Hydrologic Setting

Regional Extent

The Northern Atlantic Coastal Plain runs along the east coast of the United States from Long Island, New York, to the northern half of North Carolina (Figure 2; Trapp and Horn, 1997). Aquifers within the system extend out eastward into the Atlantic Ocean, but for practical purposes, most consider the shoreline as the eastern edge. To the west, the system reaches the Atlantic Seaboard Fall Line, where the Piedmont and Atlantic coastal plain meet. The NACP eventually grades southward where it meets the Southern Coastal Plain aquifer system.

Figure 2. Areas highlighted in yellow represent the Northern Atlantic Coastal Plain aquifer system. The green line indicates the location of the cross section in Figure 12 (U.S. Geological Survey, 2018).

Regional Climate

The aquifer system extends across 6 states, yet weather conditions are similar throughout. The Northern Atlantic Coastal Plain experiences climates ranging from humid and temperate, to subtropical (Denver, et al., 2014). The average annual temperatures span from 62 degrees Fahrenheit in the south to approximately 54 degrees Fahrenheit in the north. The average annual rate of precipitation throughout the area from 2005 to 2009 was around 45 in/yr (Masterson, et al., 2013). Precipitation is distributed fairly evenly throughout the year but will be more intense during warmer months when tropical weather systems and thunderstorms occur (Denver, et al., 2014). Precipitation is a major constituent of recharge for the system.

As climate change continues to become a growing issue throughout the world, it has the potential to greatly affect the climate within the aquifer system and as a consequence, affect the amount and availability of the groundwater. The majority of climate models indicate temperatures will be warmer in the future as well as precipitation being more variable or decreasing (Masterson, et al., 2016). These two conditions may lead to severe droughts and rising sea levels. During droughts, the demand for groundwater increases, but recharge to aquifers by precipitation decreases. This imbalance often causes over pumping groundwater within aquifers. Rising sea levels can also negatively affect an aquifer system. Coincidentally, recent analyses of the rising sea levels have found the Mid-Atlantic region along the east coast has higher rates of sea level rise than anywhere else along the coasts of the United States (Masterson, et al., 2016). Rising sea waters can increase the potential for saltwater intrusion within aquifers and cause groundwater inundation of structures such as basements and subway systems. Climate change has the ability to increase as well as decrease the amount of water stored within the NACP aquifer system.

Recharge

Approximately 81,000 Mgal/d of freshwater enters the NACP aquifer system by recharge from precipitation, wastewater, and leaky water-supply and sewer lines, with precipitation acting as the primary source (Masterson, et al., 2013). Average recharge rates are usually higher in the northern portion of the system and lower in the southern portion (Figure 3). As previously stated, the average annual rate of precipitation between 2005 and 2009 in the NACP area was approximately 45 in/yr, which is equal to 62,000 Mgal/d of input. Out of the 62,000 Mgal/d of potential input from rain, only 19,600 Mgal/d (31%) reaches the aquifer system as recharge while the remaining 69% is lost to evaporation, transpiration, and surface runoff. The NACP aquifer system consists of shallow unconfined and deeper confined aquifers, but the majority of the recharge only enters the unconfined aquifer (Masterson, et al., 2013). Only 2% of recharge reaches the deep confined layers where they crop out near the Fall Zone in the western part of the system.

Wastewater from centralized sewage treatment facilities and onsite domestic septic systems can also be an important form of recharge to the aquifer system (Masterson, et al., 2013). The majority of wastewater from these facilities end up in surface waters nearby without ever touching groundwater, but as much as 70 Mgal/d enter the shallow, unconfined aquifer due to leaking lines from old public-supply distribution and sewer systems.

Figure 3. The average amount of recharge the aquifer system receives in inches per year (Masterson, et al., 2013).

Figure 4. Distribution of major streams and rivers within the NACP region (Masterson, et al., 2013).

Relationship to Surface Waters

Groundwater is a major source of input to the streams and rivers within the Northern Atlantic Coastal Plains region (Figure 4). 50% to 90% of freshwater that enters the streams originate from the aquifers, with 99% of the water discharging from the surficial (upper most unconfined) aquifer (Denver, et al., 2014). It is estimated that approximately 18,600 Mgal of freshwater enters the NACP region per day from streams that originate elsewhere (Masterson, et al., 2016). Once the streams enter the region, flow increases by 3,800 Mgal/d from the additional groundwater discharge coming from the NACP aquifers. It can take a few years for groundwater to move through the surficial aquifer and discharge to streams, but it can take hundreds of years for water that originates within the confined aquifers to do the same (Denver, et al., 2014). Groundwater within the NACP aquifer system also discharges to estuaries and the Atlantic Ocean. Some of the major estuaries the groundwater flows into include the Delaware Bay, Chesapeake Bay, and the Albemarle Sound. It is not possible to make a direct measurement of coastal groundwater discharge, but in 2005, the total amount of outflow to tidal waters by major river systems was 30.5 billion gallons per day (Masterson, et al., 2013).

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