Natural arsenic-rich spring waters discharging from the Austin Chalk, North-Central Texas, USA: Mineral and chemical evidence of pyrite oxidation followed by reductive dissolution of neo-formed Fe(III) oxides/oxyhydroxides

Abstract

The chemical composition of acidic springs with elevated arsenic (As) concentrations discharging from the Cretaceous Austin Chalk in North-Central Texas, USA, along with the mineralogy of ochreous precipitates associated with these springs, was investigated to evaluate the source of the arsenic. Water samples were collected from the springs for chemical analysis and bedrock (i.e., Austin Chalk) and local soil samples were taken for mineralogical and chemical analyses. The mean (±1σ) pH of the spring waters is 6.3 ± 0.68. The spring waters also exhibit low redox potentials (Eh = −100 ± 70 mV), high iron (Fe) concentrations (40 mg kg−1), elevated As concentrations (up to 105 μg kg−1), moderate total dissolve solids (TDS) values (780 mg kg−1 ≤ TDS ≤1380 mg kg−1), and a sulfurous odor. Arsenic concentrations are highest where the springs discharge, but subsequently decreases with flow beyond the spring orifices. Scanning electron microscopy (SEM) of samples of the Austin Chalk from the field site revealed that pyritization of planktonic and benthic foraminifera tests is a common feature of these aquifer rocks, and that the pyrite occurs largely as framboids. Electron microprobe analysis of the pyrite grains indicate that early diagenetic forms consist chiefly of Fe and sulfur (S) in nearly stoichiometric proportions (i.e., ∼45 and 50 wt % respectively, for Fe and S), whereas late diagenetic and/or weathered forms are largely Fe (∼60 wt %) with minimal S contents (∼0.1 wt%). Arsenic is distributed evenly in both phases (∼0.2 wt %) but is absent in the surrounding carbonate matrix. The spring water compositions along with the mineralogy of the aquifer rocks and the ochreous precipitates are consistent with the source of the elevated dissolved As concentrations being the result of oxidation and chemical weathering of As-bearing pyrite that occurs within the Austin Chalk and overlying soils/sediments as recharging meteoric water reacts with these materials. We employ a simple 1-D reactive transport model that supports As-bearing pyrite oxidation by O2(aq) in recharging meteoric waters, adsorption of mobilized As onto the resulting neo-formed Fe(III) oxides/oxyhydroxides, followed by reductive dissolution of the neo-formed Fe(III) oxides/oxyhydroxides and re-release of the adsorbed into the groundwater as the likely geochemical processes responsible for the elevated As concentrations measured in the studied springs.