Occurrence and mobility of molybdenum during managed aquifer recharge in carbonate aquifers

Abstract

Molybdenum (Mo) is an essential trace element for human health, although a high intake can be detrimental to humans. The risk of an increased Mo intake from drinking water is generally low. Still, interactions of groundwater with sediments enriched in naturally occurring (geogenic) Mo can result in Mo concentrations that exceed recommendations for drinking water. Such interactions are triggered primarily by anthropogenic disturbances of the naturally prevailing physicochemical conditions in the aquifer. An intensive release of Mo in carbonate aquifers is particularly problematic as large proportions of the world's population depend on carbonate aquifers as a drinking water resource. For example, the population of Florida derives most of its drinking water from the Floridan Aquifer System (FAS), the principal karstic carbonate aquifer in Florida and one of the most productive aquifers in the world. However, the cyclic injection, storage and recovery of oxygenated water in the anoxic FAS during the operation of Aquifer Storage and Recovery (ASR), which is the most commonly applied technique of managed aquifer recharge (MAR), caused the release of Mo in groundwater at several locations in Florida. Exceptionally high concentrations were observed during the operation of ASR in the Eocene Avon Park Formation (APF) as part of the FAS, with concentrations of Mo exceeding the recommendations for drinking water several times. Since ASR operations are important for drinking water supplies in Florida and many other parts of the world, this study investigated the occurrence and main source of Mo in the carbonate aquifer matrix and the processes that affect the mobility of Mo in groundwater to improve the risk assessment of Mo release during MAR. The findings are based on laboratory-scale geochemical analyses and reactive transport modeling on carbonate aquifers covering the APF but are also transferable to similar carbonate aquifers worldwide.