Thursday, July 23, 2015

Restoring the Coastal Plain Aquifer

from USGS

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). At the most recent meeting of the Potomac Watershed Roundtable, Michael Alter PG and Jason Early PG of Clear Creek Associates gave a presentation about utilizing artificial aquifer recharge for managing the sustainability of water resources in the Coastal Plain.

Overdrawing a groundwater aquifer can have many negative impacts on the regional hydraulic balance- a reduction in discharge to surface water at some other location, an increase in recharge from surface water, a loss of storage in the aquifer by falling water table or a loss of an aquifer by salt water intrusion or some combination of these effects. To remain a renewable resource the amount of groundwater removed from an aquifer needs to match the recharge rate. The Geologists from Clear Creek Associates suggested that Virginia increase the recharge rate of the groundwater aquifer using a recharge well. What we consume much be replaced. We either consume less groundwater or we utilize artificial recharge of the aquifer.

The rate of groundwater withdrawal from the Virginia Coastal Plain is currently unsustainable. The withdrawal rate of groundwater increased continuously during the 20th century. By the 2003 the withdrawal rates from Coastal Plain aquifers in Virginia totaled approximately 117 million gallons per day. As a result, groundwater levels had declined by as much as 200 feet near the large withdrawal centers of West Point and Franklin, Virginia the home of paper mills.

Since 2003 the water level has continued to fall despite the Virginia Department of Environmental Quality (VA DEQ) regulating groundwater withdrawals in the Virginia Coastal Plain through the VA DEQ Groundwater Withdrawal Permit Program. The Gravity Recovery and Climate Experiment (GRACE) satellite mission from NASA that has been collecting data for more than a decade confirmed this. The GRACE satellites measure monthly changes in total earth water storage by converting observed gravity anomalies measured from space. The researchers have recently published the analysis the 10 years of GRACE data (2003 to 2013) to quantify groundwater use, resilience and stability over time.

Despite the water rich climate, the Atlantic Coastal Plain aquifer is under stress and is being used beyond it recharge rate. It is only a matter of time until areas within the historic boundary of the aquifer subside and begin to go dry and subside. These days recharge of groundwater through spreading basins, pits, and injection or drainage wells is becoming more widely practiced. However, there are many challenges to recharging groundwater. The first is geologic.

In the Coastal Plain, groundwater is present in pores between the sediment grains with the bottom surface of the Coastal Plain aquifer in Virginia resting on the top surface of basement bedrock, which serves as the lower boundary of the entire Coastal Plain aquifer system. The top surface of the Potomac aquifer in Virginia slopes generally to the east and consists of mostly of layers of clay that are not entirely continuous, allowing some interchange between the surface and the Coastal Plain Aquifer. Most of the recharge of the aquifer occurs at the Fall Line. Of an estimated 10 inches per year of recharge at the water table along the Fall Lone, approximately 9 inches per year remains perched above the clay layer and travels only a short distance before being discharged into to nearby streams and rivers (McFarland, 1997, 1999). Only the remaining 1 inch per year manages to enter the regional flow system as recharge to deeper, down gradient parts of the Coastal Plain Aquifer.

The predominant geology of this area of Virginia would make artificial recharge almost impossible using anything except for recharge using injection wells directly into an aquifer. Artificially recharged water must first move through the clay zone and the only effective method is to use a recharging well. The Geologists from Clear Creek Associates utilized the groundwater system model developed by the USGS to simulate the effect of a 10 million gallon a day recharge well to inject reclaimed water directly into the groundwater aquifer.
from USGS
Though this was only an exercise in exploring ideas, waiting too long may make the task impossible. Areas where ground subsidence caused by excess withdraws of groundwater from the fine-grained compressible confining beds of sediments cannot be recharged. In addition, care must be taken in determining the ideal location for a recharge well; geological characteristics such as faults with significant offset, folds, and extensive coarse- or fine-grained sedimentary geological units can control both groundwater flow and the fate of water from artificial recharge. Groundwater is not an underground bathtub of water, the site specific geology and regional hydrology will determine the ability to recharge the aquifer and where that recharge must take place.

In addition, if we are to solve our water sustainability problems using artificial recharge we must protect the water quality of the Coastal Plain Aquifer from possible contamination. If we recharge the groundwater aquifer with treated waste water we are potentially introducing precursors of disinfection byproducts, trace Pharmaceuticals and personal care products and many other contaminants into our groundwater aquifers. Before we use any source of water to recharge the groundwater aquifer to maintain the supply of available water, we need to better understand what contaminants (and emerging contaminants) that survive treatment and are carried in the wastewater and other water supplies. The groundwater aquifer serves to dilute the wastewater contaminants that survive treatment, but we need to be honest and informed about what we are putting into or leaving in what is ultimately our drinking water supply.

Reducing water use in the region to a sustainable level for the Coastal Plain Aquifer seems an impossible task and would be economically devastation. We are left with either adding reservoirs and surface water systems or utilizing the water store capacity of the groundwater aquifer. We cannot wait too long to take action.

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