Thursday, June 7, 2018

Water Level Shows Seasonality and Rain’s Impact on Wells

The recent rains in this part of Virginia not only allowed me to find four leaks in my roof where the solar panel rack was not flashed and the black jack finally failed, but also restored the groundwater aquifer to 8 feet below grade after a dry winter when levels fell to 12 feet below grade. If your water is supplied by a well, you need to be aware of the condition of the groundwater aquifer that supplies your well and live within your water resources. There are dry years and wet years and water will vary, though it is not always obvious.

The groundwater aquifer you tap for water is not seen, but you still need to be aware of your water budget and live within it. The daily household water needs here in Virginia is about 75 gallons/day per person according to the U.S. Geological Survey. 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 as you can see in the graph above. 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 dry spells when there is little or no rain.

The quantity and quality of ground water in Prince William County varies across the county depending on the geologic and hydrogeologic group you are in. Within the county there are four distinct geologic provinces: (1) the Blue Ridge, (2) the Culpeper Basin, (3) the Piedmont, and (4) the Coastal Plain. The U.S. Geological Survey divides the four geologic provinces of the county into seven hydrogeologic groups based on the presence and movement of the ground water calling them groups: A, B, B1, C, D, E and F. The age of the groundwater in your well is dependent on the hydrogeologic group.

Direct determination of the groundwater level in your well requires a water level meter which most of us do not have, but a less direct indication of the status of your well might be obtained from a proxy well. The U.S. Geological Survey, USGS, maintains a group of 171 groundwater monitoring wells in Virginia that measure groundwater conditions daily and can be viewed online. One of the Virginia wells is just up the road from me in the same hydrogeologic group and the ten year history of the well can be seen above. The seasonality of groundwater wells can be clearly seen in the graph.

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 several extreme lows.

This well is in hydrogeologic group B in the northwestern part of Prince William County and consists of sedimentary rocks of the Culpeper Basin. The predominant rock types are conglomerates, sandstones, siltstones, shales, and argillaceous limestones. This is a fractured rock system with moderate to excellent water-bearing potential with very little overburden. The highest reported yields in the county are from wells located in hydrogeologic group B and this is where I live. The downside to this formation is that the hydrogeologic group 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. Another potential problem is in an extended drought there is limited storage, recharge is quick, though. As you can see below in hydrogeologic group B, the storms of this past April and May are clearly visible in the well monitoring data.

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