Grosbois C, Schäfer J, Bril H, Blanc G, Bossy A
Université de Limoges, 123 Av. Thomas, Groupement Recherche Eaux-Sols-Environnement, IFR 145 GEIST, 87060 Limoges cedex, France.
Sci Total Environ. 2009 Mar 1;407(6):2063-76. doi: 10.1016/j.scitotenv.2008.11.012. Epub 2009 Jan 3.
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.
上伊勒河（法国西南部）流经法国第二大高产金矿开采区。在一个水文年期间，对上伊勒河地表水溶解态的砷（As）、氯离子（Cl⁻）、铬（Cr）、铁（Fe）、锰（Mn）、钼（Mo）、硫酸根离子（SO₄²⁻）、钍（Th）和铀（U）进行了高分辨率调查，旨在更好地了解矿区下游微量元素进入河流系统的途径。溶解态砷（浓度高达35000纳克/升）和钼（浓度高达292纳克/升）比区域溶解态背景值高出约3倍，且与流量呈负对数关系。溶解态铬（浓度高达483纳克/升）、钍（浓度高达48纳克/升）和铀（浓度高达184纳克/升）的浓度随流量增加。结合了地表水摩尔比、地球化学背景以及雨水和地下水数据之间的地球化学关系。不同元素组的对比行为可以通过一个包含三个季节成分的情景来解释：（i）高流量成分中砷和钼的浓度较低，但铬、钍、铀的浓度较高。这归因于诸如水土相互作用、大气输入、基岩和河床沉积物风化等扩散源。尽管该成分可能也包括硫化物矿脉风化的贡献，但该信号被稀释所掩盖。（ii）一个低流量成分中硫酸根离子、铁、砷和钼的浓度较高，铬、钍和铀的浓度适中。该成分归因于点源，如矿井巷道废水、采矿废料风化以及矿床中自然和/或采矿诱导的硫化物氧化产生的地下水输入。（iii）第二个低流量成分显示出高砷加钼浓度，同时硫酸根离子、铁、铬、钍和铀的浓度极低，这可能反映了相邻含水层中通过氢氧化铁形成对微量元素的清除作用。地表水中铁、铬、钍和铀的减少支持了这一点。通量估计表明不同元素对年溶解通量有不同的影响。径流可能占上伊勒河年溶解态砷、钼、钍和铀通量的主要部分。与硫化物氧化相关的输入分别占年砷通量和钼通量的约30%和约24%。在低流量期间，氢氧化铁的形成强烈保留了铬、钍和铀，限制了它们在地表水中的溶解浓度。如果这个过程最终可能降低砷的迁移性，那么它对地表水溶解态砷浓度的影响可能有限，或者/并且可能被硫化物氧化过程中砷的释放所抵消。
