Slowing Land Subsidence in Hampton Roads, VA with Wastewater
Hampton Roads, Virginia is the second largest population area threatened by sea level rise in the country. Southeast Virginia is predicted to be inundated with an additional one to three plus feet of sea level by 2060. Forcing as many as 170,000 residents to relocate. About half of the previous sea level rise can be accounted to land subsidence, and this proportion is not expected to decline. (1) Land subsidence can be caused by many things. Aquifer compaction is the major cause, this occurs when enough water is withdrawn from the aquifer and the overlying soil compacts the empty pore spaces.
The Hampton Roads Sanitation District is experimenting with a new method of injecting treated wastewater into the underground aquifer as a means to minimize the impacts of land subsidence for the the area. Recharging the aquifer could help completely offset the ground sinkage since the final project would be injecting more water than is needed by the local population. Previous studies have found a correlation between groundwater usage and land subsidence. When the paper mill in Franklin, Va closed, the ground was found to have risen 27 mm in about ten years. (2)
Subsidence in this area can be caused by isostatic subsidence, the continent trying to rebalance itself from the weight loss of melted glaciers, or groundwater withdrawal. Ground water withdrawal is the leading cause. In the Hampton Roads area, wells are pumping 100 million gallons of water a day out of the Potomac aquifer that has no natural water recharge source. Small businesses and municipalities use 60% of the water pumped out. The Hampton Roads Sanitation District serves 1.7 million people and treats 249 million gallons of wastewater a day and hopes to be able to put 120 million gallons of treated wastewater back into the aquifer.
At the York River Treatment Center, staff are currently testing two different methods, reverse osmosis and carbon filtering, to purify wastewater to drinking water standards before it would be injected into the Potomac aquifer. Reverse osmosis is using a very tight membrane to filter the water. It cleans 85% of the water, but 15% is transformed into a concentrated brine that needs to be disposed of. Carbon filtering passes the wastewater through two carbon filters that use microbes to digest the waste. Testing the two methods started in September 2016, with plans to continue the study for an additional 15 months. Carbon filtering currently seems to be the better option as it has a lower carbon footprint, doesn’t create a brine, and is more cost efficient.
Once the water is purified, it is ready to be injected into the Potomac aquifer. However, matching the geochemistry of the aquifer at the injection site is crucial. This means that the treated wastewater would need to have a similar chemical composition and pH of the aquifer water. All of the drill sites will test the groundwater and then adjust the treated wastewater to match.
Hampton Roads Sanitation District plans to have seven injection facilities that would include wells drilled deep enough to pump the treated wastewater into the aquifer. The first pilot facility has plans to break ground in January 2017 and finish construction by early 2018. Ted Henifin, Hampton Roads Sanitation District general manager, wants to be “full scale between 2020 and 2030.” (3) The ultimate goal would be a facility that could treat and inject 120 million gallons of wastewater a day. If actualized, this facility will support the largest groundwater recharge effort in the country.
Injecting treated wastewater would also reduce the pollution going into the Chesapeake Bay. Henifin predicts that purifying and reinjecting the wastewater could reduce nitrogen and phosphorous going into the Bay by “90 to 95%.”(4) If successful, Hampton Roads could serve as an innovative model for other municipalities to meet their TMDL goals for the Bay. The Hampton Roads Sanitation District has already spent $750 million to meet state requirements of nutrient reduction. This project might prove to be the cheaper alternative in the long run to mitigating sea level rise and nutrient levels in the Bay.
1.Tompkins, Forbes, and Christina Deconcini. “Sea-Level Rise and Its Impact on Virginia.”
(n.d.): n. pag. Wri.org. World Resources Institute, June 2014. Web. 14 Dec. 2016.
2. Mayfield, Dave. “The 2,000-foot Hole in the Ground That’s Important in the Battle against
Sea Level Rise.” Virginian-Pilot. The Virginian-Pilot, 03 Dec. 2016. Web. 12 Dec. 2016.
3.Fears, Darryl. “Hampton Roads’ Solution to Stop the Land from Sinking? Wastewater.” The
Washington Post. The Washington Post, 20 Oct. 2016. Web. 12 Dec. 2016.
4.Mayfield, Dave. “Waste Not the Wastewater: Hampton Roads Sanitation Agency Fast-tracks
Plan to Turn Wastewater into Drinking Water.” Virginian-Pilot. The Virginian-Pilot, 21 July 2016. Web. 12 Dec. 2016.