Monday, February 26, 2018

Drought in Virginia

In their weekly report last week the U.S. Drought Monitor produced by the National Drought Mitigation Center at the University of Nebraska-Lincoln, the United States Department of Agriculture, and the National Oceanic and Atmospheric Administration. reported that the precipitation that fell in the previous week, but dry conditions continued in the Southeast. Moderate drought coverage in south-central Virginia was reduced because of increasing streamflow and lessening mid-range precipitation deficits. At this time over 12% of Virginia was in drought and almost 60% of the Commonwealth was experiencing abnormally dry conditions. Continued light rains over the weekend may begin to alleviate this as happened last year. 



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. The U.S. Geological Survey (USGS) monitoring well up the road from my home in the Culpeper Basin geology has been showing a worrying trend over the past decade. 


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 last data set from the Hydrological Sciences Branch of NASA using the now dead GRACE-2 satellite 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. Most of Prince William County in drought (except for my neighborhood), but once more 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 southeast corner of the county in the Coastal Plain and the northwestern corner of Prince William County down the hill from Bull Run Mountain are the only areas in the county not currently in drought. The geology near Bull Run Mountain consists of 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. So I keep my eyes on drought.

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