Petrographic and mineralogical analyses combined with sequential extractions and leaching experiments as a function of pH were performed on black clayey sediments fulfilling karsts in the Hydrogeological Experimental Site (HES) of Poitiers (France) to investigate the behavior of arsenic and selenium in a fractured limestone aquifer.

Sequential extractions showed that arsenic is mainly associated with pyrite (about 35%) and secondary iron oxyhydroxides (around 13%), along with a substantial exchangeable fraction (about 13%). The soluble fraction and the fraction associated to organic matter are ∼2% and ∼5%, respectively. The distribution of selenium is mainly pyritic (around 39%) or associated with organic matter (about 18%). Its association to secondary iron oxyhydroxides minerals is low (around 2%), whereas its soluble fraction is around 5%.

SEM analyses revealed the presence of arsenic “hot spots” into euhedral pyrite crystals surrounded by a halo of iron oxyhydroxides resulting from their alteration, and both are enriched with arsenic. Selenium has a similar pyritic origin but after alteration, it is predominantly associated with organic matter.

Despite different distributions, the leaching experiment as a function of pH showed that the mobilization of arsenic and selenium overlapped below pH 2 and above pH 8. The main differences were observed between pH 2 and 8 with a plateau at 5% of released selenium, whereas the amount of mobilized arsenic continuously decreased. The pH-dependence of both elements is attributed to the partial dissolution of pyrite in acidic conditions combined with desorption processes at higher pH values.

在普瓦捷（法国）的水文地质实验点（HES）对充填岩溶的黑色粘土沉积物进行岩相学和矿物学分析，结合顺序提取和浸出实验作为pH的函数，以研究石灰岩含水层破裂后砷和硒的行为。 。

连续萃取表明，砷主要与黄铁矿（约35％）和仲氢氧化铁（约13％）有关，同时还有大量可交换馏分（约13％）。可溶性部分和与有机物有关的部分分别为〜2％和〜5％。硒的分布主要是黄铁矿（约占39％）或与有机物有关（约占18％）。它与羟基氧化铁矿物质的缔合度较低（约2％），而其可溶部分约为5％。

SEM分析表明，存在于正铁矿黄铁矿晶体中的砷“热点”，由于其改变而被羟基氧化铁的光晕所包围，并且都富含砷。硒具有类似的黄铁矿来源，但是在改变后，它主要与有机物有关。

尽管分布不同，但浸出实验随pH的变化显示砷和硒的迁移在pH 2以下和pH 8以上重叠。在pH 2和8之间观察到主要差异，在5％的硒释放水平上达到稳定，而动员的砷量不断减少。两种元素的pH依赖性归因于黄铁矿在酸性条件下的部分溶解以及在较高pH值下的解吸过程。