relationship of springs and groundwater
Source: USGS - Ground Water and Surface Water: A Single Resource
When it rains... where does the water go?
Some of it runs off the surface into a nearby creek. In urbanized areas, where pavement and roof tops cover the natural ground surface, water might run down a gutter into a stormwater pond, then drain out the outlet into a creek.
Runoff from parking lots occurs in rural as well as urban areas - even in "natural areas" that have made accomodations for visitors. When it rains on the parking lot at Leesylvania State Park, the water runs directly from the impervious pavement into the Potomac River. Ultimately, the runoff carries whatever car fluids that the raindrps absorbed from the parking lot all the way to the Chesapeake Bay and the Atlantic Ocean.
In undeveloped areas, water hits tree leaves, soaks into the soil, and goes underground into aquifers. "An aquifer is a geologic formation, group of formations, or part of a formation that contains sufficient saturated, permeable material to yield significant quantities of water to wells and springs," according to the Principal Aquifers of the 48 Conterminous United States, Hawaii, Puerto Rico, and the U.S. Virgin Islands description from the US Geological Survey.
Where a stream valley has eroded down into the top of the aquifer, water emerges at a spring. Water that goes into the ground comes back to the surface, at natural seeps or pumped back to the surface through wells drilled down into an aquifer.
An aquifer is similar to a sponge. Typically, a sponge will be dry at the top, where pores are filled with water. Further down, water saturates the spaces between the pores. When you drill a well downinto an aquifer and cross from air-filled to water-filled pores between soil particles, you are crossing the "water table." The water table is the top level of the soil where a well will begin to fill with water. A simplistic explanation - above the water table, soil is dry. Below the water table, soil is wet.
Depending upon the rock type, aquifers can absorb and "leak" a lot of water, or just a little. After a rainstorm in Northern Virginia, the sandstone bedrock west of Centreville (the Triassic Basin that formed when the Atlantic Ocean first opened) will have plenty of water between the grains of sandstone - but the basalt layers in the Triassic Basin stay very dry. The basalt is not porous. The grains of iron, calcium, and other minerals in the basalt are packed tightly together, without gaps for intrusion of water. In addition, the basalt is not fractured, so water does not seep into many cracks.
Most people living in the Rural Area of Prince William County, away from water and sewer services provided in the Development Area, have to drill wells to obtain drinkling water. Before building a house away from municipal water systems, it is wise to drill a well and make sure there is enough water. Water does not flow quickly though all sediments, or through fractures in all bedrock. Wells drilled in the Blue Ridge crystalline rocks (or in a basalt flow in a Triassic Basin) may generate less than the 3-5 gallons/minute usually required to obtain a mortgage on a new house. In contrast, limestone in the Shenandoah Valley is porous, and some wells there can flow at hundreds of gallons/minute.
A well driller might encounter the water table at 20 feet, but typically wells are drilled 70-400 feet deep. This allows drillers to intersect multiple layers of water-soaked rock. With enough water oozing from the ground into the well through holes in the casing, a typical house could pump 5 gallons/minute from a well for multiple showers/cooking in the morning without emptying the well completely. In the course of a day, a well will recharge with water from adjacent rock formations before a family comes home in the evening and pumps water from the well into the house again.
For water quality purposes, drinking water wells are sealed (or "cased") for the top 30 or so feet. An iron pipe inserted into the well is not perforated in the top 30 feet, blocking the surface water with fertilzer, dog poop, oil from cars, etc. from entering the well. Drinking water wells must also be located a certain distance away from septic tanks, where human waste is converted by bacteria into harmless gases and fluids.
Drinking water from most wells is not chlorinated or disinfected with ultraviolet light, and well water is not flouridated either. Well water has fewer non-natural chemicals, which is a benefit if you keep fish in an aquarium. Well water may include so much calcium (creating "hard water") or sulfur that homeowners need to buy water conditioning systems. The cost of a water softening system, requiring several bags of salt each month, can be equal to the cost of municipal water.
When it rains on field or forests in Virginia, most of the raindrops hit tree leaves and then go into the ground. Whatever the raindrops absorbed in their travels is carried underground. In some cases, the aquifer may be contaminated - but in many cases, the soil particles trap the contaminant until bacteria, ultraviolet rays, or other natural factors decompose the molecules.
Government agencies are encouraging developers to direct runoff to seep into groundwater rather than to surface streans. The Chesapeake Bay Preservation Act regulations ("Ches Bay regs") require a 100-foot wide buffer on either side of perennial streams in most of Tidewater Virginia. Buffers of undisturbed vegetation along streams allow sediment to be trapped by grass/leaves on the ground before runoff carries the particles to the stream. Without buffers, sediment clogs streams while also carrying excessive amounts if nutrients (nitrogen and phosphorous) into waterways.
Low Impact Development (LID) projects are now designed to minimize surface runoff, by diverting and retaining rainfall so it soaks into the ground. Depressions filled with permeable soil or even rock (french drains) trap runoff from roofs and parking lots. If LID projects are designed correctly and maintained, the runoff soaks into the ground slowly rather than flows to a nearby stream. LID projects can fail where there is too much clay in the soil, or silt/debris accumulating in a depression blocks rainwater from seeping into the ground.
One potential long-term impact of diverting surface water underground: modern pollution may be pushed into the groundwater, polluting underground aquifers. Plumes of polluted groundwater from leaking underground storage tanks at gas stations, or from improper disposal of hazardous waste in deep wells, can create serious environmental impacts for many, many years.
Due to the slow flow of groundwater, a raindrop may spend centuries or even thousands of years underground before re-emerging at the surface. The Northern Neck Bottling Company, located in Montross in Westmoreland County, used to claim the Carver's Original Ginger Ale were special in part because "The water originates in the Blue Ridge Mountains of Virginia and is drawn from a 650-foot deep artesian well at the plant."1
