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Geology and Groundwater
Maryland can be divided into five physiographic provinces. The geology underlying each province largely dictates the unique topography of each. In a similar way, the underlying geology dictates the types and quality of the aquifers in each region.
An aquifer is an underground reservoir that stores and yields groundwater in economically usable quantities.
In general, west of the Fall Line, groundwater is found in fractures in consolidated rock, or in weathered rock called saprolite. The consolidated rock may be igneous, metamorphic, or consolidated sedimentary rock. Limestone aquifers occur locally in the Ridge and Valley province (including the Hagerstown Valley) and are formed by fractures or solution cavities.
East of the Fall Line, the geologic units consist almost entirely of unconsolidated sediments (sand, gravel, silt, and clay), and groundwater is found in void spaces between sand and gravel particles.
The Hydrologic Cycle
Groundwater is derived almost entirely from precipitation. It is the portion of the precipitation (rain or melting snow) that moves from the land surface into the soil by infiltration and then into underground storage and circulation. Of the water that falls on the land surface only about 25% enters the groundwater system. The direct surface runoff is the portion of precipitation which has not gone underground, but runs over the surface as streams. Total runoff includes groundwater that discharges into streams, maintaining their baseflow.
Evapotranspiration during the growing season removes a significant amount of groundwater in the shallow system and sends it back into the atmosphere. For this reason, much of the groundwater recharge for the year takes place during the fall and winter months.
The hydrologic cycle can be partly summarized by the following balance equation:
P = R + ET + S,
P - precipitation,
R - surface and groundwater runoff,
ET - evapotranspiration; and
S - changes in storage (surface, soil, and groundwater reservoirs)
Groundwater flow paths vary greatly in length, depth, and travel time from points of recharge to points of discharge in the ground-water system.
A convenient way of showing the rate is in terms of the time required for groundwater to move from different parts of a recharge area to the nearest discharge area. The time ranges from a few days in the zone adjacent to the discharge area to thousands of years (millennia) for water that moves from the central part of some recharge areas though the deeper part of the groundwater system.
Water from the deeper confined aquifers in Maryland's Coastal Plain may not have seen the light of day for tens to hundreds of thousands of years.
Unconfined and Confined Aquifers
A groundwater reservoir is a confined aquifer (also called an artesian aquifer) if it is sandwiched between two impervious zones, or confining units. The unconfined aquifer (also called the water-table aquifer) is the uppermost saturated layer. The surface of the water table is open to the atmosphere.
In a confined aquifer, the water level in a well rises above the top of the aquifer. The potentiometric surface is the level to which the groundwater would rise in a well completed in the aquifer. If the water level rises above land surface such that the water flows from an open well, the well is called a flowing artesian well. Recharge to a confined aquifer may occur far away, where the water-bearing unit is hydraulically connected to the surface.
The figure shows two wells. Well "A" penetrates the unconfined aquifer and its water level reflects the water table. Well "B" penetrates the confined aquifer and is flowing because its potentiometric surface is above land surface.