Recent calcite spar in an aquifer waste plume: A possible example of contamination driven calcite precipitation

This paper documents possible modern (within the last 50 years) calcite cementation within an unconfined contaminated aquifer at the Test Area North (TAN) on the Idaho National Engineering and Environmental Laboratory site. Inferred modern calcite occurs at the distal tips of the latest generation of calcite cement and has bright luminescence when examined by cathodoluminescence. Luminescent distal tips have elevated Mn values (up to 6400 ppm) when compared with other TAN calcite cement (< 200 ppm). Additionally, speciation modeling with PHREEQE indicates that modern TAN groundwater exhibits a slight supersaturation with respect to calcite (calculated saturation indexes range from 0.2 to 0.5). The observed abundance of luminescent distal tips (< 1-38 μg calcite/g total sample) is significantly less than the potential calcite precipitation (250-1100 μg calcite/g total sample) indicated by laboratory microbial activity measurements; a finding consistent with previous studies. Finally, while our results do not elucidate the specific mechanism(s) responsible for the elevated Mn present in some TAN calcite, we do demonstrate at least an indirect microbial involvement for Mn enrichment. Therefore, it is conceivable that geochemical signatures indicative of distinctive redox conditions can be recognized in recently formed calcite from other contaminated groundwater systems. (C) 2000 Elsevier Science B.V. All rights reserved.