A falling Static Water Level in Prince William

Recently, when I replaced my pump, I took the opportunity to have my static water level measured. Monticello Pump Service used a sonic water level meter to measure the water level. Sonic meters are often accurate to within 1/10of a foot, but that was good enough for my purposes. The measured level was 43 feet below grade. Sixteen years ago when the well was installed the static water level was 30 feet below grade. The recharge for the well at completion was 60 gallons per minute and is still quite powerful as demonstrated by my running the hoses to clear the chlorine from the system for 16 hours without a problem. What the falling water level appears to be indicating is that the groundwater table is falling.

The water level in a groundwater well usually fluctuates naturally during the year. Groundwater levels tend to be highest in the early spring in response to winter snow melt and spring rainfall when the groundwater is recharged. Groundwater levels begin to fall in May and typically continue to decline during summer as plants and trees use the available shallow groundwater to grow and streamflow draws water. Natural groundwater levels usually reach their lowest point in late September or October when fall rains begin to recharge the groundwater again.

However, groundwater levels can be affected by how many other wells draw from the aquifer, how much groundwater is being used in the surrounding area for agricultural, industrial or commercial use, or how much groundwater is being recharged. Development of an area can impact groundwater recharge. Land use changes that increases impervious cover from roads, pavement and buildings does two things. It reduces the open area for rain and snow to seep into the ground and percolate into the groundwater and the impervious surfaces cause stormwater velocity to increase preventing water from having enough time to percolate into the earth, increasing storm flooding and preventing recharge of groundwater from occurring. Slowly, over time, this can reduce groundwater supply and the water table falls.

Increasing population density as we have been doing here in the Rural Crescent of Prince William County increases water use. Significant increases in groundwater use and reduction in aquifer recharge can result in the slowly falling water levels that indicate that the water is being used up. Unless there is an earthquake or other geological event groundwater changes are not abrupt and problems with water supply tend to happen very slowly as demand increases with construction and recharge is impacted by adding paved roads, driveways, houses and other impervious surfaces. That appears to be what is happening in this area of Prince William County.

The U.S. Geological Survey, USGS, maintains a group of groundwater monitoring wells in Virginia that measure groundwater conditions daily and can be viewed online. Only one of the Virginia wells is within the Rural Crescent. That well is in the northwest portion of the Rural Area just west or Route 15 in the Culpeper groundwater basin not too far from my home. Daily monitoring data available from that well go back to 2004 (other records exist covering 39 years 1975-2014, but are no longer available on line). What can be seen in the graph below is the slow decline in the water level despite not experiencing any significant droughts since 2008 and having the wettest year on record in 2018. The decline is modest over this period, but will continue and get worse over time especially if demand for groundwater is increased and impervious surfaces continue to grow, reducing recharge.

USGS 49V

In the monitoring well the fall lows have been getting lower and the recharge even in 2018, the wettest year on record, did not reach the level of recharge during a drought in 2007-8. We appear to have a problem in this area. There is no information available in any other area of the Rural Crescent. Prince William County needs to engage the USGS and DEQ to study the groundwater in Prince William County expanding on the study they are performing for Fauquier County.

Little is known about the sustainability of our groundwater basins, but potential problems are still at a manageable stage. Groundwater models and data from more monitoring wells can help develop a picture of the volume of the water within the groundwater basin and at what rate it is being used and at what rate it is being recharged. We need to know if the current and planned use of our groundwater is sustainable even in drought years. We need to understand how ground cover by roads, parking lots and buildings will impact groundwater recharge and what level of groundwater withdrawals are sustainable on site to determine if a proposed change in land use or additional use of groundwater is sustainable before it is granted. Without coordinated and proactive management, the aquifers supplying our region will be depleted.