Geological characterization and provenance of As-enriched aquifers in the Indus basin Pakistan: Tracing arsenic source from zircon trace element geochemistry and UPb isotope data

Geochemical surveys conducted on groundwater within the Punjab floodplains revealed high contamination of Arsenic (As). Although the spatial distribution of As and the mechanism of its release have been largely studied and understood however uncertainties still remain regarding its source and derivation. In this research, sediment samples from four boreholes were collected to study As contamination and main sediment phases that host As. To find the source of As-contaminated sediments, zircon trace element geochemistry and UPb isotope analyses were conducted on aquifer sediment samples collected from the Indus basin floodplain. Two lithofacies associations were identified in vertical lithological sequences: (1) a fine-grained over-bank layer and (2) a sandy channel-fill layer. Mineralogy of aquifer sediments is predominantly composed of quartz (30-57 %), plagioclase (4-25 %), mica (3-10 %), and trace amounts of pyrite and hematite. Geochemical analyses indicate biotite, chlorite, amphibole, garnet, and magnetite as the principal host of As. UPb isotope analysis, performed on zircon grains extracted from selected borehole samples from Ravi River floodplain, yield ages in the range from 1784 to 51 Ma, exhibiting several dominant peaks at 929 Ma and 307 Ma. The age data indicate sedimentary provenance from mixed magmatic lithologies that are exposed in the Himalayan mountain range in north Pakistan. Similarly, UPb ages of zircons from Chenab River floodplain showed a range from 1902 to 12 Ma with dominant peaks at 2000 Ma and 800 Ma, also indicating a similar sediment source. Trace element data of oscillatory zoned zircon grains suggest their formation mostly from igneous rocks and discriminate their source from crustal origin, mainly from magmatic rocks. The association of As-bearing sediment of the aquifer to a particular and known magmatic unit within the Himalayan region is complex because the sediments could have been derived from a vast area and may contain a mixed parental source.