Hydrochemistry of a coastal sedimentary basin: evidence from the Lower Kutai Basin, Indonesia

Groundwater is a vital freshwater resource in coastal regions, where 38 % of the global population currently resides. The hydrochemistry of abstracted groundwater in low-lying deltaic regions can pose a risk to human health, especially where monitoring of groundwater quality is limited. This study investigates new evidence of the hydrochemistry of shallow (depths of <250 m) groundwater in the Lower Kutai Basin (LKB) where Indonesia's new capital, Nusantara, is situated. Shallow groundwater is predominantly (67 out of 73 samples) fresh with a median total dissolved solids of 197 mg/L and hydrochemical facies are dominated by the bicarbonate anion. In this coastal sedimentary basin, high concentrations of iron (median = 5.4 mg/L) and manganese (median = 138 μg/L) that exceed WHO drinking-water guidelines reflect widespread reducing conditions in shallow groundwater, promoted by sluggish flow under low hydraulic gradients (primarily <0.002). Stable isotope ratios (δ¹⁸O, δ²H) indicate that inland fresh groundwater, traced to heavy rainfall, becomes isotopically heavier and more saline toward the coast. Although the hydrochemical conditions favoring arsenic mobilization mirror those of Asian megadeltas, arsenic concentrations in the shallow groundwater of the LKB are generally low (median = 0.5 μg/L).