Sunday, June 23, 2024

There are Groundwater Problems

 I recently received the following (edited) query. Michelle Trenum suggested I contact you regarding our neighborhood, Mackenzie Meadow in Nokesville. I wanted to see if you had any information on Nokesville‘s water table, or anything I could present to the residents at our HOA meeting. The neighbor right next to me is having well issues. Apparently he has the most shallow well in our neighborhood. I believe he said it was just under 300 feet in depth, and he seems to run out of water when the neighbor on the other side of him  uses a lot of water at once. Meanwhile, there are neighbors with irrigation systems to water their lawns daily encompassing around half of their 10 acre lot . A few years ago a neighboring farm told me that after Mackenzie Meadow was built, they could no longer water their fields without running out of water. Any information you can provide, I would greatly appreciate it.

Groundwater is the moisture and water that exists in the spaces between rocks, the pores in the soil and fractures in the geology-the invisible portion of the water cycle. Groundwater is renewed through precipitation infiltrating into the ground, though seasonal, but can be extracted year-round. Provided that there is adequate replenishment, and that the source is protected from pollution, groundwater can be extracted indefinitely and can be robust in the face of drought. However, groundwater is not unlimited.

Increase the amount of groundwater extracted beyond what is replenished, then slowly over time the aquifer is used up, the water level falls and wells go dry. Development adds people, businesses and industry. All need water- increasing the demand for water while adding roads and buildings that prevent the infiltration of precipitation into the ground.  Essentially, reducing the replenishing (recharge) of the aquifer while increasing the demand for water. This potentially unsustainable combination. Increase water use or reduce recharge by eliminating forested areas and replacing them with compacted soils (lawns that need to be watered), pavement, buildings and over time the aquifer will become exhausted.

Groundwater is both used for water supply and serves to support steam flow between rain storms. Groundwater comes from rainwater and snow melt percolating into the ground. Typically, the deeper the well (thousands versus hundreds of feet) the further away is the water origination and the older the water. The groundwater age is a function of local geology, the amount of precipitation and the rate that water is pumped out of the aquifer. Geology also determines the ease with which water and contaminants can travel through an aquifer and the amount of water the land can hold. The land surface through which groundwater is recharged must remain open and uncontaminated to maintain the quality and quantity of groundwater.

We do know that groundwater availability varies by location even within Prince William County (Nelms and Richardson, 1990) . Precipitation and soil type determines how much the shallower groundwater is recharged annually. The volume of water that can be stored is controlled by the reservoir characteristics of the subsurface rocks. A significant portion of Nokesville is Hydrogeologic group C . The rocks of group C are Early Jurassic in age and include: the Mount Zion Church, Hickory Grove, and Sander Basalts; an unnamed diabase; and thermally metamorphosed rocks.

Rocks within hydrogeologic group C tend to have generally poor water-bearing potential because of the wide spacing between fractures, mineralization of fractures, and random fracture orientations. Better yields have been obtained from wells finished in areas where the diabase is intersected by cross-strike lineaments (Nelms and Richardson, 1990, p. 25) and in areas underlain by basalt which also exist in Nokesville.

Water resources are sustainable when the water used on average does not exceed the recharge to the aquifer.  To use groundwater sustainability requires adequate measurements and observations over years. Though the U.S. Geological Survey, USGS, maintains a group of groundwater monitoring wells in Virginia that measure groundwater conditions daily, only two are in Prince William County. One in Prince William Forest Park and one of the within the former Rural Crescent in the siltstone Hydrogeologic group B.

The water level in a groundwater well usually fluctuates naturally during the year. Groundwater levels tend to be highest in the early spring (my most recent reading) 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 residential, agricultural, industrial or commercial use, or how development has impacted groundwater recharge. Development typically increases impervious cover from roads, pavement and buildings.  This 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.

The well below is the one located in the northwest portion of the Rural Area just west or Route 15 in the Culpeper groundwater basin it is about 18 miles from Mackenzie Court, and in a different Hydrogeologic group and would not be representative of the groundwater in Nokesville.  Daily monitoring data available from that well go back decades and the groundwater level was fairly stable until around 2004. 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 and impervious is not managed. This area of Prince William County appears to have a slowly growing problem.

USGS Well 49V

The monitoring well in Prince William Forrest Park is in an overburden aquifer and tends to reflect precipitation since the area around the well has not increased use or development during the data period. Prince William County needs more information about groundwater.

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 ensure all residents of Prince William County will continue to have water. 

It is unreasonable to think that the approximate16,000 wells supplying households and the Evergreen Water System could somehow all connect to public water supplies from the Potomac River or Occoquan Reservoir many miles away. The tens of millions of dollars it cost to do this would have to be borne by the private well owners. The PW Board of Supervisors to fund the groundwater study as quickly as possible to ensure the continued availability of water for all our residents.


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