172-14 Geogenic Molybdenum in Carbonate Aquifers - Implications for Drinking Water and Aquifer Management

Session: Environmental Geochemistry and Health (Part II)

Presenting Author:

Author:

Abstract:

While point-source contamination scenarios with molybdenum (Mo) are more commonly studied, geogenic (natural) Mo release from aquifer matrices is an underrecognized drinking water hazard, particularly in poor urban and rural settings where supply is decentralized and rarely tested. Carbonate aquifers are especially prone to this risk, as Mo is known to accumulate in marine sediments.

Integrating detailed characterization of aquifer matrix mineralogy, sequential chemical extractions, groundwater geochemistry, ³H/³He age dating, and reactive transport modeling, more than 10 years of work on the Floridan Aquifer System (USA) demonstrates that Mo is hosted not in pyrite or other sulfide minerals — despite Mo's chalcophile character — but in sulfurized organic matter within the aquifer matrix. Alteration of hydrologic flow paths due to faulty well installation, pumping-induced water table decline, and managed aquifer recharge (MAR) injection of oxic water drives a rapid shift from high-pH reducing to low-pH oxidizing conditions, releasing Mo to groundwater at concentrations up to 5,050 µg/L — far exceeding the EPA health advisory of 40 µg/L. This contamination occurs at scales too fine to be captured by regional or national groundwater monitoring networks, leaving affected wells undetected for years.

These findings have direct implications for drinking water safety, well siting, and aquifer management, leading to three recommendations: (1) reconnaissance-scale Mo monitoring targeted at organic-carbon-rich zones within carbonate aquifers, (2) mandatory testing of suspected aquifers before new well installation, and (3) explicit inclusion of Mo in risk assessments for managed aquifer recharge projects, where injected oxic water can trigger the same redox-driven release mechanism.

© Copyright 2026 The Geological Society of America (GSA), all rights reserved.