A comparison of the geochemical response to different managed aquifer recharge operations for injection of urban stormwater in a carbonate aquifer

Abstract Managed aquifer recharge (MAR) is increasingly being considered as a means of reusing urban stormwater and wastewater to supplement the available water resources. Subsurface storage is advantageous as it does not impact on the area available for urban development, while the aquifer also provides natural treatment. However, subsurface storage can also have deleterious effects on the recovered water quality through water–rock interactions which can also impact on the integrity of the aquifer matrix. A recent investigation into the potential for stormwater recycling via Aquifer Storage Transfer and Recovery (ASTR) in a carbonate aquifer is used to determine the important hydrogeochemical processes that impact on the recovered water quality. An extensive period of aquifer flushing allows observation of water quality changes under two operating scenarios: (1) separate wells for injection and recovery, representing ASTR; and (2) a single well for injection and recovery, representing Aquifer Storage and Recovery (ASR). Calcite dissolution produces the dominant inorganic chemical change to the quality of stormwater following storage in the carbonate aquifer; independent of the mode of MAR operation (ASTR or ASR). The magnitude of calcite dissolution in response to the injection of urban stormwater exceeds that previously reported within the same aquifer using both stormwater and reclaimed wastewater. While some dissolution is induced in response to redox processes, the primary influence on dissolution is the reactivity of the source water itself as indicated by the sub-saturation with respect to calcite. Cation exchange is evident through the ASTR mode of operation, producing a marginal increase in the Na concentration during freshening of the storage zone. This increase in Na concentration is not evident in the ASR mode as cation exchange is limited to the initial pore flushes of the storage zone. Aquifer storage provides some treatment through nutrient removal, mainly through removal of ∼35% of the dissolved organic C (DOC). DOC removal is greatest when the MAR operation involves separate injection and recovery wells as ASTR allows enhanced removal by sorption. ASR can lead to nutrient recycling around the injection well which can produce water quality that is atypical of the quality in the bulk of the storage zone. Water recovered from ASR shows some removal of DOC through microbial oxidation coincident with removal of O2 from the source water, which is considered to be a sustainable process during subsurface treatment. During ASTR, enhanced DOC removal is attributed to adsorption, but as with cation exchange, this removal may be limited to the initial period of aquifer conditioning. Oxidation of pyrite is evident during the initial stages of injection until the pool of reactive pyrite within the storage zone is consumed, affecting the quality of water recovered via the ASTR mode only.

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