Downstream composition changes of acidic volcanic waters discharged into the Banyupahit stream, Ijen caldera, Indonesia

The crater lake of Kawah Ijen volcano contains extremely low pH (<0.4) wate rs with high SO4 (similar to 70000 mg/kg), Cl (similar to 21000 mg/kg), F ( similar to 1500 mg/kg), Al (similar to 5000 mg/kg), Fe (similar to 2000 mg/ kg) and trace metal (Cu similar to 0.5, Zn similar to 4, Pb similar to 3 mg /kg) contents. These brines seep outward through the western crater rim and reappear on the other side as streamlets, which form the headwaters of the Banyupahit stream. The Banyupahit first mixes with fresh rivers and therma l springs in the Ijen caldera and then irrigates a coastal agricultural pla in which is 30 km from the summit crater. We discuss the downstream composition changes affecting the Banyupahit wate rs by using stable isotope, chemical and mineralogical data collected from sites along the stream length. The saturation of the stream waters with res pect to minerals was evaluated with SOLVEQ and WATEQ4F and compared with th e geochemical observations. An aluminous mineralogy (alunogen, pickeringite , tamarugite and kalinite) develops in the upper part of the Banyupahit due to concentration of the headwaters by evaporation. Downstream attenuation of dissolved element concentrations results principally from dilution and f rom mineral precipitation. The stream pH changes from similar to 0 at the s ource to >4 close to the mouth. The delta D and delta(18)O values and the r elative SO4-Cl-F contents of the Banyupahit waters indicate that the tribut aries are mostly meteoric. Dissolved SO4 in the acidic stream come only fro m the crater lake seepages and are not involved later in microbially mediat ed reactions, as shown by their delta(34)S and delta(18)O values. Re-equili bration of the stream SO4 oxygen-isotope composition with H2O from tributar ies does not occur. Calcium, SiO2, Al, Fe, K and SO4 behave non-conservatively in the stream wa ters. Gypsum, silica (amorphous or poorly ordered), a basic aluminum hydrox ysulfate (basaluminite?), K-jarosite and amorphous ferric hydroxide may exe rt a solubility control on these elements along the entire stream length, o r in certain stream sections, consistent with the thermochemical model resu lts. Downstream concentration trends and mineral saturation levels suggest that precipitation of Sr-, Pb-rich barite and celestite consume Ba, Sr and Ph, whereas dissolved Cu, Pb and Zn may adsorb onto solid particles, especi ally after the junctions of the acidic stream with non-acidic rivers. We ca lculated that significant fluxes of SO4, F, Cl, Al, SiO2, Ti, Mn and Cu may reach the irrigation system, possibly causing serious environmental impact s such as soil acidification and induration. (C) 2000 Elsevier Science B.V. All rights reserved.