When it rains... where does the water go? In urbanized areas, where pavement and roof tops cover the natural ground surface, raindrops might run down a gutter or off a paved road into a stormwater pond, then drain from the pond's outlet into a creek.
Runoff from parking lots occurs even in natural areas, such as state parks. When it rains at the marina in Leesylvania State Park, water runs directly from the impervious parking lot asphalt directly into the Potomac River, while much of the rainwater hitting the entrance road (Daniel K. Ludwig Drive) first flows into Powell's Creek before reaching the river. The surface runoff will absorb dirt particles (and fluids that have dripped from cars and trucks), making the roads and parking lots look clean after a storm - but carrying the dissolved pollution downstream towards the Chesapeake Bay.
In undeveloped areas within the park, water hits tree leaves, soaks into the soil, and goes underground. On a hot summer day, the water under the surface may stay only briefly in the pore spaces between soil particles, before evaporating directly back into the atmosphere or indirectly after being sucked into the root system of plants ("evapotranspiration").
Virginia soils can keep water for up to two weeks, before evapotranspiration sucks all the moisture from near the surface. Suburban gardeners and farmers across the state are well aware of the days between rains. As one farmer put it:1
If rainfall is sufficient, however, raindrops may seep below the root zone and accumulate under the surface in a layer of soil or rock, to become groundwater rather than surface runoff or transpiration. Water seeping underground does not create a lake or pond under the surface; instead, the drops fill up the pores between particles of rock. Rock underground is rarely so solid that there are no pores, and there are normally many connections between pores that make the subsurface permeable. Even igneous rocks typically have fractures or joints through which water can move, though slowly and in limited quantities.
At Nottoway Park (near the Vienna Metro station), the depth to groundwater averages 16 feet - so plant roots are sucking up the water trapped only temporarily in the soil between rains, while the groundwater in the completely-saturated zone can be much deeper.2
Underneath the Fairfax campus of George Mason University, the metamorphic bedrock has weathered and become a rotted "saprolite" through chemical weathering as feldspars (such as NaAlSi3O8, KAlSi3O8, CaAl2Si2O8) and other minerals have absorbed water and altered to clay. In the Piedmont bedrock of Fairfax County, between Centreville and I-95, the igneous rocks have decomposed to great depth, creating pores between the particles and making it easier to drill a well to the unweathered bedrock.3
A zone of water-saturated rock is called an aquifer if enough water accumulates underground in the pores between rock particles to supply a natural spring or an artificial water well. Depending upon the rock type, aquifers can absorb and release a lot of water... or just a little.
After a rainstorm in Northern Virginia, the sandstone bedrock west of Centreville (the Mesozoic Basin that formed when the Atlantic Ocean first opened) will absorb substantial water between the grains of sandstone. However, the soil that formed above basalt intrusions in the basin stay relatively dry, because the igneous rock is not very porous. The grains of iron, calcium, and other minerals in the basalt are packed tightly together, without gaps for water molecules. In addition, the basalt has few fractures, so water does not seep into many cracks.
|The quality as well as the quantity of groundwater is significant to users. In densely-settled areas such as Alexandria, utilities treat raw water so it meets Safe Drinking Water Act standards and then pipe drinking-quality water to customers. Most people living in the Rural Area of Prince William County, away from water and sewer services provided in the Development Area, drill individual wells to obtain drinkling water for widely-separated houses - and rarely treat the water, except to replace calcium with sodium (salt is used to soften the water, reducing its "hardness") and occasionally to remove excessive dissolved iron.
For water quality purposes, water wells supplying drinking water to rural houses are sealed (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 its load of diossolved 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 flushed away from rural homes is converted by bacteria into harmless gases and fluids.
Locating where to drill a well requires an understanding of local geology, as well as local/state requirements for distance from structures and septic systems. Some owners of rural lots even hire people with a reputation for "dowsing" or "witching" where water will be encountered underground. In theory, a forked branch or other tool, when held correctly by a skilled dowser as he/she walks over a property, will bend towards the ground whenever there is a strong signal of water below the surface.
All bedrock is not the same; rainwater does not flow quickly through the surface into all sediments, or through fractures in all bedrock. Wells drilled in the Blue Ridge crystalline rocks in western Fauquier county, or in a basalt flow in the 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. (Before building a house away from municipal water systems, it is wise to drill a well and make sure there is enough water...)
dowsing for groundwater
Source: US Geological Suvey - Water dowsing
Groundwater is stratified underground in a pattern similar to a sponge that has not been used for a day. Typically, such a sponge will be dry at the top, where pores are filled with air. Further down, water molecules saturate the spaces between the pores. In soil (and in caves), the dry zone at the top is the vadose zone. The area of an aquifer where pores are filled with water is the phreatic zone. In a cave, the surface of an underground lake will show very clearly the beginning of the phreatic zone.
When you drill a well down into 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 fill with water. A simplistic explanation - above the water table, soil is dry. Below the water table, soil is wet. Reality is more complicated, of course. There is a capillary fringe above the water table, where small pores may be wet and large pores may be dry due to the physics of surface tension and capillary action.
A well driller might encounter the water table at 20 feet, but typically wells intended to supply single family residences are drilled 70-400 feet deep. To supply the town (now city) of Fairfax before it implemented a water supply system based on surface water (Goose Creek and Beaverdam Creek, in Loudoun County), municipal wells were drilled from 165 to 809 feet deep and yielded 15 to 125 gallons/minute.4
Drilling deep allows drillers to intersect multiple layers of water-soaked rock, or to intersect enough vertical distance in one aquifer to supply the minimum required. With enough water oozing from the ground into the well through holes in the well casing, a typical well is designed to supply at least five gallons/minute - enough water for for multiple showers/cooking in the morning without emptying the well completely. A deep well also serves as a storage tank. 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.
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.
Where a stream valley has eroded down to the level of the water table at the top of the aquifer, water emerges at a spring. If the water table drops during the summer, when evapotranspiration increases, a spring may dry up, only to re-appar in the winter once the water table rises again. Water levels can fluctuate 15 feet. Water that goes into the ground may also be pumped back to the surface through artificial wells drilled down into an aquifer - and if the well is not deep enough, it too may go dry in late summer:5
Groundwater that emerges from springs/wells started as surface water. Whatever the raindrops absorbed in their travels through the atmosphere and into the ground will be carried underground. The aquifer may be contaminated with gasoline or heavy metals from automobile brake particles on pavement, or from bacteria from animal feces - but in many cases, the soil particles will trap the contaminants.
For organic contaminants, bacteria, ultraviolet rays, or other natural factors may decompose the molecules and clean the groundwater before it emerges at a spring/well. In limestone karst areas, however, the pores and permeability underground may be sufficient for contaminants (even items as large as corn cobs) to migrate underground. If a sinkhole is used as a garbage dump, then the waste is likely to reappear at a spring nearby.
Government agencies now encourage developers to direct runoff to seep into groundwater rather than to surface streams. 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.
East of the Fall Line, sediments have been deposited since the the Cretaceous Period, as much as 145 million years ago. The Coastal Plain east of I-95 was underwater for most of the time between the Cretaceous Period through the Pliocene Period, 3 million years ago, with unconformities reflecting periods when sea levels dropped.6
The geologic formations deposited since the Cretaceous vary in their capacity to store water and serve as aquifers for human use:7
One description of the Mid-Atlantic Coastal Plain noted:8
West of the Fall Line, the recharge area (the source of the groundwater, where raindrops seeped below the surface) will be located close to a natural spring or human-drilled well. Even in the Blue Ridge, igneous rocks are sufficiently fractured to allow surface waters to seep into the ground, and be pumped back to the surface wherever a well is drilled. If the elevation of the recharge area is higher than the elevation of a well, then water will flow under pressure out of an artesian well without requiring any pumping.
Due to the slow flow of groundwater through most porous and permeable rock formations, a raindrop may spend decades or even thousands of years underground before re-emerging at the surface. Groundwater extracted at Indian Head, Maryland (across the Potomac River from Quantico Marine Corps Base in Prince William County) had been underground only about 35 years.9
Folk tales of long-distance underground transport are overstated:10
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:11
Perhaps... but water that seeped into Piedmont formations would emerge at springs when encountering low elevations at the Fall Line. For mountain raindrops to get 650 feet under the Northern Neck, raindrops falling on the Blue Ridge would have to flow down the Potomac/Rappahannock rivers, then seep underground near the Fall Line. However, there is a grain of truth to the claim that Northern Neck wells may draw from sources far away.
Aquifers in the Coastal Plain may be recharged from as much as 20-50 miles away, at the Fall Line. Rainfall can seep into the Potomac Formation and other geologic units where they are exposed to the surface near the Fall Line. Raindrops from Richmond can flow eastward, downward - and slowly - to end up in an aquifer hundreds of feet beneath the surface at Hampton Roads.
Much of the water in the Lower Potomac Aquifer was trapped when the sediments were first deposited in the Cretaceous Period. At Suffolk and Newport News, for example, the Upper/Middle/Lower Potomac aquifers will be largely isolated from the surface by a series of confining beds; recharge by modern rainwater will be difficult.
Recharge rate is a significant policy issue. If deep aquifer isolation is complete, then groundwater pumped to the surface from such sources will not be replenished. If groundwater is not renewable, then pumping from deep aquifers should be managed as a one-time mining process, comparable to extraction of coal or quarrying/crushing basalt for construction projects.
Obviously the municipal and industrial wells in Hampton Roads, the Peninsula, the Middle Peninsula, the Northern Neck, and the Eastern Shore do not dry up quickly once pumping starts. Some recharge must be occurring, either via transport from the Fall Line exposures or directly from the surface.
conceptual model of shallow aquifer recharge and underground flow in southern part of Virginia Beach showing how "confining units" can be fragmented and allow rainwater to soak underground through Columbia aquifer into Yorktown-Eastover aquifer... and how saltwater can intrude and affect shallow wells
Source: US Geological Survey (USGS), Conceptual Hydrogeologic Framework of the Shallow Aquifer System at Virginia Beach, Virginia, Water-Resources Investigations Report 01-4262 (Figure 13)
Confining units in the Coastal Plain are not 100% impervious. Some fluids do migrate vertically between layers, so rain that falls on the Coastal Plain could end up in the aquifers under the surface. However, the confining units do limit the speed and volume of "leakage" between different aquifers. So long as rainfall exceeds withdrawal, and the ground surface remains pervious, then freshwater will move underground due to gravity. The freshwater is lighter than saltwater, so freshwater will "float" on top of saltwater underground.
sediments form aquifers and confining layers, typically with heavier saltwater underneath freshwater
Source: US Geological Survey (USGS), Effects of Withdrawals on Ground-Water Levels in Southern Maryland and the Adjacent Eastern Shore, 1980–2005 (Scientific Investigations Report 2007–5249, Figure 3)
However, if wells pump freshwater out of an aquifer faster than that aquifer is recharged from the surface or adjacent freshwater zones, then saltwater or brackish water in the Chesapeake Bay estuaries may intrude further into aquifers underneath the Coastal Plain. Drinking water/industrial wells also can be affected if freshwater recharge is altered by stormater systems that carry rain quickly to streams, rather than allow the rainwater to soak into the ground:12
Groundwater may be a renewable resource, but an aquifer can be drained faster than it recharges naturally. Based on legislation passed by the General Assembly in 1992, the Virginia Department of Environmental Quality (DEQ) has designated two Ground Water Management Areas, one on the Eastern Shore and one west of the Chesapeake Bay. State permits are required to withdraw more than 300,000 gallons/month from wells in a designated Ground Water Management Area.
There is plenty of water in the Atlantic Ocean and Chesapeake Bay to recharge aquifers in the area, but salt water is less desirable for drinking, irrigation, or industrial uses. In Virginia's two Ground Water Management Areas, DEQ has the authority to denying or limit requests for large groundwater withdrawals. That allows the state officials to minimize the potential of salt water from the Chesapeake Bay/Atlantic Ocean intruding into an aquifer.
Brackish groundwater is converted to drinking water quality by desalination plants in James City County, Gloucester County, and the cities of Chesapeake, Newport News, and Suffolk. Desalination is typically the most expensive way to generate drinking water from seawater, which is why desalination plants built during droughts in Australia and California were monthballed after rains returned, but desalination is cost-effective when the groundwater is contaminated by just a small percentage of salt.13
In southeastern Virginia, there is a natural inland saltwater wedge where groundwater far west of the Atlantic Ocean is unexpectedly salty. One theory presumed the aquifers were just slow to flush out saltwater from the past, when sea levels were higher and all of the groundwater underneath that part of the Coastal Plain was seawater.
In the 1990's, the discovery of a 35-million year old impact crater in the lower Chesapeake Bay provided a new explanation. The inland saltwater wedge was created when a meteor/comet (bolide) dug a 6,000-foot deep crater. Seawater instantly vaporized, creating a brine (1.5 times as salty as normal seawater) that was trapped as debris flung up by the impact quickly re-filled the hole. The impact also cracked the confining layers in sedimentary formations for some distance, providing channels today for seawater to intrude vertically and inland.14
In addition to potentially-excessive groundwater withdrawals by municipal drinking water wells, primary concerns in Ground Water Management Area include industrial water use by paper mills at Franklin and West Point, and by chicken processing plants or facilities supporting offshore energy development on the Eastern Shore. Groundwater withdrawal at the West Point paper mill has lowered groundwater levels as much as 100 feet, and created a "cone of depression" with a radius of about 29 miles around the withdrawal point. At Franklin, the land itself has subsided an inch, as sediments compacted after water was extracted.15