VOLCANOGENIC POLLUTION BY ACID WATER DISCHARGES ALONG CIWIDEY RIVER, WEST JAVA (INDONESIA)

Active and recently extinct volcanoes surrounding the catchment area o f the upper Citarum River (West Java) are the source areas of surface waters with high concentrations of natural contaminants. Ciwidey River , one of its tributaries, originates on the flanks of Patuha, a Quater nary volcano which contains an acid (pH < 1) crater lake having high c ontents of sulphur and chlorine, and where various other hydrothermal manifestations are active, We present the results of a geochemical stu dy carried out in 1995-1996 to assess the impact of volcanogenic pollu tion along this river, whose natural flow is disturbed by the use of w ater for irrigation and other purposes. Concentrations of dissolved ma jor and trace elements in the Ciwidey River show peak levels associate d with the influx of acid streams, Downstream decreasing trends are at tributable to dilution and other attenuation processes commonly observ ed in acid mine drainage. The main contaminating streams originate fro m two different sources: (1) a flank spring producing water which is s uspected to contain a portion of leaking acid lake water; and (2) arti ficial sulphur-mud deposits which are the remains of sulphur productio n from crater-lake sediments that were exploited in the first half of this century. Fluctuations in pH and dissolved metals observed along t he river during repeated sampling campaigns are correlated with season al successions of wet and dry periods, but the interference in the dra inage pattern has modified the original natural variations. The river water shows lowest pH values and remains acid along the greatest dista nce during the rainy season. Downstream decreases in acidity, which ar e largely induced by dilution with neutral water, determine to a signi ficant extent whether potentially harmful elements remain dissolved or are removed by precipitation or sorption onto solid phases. The seaso nal variability in discharges and acid input, combined with human inte rference in the river flow thus creates highly dynamic conditions for mobilisation and fixation of the volcanogenic pollutants, Among the el ements studied, aluminium, arsenic, boron, iron and manganese reach co ncentrations which deserve attention in view of the potential conseque nces for the human environment. In general, the volcanogenic pollutant s return to baseline levels before the confluence with the Citarum Riv er at some 30 km from the focal point of the acid discharges. It is ex pected that only the most conservative dissolved elements could cause detectable changes resulting from the influx of the Ciwidey River wate r. However, ephemeral peak discharges cannot be excluded, and may have an impact which remained unnoticed during our sampling campaigns. (C) 1998 Elsevier Science B.V. All rights reserved.