ABSTRACT
The subwatershed studied drains a non-exploited area of the St-Yrieix-la-Perche gold mining district (French Massif Central) and it is located on an arsenic (As) geochemical anomaly. In this context, it is important to know the geochemical processes involved in the transfer of As from solid environmental compartments to the aquatic system. The stream showed a temporal variation of dissolved As (As(d)) content from 69.4 µg.L(-1) in the low flow period to 7.5 µg.L(-1) in the high flow period. Upstream, ground- and wetland waters had As(d) concentrations up to 215 and 169 µg.L(-1), respectively. The main representative As sources were determined at the subwatershed scale with in-situ monitoring of major and trace element contents in different waters and single extraction experiments. The As sources to stream water could be regrouped into two components: (i) one As-rich group (mainly in the low flow period) with groundwater, gallery exploration outlet waters and wetland waters, and (ii) one As-poor group (mainly in the high flow period) with rainwaters and soil solutions. In the soil profile, As(d) showed a significant decrease from 52.4 µg.L(-1) in the 0-5 cm superficial soil horizon to 14.4 µg.L(-1) in the 135-165 cm deep soil horizon. This decrease may be related to pedogenic processes and suggests an evolution of As-bearing phase stability through the soil profile. Quantification of As(d) fluxes at the subwatershed scale showed that groundwater was the major input (>80%) of As(d) to surface water. Moreover, natural weathering of the As-rich solid phases showed an impact on the As release, mainly from superficial soil horizons with runoff contributing about 5% to As input in surface water.
ABSTRACT
The Upper Isle River (SW France) drains the second most productive gold-mining district of France. A high resolution survey during one hydrological year of As, Cl(-), Cr, Fe, Mn, Mo, SO(4)(2-), Th and U dissolved concentrations in surface water aimed to better understand pathways of trace element export to the river system downstream from the mining district. Dissolved concentrations of As (up to 35000 ng/L) and Mo (up to 292 ng/L) were about 3-fold higher than the regional dissolved background and showed a negative logarithmic relation with discharge. Dissolved concentrations of Cr (up to 483 ng/L), Th (up to 48 ng/L) and U (up to 184 ng/L) increased with discharge. Geochemical relationships between molar ratios in surface water, geochemical background as well as rain- and groundwater data were combined. The contrasting behavior of distinct element groups was explained by a scenario involving three seasonal components: (i) The high flow component is poorly concentrated in As and Mo but highly concentrated in Cr, Th, U. This has been attributed to diffuse sources such as water-soil interactions, atmospheric inputs, bedrock and bed sediment weathering. Although this component probably also includes a contribution by weathering of sulfide veins, this signal is masked by dilution. (ii) One low flow component presents high SO(4)(2-), Fe, As and Mo and moderate Cr, Th and U concentrations. This component has been attributed to point sources such as mine gallery effluents, mining waste weathering and groundwater inputs from natural and/or mining-induced sulfide oxidation in the ore deposit. (iii) A second low flow component showing high As plus Mo concentrations associated with very low SO(4)(2-), Fe, Cr, Th and U concentrations, probably reflects trace element scavenging by ferric oxyhydroxide formation in the adjacent aquifer. This is supported by the decrease of Fe, Cr, Th and U in surface waters. Flux estimates suggest contrasting element-specific impacts on annual dissolved fluxes. Runoff may account for the major part of annual dissolved As, Mo, Th and U fluxes in the Upper Isle River. Inputs related to sulfide oxidation respectively contributed approximately 30% and approximately 24% to annual As and Mo fluxes. The formation of ferric oxyhydroxides strongly retained Cr, Th and U during the low flow, limiting their dissolved concentrations in surface waters. If this process may eventually decrease As mobility, its impact on dissolved As concentrations in surface water may be limited or/and counterbalanced by As release during sulfide oxidation.
