|Yellow is dry, tan is drought 2/21/2017|
In their weekly report last week the U.S. Drought Monitor published by the National Oceanic and Atmospheric Administration (NOAA) and Department of Agriculture reported that the precipitation that fell in the previous week continued to reduce long-term drought in California and contracted drought in the Southern Plains, but dry conditions in the Mid-Mississippi Valley, Southeast, and Mid-Atlantic expanded drought. At this time over 12% of Virginia was in drought and over half the Commonwealth was experiencing abnormally dry conditions. The dry conditions expanded beyond Virginia in the Chesapeake Bay area; and drought expanded eastward across northeast Maryland, northern Delaware, and southern New Jersey.
Rainfall and snow melt are the water that flows to the rivers and streams of the watershed, but also percolates into the ground and recharges the groundwater. Private drinking water wells draw their water from groundwater and over 20% of Virginians depend on private wells for their drinking water. Geology, climate, weather, land use and many other factors determine the quality and quantity of the groundwater, so I keep my eye on the precipitation. Prince William is unique in having four distinct geologic provinces that come together in the County: (1) the Blue Ridge, (2) the Culpeper Basin, (3) the Piedmont, and (4) the Coastal Plain. Last fall the groundwater level in the U.S. Geological Survey (USGS) monitoring well up the road from my home in the Culpeper Basin geology recorded its lowest level in 86 years and we were not even officially in drought at that time. That’s when I began to worry about my water supply.
It is concerning that the seasonal lows are getting lower. This is a sign that the present groundwater use may not be sustainable. No studies have been done that attempt to quantify what the total available water is within the county or any of the geologic provinces where residents depend on groundwater for drinking water supply. Thus, it is impossible to know if the overuse or diminished recharge of the aquifer is critical yet. According to studies by a group of researchers at the University of California, Irvine, the University of Texas, and the Hydrological Sciences Branch of NASA at GSFC using satellites to perform real world groundwater monitoring Virginia’s aquifers are under stress. That means that we are using up the groundwater faster than it is recharging. That is exactly what would cause an 86 year low level of groundwater when we were not yet in a drought. Now with all of Prince William County in drought, there is no relief in sight though a wet spring could alleviate the drought.
My well draws water from an unconfined aquifer. A water-table, or unconfined, aquifer is an aquifer whose upper surface is the water table, and is at atmospheric pressure. The water table rises and falls with moisture content that is contained in the soil, and right now the area is in drought and the water table level is falling. Water-table aquifers are usually shallower than confined aquifers and because they are shallow, they are impacted by drought conditions much sooner than confined aquifers.
The northwestern part of Prince William County down the hill from Bull Run Mountain consists of sedimentary rocks of the Culpeper Basin. The predominant rock types are conglomerates, sandstones, siltstones, shales, and argillaceous limestones. When there is rain and snow melt, this geology tends to have wonderful water-bearing potential because it is a fractured rock system with very little overburden. The highest reported yields in the county are from wells in this geology. The downside is that this area is susceptible to contamination- the fractures that carry water can easily spread a contaminant and without adequate overburden spills could flow to depth through a fracture; and there is limited water storage within the fractured rock system. An extended drought could significantly impact my well and the other wells in this area.