Thursday, October 6, 2016
Is My Groundwater Being Used Up?
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 U.S. Geological Survey (USGS) groundwater is the provides 38% of public water supplies in our country. In addition, groundwater is the sole source of drinking water for more than 97% of the rural population who are not connected to city or community water systems. I am one of the 46 million Americans who depend on a private well for their water, so I care very much about groundwater, its sustainability and its protection.
My well draws on an unconfined aquifer. A water-table, or unconfined, aquifer is an aquifer whose upper water surface (water table) is at atmospheric pressure, and thus is able to rise and fall with moisture that is contained in the earth. Water-table aquifers are usually shallower than confined aquifers are. Because they are shallow, they are impacted by drought conditions much sooner than confined aquifers. A confined aquifer is an aquifer below the land surface that is saturated with water. Layers of impermeable material are both above and below the aquifer, causing it to be under pressure so that when the aquifer is penetrated by a well, the water will rise above the top of the aquifer.
The water level in the aquifer that supplies a well does not always stay the same. Droughts, seasonal variations in rainfall, and pumping affect the level of the water table. If a well is pumped at a faster rate than the aquifer around it is recharged by precipitation or other underground flow, then water levels in the well can fall. This is what happens during times of drought and happened this summer when there was little or no rain in our little micro-climate. A well is said to have gone dry when the water level falls below the pump intake. This does not mean your well will never have water in it again, as the water level may come back through time as recharge increases. If drought has caused the water level to fall, then precipitation can restore the well.
There are other forces that can impact the recharge of a well. Land use changes that significantly increase impervious cover and stormwater velocity can prevent water from soaking into the earth and reduce recharge of the groundwater making existing wells more susceptible to drought. Significant increases in groundwater use for irrigation of crops or playing fields, or commercial or industrial purposes can overtax and aquifer and dry out neighboring wells. Unless there is an earthquake or other geological event groundwater changes are not abrupt and problems with water supply tend to happen slowly as demand increases with construction and recharge is impacted by adding paved roads, driveways, houses and other impervious surfaces.
The water level in a groundwater wells naturally fluctuates during the year. Groundwater levels tend to be highest in the early spring after winter snowmelt 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. It is concerning that the monitoring well recorded its lowest level in 86 years.
The natural fluctuations of groundwater levels are most pronounced in shallow wells that are most susceptible to drought. Older wells tend to be shallower. However, deeper wells may be impacted by an extended drought and take longer to recover. My well is fairly shallow in a fractured rock system with little overburden. During dry periods, I can watch the water level fall. The chart below is from a nearby USGS monitoring well.
Private wells draw their water from groundwater. Geology, climate, weather, land use and many other factors determine the quality of the groundwater; and the water level in your well depends on a number of things, such as the depth of the well, the type (confined or unconfined) of aquifer the well taps, the amount of pumping that occurs in this aquifer, and the amount of recharge occurring. Within Prince William County Virginia there are four distinct geologic provinces: (1) the Blue Ridge, (2) the Culpeper Basin, (3) the Piedmont, and (4) the Coastal Plain. 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. This geology tends to have moderate to excellent 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.
It is concerning that the seasonal lows are getting lower. This is a sign that the present groundwater use is not sustainable. Since we do not know what the total available water is, it is impossible to know how critical the overuse or diminished recharge of the aquifer is. According to studies by a group of researchers at the University of California, Irvine, the University of Texas, and the Hydrological Sciences Branch at NASA 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 an 86 year low level of groundwater is telling us.