Abstract

Generally, the disposed water from a geothermal field contains Cl, B, Fe, Zn, S, As and Hg. Among these S, As and Hg are the most dangerous contaminants of naturally occurring inorganic materials in the geothermal production test, such as in well Dieng-8. This well, with a total depth of 1,866 m, is one of the twenty-six wells drilled at this time in the Dieng Geothermal Field, Central Java. Before disposing to the river, the water is kept in the stabilizing pond to let the toxic compounds settle out.

Some samples were taken in the morning (09.00), mid-day (12.00) and afternoon (16.00) at three different points : inlet, centre, and outlet of the pond, to check the contaminants concentration distribution. Samples were also taken in the same hour upstream and downstream of the river to detect the influence of the mixing. The concentrations are determined in the laboratory and theoretically calculated using material balance method to obtain information whether the difference is significant.

Measurement results indicate that the effort to decrease the contaminant concentrations in the pond is not significant, supported by the fact that the concentrations downstream are higher than upstream. This means that environmental quality management is needed, because the water is used for agriculture and consumed by local people in Ngandam village. The contaminant concentrations in the river should be below the limit of the national water quality standards.