Vulnerable Area

There are some areas where the geologic setting makes the groundwater more vulnerable to contamination than others. Vulnerability is considered here to be the relative ease of downward migration of a pollutant from its release at the surface. This is dependent in part on the permeability and thickness of the soil material overlying the groundwater, which can be inferred from geologic maps. There are two conditions that indicate relatively high vulnerability to contamination: permeable deposits of mostly sand and gravel (and to a lesser extent, silt), and unconsolidated material less than 50 feet thick.

Highly vulnerable areas of permeable geologic materials include: alluvium; valley-train and outwash plain sediments; muck, peat and marl paludal; eskers, kames, and kame complexes; solian sand and silt; beach and shoreline deposits; sandy lacustrine sediments; and valley-train sand and gravel overlain by thin lacustrine or alluvial deposits. Where the unconsolidated deposits are shallow, vulnerability to groundwater contamination also exists. This is because there is relatively less material to slow contaminant migration into bedrock and bedrock aquifers or into groundwater in the non-bedrock material. Since the smallest contour interval on maps showing the thickness of unconsolidated deposits statewide is 50 feet, areas with less than 50 feet of this material are considered highly vulnerable.

Sinkhole An area of the state where bedrock conditions are uniquely vulnerable to contamination of groundwater is in karst or sinkhole topography. When limestone bedrock appears close to the surface, it often contains sinkholes, caves, solution channels, and cave streams. Surface contaminants can rapidly enter and move into groundwater via this method.

Soil survey reports are available from Cooperative Extension Service, Natural Resource Conservation Service (NRCS) or Soil  and Water Conservation District (SWCD) offices for individual counties in most states, and is even available online in a few. The information available in those references is very detailed compared to statewide geologic and groundwater data. Knowledge of soil permeability, parent material, drainage, limitations on use, depth to water table, and other factors can be obtained from a soil survey. For site specific planning to identify conditions where the groundwater is vulnerable to contamination, the soil surveys and assistance from soil scientists can be very useful. By contrast, the areas vulnerable to contamination based only on geologic information should be interpreted on a broader scale and not for site specific decisions.

Geological and Hydrogeological Data

Geological and hydrogeological data are necessary to understand the sensitivity of an area to groundwater contamination. Basic information needed may include, but would not be limited to:
Nature of the overburden soils.
Depth of bedrock.
Bedrock characteristics, such as faulting and fracturing, which could influence groundwater flow.

Necessary hydrogeological information includes:
Depth to groundwater.
Groundwater flow direction and velocity estimates.
Potential receptors, including streams, rivers, lakes, wetlands, and deeper confined aquifer systems.
Groundwater quality and aquifer classification as designated by the Environmental Protection Agency (EPA) or a state agency.
Local groundwater uses.
Location of any nearby aquifer recharge areas.
Upgradient sources of groundwater contamination.

Sources of Hydrogeological Data

Many agencies may already have some basic site information from soil borings conducted during previous construction or from past hydrogeological investigations. Additional sources of existing information may include:
U.S. Department of Agriculture (USDA) Soil Conservation Service (SCS).
U.S. Geological Survey (USGS).
Federal and state regulatory offices.
Local well drillers.

This information should be used with some caution, however, since regional hydrogeology may not always apply to an individual site.

Depending on the complexity of the local hydrogeology and the availability of usable information, fieldwork may be necessary to supplement existing data. New wells may have to be drilled to obtain the needed information.

Groundwater Models

Groundwater vulnerability models and groundwater flow models may be useful in estimating the vulnerability of groundwater to contamination.

The model should be selected and used based on a particular site's needs. As with any modeling, the quality of the predicted output depends on the quality of the underlying assumptions and input data.

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