Integrated Environmental Assessment of BKME Discharged to a Mediterranean River

Abstract

Abstract The Mediterranean climate of Central Portugal produces a bimodal precipitation pattern such that effluent dilution is high during the winter rainy period and low in summer. An integrated assessment strategy was used to investigate the effect of this seasonally distinct flow regime on the environmental effects of a largely eucalyptus-derived, bleached kraft mill effluent (BKME) that receives secondary treatment before discharge to the Vouga River. The assessment included the triad of various water chemistry measurements, laboratory bioassays, and field surveys of benthic invertebrate and periphytic diatoms. The BKME discharge increased water temperature, pH, colour, suspended solids, conductivity, chemical oxygen demand, plant nutrients (nitrogen and phosphorus) and AOX content in the downstream receiving waters during both seasons. However, differences were much more pronounced in summer during the natural low flow period. Ecotoxicity tests revealed a significant effluent effect only in summer. Attached diatoms exhibited reduced species diversity at effluent-affected sites only in summer. Invertebrate densities and diversity were reduced downstream of the effluent in both seasons, but more so in summer. The effluent discharge often increased downstream water temperature by 10 to 15°C, with summer temperatures regularly exceeding 30 to 35°C.