Molybdenum Mobility During Managed Aquifer Recharge in Carbonate Aquifers

Abstract

The mobility of molybdenum (Mo) in groundwater systems has received little attention, although a high intake of Mo is known to be detrimental to human and animal health. Here, we used a comprehensive hydrochemical data set collected during a multi-cycle aquifer storage and recovery test to study the mechanisms that control the mobility of Mo under spatially and temporally varying hydrochemical conditions. The model-based interpretation of the data indicated that the initial mobilization of Mo occurs as a sequence of reactions, in which (i) the aerobic injectant induces pyrite oxidation, (ii) the released acidity is partially buffered by the dissolution of dolomite that (iii) leads to the release of Mo with highly soluble sulfurized organic matter prevailing between the intercrystalline spaces of the dolomite matrix or incorporated in dolomite crystals. Once released, Mo mobility was primarily controlled by pH-dependent surface complexation reactions to the sediments and, to a lesser extent, the capture by iron sulfides (FeS). In the studied system, Mo mobilization could be effectively mitigated by reducing or eliminating pyrite oxidation, which decreases the likelihood of dolomite dissolution and associated Mo release.

Keywords: dissolution; dolomite; managed aquifer recharge; molybdenum; pyrite.

Figure 1

Structure in the subsurface of the Orange County ASR facility. All depths are referenced below land surface (bls), approximately 80 feet NGVD. Depths according to BFA, BFA et al., and BFA.

Figure 2

Concentration snapshots for a synthetic tracer (A) and chloride at 385 days (B) and 566 (C) days after the start of the ASR operation. The solid light grey lines indicate the injection and observation wells ASR-1, LFMW-1, and LFMW-2. The dashed light grey line indicates a part of LFMW-2 that was not considered in the transmissivity-weighted concentration calculations (see Supporting Information). Transmissivity-weighted chloride concentrations in the injection and monitoring wells during injection (blue background), storage (white background) and recovery (red background) (D). Observation data were provided by FDEP.

Figure 3

Comparison of the reactive/non-reactive transport simulations and observation data at monitoring well LFMW-1 over all seven ASR cycles (A). Processes influencing the buffering of the pH value at LFMW-1 (B). The observed data were provided by FDEP.

Figure 4

Correlation of Mo with Ca and Mg at ASR-1 and LFMW-1 during the storage phase of ASR cycle 3 (386–430 days). The data were provided by FDEP.

Figure 5

Influences of dolomite dissolution and the addition of adsorption on mineral surfaces, the incorporation into FeS and the occurrence of powellite on the mobility of Mo. The black dotted line indicates the recommended upper limit of 70 μg/L Mo in drinking water. The observed data were provided by FDEP.