More on the Loudoun Groundwater Assessment

The Agriculture Work Group of the Loudoun County Preservation and Conservation Coalition issued their groundwater report last month. At that time I talked about the overall conclusions: The Agriculture Work Group was alarmed not only at the drought conditions that have plagued the small well systems and resulted in water restrictions the past few summers, but also in the falling water levels in most areas. They created the report to heighten awareness of the need for proactive groundwater planning. I believe that their report demonstrates the need for Prince William County to move forward with their groundwater study and in building a network of monitoring well. Only with decades of data can the trends in water supply be measured, and yet we are already seeing troubling signs.

I would like to highlight and excerpt some sections of the report to argue for action by Prince William County moving forward with their own study and installing a well monitoring network. Before we do that, let’s cover a couple of basic points first. Groundwater is water beneath the surface of the earth. It is one of our Nation's most important natural resources and is often taken for granted. According to the USGS: “Contrary to popular belief, groundwater does not form underground rivers. It fills the pores and fractures in underground materials such as sand, gravel, and other rock, much the same way that water fills a sponge. If groundwater flows naturally out of rock materials or if it can be removed by pumping (in useful amounts), the rock materials are called aquifers.

Groundwater moves slowly, typically at rates of 7-60 centimeters (3-25 inches) per day in an aquifer. As a result, water could remain in an aquifer for hundreds or thousands of years. Groundwater is the source of about 40 percent of water used for public supplies and about 39 % of water used for agriculture in the United States.”

While groundwater is a renewable resource it is NOT unlimited. The water on earth is finite. We do not know how much water we have in the Culpeper basin, which underlies much of the western part of Prince William County, nor do we know what the sustainable rate of ground water use is. We can only hope that the Culpepper Basin is adequate to sustain the former rural crescent and the headlands of the streams in Prince William County in the next drought.  We do know that overuse of groundwater and changing land use are the biggest threats to the sustainability of our water supply.

Based on the 2023 Water Resources Monitoring Data Summary Report (Loudoun County, 2023), Loudoun County currently records groundwater levels at 20 dedicated monitoring wells (Figure 10). Seventeen (17) of these wells are operated by the Department of Building and Development and three are operated by USGS. To date over 490,000 readings have been recorded. These wells are dedicated for monitoring purposes and are not pumped.

 Assessment of the Groundwater Supply in Loudoun County      

In their report on Loudoun County ground water, the third USGS well described is located in Prince William County, just south of the Loudoun County line in the Bull Run watershed. It has collected data at this location since 1968.  The water level in this well has been declining about a foot a decade since 2000; however, it is not in a location of heavy development-yet. The Leesburg USGS well, in an area of heavier development, is declining about 3 feet per decade.

Loudoun also mentions there have been unconfirmed reports of private wells that dried up throughout the county (as there has been in Prince William). Many farmers had to start feeding hay to livestock as early as September 2024 to compensate for dried up pastures and a number of farmers began hauling water to livestock to compensate for dried up streams. To date, the Loudoun County Department of Health has identified 235 replacement wells that have been drilled in Loudoun since 2000.

Not only is the groundwater level in the monitoring wells falling, there are also other troubling signs with groundwater in the county. Records from the Loudoun County Department of Health indicate that numerous wells that were drilled with no water (i.e., dry hole) or went dry over time and were replaced. These outcomes, presumably, are due to declining water levels. There are also hundreds of wells designated as “Dry Hole” where records are less specific prior to 2001. Wells are classified as; wells that were reported to have been drilled dry, wells that are classified as Dry Hole (mostly recorded prior to 2001), and wells that are a replacement to an existing supply (presumably a shallow well or spring).

Figure 17 below shows that fundamental changes in groundwater availability have taken place in which replacement wells were required. New subdivisions exacerbate the  decline of the water table and have caused more shallow existing wells in older homes to go dry. Prince William County can expect to see more of that as they continue to increase housing density in the western portion of the county where well use is common.

 

Assessment of the Groundwater Supply in Loudoun County

In Loudoun County as of 2023, over 3,200 (almost 14%) wells have been abandoned for various reasons. Reasons include wells were no longer needed as a community water supply line was added and wells were abandoned because they have gone dry.

Groundwater wells in the Piedmont region typically intersect several fracture zones. These fracture zones are often 5 to 10 feet thick and the primary fracture zones often occur between 200 and 600 feet deep.

Climate models continue to predict that a warming world will lead to higher humidity, because warmer air evaporates more water from Earth’s surface and the air can hold more moisture (Stillman, 2024). As warming persists, evaporative and transpirative demand increases. An integrated surface water groundwater hydrologic model has been developed to evaluate the sensitivity of shallow groundwater resources to warming across most of the United States (Condon, et al., 2020)

The study  cited above shows that warming results in groundwater storage losses. In addition to global and continental scale investigations, it is important to note that researchers are also addressing regional impacts of climate change in a model analysis of groundwater in eastern Virginia (Culver, et al., 2025). There are two reasons for citing this research. First, is that other researchers acknowledge that climate change is affecting water resource availability in eastern Virginia now, not centuries from now. Second, is that the researchers believe that precipitation variability that results in more drought conditions is causing them to reassess the sustainability of groundwater resources for water supply within a Virginia Groundwater Management Area and this is likely to impact the Culpepper Basin and surrounding surface water flow volume.

Mankind’s hand in changing the land surface impacts water resources. Land use changes that increase impervious cover, add more suburban lawns, roadways, buildings, pavement and eliminates woodlands reduces groundwater recharge and increases stormwater flooding. Land use changes also potentially increases the use of groundwater by adding more homes and businesses (like Amazon who reports drawing millions of gallons of groundwater in the Manassas area).

Slowly, the changes in land use change the ecology of the watershed and can reduce the water supply over time. As groundwater levels fall, perennial steams that feed the rivers become intermittent during dry periods like the last few summers. I believe this is what is happening in the area of the Bull Run Mountain Conservancy where for the second summer in a row, what where perennial streams have stopped flowing in the summer.

Generally, groundwater in the Culpeper Basin is renewed each year through precipitation. The water stored in the watershed has always been able to provide adequate water in droughts because historically the withdrawal of water was within the average recharge rate. However, the only nearby USGS groundwater monitoring well is no longer stable. The water level has been slowly falling since before the last drought- despite a series of wet years.

Groundwater recharge through precipitation requires adequate area for infiltration; control of sheet flow created by roads and paved areas, as well as protecting the most geologically favorable infiltration points. In a natural environment much of the precipitation soaks into the ground. Some water infiltrates deep into the ground and replenishes aquifers, which store huge amounts of freshwater for long periods of time. Some infiltration stays close to the land surface and can seep back into rivers, creeks, and ponds through the hyporheic zone.

Maintaining open areas provides areas of groundwater recharge. According to the U.S. Environmental Protection Agency, impervious cover levels of less than 5%-10% can significantly impact watershed health increasing stormwater runoff and reducing groundwater recharge. When runoff volume increases, runoff velocity increases, and peak storm flows increase and you get flooding with soil erosion, fast moving stormwater carrying contamination and reduced or eliminated water infiltration into groundwater. The groundwater is essential as the base flow to the streams and rivers that feed the Occoquan Reservoir during the dry months. Is this a little hint of the beginning of the end.

The bottom line is our water supply may not be sustainable and is certainly not sufficient when increasing impervious surfaces and increasing demand by adding data centers and population. We are headed for disaster, but so slowly that we can continue ignore all the signs until disaster is upon us.