Aquifers in Maryland

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 and bedding-plane partings in consolidated rock. The consolidated rock may be igneous, metamorphic, or consolidated sedimentary rock. Unconsolidated overburden covers the rock in most places, and the water table may occur above or below the overburden-rock interface. Limestone aquifers occur locally in the Piedmont and Valley and Ridge, and extensively in the Hagerstown Valley.

East of the Fall Line, the geologic units consist almost entirely of unconsolidated sediments overlying consolidated basement rock, and groundwater is found in void spaces between sand and gravel particles.

Coastal Plain Region

Coastal Plain sediments thicken from northwest to southeast, increasing from a few feet at the Fall Line to greater than 7,000 feet at Ocean City. Most aquifers in the Coastal Plain are overlain by low permeability clay layers (forming confined or artesian aquifers) which prevents surface contaminants from reaching the aquifers. In the Coastal Plain physiographic province, groundwater flows through pore spaces between sand and gravel particles – known as intergranular flow. Wells in the Coastal Plain commonly have moderate to very high yields. Groundwater levels are not affected signficantly by short-term variations in climate (droughts and wet periods), but typically do reposnd readily to increased groundwater withdrawals.

Fractured Rock Region

In the Piedmont physiographic province, groundwater occurs in rock fractures under unconfined conditions. Most groundwater is stored in the weathered bedrock, or saprolite, which overlies the solid rock. Fractures tend to decrease in size and number with increasing depth. Well yields are generally not increased by drilling deeper than about 300 feet. Fracture widths are generally less than a tenth of an inch. Groundwater systems tend to be localized – flow is from the inter-stream divides towards the nearest perennial stream and there is very little inter-basin ground-water flow. Well yield can be literally hit or miss, depending on if you tap into a fracture system or not. In the Maryland Piedmont, wells drilled in valleys generally have greater well yields than those drilled in uplands or slopes. This is partly because the valleys develop along fracture zones, and also because the water table is closer to the surface (where the fractures are more numerous and wider). During drought, particularly if drought occurs during recharge periods (late fall and winter), water tables can decline.