Microbial geochemistry of groundwater in deep aquifers, central and east-central Texas

Abstract

Groundwaters from 40 municipal and monitoring wells in Texas were collected for the study of water chemistry and microbiology in the deep subsurface. Carbon isotopic compositions (i.e. $deltasp{13}$C) of dissolved inorganic carbon (DIC) ranged from $-$17.6 to $-$6.6$perthous$ for the Yegua, Sparta, Queen City, and Wilcox-Carrizo aquifers in the east-central Texas and from $-$12.5 to $-$1.2$perthous$ for the Edwards and Trininty aquifers in central Texas. Organic matter oxidation appeared to be the dominant process leading to low $deltasp{13}$C values of DIC in the Edwards and Trininty aquifers. Dissolved methane from the Yegua, Sparta, Queen City, and Wilcox-Carrizo aquifers had variable concentrations (0.04 to 1626 $mu$M) and mostly low $deltasp{13}$C values (${<}{-}$50$perhous$), indicative of microbial origin. Dissolved methane in the Edwards and Trinity aquifers had generaly low concentrations ($<$5 $mu$M) and high $deltasp{13}$C values (${>}{-}$35$perthous$), indicative of microbial origiin. Dissolved methane in the Edwards and Trinity aquifers had generally low concentrations ($<$5 $mu$M) and high $deltasp{13}$C valuses (${>}{-}$35$perthous$), indicative of methane oxidation. All the samples exhibiting carbon isotopic evidence for methane oxidation had low methane concentrations and high sulfate contents, suggesting an association between methane oxidation and sulfate reduction. Total bacteria ranged from 449 to 6.7 $times$ 10$sp4$ cells/ml and were generally low ($<$2000 cells/ml) in deep well waters ($>$410 m). Populations of methanogens and sulfate reducers varied between 0 and 15 cells/ml and methanotrophs varied between 0 and 455 $pm$ 458 cells/ml. Methanogens were more frequently found in the high-methane waters of the Yegua, Sparta, Queen City, and Wilcox-Carrizo aquifers that in the generally low-methane waters of the Edwards and Trinity aquifers, supporting the geochemical evidence that methanogenesis may be presently active in the Yegua, Sparta, Queen City, and Wilcox-Carrizo acquifers. Higher numbers of sulfate reducers correlated with higher methane $deltasp{13}$C values, supporting the geochemical evidence that methane oxidation may be coupled to sulfate reduction in these waters. Methanotroph abundance correlated with concentration of dissolved methane, suggesting that aerobic methanotrophy in Texas aquifers may be limited by the availability of methane.