encountered below land surface and is not capped by a low  permeability zone (confining unit). It has several very important functions. It is the zone that provides recharge to the deeper confined aquifers. Also, the unconfined aquifer is the primary zone that provides much of the base flow to streams and rivers (discharge). Lastly the unconfined aquifer is the zone where many pollutants breakdown and are attenuated, keeping the environment and public health protected. Within the Coastal Plain, the unconfined aquifer is named the Surficial Aquifer (Figure 3).

Figure 3: Generalized cross section of aquifers underlying the southern Coastal Plain study area
(Scotland through Columbus Counties) of North Carolina (modified after Smith and others, 1996).

The aquifers are named according to the stratigraphic units. In some areas, a confining unit does not cap the first productive aquifer (encountered below the Surficial Aquifer). In such an instance, the aquifers are separated into two distinct units based on their lithology. As depicted in figure 3, the number and thickness of aquifers underlying the study area increases from west to east. The depth to bedrock at the Scotland-Richmond County line is 200250 feet, where the unconfined aquifer and just one confined aquifer are identified (Figure 4). At the Columbus-Brunswick County line where the depth to bedrock is about 1,200 feet, in addition to the unconfined aquifer, there are at least 4 different confined aquifers (Figure 5).

The southern Coastal Plain groundwater system is recharged mostly by precipitation. Precipitation falls to the ground surface and takes one of three paths. Most of the water (6070%) will be utilized within the soil zone by a process known as evapotranspiration. This is simply the loss of water by evaporation from the soil and transpiration from plants. Only about 10% is transported directly to the streams and rivers by over land flow. The remaining 20-30% moves through the soil zone and recharges the aquifer system. Figure 6 illustrates this concept by using an average annual water budget of 50 inches of rainfall for the Coastal Plain. This conceptual model assumes groundwater is not being withdrawn by wells and that the amount of water entering the system is equal to the amount leaving the system (in equilibrium).

Figure 6: Example water budget for the Coastal Plain of North Carolina.

The average rainfall amount was derived from a map of statewide annual average precipitation, which is based on historical data (Figure 7). The average precipitation for the study area ranges from 37.555 inches per year.

It has been estimated for northeast North Carolina that about 6070% of the water that flows through stream and river channels originates as groundwater (Wilder, et al., 1978). This process is known as groundwater discharge. Continuous groundwater discharge is the reason our streams and rivers continue to flow during periods of little or no rainfall. The unconfined aquifer acts as an underground reservoir slowly releasing water to supply the base flow for streams, and also to the deeper confined aquifers below.

The time interval between recharge and discharge in the unconfined and semiconfined aquifer system can be anywhere from days to hundreds or thousands of years. This retention time not only supports stable surface water stream flow, it also allows time for contaminants to break down or be retained by the aquifer matrix. These natural contaminant transformations have been the focus of many scientific studies. However, some types and locations of contamination are too much of a threat to public health and the environment to allow the aquifer system to remediate itself. When this occurs, those responsible are required by state regulations to seek alternative remediation strategies. The delicate balance between groundwater quality and land uses that alter that quality is difficult to maintain.

Figure 7: Map of average annual precipitation in North Carolina.

To better understand groundwater recharge and discharge, the terrain of the Coastal Plain can be divided into three regions. The wet flat uplands (upland flats) are areas such as Green Swamp, Colly Swamp or the Bladen Lakes Area, which are poorly drained (Figure 8). These areas are zones of recharge as the movement of groundwater is primarily vertical downward away from the recharging wetlands (Heath, 1997). In these areas, man made contaminants will impact both the Surficial Aquifer and any unconfined primary aquifer beneath. Because the unsaturated zone above the water table is fairly thin or nonexistent in these areas, there is little beneficial degradation of contaminants and wastewater introduced into these areas.

Figure 8: Groundwater flow paths in zones of recharge and discharge.

The dry flat uplands and sloping uplands (valley slopes) of the area are situated on the margins of the main recharge areas (Figure 8). In these areas there is some recharge to the Surficial and shallowest confined aquifer, however much rainfall on these areas is lost to evapotranspiration (Heath, 1997).

Groundwater in these areas is moving with more of a horizontal than vertical component. The impact of releases of contaminants or wastewater in these areas affects mostly the upper portion of the Surficial Aquifer. In these areas, the unsaturated zone above the water table imposes beneficial degradation of waste and contamination moving toward the water table.

The streams and flood plain areas (valley bottoms) of the region are areas where groundwater is discharging or moving toward the land surface (Heath, 1997). As a result, contaminants and wastes introduced to these areas remain close to or on the land surface. However, in these areas, wastes or contaminants transported in groundwater as a result of activities in recharge areas returns to the land surface to be discharged into creeks and streams.

On figure 6, note that out of 50 inches of precipitation, only one inch recharges the deeper confined aquifers. This is not to say that one inch of recharge occurs within the confined aquifers everywhere across the coastal plain, but it is an estimated average that illustrates the limited quantity of water and the large amount of time that deep aquifer recharge requires (Harden, et al., 2003). Recent studies in the central Coastal Plain indicate that one inch of recharge to the deeper aquifers within the Coastal Plain may be an over estimate.

The confined aquifers within the Coastal Plain have historically been very popular choices for public water systems, municipalities, industries and agriculture to tap because of their high yield and excellent water quality. The NC Division of Water Resources has shown in hydrographs for the Cretaceous Aquifer system that water levels in Bladen County have been declining since the 1970’s (DWR, 2002). If all water users in a region rely on the same source, withdrawing water at rates that exceed recharge, the water supply will eventually be depleted.

For the purpose of illustration, if one inch of recharge per year reaches the confined aquifer that a hypothetical municipality is drawing water from, the recharge is equal to 27,154 gallons of water for every acre of land area. This acrevolume of water would only supply one average household with water for 2.47 months. To put it another way, each household would require a lot equal to 4.86 acres to balance annual recharge and withdraw. If withdrawal exceeds recharge, depletion of the aquifer gradually occurs. Very few towns or municipalities have the minimum lot size needed to meet these criteria. This example does not consider groundwater used for commercial and industrial use.

Figure 9: Recharge (inches/year) of the Surficial Aquifer based on soil type.

The unconfined aquifer receives an average of 12 inches of recharge per year (Figure 9). One inch is estimated to continue its downward migration into the confined aquifers below, with 11 inches remaining, slowly discharging to local streams. If the unconfined aquifer were to be utilized for the municipality’s water supply, this annual amount of water available for use is 11 times greater than the recharge received by the confined aquifer.

In parts of the Coastal Plain, the deeper confined aquifers are overused. Exceeding the safe yield (Heath, et al., 2003) has created a need for alternative water supplies and is the reason why increasing demand may be placed on the shallow confined and semiconfined aquifers. Because of the water demand, water quality protection and increased utilization of groundwater derived from the shallow unconfined and semiconfined aquifers will become more important into the future.