Elevated arsenic concentrations (up to 19 μg L^-1) occur in private groundwater wells in fractured bedrock aquifers close to the contact between Silurian greywackes of the Longford-Down Terrane, and the Palaeogene Slieve Gullion and Carlingford igneous complexes in NE Ireland. Palaeogene basaltic intrusions intersected in two drill cores were found to contain up to 80 mg kg^-1 arsenic; concentrations that are elevated compared with the global averages for basalt. Fine grained (c. 5 μm) disseminated sulfarsenides, associated with cobalt and nickel were identified in the basalts as the primary sources of groundwater arsenic in this fractured bedrock aquifer system. Many of the high-As waters exhibit relatively high pH, arguing against a simple single-stage sulphide oxidation model for arsenic solubilisation. This observation coupled with the widespread presence of iron oxyhydroxide coatings on natural rock fractures intersected in the drill cores suggests a multi-stage As mobilisation process. This interpretation envisages sulfarsenide mineral oxidation, adsorption of dissolved As to iron oxyhydroxide mineral surfaces, and finally desorptive release of oxyanion As species from these surfaces as waters evolve to higher pH as a result of water-rock reactions.

升高的砷浓度（高达19微克大号^-1）发生在私人地下水井在断裂基岩含水层靠近朗福德向下地体，和古近格利恩山和卡林福德火成岩络合物在NE爱尔兰志留硬砂岩之间的接触。发现相交在两个钻芯中的古近纪玄武岩侵入体含有高达80 mg kg ^-1砷; 与玄武岩的全球平均水平相比有所升高。在该裂隙基岩含水层系统中，在玄武岩中发现了与钴和镍相关的细粒（约5μm）弥散的亚砷化物，是地下水砷的主要来源。许多高砷水表现出较高的pH值，这与简单的单级硫化物氧化模型（用于砷溶解）有关。这一观察结果加上在钻芯中相交的天然岩石裂缝上广泛存在的羟基氧化铁涂层，表明了多阶段的As动员过程。这种解释设想了硫化砷矿物质的氧化，将溶解的As吸附到氢氧化铁矿物质的表面，