Thursday, May 23, 2019

In 2018 the Water Table Rose 3 feet in Fairfax

Climate projections predominately forecast that Virginia will become wetter and warmer. Last year’s excess rain caused to an extent by an El Nino and other weather may have been a preview. I moved to Virginia from California for the water so I was more pleased than not when the measured total precipitation inches in my yard was almost 71 inches. The rain was somewhat less about 66-67 inches in Fairfax, just a few miles northeast.

The result of all that rain was that the water table, the level of groundwater beneath the surface rose three feet in Fairfax county. The water table 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. 

from USGS

As you can see in the chart above, the peak ground water in May 2017 was three feet below the peak groundwater level in May 2018 and unusually, the winter levels of groundwater climbed another foot. The result of groundwater level, the water table, going from 12 feet below grade to 9 feet below grade is often a wet basement. Especially, if a sump pump failed.

The American Society of Home Inspectors estimates that 60% of U.S. homes have wet basements. Even if you had not previously had any problems in the past a three foot rise in the water table, from a very wet year like we’ve just can suddenly cause a previously dry basement to become wet.

Water or moisture in a basement can come from three sources: seepage of groundwater, condensation and rain. Condensation often occurs where cold meets warm air. Basements often get wet when rainwater runs toward the walls of houses from roofs, yards and driveways and infiltrates, but can also result when the rain causes the water table to rise as happened last year. So, if you are in Fairfax and had a basement moisture problem last year, you are not alone. As you can see below, the high water table continued into the spring of this year. Check your sump pumps to make sure they are working, and check your gutters and downspouts to make sure they are not blocked. Clean out the gutters, repair or replace any damaged gutters and extend your downspouts away from your house. Water from down spouts should be directed away from the house, discharging at least a few feet from the foundation. With a little effort you can keep your basement dry.  

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