Trace metal cycling in karst aquifers subject to periodic river water intrusion

Abstract River water that enters critical zone aquifers can deliver metals and alter groundwater redox potential, resulting in precipitation and/or dissolution of metal oxides. These effects on metal concentrations should be particularly dynamic in karst critical zones where surface water and groundwater are linked through preferential flow paths (e.g., caves and conduits). Metals can be either toxins or micronutrients and thus their concentrations are important to water quality. To evaluate trace metal dynamics in karst critical zones, we measured Pb and Sr isotopic ratios and major and trace metal concentrations of Fe,Mn-oxide deposits collected from two water-filled cave networks (Madison Blue and Peacock springs) in the carbonate upper Floridan aquifer (Florida, USA). We also measured dissolved and total metal concentrations in water from conduits of both cave networks during three river water intrusion events and subsequent flood recessions. The Fe,Mn-oxides contain anthropogenic Pb that increases in concentration from interior to exterior of the oxides, indicating they formed and continued to grow over the past several decades. Sr isotope ratios of the Fe,Mn-oxides are intermediate between flood water and groundwater values and indicate they form during intrusion events. This result is corroborated by variations in Fe and Mn concentrations in water during intrusion and recession. The Fe,Mn-oxide interiors are more altered than exteriors as shown by depletion of Mn concentrations and the lack of a positive Ce anomaly in the interior of the Fe,Mn-oxides. Most trace metal concentrations correlate with Mn concentrations suggesting remobilization of trace metals over time periods longer than individual intrusion events. Cycling between accumulation and dissolution of Fe,Mn-oxides presents challenges for evaluation and remediation of trace metal contamination of carbonate critical zones.