Sunday, October 23, 2022

The Coastal Plain Aquifer


The Northern Atlantic Coastal Plain aquifer system consists of six regional aquifers and extends from Raritan Bay, N.J. to the North Carolina-South Carolina State line. The crystalline rocks of the Piedmont Physiographic Province at the Fall Line mark the western limit of the aquifer. The eastern limit of the aquifer system is, for all practical purposes, the shoreline. The northern part of the Atlantic Coastal Plain is underlain by a wedge-shaped mass of semi-consolidated to unconsolidated sediments that thickens toward the ocean and is topped by a layer of crystalline rock.

The thickness of the sediments vary. At the New Jersey coastline they are about 4,000 feet, but the sediments reach as much as 8,000 feet along the coast of Maryland and 10,000 feet along the coast of North Carolina. The sediments consist of lenses and layers of clay, silt, and sand, with minor amounts of lignite, gravel, and limestone. The sand, gravel, and limestone make up the water bearing aquifers; some are traceable over long distances, whereas others are only local.

The aquifers are separated by confining units of clay, silt,and silty or clayey sand. Although water moves more readily through the aquifers than through the confining units, water does leak very slowly through the confining units, especially where they are thin or where they contain sand; the aquifers, therefore, are hydraulically interconnected to some degree.

A series of clay and silt confining layers separate the regional aquifers that are used for water supply (Masterson and others, 2015). Recharge enters the aquifer mostly from the outcrop areas in the landward part of the aquifer system, but some limited recharge comes from downward leakage through confining units.

The surficial aquifer is the uppermost aquifer in the aquifer system.  This aquifer consists of unconsolidated, locally gravelly sand, mostly of Quaternary age. Although a thin blanket of unconsolidated sediments makes up the uppermost Coastal Plain beds over wide areas, these sediments generally yield small volumes of water to rural and domestic wells. Water in the surficial aquifer is especially susceptible to contamination by human activities and saltwater intrusion. 

The Chesapeake aquifer underlies the surficial aquifer in most places, but the two aquifers are separated by a clayey confining unit that significantly slows the downward movement of groundwater, though does not prevent it. The Chesapeake aquifer consists mostly of sand beds of Miocene age.

The Piney Point-Castle Hayne aquifers and the Potomac-Magothy aquifers (which include Mago[1]thy, Potomac-Patapsco, and Potomac-Patuxent aquifers) are aquifers in the Northern Atlantic Coastal Plain aquifer system that are the primary sources of groundwater for public supply (Masterson and others, 2015). The Potomac aquifer in Virginia comprises part of the regional Potomac aquifer system, which also includes the Potomac-Patapsco aquifer and Potomac-Patuxent aquifer in Maryland, Delaware and New Jersey.

The Potomac aquifer is a confined aquifer that once formed artesian wells prior to being de-pressurized. The Potomac aquifer is several thousand feet thick for much of the Coastal Plain and contains hundreds of trillions of gallons of pressurized water. Unfortunately, what once seemed like a vast never-ending resource is being overused. In Virginia approximately 155million gallons of groundwater is pumped from the Potomac aquifer each and everyday.

The first hint of a problem was a drastic reduction the pressure of the confined water. Water no longer rose to the surface without lift pumps. Then the groundwater level began to fall. Deeper wells were needed to access water. This was followed by aquifer compaction – and now, land subsidence, saltwater intrusion and increased vulnerability to sea level rise.

This groundwater provides much of the drinking water in the Hampton Roads area. There is only Beaverdam Lake reservoir on the Coastal Plain of the Middle Peninsula that supplies drinking water, and no drinking water reservoirs on the Northern Neck. North of the York River almost all of the public and private drinking water comes from groundwater.  The rate of groundwater withdrawal from the Potomac Aquifer is currently unsustainable.

Groundwater in the Coastal Plain region in eastern Virginia is being used up. This has been confirmed by measurements of groundwater levels, modeling of the aquifer system by the U.S. Geological Survey (USGS) and measurements of changes in gravity by the National Aeronautics Space Administration (NASA). Reducing water use in the region to a sustainable level for the Potomac Aquifer would be economically devastating and quite frankly, an impossible task. You can’t take away water without a fight. We are left with either adding reservoirs and surface water systems or utilizing the water storage capacity of the groundwater aquifer. Like many parts of the country and Northern Virginia, the Hampton Roads area has turned their sights on reusing wastewater to supplement the drinking water supply. Their plan is to utilize the existing storage in the groundwater system.  

For 40 years Los Angeles County has recycled the water from wastewater treatments plants. This water from both secondary and tertiary treated wastewater is discharged into spreading basins on the surface to recharge groundwater. Groundwater recharge can be done by surface spreading or direct injection wells. It has long been know that soil filtration improves water quality and soil column studies with secondary effluent from wastewater treatment has shown dissolved organic carbon removal of about half by percolation through 20 feet of various soil types. However, the 40 years experience has found trace contaminants from disinfection by products in the groundwater.

Recharging an aquifer has lower capital costs than dam and reservoir construction, but carries risks and challenges.. The first challenge is geologic. The predominant geology of this area of Virginia makes the usual methods of artificial recharge- spreading basins almost impossible. Without using injection wells, to deliver water  directly into an aquifer it would not be practical. Artificially recharged water must first move through the clay zone and the only effective method is to use a recharging well. This method has risks, big risks. We are potentially introducing trace contaminants, precursors of disinfection byproducts, trace Pharmaceuticals and personal care products and many other unknow contaminants  from our modern world into our groundwater aquifers. Before we use reclaimed wastewater to recharge the groundwater aquifer to augment supply, we need to fully understand what contaminants (and emerging contaminants) survive treatment and are carried in the wastewater to the aquifer.

Enter the Sustainable Water Initiative for Tomorrow (SWIFT) project. It proposes to replenish the Potomac aquifer, eastern Virginia’s primary groundwater supply, with purified water. This purified water would be treated to be compatible with the existing water in the aquifer to ensure seamless integration into the system and introduced by recharge wells drilled at seven of the 13 Hampton Roads service District wastewater treatment plant sites. Recharge wells store water for future use by placing it deep underground into formations below the shallow soil layer.

This is a great concept, but the question remains is it possible to do safely in practice in Virginia? Currently, wastewater (sewage) travels through multiple levels of treatment at Hampton Roads Service District, HRSD’s, 13 wastewater treatment plants to ensure it meets regulatory discharge levels of particular contaminants that are measured and are protective of aquatic life and public health. With the SWIFT project, the treated wastewater will undergo additional treatment procedures in the Advanced Water Treatment Process to treat it even further in order to meet stringent drinking water standards. Right now, at the research center in Suffolk, Virginia a million gallons of day of treated wastewater is being further treated to meet the higher drinking water standards and pH and oxygen levels of the aquifer and is injected at low pressure to a well with open slotting between 500 and 1,400 feet below grade into the aquifer.

The groundwater aquifer serves to dilute the trace contaminants that survive the treatment plant, but we need to be honest and informed about what we are putting into or leaving in what is ultimately our drinking water supply. The water of the Potomac Aquifer has been protected for a millennium from man’s arrogance and lack of knowledge. Now we are injecting what we believe to be clean water directly into this water body. With plans to inject a million gallons of reclaimed water a day.

Maybe we should pump this water directly into the drinking water distribution system in Hampton Roads for a few decades to make sure there are no unexpected consequences before we pump a million gallons a day into the aquifer. The solution to pollution may not be dilution.

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