Characterization of acid river dilution and associated trace element behavior through hydrogeochemical modeling: A case study of the Banyu Pahit River in East Java, Indonesia

Kawah Ijen volcano in East Java, Indonesia emits hyperacid (pH ≈ 0) brines rich in toxic elements including F, Al, Cd and Tl, which are transported downstream by the Banyu Pahit River, which is eventually used to irrigate farmland on the Asambagus Plain. The fate and behavior of major and trace elements are investigated, in the region of greatest change to the Banyu Pahit River, where thermal springs and the neutral Kali Sat and Kali Senggon Rivers increase its discharge 6-fold and pH increases from 1.9 to 4.5 with the development of abundant precipitates. The conservative behavior of Cl, F and SO4 allows determination of the proportions of the mixing constituents at each confluence. Comparing a mass balance model based on the resulting proportions with measured concentrations demonstrates conservative behavior for most cations, with the exception of Si, Zr, Sn and Ba, which precipitate as amorphous silica, barite, and Zr and Sn phases, respectively. Iron is added as particulate Fe-(oxy)hydroxide in mixing with thermal spring outflow, and dissolves in the lower pH water. This also contributes Co, Cd, Mn, Y, and the rare earth elements, which are desorbed from the Fe-(oxy)hydroxide upon entering the acid water. Subsequent saturation in Fe-(oxy)hydroxide after mixing with the neutral rivers leads to a reverse response. Still, element behavior is close to conservative, which means that the volcanogenic toxic element load is almost entirely transferred to the Asambagus Plain with ensuing environmental and health impacts. This study shows that compositions of these acid waters can be understood and reproduced in a thermodynamic model, but only when fine-tuned using measured compositions and field observations. Therefore, the model’s utility in assessing the fate of toxic elements and in planning the environmental mitigation is limited.

► The Kawah Ijen Volcano, Indonesia emits hyperacid toxic brines which are transported downstream by the Banyu Pahit River.
► Thermal springs add particulate Fe-(oxy)hydroxide which partly controls element concentrations, increasing Fe and metals.
► Upon mixing with neutral rivers, the Banyu Pahit precipitates Fe-(oxy)hydroxides and scavenges elements onto precipitates.
► Downstream the Banyu Pahit contains 15% hyperacid water, with the remainder from neutral rivers.
► The volcanogenic toxic load is almost entirely transferred downstream to the Asambagus Plain.