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Frequently Asked Questions - The Hydrology of Groundwater

Questions and answers are original compositions or are compiled from any available sources and credit is given where appropriate. New material will be added as needed. Contributions are welcome.

The Hydrology of Groundwater

What is Groundwater?

Groundwater is water that flows or seeps downward and saturates soil or rock, supplying springs and wells. Groundwater often begins as precipitation and soaks into the ground where it is stored underground in rock crevices and in the pores of geologic materials (these are aquifers), the same way as water fills a sponge. The upper surface of the saturated zone is called the water table.

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How important is Groundwater?

Groundwater, which is found in aquifers below the surface of the Earth, is one of the Nation's most important natural resources. Groundwater is the source of about 38 percent of the water that county and city water departments supply to households and businesses (public supply). It provides drinking water for more than 97 percent of the rural population who do not get their water delivered to them from a county/city water department or private water company.

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What is an aquifer?

An aquifer is a geologic formation that can store and transmit water to wells, springs and some streams. An aquifer is more like a sponge than an underground river: geologic materials have connected pores that allow water to move from one space to another, but unless the rock is fractured, water does not move through large, hollow tunnels at rapid rates. Wells can be drilled into aquifers and water can be pumped out. Precipitation adds water (this is recharge) into the porous rock of the aquifer. The rate of recharge is not the same for all aquifers, though, and that must be considered when pumping water from a well. Pumping too much water too fast draws down the water in the aquifer and might eventually cause a well to yield less water or run dry. Pumping your well too fast or too often might also cause your neighbor's well to run dry if you both are pumping from the same aquifer. Aquifers can be quite extensive, possibly stretching for tens of miles, feeding hundreds of Groundwater wells and streams. This is why usage of your well can influence other people miles away.

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What is the difference between a confined and a water-table (unconfined) aquifer?

A confined aquifer is an aquifer below the land surface that is saturated with water. Layers of impermeable material are both above and below the aquifer, causing it to be under pressure so that when the aquifer is penetrated by a well, the water will rise above the top of the aquifer. A water-table, or unconfined, aquifer is an aquifer whose upper water surface (water table) is at atmospheric pressure, and thus is able to rise and fall. Water-table aquifers are usually closer to the Earth's surface than confined aquifers are, and as such are impacted by drought conditions sooner than confined aquifers.

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What is an artesian well?

An artesian well taps a confined aquifer. This aquifer is a water-bearing geologic material below ground that is surrounded by other rock or material that does not allow water to pass through easily. The surrounding material may add pressure on the aquifer, and water in this aquifer can be pushed up the well, sometimes all the way to the surface, creating a flowing well.

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What is the difference between consolidated and unconsolidated sediments?

Consolidated sediments are materials that have been metamorphosed or cemented together, like limestone and sandstone. Much of the western portion of Maryland that includes the Piedmont, Valley and Ridge, Blue Ridge and Appalachian Plateau consists of consolidated sediments. Groundwater flows through fracture networks in these consolidated sediments. Unconsolidated sediments are sediments ranging from clay to sand to gravel, with connected pore spaces that allow Groundwater to be stored and transported. The Coastal Plain consists mainly of unconsolidated sediments. Coastal Plain confined aquifers are the main source for municipal water supplies on the Delmarva Peninsula and the western shore of Maryland.

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How does the water level in my well change?

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 height of the underGroundwater levels. 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 be lowered. This is what is happening during drought periods. The water level in a well can also be lowered if other wells near it are withdrawing too much water.

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What determines when a well will go dry?

A well is said to have gone dry when water levels drop below a pump intake. This does not mean your well will never have water in it again, as the water level may come back through time as recharge increases. The water level in your well depends on a number of things, such as the depth of the well, the type (confined or unconfined) of aquifer the well taps, the amount of pumping that occurs in this aquifer, and the amount of recharge occurring. Wells screened in unconfined water table aquifers are more directly influenced by the lack of rain than those screened in deeper confined aquifers. A deep well in a confined aquifer in an area with minimal pumping is less likely to go dry than a shallow, water-table well.

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How do I find out if my well will go dry?

Wells screened in unconfined water table aquifers are more directly influenced by the lack of rain than those screened in deeper confined aquifers. This means that it may be more likely for the water level in wells screened in the water table to drop below the pump level and prevent water from being obtained. This does not mean that wells in a confined aquifer will not go dry, as they are also influenced by pumping rates and lack of recharge.

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Why doesn't a drought go away when it rains?

There are a few reasons for this. During a drought, a lot of rain is needed to make up for water deficits , and to fill our reservoirs and recharge our aquifers. During hot summers, a lot of rain gets evaporated before getting the chance to replenish our water supplies. In terms of replenishing our Groundwater supplies, it may take a long time for water that falls on the Earth's surface to reach and recharge the aquifers. The deeper the aquifer, the longer it takes for rainfall to get there.

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How does water reach my home?

All of the water that we use in our homes comes from either a Groundwater source, such as a well, or from a surface-water source, such a river, lake, or reservoir. Precipitation falls on the Earth's surface and eventually adds water (recharge) into an aquifer. This water may be pumped into your home from a well that taps into the aquifer. If your water source is a reservoir, precipitation and other surface water collects in the reservoir. This water is piped to homes from a public supplier. According to U.S. Geological Survey Circular 1200 "Estimated Use of Water in the United States in 1995", Maryland Department of the Environment, and Delaware Department of Natural Resources and Environmental Control, the total 1995 Groundwater withdrawals for Delaware were 110 million gallons per day, with 66% of the total population using groundwater. For Maryland, the total 1995 Groundwater withdrawals were 246 million gallons per day, with 31% of the total population using Groundwater.

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What can I do to conserve water being drawn from aquifers and reservoirs?

The Maryland Department of Environment (MDE) recommends the following water conservation tips:

Low-income or senior citizens can call 800-492-7127 to get info on assistance for fixing leaks or installing low-flow shower heads.

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