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water in recharge basins, injection recharge wells, or increasing infiltration. Inadvertent <br />recharge also occurs as a consequence of water use and land management practices. <br />Depending on the technology employed, planned recharge operations can transmit water <br />to shallow groundwater or deeper aquifer formations. Groundwater in shallow aquifer <br />units containing or connected to the water table is considered unconfined because water <br />can percolate from the land surface to the saturated groundwater zone. The unsaturated, <br />or vadose zone, between the surface and the groundwater zone, is comprised of materials <br />that contain a mixture of air and water within open pore spaces. These zones often vary <br />spatially and can change over time according to recharge and drawdown rates. <br />Groundwater in deeper confined aquifer units is separated from an unconfined water table <br />system by one or more impermeable or semipermeable geologic layers. Confined aquifers <br />are frequently under hydrostatic pressure and, in some instances, can force water upward <br />to produce artesian wells or springs. Confined aquifers are often viewed as underground <br />reservoirs, although very deep aquifers may be physically or economically inaccessible. <br />Groundwater extraction at rates that exceed the effective recharge rate is called overdraft <br />and is equivalent to groundwater "mining" as a nonrenewable resource. The "safe yield" <br />can imply groundwater extraction at a rate equal to recharge; however, the assumptions <br />applied (e.g., time period) are important to assess the potential for short -term drawdown <br />impacts or determine a yield rate that is realistic as a long -term sustainable condition. <br />Artificial Recharge Methods <br />The method applied in planned recharge depends on hydrogeologic conditions, the costs <br />and value of water, and objectives for recharge. Recharge operations generally fall into <br />two basic categories— surface infiltration and direct well injection. <br />Surface infiltration systems are passive processes that include dedicated facilities such as <br />spreading basins and watershed measures employed to induce additional recharge through <br />streambeds or in ponds. Infiltration sites require permeable soils (sandy loams, sands, and <br />gravels) that promote high rates of infiltration and percolation to groundwater. Although <br />infiltration can eventually contribute to deeper aquifers, surface infiltration operations are <br />primarily used to recharge shallow groundwater sources. In some cases, land management <br />practices are applied to enhance infiltration by capturing precipitation and detaining runoff <br />to allow more time for infiltration and percolation to the groundwater system. <br />The interaction of water with soils during infiltration can also reduce common biological <br />and chemical pollutants from water. These processes, collectively termed "Soil- Aquifer <br />Treatment" (SAT) are generally considered to improve the quality of water sources such <br />as storm runoff and reclaimed wastewater (NAS, 1994). The appropriate application and <br />effectiveness of SAT processes are topics of ongoing research interest, particularly for <br />wastewater reclamation and reuse in the and Southwestern United States. <br />1.11 High Plains States Groundwater Demonstration Program <br />