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IMPACTS ON UNDERGROUND ECOLOGY: The impacts on underground ecology can be discussed in terms of microbial ecology and related geochemical interactions. The microbial activity within an aquifer is strongly related to the redox potential of the aquifer. Aquifers that contain dissolved oxygen are considered to be aerobic and have a high redox potential similar to aerobic surface waters. Aquifers that are depleted in molecular oxygen but contain chemical oxygen in the form of nitrate are considered to be anoxic and have an intermediate redox potential. Aquifers that are anaerobic have low redox potentials and a variety of electron acceptors might be used including sulfate, carbon dioxide, iron or manganese. The impact of groundwater recharge on underground ecology depends on the prior redox potential of the aquifer, the type of water that is used for recharge and the method of recharge. Both microbial and geochemical interactions are difficult to predict without conducting tests to evaluate the interactions prior to groundwater recharge.

Groundwater recharge using surface waters containing biodegradable organics or reclaimed water will generally result in depletion of molecular oxygen as the groundwater is recharged. When recharge basins are used, high levels of aerobic microbial activity consume oxygen at the soil/water interface resulting in anoxic or anaerobic conditions below the recharge basin and in areas dominated by the recharged water. When reclaimed water is used, there is generally some residual nitrogen in the form of nitrate and the aquifer becomes anoxic. The presence of some nitrate will prevent the development of anaerobic conditions and the potentially undesirable effects of anaerobic conditions on groundwater quality. Since many soil microorganisms are facultative, the enzymatic activity of soil microorganisms is similar under either anoxic or aerobic conditions and long term transformations of organics should proceed in similar fashions under either aerobic or anoxic conditions. Anoxic conditions dominate many aquifers influenced by recharge activity in the Southwestern United States.

Anaerobic conditions in an aquifer influenced by groundwater recharge can develop as a consequence of groundwater recharge and pre-existing anaerobic conditions are likely to remain anaerobic after groundwater recharge. An aerobic or anoxic aquifer can become anaerobic if the applied water for groundwater recharge is depleted in molecular oxygen and nitrate while containing some residual biodegradable materials. This situation is most likely to occur if reclaimed water or poor quality surface water is applied for extended periods to recharge basins without drying cycles to permit for reaeration of soils and subsequent nitrification of adsorbed ammonia. This has been observed under a combined wetlands/percolation system where anaerobic conditions develop in the wetland sediment and persist in the underground environment (Nahar et al., 1998). If an aquifer is naturally anaerobic, the biogeochemical mechanisms for the development of anaerobic conditions tend to prevail throughout the aquifer. Therefore, when oxygen or nitrate is introduced into an anaerobic aquifer, the oxygen or nitrate is rapidly consumed through a biogeochemical pathway that maintains anaerobic conditions in the aquifer. This has been observed for both groundwater recharge and for bank filtration when aerobic surface waters are drawn through an anaerobic aquifer. Anaerobic aquifers tend to have elevated concentrations of sulfide, reduced iron and manganese that can have a negative impact on water quality. When these reduced elements are exposed to oxygen or strong oxidants such as disinfectants, they are oxidized to forms that precipitate and can foul distribution systems. Therefore, groundwaters extracted from anaerobic aquifers typically require treatment for the removal of reduced compounds prior to distribution.

When oxygen or strong oxidants such as chlorine are introduced to anaerobic aquifers, rapid oxidation of reduced compounds occurs at the point of contact between the recharge water and the anaerobic aquifer. This can result in the accumulation of precipitates and clogging of the aquifer. For recharge basins, the interfacial area between the recharge water and the anaerobic aquifer is large preventing a concentrated accumulation of precipitates. When direct injection is used, clogging around the wells can be a serious problem.

CONTAMINATION OF SOIL AND GROUNDWATER: Groundwater recharge will impact pre-existing contaminated conditions and geochemical interactions between recharge waters and soils can also impact groundwater quality. If recharge basins are located above contaminated soils, there is a strong possibility contaminants will rapidly migrate into the groundwater resulting in groundwater contamination when the contaminants were previously present only in the soil. In some cases, natural migration of contaminants in the soil could take decades or centuries before groundwaters will be impacted by the contaminants and natural attenuation could greatly reduce the future impact on groundwater quality. In such cases, groundwater recharge should not be practiced or thorough soil remediation should be completed prior to groundwater recharge. Contaminated soils adjacent to recharge basins could also be affected by lateral flows or rising groundwater tables resulting in groundwater contamination. In Mesa, Arizona, USA, groundwater recharge with reclaimed water in an area of contaminated soils has resulted in no negative impact on groundwater quality since the soils above the groundwater were successfully remediated by vapor extraction. The lack of impacts on groundwater quality is surprising since the groundwater table has risen over 10 meters since 1990. Groundwater recharge will also influence local gradients and groundwater flow patterns. Therefore, the consequences of groundwater recharge on groundwater contamination in the vicinity of a recharge project must be considered. A recharge project could cause movement of contaminated groundwater towards a potable well resulting the loss a potable well and a contaminated groundwater plume that is more difficult to contain. Therefore, the siting and planning of groundwater recharge systems should always consider pre-existing contaminated conditions, particularly when the contaminants are mobile organics or inorganics.

Geochemical interactions between soils, aquifer materials and recharge waters can dictate final water quality. In Orange County, California, USA, recharge waters low in total dissolved solids are used and after several years of contact with native aquifer materials, the recharged groundwater comes to equilibrium with the native aquifer materials resulting in a significant increase in the total dissolved solids of the recharged groundwater. Similar to the case for anaerobic aquifers, the recharged groundwater does not improve the quality of the receiving aquifer. When groundwaters are recharged that are higher in total dissolved solids than the receiving aquifer, the recharged groundwater tends to maintain the higher total dissolved solids concentration since the total dissolved solids are stable and often do not interact to form precipitates with the native aquifer materials. Ion exchange reactions with clays can result in an exchange of dissolved sodium for calcium when the recharge waters have high levels of sodium and low levels of calcium. This effect does not significantly impact water quality unless dispersed clays migrate to recovery wells.

The impact of geochemical interactions with native soils and soils strongly influenced by prior agricultural activities can result in short-term groundwater contamination. When vadose zone soils in arid regions have not been contacted with significant quantities of water for many years, salts and minerals on the soil surface will rapidly dissolved when exposed to recharging groundwater. This can result in temporarily high concentrations of natural contaminants such as fluoride, arsenic, barium, lead or chromium (Johnson et al., 1999). These contaminants can be present at concentrations above maximum contaminant levels, however, the contaminants are only present as a spike of elevated concentrations. After longer-term storage of the groundwater, dispersion will reduce the contaminant concentrations to acceptable levels before the groundwater is recovered. Similarly, salts and nitrate accumulated in the vadose zone from prior agricultural activity can be rapidly dissolved as a consequence of groundwater recharge. Since agricultural activity can result in large quantities of salts and nitrates accumulated in the vadose zone, the impact of recharging groundwater with might not be a short-term effect. In most cases, the receiving groundwater already has elevated levels of salts and nitrates from the prior agricultural activities and groundwater recharge will eventually reduce the salinity and nitrate concentrations in the receiving aquifer. This phenomena has been observed at several sites in the Southwestern US where nitrate and salinity concentrations increased for time periods ranging from months to several years after groundwater recharge was initiated before beginning to decrease.

SUMMARY AND CONCLUSIONS: Groundwater recharge should be considered as an integral part of water resources planning. The advantages of groundwater recharge far outweigh the potential disadvantages of groundwater recharge. Groundwater recharge provides greater flexibility in water resources planning and groundwater banking systems provide economic incentives for groundwater recharge projects. Prevention of land subsidence is difficult even when systems for groundwater banking have been developed. As with most water resources projects, the development of infrastructure for the conveyance of water for groundwater recharge projects can be prohibitively expensive. Consideration of groundwater recharge in the initial stages of water resources planning will reduce large expenditures by optimizing the location of projects.

ACKNOWLEDGEMENTS: The author would like to thank Herman Bouwer of the United States Department of Agriculture Water Conservation Laboratory for his advice and mentorship. The author also wishes to thank Joerg Drewes of Arizona State University and Margaret Nellor of the County Sanitation Districts of Los Angeles County.

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