Are the stream macrobenthos impacted by the wastewater from rubber factories?

Abstract

We assessed how the wastewater generated from raw rubber factories affected the water quality parameters and how such changes influenced the stream macrobenthic assemblages in some streams in the wet zone of Sri Lanka. For this assessment, water quality parameters viz. DO, COD, BOD5, conductivity, TDS, T, pH, and OMC in the sediment were measured, and the macrobenthic fauna were sampled during the dry season at six sampling sites established based on judgemental sampling technique viz. rubber factory wastewater effluent canal (site A), point of wastewater discharge in the stream (site B),  50 m upstream site from site B (site C), 50 m downstream site from site B (site D), 100 m downstream site from site B (site E), and 150 m downstream site from site B (site F) following standard field sampling techniques. Secondary research data (dry season) from two other streams subjected to rubber factory wastewater effluents, namely Rakwatte Ela (2001) and Gurugoda Oya (2011), were also used for this assessment. Results revealed that the COD, BOD5, OMC, conductivity, and TDS levels were elevated, and the DO level was reduced significantly in the highly polluted A and B sites in all three streams. Parallel to them, the macrobenthic diversity decreased significantly (p<0.5; ANOVA) at these two sites. However, the complete opposite was observed at the furthermost downstream site F, where the water quality parameters and the microbenthic composition became almost the same as that in the upstream control site C. It is evident that the changes made to the stream water quality and the macrobenthic assemblages by the rubber factory wastewater are never permanent and disappear within a relatively short stretch of 150 m along the streams, most probably due to the dilution of wastewater along the stream. The pollution-tolerant tubificids and chironomids were bioindicator candidates to detect such changes where they became highly abundant at the highly polluted wastewater effluent canal (i.e., Site A) and point of wastewater discharge in the stream (i.e. Site B), but their abundance gradually decreased along the downstream sites probably due to wastewater dilution by the fresh water supply from the stream.