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The implementation of Managed Aquifer Recharge (MAR) techniques, such as Aquifer Storage and Recovery (ASR) and Artificial Recharge (AR), is being constrained by the recent finding of trace-metal mobilization. The introduction of treated surface waters into anoxic aquifers leaches metals from native minerals, thereby leading to the mobilization of arsenic, molybdenum, iron and other trace metals. A January 2006 change to the Primary Drinking Water Standard (PDWS) has caused many Florida ASR facilities to be out of compliance with respect to arsenic. In an effort to offset groundwater withdrawals, and reduce the impact of saltwater intrusion, several of Florida’s water management districts are planning large-scale AR projects, which may also generate arsenic concentrations in the aquifer above the 10 µg/L limit. The viability of these critical water management tools has been impacted by this technical challenge. To evaluate trace-metal mobilization during MAR we have initiated a 3-year research program that will confirm the geochemical response that occurs during ASR and AR; identify the mechanism controlling the fate and transport of arsenic and predict the long-term impact of arsenic mobilization through numerical modeling. A continuous core of the primary storage interval for southwest Florida ASR sites, the Suwannee Limestone, has been collected. Intact core column experiments are underway to support the development of 3-D reactive transport models. The reactive transport code PHT3D has been selected to simulate ASR and AR processes. This model couples the contaminant transport model MT3DMS with the geochemical model PHREEQC. A model calibration to existing ASR datasets is used to simulate long term (10 years) operation of ASR and AR systems. The model may also be used to investigate ASR and AR operational approaches for managing arsenic mobilization. |