First investigations on arsenic content in ultramafic rocks' alterites from Nickel mines, implications for surface waters quality in ultramafic watersheds (New-Caledonia)

Kévin Schneider (1) , Monika Le Mestre (1) , Isabelle Desriaux (1) , Peggy Gunkel-Grillon (1)

1 ISEA - Institut de sciences exactes et appliquées

Arsenic is a well-known toxic metalloid and main species in water are arsenate As(V) and arsenite As(III) more toxic. In small tropical islands, tap water is often provided from surface water and the arsenic risk, widely studied in groundwater, is poorly studied in surface water. In New-Caledonia, 61% of the water comes from surface water and opencast nickel mines generate the dispersal of large quantities of ultramafic rocks' alterites rich in iron oxide-hydroxide. Arsenic contents in surface waters exceeding the World Health Organization recommended 10µg.L-1 value, were noted downstream nickel mines. Many studies illustrated the high affinity of arsenite and arsenate for pure or natural iron oxides and hydroxides. Arsenic could therefore be transported by ultramafic rocks' alterites displaced due to mining activities. To evaluate the arsenic risk for surface water in ultramafic catchments, alterites samples were collected in the vicinity of three active mines. They were analyzed for their total and exchangeable arsenic content. Their arsenic adsorption capacity was studied thanks to batch experiments. Results indicate that arsenic is present in all the samples with total contents ranging 0.20 to 5.14mg.kg-1 even though no As minerals occurring in nature were detected. The present work shows that ultramafic rock's alterites contain arsenic even though the primary mineralogical source of As is unknown. No exchangeable arsenite was detected but the median value for exchangeable arsenate is 0.16mg.kg-1 representing 10.3% of the total As being easily mobile. Adsorption experiments revealed that their maximum adsorption capacity for arsenate and arsenite was 1.05 mg.kg-1. This study reveals that ultramafic rock's alterites are a source of arsenic to surface water in the form of suspended particulate matter with a significant proportion easily soluble. These results therefore have implications for the survey of surface waters in ultramafic watersheds in New-Caledonia and more broadly in small tropical islands.

Mots clés: Ultramafic rocks' alterites; Inorganic arsenic; exchangeable arsenate; arsenic and arsenate adsorption capacity; ultramafic watershed; surface water

Domaines: Sciences de l'environnement; Chimie inorganique; Chimie analytique

Kévin Schneider, Monika Le Mestre, Isabelle Desriaux, Peggy Gunkel-Grillon. First investigations on arsenic content in ultramafic rocks' alterites from Nickel mines, implications for surface waters quality in ultramafic watersheds (New-Caledonia). Environmental Chemistry Letters, 2020, ⟨10.1007/s10311-020-01009-6⟩. ⟨hal-02888355⟩