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

Abstract

The sources of sulfate in an aquifer system, and its formation/degradation via biogeochemical reactions, were investigated by determining sulfate isotope ratios (δ³⁴S(SO₄) and δ¹⁸O(SO₄) 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 δ³⁴S(SO₄) and +4.9‰ to +17.8‰ for δ¹⁸O(SO₄) 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 δ(34)S(SO₄) (+2.3‰ to +7.6‰) with low and varied δ¹⁸O(SO₄) (+4.9‰ to +12.9‰) compositions, (2) high and varied δ³⁴S(SO₄) (+10.2‰ to +33.4‰) with high δ¹⁸O(SO₄) (+12.4‰ to +17.3‰) compositions, and (3) low δ³⁴S(SO₄) (< +6.1‰) with high δ¹⁸O(SO₄) (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 δ¹⁵N(NO₃) 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.