Proposed Groundwater Study in Prince William County

The PW County Department of Public Works has submitted a proposal to the Prince William County Board of Supervisors for Groundwater Study to address concerns with the impact of future developments and the sustainability of groundwater as a water supply.

The Department of Public Works proposes to have the U.S. Geological Survey (USGS) create a Soil Water Balance Model to analyze the impact of development on groundwater recharge to “predict the amount of recharge available for water resource planning for projected changes in land use and land cover. “ That is an essential component, but not enough.

Approximately 15% of Prince William County depend on groundwater for their drinking water. This includes the about 8,000 dwellings in the Rural Crescent, the Evergreen Water District and the other homes in the county on private wells. Though, public water is available to the county population, delivering public water to the homes in the Rural Crescent would cost tens of thousands of dollars per household to run laterals from water mains and untold amounts of money to bring the water mains throughout the rural area. Water mains cost $400-$1,000 per foot and there are miles upon mile of piping that would have to be paid for along with lift stations and other distribution infrastructure by  other water users. In addition, as Mr. Smith points out in his memo to the Board of Supervisors, the water supply is only projected to be adequate until around 2040. The pubic water supply is not unlimited.

 As the Department of Public Works correctly points out there are two monitoring well in Prince William County that track groundwater levels. One well in the Culpeper Basin near the Loudoun County Line and one well in Forest Park in the Potomac Aquifer in the Coastal Plain. The data is publicly available for both wells, but they are not part of any state or local program of monitoring. There are six hydrogeologic groups in Prince William County, and various formations within the two major aquifers. Groundwater is not uniform. Measuring water level 10 or 20 miles away tells you nothing about groundwater conditions in a nearby area.

Two wells are not adequate to know if the groundwater supply is sustainable. The information at the Department of Health is based on reports from homeowners when they need a permit to drill a new well. Someone with well problems would call a well driller to see of they can solve it. The Department of Health would only hear about a well that completely failed and a new drilling permit was needed, where lowering the pump or deepening the well did not help. Groundwater problems are decades in the making, they are very slow and gradual. The Department of Health would be a lagging indicator of a groundwater problem. By the time complaints occur, the problem would be past solution.

Despite what Mr. Smith says in his memo, the groundwater level in the one well in the Rural Crescent has been falling for about 15 years. After recovery of the drought in 2006- 2008, the spring highs in the groundwater level have decreased and the annual lows have gotten lower. You can see in the graph below (the same one in Mr. Smith’s memo) that within the seasonal cycle of groundwater levels that the seasonal highs have been falling slightly (slightly more than a foot) and the seasonal lows are lower. The water level is visibly decaying at a time that Prince William County has experienced the wettest years on record. This is a warning that we are using up the groundwater at least in that one well. It may not be characteristic of the entire county. There needs to be more monitoring to know if water is being used sustainably.

The USGS states: “A natural consequence of groundwater withdrawals is the removal of water from subsurface storage, but the overall rates and magnitude of groundwater depletion in the United States are not well characterized.” Based on a series of studies “Estimated groundwater depletion in the United States during 1900–2008 totals approximately 1,000 cubic kilometers (km3). Furthermore, the rate of groundwater depletion has increased markedly since about 1950, with maximum rates occurring during the most recent period (2000–2008) when the depletion rate averaged almost 25 km3 per year (compared to 9.2 km3 per year averaged over the 1900–2008 timeframe). The depletion in the USGS well in the rural areas only began to occur since 2004.

Prince William County needs to expand the scope of work for the Groundwater study to include sustainability. The residents of the Rural Area cannot afford to pay for the connecting of their homes to the public water supply (that itself is only projected to remain adequate until 2040) and the other residents of the county should not have to pay to buildout the infrastructure to bring water mains throughout the rural area. Prince William County is required to plan for sustainable groundwater to supply all existing and future residents of the rural areas, let’s make sure we are doing it.

from USGS

A consequence of using groundwater is depletion, but the overall rates and magnitude of groundwater depletion in the United States are not well characterized. The map below from a USGS study shows long-term cumulative depletion volumes in 40 separate aquifers in the United States, bringing together information from the literature and existing studies. According to the USGS the rate of groundwater depletion has increased markedly since about 1950, with maximum rates of depletion beginning to occur in this century when the depletion rate averaged three times the rate in the 20th century.