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Ground Water Recharge Using Waters of Impaired Quality
CONDITIONS INFLUENCING PRETREATMENT
The soil and aquifer properties, recharge method, type of wastewater, and ultimate destination and intended use of the recovered ground water collectively dictate the degree of pretreatment required before recharge. In addition, recharge rates, regardless of the method used, depend to some degree on the quality of the source water that is recharged. Cost-effective recharge operations are achieved through tradeoffs between maximizing recharge rates and minimizing treatment costs.
The selection of a wastewater treatment process depends on the characteristics of the wastewater, the required effluent quality, and the cost of the selected treatment option. When considering a water source as a potential for recharge, the likely variety and concentration of contaminants also need to be considered. The use of waters of impaired quality as sources for recharge has raised questions about the level of pretreatment necessary prior to recharge. The most conservative approach is to assume that passage through the soil to the aquifer and through the aquifer to the withdrawal location provides no treatment and that pretreatment processes therefore should improve the source water to the quality level needed by the end user. This approach, however, can lead to expensive systems. Soil-aquifer processes can be counted on to provide treatment benefits.
The ideal porous medium for an SAT operation is one that allows rapid infiltration and complete removal of all constituents of concern. Unfortunately, no such medium exists because the attributes required to achieve one goal hamper the achievement of the other. In surface soil, coarse-textured materials are desirable for infiltration because they transmit water readily; however, the large pores in these soils are inefficient at filtering out contaminants, and the solid surfaces adjacent to the main flow paths are relatively nonreactive. In contrast, fine-textured soils are efficient at contaminant adsorption and filtration, but they have low permeability and their small pores clog easily. Structured soils con-mining biological channels (e.g. worm holes or root holes) or cracks are permeable, but the large flow paths completely dominate the movement of material and much of the matrix is bypassed. The best choice for an SAT soil is therefore a compromise, such as a fine sand or a sandy loam with relatively little structure (Bouwer, 1985).
Nature of Recharge Operation
Of the two basic methods of artificial recharge—surface infiltration and well recharge—well recharge requires water of much higher quality. This is particularly true where an aquifer composed of granular rocks is to be recharged.