Spent geothermal water discharge to rivers: Risk or environmental benefit?

Geothermal waters utilization is generally considered an environmentally friendly compared to non-renewable sources. However, the discharge of spent geothermal waters, legally classified as wastewater, poses potential environmental burdens. This study aimed to determine the extent to which treated spent geothermal waters impact the quality of the river into which they are discharged. Analyses were conducted on the effluent prior to its mixing with the receiver, and on the receiving waters both downstream and upstream of the effluent outflow. Additionally, aquatic macroinvertebrates were sampled at the outflow, 100 m downstream, and 150 m upstream. The results revealed no statistically significant differences in most analysed parameters (temperature, pH, chlorine, nitrites, phosphorus, and BOD) between the reference section and the section below the outflow. A total of 4519 aquatic macroinvertebrates were collected during the field survey. Multi-metric multidimensional scaling indicated significant differences in assemblages between the sections just upstream of the outflow and those above and below, which showed no variation in taxonomic composition. The lowest values of diversity and biotic indices occurred in the site where the effluent was discharged closest to the bank (1 m) and in the transect 3 m from the bank. In contrast, the transect furthest from the bank (5 m) exhibited parameters comparable to the reference section. No significant differences were observed for most parameters between the sections 100 m below and 150 m above. The findings suggest that the discharge of treated effluent into the river has a localized impact confined to the immediate vicinity of the outflow and does not extend over a broader gradient. The results highlight that with sufficient wastewater treatment and a hydromorphologically diverse receiving body, the negative impacts of geothermal discharge on river biodiversity are mitigated. This study provides a novel examination of a relatively under-applied approach.