Arsenate As(V) and arsenite As(III) are toxic forms of arsenic in waters. The risk of As toxicity is high in small tropical islands because tap water is often provided from surface water, and As contamination of surface water is rarely studied. For instance, the risk is high in New Caledonia because 61% of the water comes from surface water, and nickel mines induce the dispersal of large quantities of ultramafic rocks’s alterites rich in iron oxide-hydroxide, which are known to be associated with arsenite and arsenate. Levels exceeding the World Health Organization standard of 10 µg L⁻¹ have been detected in rivers downstream of Ni mines, yet there is a lack of systematic assessment. Here, we analyzed total and exchangeable As in alterites collected near three active mines. Arsenate and arsenite adsorption capacity was studied using batch experiments. The results show that alterite contains total As contents ranging from 0.20 to 5.14 mg kg⁻¹, yet the primary mineralogical source of As remains unknown. No exchangeable arsenite was detected. Exchangeable arsenate amounted to 0.16 mg kg⁻¹, thus meaning that 10.3% of the total As is easily mobile. The maximum adsorption capacity of arsenate and arsenite in mining sediments was 1.05 mg kg⁻¹. Overall, our findings reveal that ultramafic rock alterites are a source of arsenic in surface water in the form of suspended particulate matter, 10.3% of which being easily soluble.

砷砷（V）和砷砷（III）是水中砷的有毒形式。在热带小岛上，As毒性的风险很高，因为自来水通常是由地表水提供的，很少研究As对地表水的污染。例如，在新喀里多尼亚，风险很高，因为61％的水来自地表水，而镍矿则导致大量富铁氧化铁氢氧化物的超镁铁质岩石的变质散布，这些变质与亚砷酸盐和砷酸。超过世界卫生组织10 µg L ^-1标准的水平在镍矿下游的河流中已检测到，但缺乏系统的评估。在这里，我们分析了在三个活跃矿山附近收集的蚀变中的总砷和可交换砷。使用分批实验研究了砷和砷的吸附能力。结果表明，铝铁矿的总As含量在0.20至5.14 mg kg ^-1之间，但As的主要矿物来源仍然未知。未检测到可交换的亚砷酸盐。砷的可交换量为0.16 mg kg ^-1，这意味着总砷的10.3％易于移动。采矿沉积物中砷和亚砷酸盐的最大吸附能力为1.05 mg kg ^-1。总的来说，我们的发现表明，超镁铁质岩石蚀变物以悬浮颗粒物的形式存在于地表水中，是砷的一种来源，其中10.3％易溶。