Thursday, May 16, 2013

Groundwater Drought in Western Prince William

USGS monitoring well 49V1
It seemed that all of last week we had rain here in western Prince William County, my garden blooms and I am getting my garden in shape. As I was considering planting three trees that would need to be watered until they became established, I checked the water level in the U.S. Geological Survey (USGS) groundwater monitoring well up the road and was shocked at the water level. The median groundwater level for May based on 39 years of data is 9 feet below grade. The monitoring well’s groundwater level had fallen to just about 14 feet below grade. This was the lowest level of groundwater recorded for that well during May over the entire 39 years of data that had been recorded. The USGS maintains a group of 20 groundwater monitoring wells in Virginia that measure groundwater conditions daily and can be viewed online. One of the Virginia wells, 49V1 is just up the road from me in the same groundwater basin and serves as the proxy of the condition of my well.

The water level in a well usually fluctuates naturally during the year. Groundwater levels tend to be highest in the early spring in response to 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, though the lowest level ever recorded at the monitoring well up the road was in July 2011 at 15.38 feet below land surface. If the groundwater level in May is already this low, I am very worried about July and the general health of the aquifer. The natural fluctuations of groundwater levels are most pronounced in shallower wells like mine that are the most susceptible to drought.

The USGS has been using long-term groundwater monitoring data, combined with groundwater models, to improve our understanding of the storage and flow of groundwater. Whenever you pump water from a well it has to be balanced by a loss of water from storage in the groundwater aquifer. Groundwater is recharged from rain and surface infiltration from things like septic. If too much water is pumped, water tables can drop in unconfined aquifer like the one here in the Piedmont region of Virginia. The growing population and the effects of recent droughts have made the need for an updated status on the availability of the groundwater necessary and the USGS has been expanding their groundwater studies nationally.  I called the USGS Virginia Water Science Center in Richmond, Virginia and spoke to David Nelms the groundwater specialist. I happen to catch him right after the Drought Taskforce Meeting and so he was able to give me a well-considered opinion of what might be causing the low groundwater levels.

Mr. Nelms confirmed that this is the lowest groundwater level recorded in this region in May in 39 years. Though the Virginia Department of Environmental Quality, DEQ, has listed the groundwater conditions for Northern Virginia (including Prince William County) as normal, according to the USGS there is a small area in the Piedmont that just did not get enough rain last fall and over the winter to overcome the soil moisture deficit from the drought of 2012. According to Mr. Nelms this area of the Piedmont did not catch enough rainfall during the rains last fall even with Hurricane Sandy passing through. Though the water levels in the past couple of years have fallen to levels lower than recorded over the previous 39 years, the USGS did not think that anything other than a lack of rainfall was causing the low water levels. Mr. Nelms noted that the seasonal variation in groundwater levels seemed to be more extreme in recent years, and that the ownership of the property where the monitoring well is located had changed hands and could indicate a change in use could have impacted the apparent static pumping level by increasing the cone of depression if for instance the well had been used for irrigation or a sprinkler system. The USGS was not aware of any change in water use. As Mr. Nelms pointed the water level needs to be watched because it impacts not only the wells in western Prince William (including the Evergreen public water supply system), but also the surface water tributaries to the public water supply systems drawing from the Occoquan Reservoir and the Potomac River. Potomac River flow was also low for this time of year. Data from the National Weather Service’s Middle Atlantic River Forecast Center shows that the Potomac basin upstream of Washington, D.C. had a precipitation total which for the year to date is 1.5 inches below normal. We need to keep an eye on the water level and rainfall this summer.
USGS Data for 49V1

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