Zoning in an aquifer

ORP measurements are effective in delineating oxic from anoxic groundwater, but ORP measurements cannot distinguish between nitrate-reducing, Fe (III)-reducing, sulphate-reducing or methanogenic zones in an aquifer (Fig. 3.2). 

Subsurface formations can be divided into the overburden (unconsolidated) and bedrock according to its solidarity. The upper subsurface can be further divided into the unsaturated zone and the saturated zone depending on pore structure and moisture saturation. The saturated zone is the zone in which the voids in the rock or soil are filled with water at a pressure greater than atmospheric. The water table is at the top of a saturated zone in an unconfined aquifer. The unsaturated zone is the zone between the land surface and the water table, and is also called the zone of aeration or the vadose zone. The pore spaces contain water at less than atmospheric pressure, air, and other gases. This zone is unsaturated except during periods of heavy infiltration. 

The use of vegetable oil for remediation of organic contaminants in an aquifer or unsaturated zone has been smdied at Pacific Northwest Laboratory. The oil strips organic compounds from the aqueous phase or particulate matter and is then pumped out for recovery. In addition, the oil can be used as a carbon source by microorganisms, hence encouraging in situ bioremediation. The technology has not proceeded beyond bench-scale testing and is not commercially available. 

Water from different zones in the aquifer is mixed, giving an averaged concentration of contaminants in the well. Zones of interest will not be identified. 

Saturated zone. The zone below the water table in an aquifer (see unsaturated zone). 

The transport for the dissolved contaminants is considered to occur either in the vertical direction through the unsaturated zone until an aquifer is reached either in the horizontal direction, through the saturated zone flowing to an hypothetical well, where the contaminants become accessible to humans or other forms of life. The vertical flow is considered to be onedimensional. It is assumed that there is retardation during the vertical transport that is estimated assuming that the adsorption-desorption process can be represented by a linear isotherm, which means that there is a linear relationship between the radionuclides concentration in the solid and liquid phases. 

Water below the ground surface may be sorbed onto the surface of soil particles, flow downward as a thin liquid film through nnsaturated pores, flow in locally saturated pores, be held in place by capillary forces, or be stored in a saturated zone called an aquifer, as indicated in Fig. 4. Water in aqnifers is often moving as the resnlt of pressnre gradients generated by springs or by pnmping. 

Water may fall to earth and collect in a lake or stream. If it does, it eventually finds its way back to the sea. If it falls onto the land, it can form runoff dsid eventually enter a lake or stream, or it can soak into the ground and become groundwater. The porous layer of soil and rock that holds the groundwater forms a zone called an aquifer. This zone provides us with a good source of groundwater. We tap into these aquifers by using wells. 

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