Traditional development practices cover large areas of the ground with impervious surfaces such as roads, driveways, sidewalks and buildings. This is especially true for higher density and mixed use developments. This kind of development impacts the groundwater beneath the development and in the surrounding area. These paved and impervious surfaces prevent rainwater from infiltrating into the ground, causing it to run off site at velocities and volumes that are much higher than would naturally occur. According to data from the U.S. EPA, when development disturbs more than 10% of the natural land by covering surfaces with roads, driveways, walkways, patios, and homes the natural hydrology of the land is disturbed, irreparably disturbed. It may take months or even years before the impact to the aquifer becomes obvious as water resources are depleted. Rainfall cannot soak through these hard surfaces and recharge the groundwater; instead the rain water flows across the pavement picking up pollutants along the way. The storm water flows into ditches or storm drains, which typically dump the water, pollutants and debris carried in the stormwater into our streams and waterways and increasing the pollutants in the steams and rivers.
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) 24.7% of the domestic water supply in Virginia comes from groundwater- 195 million gallons a day. Groundwater is the sole source of drinking water for the population who are not connected to city or community water systems.
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 in depleted aquifers in the summer when there is little or no rain.
But 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 and overtime reducing the amount of groundwater. Significant increases in groundwater use for irrigation of crops or playing fields, or commercial 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 and this tends to mask a slowly decreasing aquifer or falling groundwater level. 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 so it is hard to see a slow and gradual loss of an aquifer even if you monitor the groundwater level. However, unless the groundwater level falls below the pump level it is typically unnoticed. It is essential for the long term sustainability of our communities that the long term impact to the aquifer be assessed before the surrounding land use is changed or developments are approved.
No comments:
Post a Comment