Geochemical stability of acid-generating pyrrhotite tailings 4 to 5 years after addition of oxygen-consuming organic covers.

Oxygen-consuming organic covers can serve as a reactive barrier to minimize sulfide oxidation in acid-generating tailings but may lead to metal mobilization in surface oxidized layers. We evaluated changes in the bio-geochemical stability of acid-generating, Cu-Ni pyrrhotite tailings 4 to 5 years after addition of a 50 cm-thick or a 1 m-thick biosolid cover planted with energy crops. The original tailings (pH 2.5) were oxidized in the first 10 to 40 cm, and goethite was the dominant sink for Cu and Ni, the main metal contaminants in these tailings. Both covers increased pH, nutrient availability, microbial activity and diversity in the oxidized tailings, and led to a reduction of water-soluble Cu, Ni, Fe and Al after 4 to 5 years of application. Changes in pH, humidity, organic C content, and redox conditions resulted in partial dissolution of jarosite and gypsum below the cover but goethite apparently remained stable. Under both covers, total Ni decreased in the oxidized layer, indicating remobilization, but Cu was retained. Significant accumulation of Cu as Cu sulfide at the oxidized/unoxidized tailings interface was detected only under the 1 m-thick cover, suggesting that the thinner cover may not sufficiently decrease the oxidizing conditions to mitigate acid mine drainage. Migration of nitrate and P down to the unoxidized tailings was observed under both covers and raises the concern of continued sulfide oxidation in unoxidized tailings. Although the implementation of thinner covers is economically more sustainable than thick covers, our results indicate that further research is required to establish their long-term suitability and performance to prevent acid mine drainage.

Contributions of natural arsenic sources to surface waters on a high grade arsenic-geochemical anomaly (French Massif Central).

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.