Use of isotope fractionation of sulfate-sulfur and sulfate-oxygen to assess bacterial desulfurication in a sandy aquifer

Abstract

The unconfined aquifer of the “Fuhrberger Feld” consists of 20–30m sands and gravelly sands with unevenly distributed reduced sulfur compounds and small lignitic pebbles. In vertical direction two hydrochemically different zones with specific solute transformations are present, an upper portion with bacterial denitrification (with reduced sulfur compounds as electron donors) and a deeper portion with hydrochemical indications of reduction of the aqueous sulfate. In the study area ten multilevel groundwater sampling wells were installed. Sampling and interpretation was based on a two-dimensional vertical-plane groundwater flownet and on the knowledge of the landuse upgradient of the multilevel wells. The concentration and isotopic composition of the sulfate at the entrance into the zone of sulfate reduction is controlled by sulfate of groundwater recharge as well as sulfate formed during denitrification. The recharge-sulfate shows clear landuse specific differences in concentration and isotopic composition: under arable land an average concentration of ≈ 60 mg SO 4l −1 with values of δ 34S close to +3‰ CDT Diablo Meteorite Standard (Can̄on) and of δ 18O about + 8‰ standard mean ocean water (SMOW) was measured, whereas under coniferous forest an average of ∼ 100 mg SO 4l −1 with δ 34 S ≈ + 2‰ and δ 18 O ≈ 4.5‰ was found. In the denitrification zone no significant sulfur isotope fractionation occurs during oxidation of reduced sulfur. The oxygenisotopic composition of the newly formed sulfate (“denitrification-sulfate”) depends on the 18O content of nitrate acting as oxygen donor and on the 18O content of groundwater in which the oxidation occurs; under certain assumptions a δ 18 O = +0.54‰ was calculated for the oxygen of the newly formed sulfate. Consequently, with increasing sulfate concentrations, i.e. with the admixture of “denitrification-sulfate”, a decrease of the δ 18O values from about +8‰ to +2‰ was observed. In the zone of sulfate reduction, both the δ 34S and δ 18O-values of the residual sulfate increase with decreasing sulfate concentrations thus proving the existence of bacterial reduction of the aqueous sulfate. This sulfate reduction follows a Rayleigh process with calculated enrichment factors of ϵ 34 = −9.7‰ for 34S and ϵ 18 ≈ −6‰ for 18O, respectively. A plot of δ 34S vs. δ 18O gives a good linear correlation with a slope close to 1: 1.4. Data on concentration and isotopic composition of sulfate and on the hydraulically derived groundwater age are used for kinetic considerations. The results of a multilevel well for depths between 10 and 17m are given as an example. The bacterial reduction can be considered as first-order reaction with a half-life between 75 and 100 years.