Pollution of Groundwater: Vulnerability

The water that individuals drink is the same water that falls in the form of rain on the fields that produce crops and graze livestock, the fertilized lawns in residential neighborhoods, and the oil-stained parking lots in major cities. Potential contaminants can be found in every rural area, in every suburban community, and on every city block.

Infiltrating precipitation moves through the vadose zone, (the unsaturated material above the aquifer ), and can transport virtually any compound with which it comes into contact. Chemicals improperly stored at the surface, animal waste (e.g., livestock operations), septic tanks, and buried waste at landfills all have the potential to contaminate infiltrating precipitation, and, ultimately, groundwater.

Transport across the vadose zone depends on the chemical characteristics of the contaminant and the composition of the zone. Certain chemicals, such as TCE (trichloroethylene), will attach to organic matter in soil. This slows the rate at which TCE will move through the vadose zone.

Contaminated recharge water will contaminate the aquifer unless there are natural barriers that can slow or stop its downward migration. Surface activities have a dramatic effect on groundwater quality. It is easy to see the link between surface activities and groundwater quality by considering where groundwater comes from (i.e., precipitation) and where it has traveled (across the land surface and through the underlying soil and rock layers).

Aquifer Sensitivity

The sensitivity of an aquifer to contamination is based on the physical characteristics of the aquifer, the overlying geologic materials, and, for a specific contaminant, its chemical characteristics. "Sensitivity" is a relative term used to describe how well an aquifer is protected from infiltrating contamination. A highly sensitive aquifer would have little or no defense, whereas an aquifer with low sensitivity would be very well protected.

A shallow, unconsolidated sand-and-gravel aquifer is highly sensitive to contamination. The physical characteristics of the aquifer permit rapid infiltration of recharge. Rapid recharge leaves little time for contaminants to degrade naturally or be adsorbed before reaching the aquifer.

Conversely, a deep, confined, layered basalt aquifer has a very low sensitivity. Infiltrating recharge could take years to reach the aquifer, allowing time for contaminants to abate or degrade.

The sensitivity of an aquifer can vary greatly, depending on geologic conditions. Fractured or faulted terrain tends to conduct recharge much more quickly than unfractured rock because fractures act as conduits for fluid flow. Hence, faulted or fractured bedrock aquifers tend to be highly sensitive. Limestone terrain that has undergone dissolution (dissolving) by groundwater often forms karst topography, which is characterized by sinkholes , caves, and rapid underground drainage. With its many conduits connecting the surface and subsurface, karst terrain makes for a highly sensitive aquifer.

Highly impermeable strata, such as silt and clay, provide a physical barrier above an aquifer. Aquifers that are overlain by thick sequences of silt and clay or unfractured bedrock tend to be less sensitive to surface activities.

Vulnerability Assessment

Two concepts have been introduced that can affect groundwater quality. The land-use activities that take place at the surface can affect groundwater quality, and the physical or geologic characteristics of the vadose zone and aquifer can provide protection from infiltrating contaminants.

Land-use activities and aquifer sensitivity are absolute terms that can be easily defined through observation and physical investigation. They are combined to define a relative term that is used to qualify the real risk to a given aquifer: vulnerability.

A vulnerability assessment defines the risk to an aquifer based on the physical characteristics of the vadose zone and aquifer and the presence of potential contaminant sources. This can be an important tool for communities and private well owners interested in protecting the long-term viability of their drinking-water source. The implementation of land-use planning or zoning overlays based on aquifer vulnerability can prevent aquifer contamination by carefully locating potential contaminant sources in areas of very low aquifer sensitivity.

For example, when a city planning board receives requests to locate an automobile service station, it poses a potentially great threat to groundwater because of the many wastes associated with vehicle maintenance and the storage and transfer of gasoline and other substances. The zoning decisionmaking process should be influenced by the relative risk of the potential source and the vulnerability assessment of the aquifer. The preferred result would place the gas station above the least-vulnerable regions of the aquifer.

How can the present generation ensure that future generations can depend on a safe, clean supply of groundwater? Assessing the vulnerability of aquifers is the first step toward careful management of groundwater resources. Implementation of groundwater protection strategies can enhance the long-term quality of an aquifer and raise public awareness of groundwater issues.

see also Aquifer Characteristics; "Chemical" entries; Clean Water Act; Fresh Water, Natural Contaminants in; Groundwater; Hydrogeologic Mapping; Karst Hydrology; Land Use and Water Quality; Land-Use Planning; Landfills: Impact on Groundwater; Pollution of Groundwater; Safe Drinking Water Act; Septic System Impacts; Supplies, Protecting Public Drinking-Water.

Jeffrey Frederick

Bibliography

Driscoll, Fletcher G. Groundwater and Wells. St. Paul, MN: Johnson Division, 1987.

Stewart, Sheree, and Dennis Nelson. Source Water Assessment Plan: Implementation of the Safe Drinking Water Act of 1996 Amendments. Portland, OR: Oregon Department of Environmental Quality and Oregon Health Division, 1996.