Evolution model of δ34S and δ18O in dissolved sulfate in volcanic fan aquifers from recharge to coastal zone and through the Jakarta urban area, Indonesia

The sources of sulfate in an aquifer system, and its formation/degradation via biogeochemical reactions, were investigated by determining sulfate isotope ratios (δ34SSO4 and δ18OSO4) in dissolved sulfate in groundwater from the Jakarta Basin. The groundwater flow paths, water ages, and geochemical features are well known from previous studies, providing a framework for the groundwater chemical and isotopic data, which is supplemented with data for spring water, river water, hot spring water, seawater, detergents, and fertilizers within the basin. The sulfate isotope composition of groundwater samples varied widely from − 2.9‰ to + 33.4‰ for δ34SSO4 and + 4.9‰ to + 17.8‰ for δ18OSO4 and changed systematically along its flow direction from the mountains north to the coastal area. The groundwater samples were classified into three groups showing (1) relatively low and narrow δ34SSO4 (+ 2.3‰ to + 7.6‰) with low and varied δ18OSO4 (+ 4.9‰ to + 12.9‰) compositions, (2) high and varied δ34SSO4 (+ 10.2‰ to + 33.4‰) with high δ18OSO4 (+ 12.4‰ to + 17.3‰) compositions, and (3) low δ34SSO4 (<+6.1‰) with high δ18OSO4 (up to + 17.8‰) compositions. These three types of groundwater were observed in the terrestrial unconfined aquifer, the coastal unconfined and confined aquifers, and the terrestrial confined aquifer, respectively. A combination of field measurements, concentrations, and previously determined δ15NNO3 data, showed that the observed isotopic heterogeneity was mainly the result of contributions of pollutants from domestic sewage in the rural area, mixing of seawater sulfate that had experienced previous bacterial sulfate reduction in the coastal area, and isotopic fractionation during the formation of sulfate through bacterial disproportionation of elemental sulfur. Our results clearly support the hypothesis that human impacts are important factors in understanding the sulfur cycle in present-day subsurface environments. A general model of sulfate isotopic evolution along with groundwater flow has rarely been proposed, due to the complicated hydrogeological research setting that causes varied isotope ratios, although its understanding has recently received great attention. This pioneer study on a simple volcanic fan aquifer system with a well-understood groundwater flow mechanism provides a useful model for future studies.

Research highlights
► The sources of sulfate and its formation/degradation via biogeochemical reactions were analyzed by determining δ34SSO4 and δ18OSO4 in dissolved sulfate in Jakarta aquifer system.
► Hydro-geological settings and human impacts are the main factors controlling the sulfate isotope values under Jakarta volcanic fan aquifer.
► The proposed conceptual models of sulfate isotopic evolution should be of benefit for future work in other aquifer.