Oxidative sulfide weathering and weak mineral attenuation controlling arsenic mobility in a crystalline aquifer

This study investigates the source-sink dynamics and hydrogeochemical controls governing arsenic fate and transport within a fractured crystalline aquifer system. The hydrogeologic framework is characterized by spatial heterogeneity in redox conditions and persistent undersaturation with respect to key Fe-, sulfate-, and Ca-bearing mineral phases. Mineralogical characterization identifies arsenopyrite and cobaltite as the primary geogenic sources; oxidative weathering of these sulfide phases drives the continuous release of arsenic. Aqueous speciation modeling reveals a predominance of As(V) and Fe(III) species, with minor As(III) occurring within reducing microenvironments. Persistent undersaturation of Fe- and As-bearing minerals, together with other attenuation-controlling phases, suggests limited potential for secondary mineral precipitation, while allowing arsenic to remain predominantly in the dissolved phase. Weak correlations between arsenic and hydrochemical gradients indicate that contaminant distribution is highly heterogeneous and controlled by the occurrence of mineralized zones. The results demonstrate that persistent arsenic mobility in fractured crystalline aquifers can occur under predominantly oxidizing conditions through sustained sulfide weathering combined with limited attenuation capacity within the aquifer system.