Origin, mobility, and temporal evolution of arsenic from a low-contamination catchment in Alpine crystalline rocks

Abstract

The reduction to 10 μg/l of the limit for arsenic in drinking water led many resource managers to deal with expensive treatments. In the very common case of arsenic levels close to the recommended maximum concentration, knowing the origin and temporal evolution of As has become of great importance. Here we present a case study from an alpine basin. Arsenic speciation, isotopic compositions of pyrite, sulfate and water, and concentrations of major and trace elements demonstrate a geogenic source for arsenic linked to the dissolution of pyrite. We provide new tools to further study As at low concentrations where many processes may be masked. The observed negative correlation between δ³⁴S_SO4 and [As] is interpreted as a Rayleigh-type sulfur-isotope fractionation during increasing pyrite dissolution. The observed positive correlation between δ¹⁸O_SO4 and As^V/As^III could help to retrieve initial redox conditions. A 3-year long monitoring at high-resolution demonstrated that drought conditions enhance pyrite dissolution whose degradation products are scavenged by recharge water. An increase in As in groundwater may result from droughts due to enhanced oxygen entry in the unsaturated zone. The 2003 European heatwave had a major effect.

Introduction

Many states lowered their limit for [As] in drinking water following the WHO guideline value of 10 μg/l [1] and costly actions are to be prepared. For water with arsenic concentration just above the limit, cost-effective solutions may not be treatment but mixing with low-arsenic concentration water. These solutions will be efficient only if they are dimensioned to the potential variations in As concentrations, and if the mechanisms able to increase [As] are evaluated. This requires that the origin of As in water is understood, and that the temporal evolution of [As] is assessed.

Smedley and Kinniburgh [1] presented a review of the source, behavior and distribution of arsenic in natural waters. Arsenic is mostly found in the environment as arsenite (As^III, reduced) and arsenate (As^V, oxidized). Its dominant forms are inorganic. Minerals commonly known to be a source for As in surface water and groundwater are arsenopyrite (FeAsS) and arsenian pyrite (Fe(S, As)₂). After mobilization from pyrite oxidative dissolution, As is often adsorbed on metal oxides and oxyhydroxides [2], [3]. Arsenic can be mobilized from this secondary source either at pH above 7 or under reducing conditions [2], [3].

Dissolution of pyrites is a complex process [4], [5], [6], [7]. Mediated by bacteria or not, it produces sulfate with distinctive sulfur-isotope ratios used to determine its sulfide origin [8]. As pyrites may contain significant amounts of arsenic (see Section 2.1), sulfur isotopes may thus be used to link [As] measured in water with a potential pyrite source. Fractionation of ³⁴S and ³²S between sulfide and sulfate and fractionation of ¹⁸O and ¹⁶O between sulfate and water have been the focus of much attention, mainly due to acid mine drainage issues [9], [10], [11], [12]. Pyrite may be oxidized by dissolved molecular oxygen or ferric iron [4]. In the first case, oxygen in dissolved sulfate is contributed by O₂ and H₂O, in the second case, oxygen is contributed by H₂O only. A transition from one mode of dissolution to the other was observed during bacteria-mediated experiments [13]. Under certain limitations, δ¹⁸O_SO4 and δ¹⁸O_H2O have the potential to decipher aerobic and anaerobic conditions or biotic and abiotic pathways [9], [11], [12], [14].

Here, we report a study of arsenic in spring water distributed in the area of Beaufort, a town of <2500 inhabitants in the French Alps. Some springs have [As] > 10 μg/l. Once this was established by the Regional Health Agency, a monthly monitoring started in 2005. The origin of As in water was unknown. For this study, we sampled spring water known to have [As] exceeding the guideline. We also collected samples from groundwater and surface water nearby. From a series of measurements of major, minor and trace element concentrations, As speciations and S–O-isotope ratios, we propose a primary source for arsenic from pyrite and mechanisms leading to its mobility. The dynamics of pyrite dissolution was studied thanks to a long-term and high-resolution monitoring of [SO₄] and water flow rate. The archives of the town of Beaufort on [As] and spring water flow rates were analyzed for their seasonal and long-term evolutions. The risk situations likely to increase arsenic concentrations in the collected spring water are presented in order to help the resource management.