Temperature-induced arsenic accumulation in groundwater from Pliocene aquifers of a semiarid continental basin

Contributions of geothermal reservoir and aquifer rock weathering to dissolved arsenic (As) concentrations remain poorly understood. This study investigated geochemical and isotopic characteristics of groundwater from the Quaternary aquifer (Quaternary-GW) and groundwater from the middle-Pliocene aquifer (Pliocene-GW) to reveal As enrichment mechanisms in geothermal groundwater. The results showed that As concentrations in Pliocene-GW were higher than in Quaternary-GW. δ7Li values varied in a wider range in Quaternary-GW than those in Pliocene-GW, and δ11B values were generally lower in Pliocene-GW than those in Quaternary-GW. Normally, δ7Li values in Pliocene-GW showed a negative correlation with the ratio of Li to Na+, and increased along the flow path, which indicated that the prolonged residence time was responsible for high δ7Li. Negative correlations between δ11B and HCO3–, CO32– and B in Pliocene-GW revealed the processes of B desorption and silicate weathering in the middle-Pliocene aquifer. The processes of B desorption and silicate weathering were enhanced by geothermal reservoir, which was evidenced from the negative correlation between δ11B and calculated reservoir temperature. Arsenic concentration increased with increasing δ7Li along the groundwater flow path in the middle-Pliocene aquifer, indicating that prolonged residence time of Pliocene-GW promoted As enrichment. Meanwhile, As concentrations were negatively correlated with δ11B in Pliocene-GW, illustrating that silicate weathering and desorption driven by geothermal conduction were important processes for As mobilization. In samples of Pliocene-GW with similar δ7Li and relatively low δ11B, As was mainly released from the original groundwater-rock interactions, especially desorption and silicate weathering facilitated by geothermal conduction. This study provides quantitative insights into As enrichment mechanisms in geothermal groundwater.