Wednesday, June 26, 2024

PWCA Tuesday Groundwater and Geology

On Tuesday evening the Prince William Conservation Alliance and Mid-county Civic Association presented a panel discussion: “Water, Water, Will There Be a Drop to Drink? The Science of Groundwater and Best Practices to Preserve Our Drinking Water Supply” with guest speakers Brad White and Sam Caldwell. Brad White is from the Department of Environmental Quality (DEQ) Groundwater Characterization Team and specializes in the Piedmont and Blue Ridge regions of Virginia. Sam Caldwell is a Hydrologist from the United States Geological Survey (USGS) who generally specializes in the areas of the Potomac Aquifer. The full recording of the panel discussion is available on the Prince William Conservation Alliance YouTube channel. Prince William Conservation Alliance - YouTube

I am only going to give you some highlights that I found interesting, though I’ve excluded an observed and fascinating  groundwater anomaly in Loudoun County, because it is in Loudoun County and requires more data and information. The only real takeaway is that groundwater often does not do what is expected by a simple understanding of a situation- geology and groundwater.

Prince William County is the county in Virginia with the most diverse geology. There are four major hydrogeologic groups: a small area of Blue Ridge Crystaline, Piedmont-Sedimentary and Volcanic, Piedmont Crystaline, and the Coastal Plain. Though all groundwater comes from precipitation, generally speaking, the groundwater in each of those hydrogeologic zones comes from different sources. The groundwater in the Blue Ridge is found in the crystalline rock fractures. In the Piedmont-Sedimentary and Volcanic geologies water come from the Culpeper Basin a large and productive aquifer that in years past had been tapped for public water supply. Now, there are only a few reported large users, but the buildout of western portion of the county has made it the sole source of water for a growing number of residents. The Piedmont Crystaline area is a far less productive groundwater area with a random fracture orientation as discovered by Nelms and Brockman. Finally, the Coastal Plain is within the confines of the Coastal Plain Aquifer.

The Coastal Plain is the only groundwater management area in Virginia. In all other areas of Virginia groundwater use is not managed or controlled. Users of more than 300,000 gallons of per month are asked to report their use to DEQ. There are no permits required on a state level and Prince William requires none.

Though it has been decades since David Nelms,  Donna Richardson and A.R. Brockman did their groundwater and geological work studying the extent of PCE contamination from the IBM spill from their former site on Goodwin Drive in Manassas, that work is the basis for all the knowledge that is available about the groundwater and geology of Prince William County. It is also, the reason that Prince William Service Authority abandoned use of the groundwater supply wells in the Manassas area (within the Culpeper Basin) for the Public Water Supply.

In 1978, IBM discovered a release of chlorinated solvent from a storage tank at their 660 acre facility in Manassas,  Virginia. They began groundwater monitoring and found chlorinated solvents specifically PCE, TCE, DCE and TCA in the groundwater. IBM installed 49 on- and 45 off-site wells. Groundwater treatment began on-site in 1985 and off-site in 1997. The PCE plume had migrated off-site towards a public well in the Prince William County Service Authority (PWCSA) system. IBM installed a treatment system at the public well in 1985, and in 2001, the PWCSA discontinued use of the well. So as you can see below the reduction in use of groundwater for pubic water supply after 1980's. 

IBM leases the well for use as part of the contaminated groundwater recovery system. This is known as a “pump and treat system” which consists of pumping contaminated groundwater from the three on-site and two off-site wells to carbon absorption tanks where the chlorinated VOCs are removed. The treated water is discharged to surface streams under a permit issued by the Commonwealth of Virginia. Since 2001, Manassas has not used  wells as a drinking water supply. However, IBM paid for the work of Nelms, Brockman and Richardson of the USGS and a settlement paid for a portion of the allocation agreement with Fairfax Water.

Currently, there are only a few reporting users of more than 300,000 gallons per month of groundwater: the IBM pump and treat on-going remediation, a user in Manassas reporting 35 million gallons per year (believed to be Amazon), and a quarry dewatering operation.  

Wells within the formations of the Culpeper Triassic Basin are the most productive in the county. The basin was formed several million years ago and is believed to have once been a closed basin. The fractured rock systems are extremely productive with limited overburden and almost direct recharge to the bedrock fractures from surface fractures. In the Piedmont Crystalline area in Mid-County, the bedrock fractures are the groundwater reserves, but the orientation varies and are not reliably present. The thick overburden can be a store of groundwater. Finally, the Coastal Plain in Prince William is a small area adjacent to the fall line where it recharges the aquifer. It is capped with a confining layer of clay that creates a pressurized system. Though an private wells are in the upper aquifer.

This final image shows the electrical conductivity of the groundwater. Generally, the higher the specific conductivity, the more ions in solutions. This is typical of more soluble rocks where the minerals are picked up by the groundwater and the water is “hard.” So a specific conductivity map is mapping hard water. 

Finally, Mr. White concluded with four points:

  • When groundwater was used for public water supply in the 1980's and 1990's the reported use from those using more than 300,000 gallons a month was higher. 
  • The historic use of groundwater for public water supply in the past has demonstrated the substantial capacity for groundwater withdrawals from the portion of the county within the Culpeper Triassic Basin.
  • The Coastal Plain portion of Prince William County is actively managed through a permitted withdrawal system in the Groundwater Management Area of the state. No such management system is in place in the rest of the county. 
  • Proactive monitoring of groundwater pressures in the Coastal Plain and Triassic Groundwater system may provide valuable baseline information. 

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