how fast the groundwater was moving (Mace et al., 1997). While many processes contribute to the fate and transport of hydrocarbons in groundwater, the overwhelmingly most important process is the microbial degradation of petroleum compounds. In virtually all aquifers the rate of microbial degradation of the hydrocarbon contaminant by the native microbial consortium is fast enough that, combined with other attenuation mechanisms (dilution, sorption, and volatilization), the plumes were attenuated within ~150 m. This realization substantially changed the remediation strategy for hydrocarbon contamination of groundwater, and the term “natural attenuation” has now entered the lexicon of the environmental professional (NRC, 2000a). However, natural attenuation is not as effective for chlorinated organic solvents such as TCE and perchlorate, where reduced rates of natural degradation allow plumes to travel several kilometers (Brusseau and Tick, 2006).

HEALTH HAZARDS OF WATERBORNE EARTH MATERIALS

Health hazards from drinking water arise from natural or anthropogenic contamination of source waters used for potable use. In particular, contamination of groundwater is dependent on the earth’s materials that host the aquifer. The physical properties of the subsurface result in significant differences in the behavior of groundwater compared to that of surface waters. For example, residence times for groundwater range from a few years to hundreds of years or more. Dilution effects, either in water or the atmosphere, are much less significant for groundwater compared to surface water systems. In addition, the absence of light eliminates the possibility of photochemical reactions, a major route of transformation in lakes or streams. The net result is that once groundwater and the subsurface geological units are contaminated, they are very difficult to decontaminate, and therefore pollution prevention is critical for maintaining sustainable groundwater resources.

Risk assessment is an essential element of effective management of groundwater resources. There are two components to the risk of pollution from groundwater—groundwater vulnerability and contaminant load. Groundwater vulnerability is the intrinsic susceptibility of the specific aquifer in question to contamination (see Table 4.1). An aquifer that is close to the surface, overlain by sandy soil, and located in an area with high precipitation rates would clearly be more vulnerable to contamination than an aquifer in an area of low precipitation that is hundreds of meters below ground surface and overlain by clay soils or other relatively impervious material.

Factors involved in the contaminant load are the type of contaminant, the amount of contaminant released, the timescale of release, and the



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