Basic Geology and Hydrogeology

This chapter discusses fate and transport of contaminants in the subsurface. Where the water-bearing zone is a confined aquifer, the height of the water column in the well represents the pressure in the aquifer. In an unconfined aquifer, the water table separates the underlying saturated zone from the overlying unsaturated, or vadose, zone. As the unconfined aquifer is defined by the location of the water table, the fluctuation of the water table over time will change the zone of saturation. When an unconfined aquifer is underlying contaminated soils, water table fluctuations over time will repeatedly bring the zone of saturation into contact with the contaminated soil, thus further contaminating the groundwater. In an unconfined aquifer, draining of some pore spaces accompanies a withdrawal of water from the aquifer. Most groundwater contamination problems occur initially in unconfined aquifers because of the lack of an overlying low permeability confining layer.