Impact of Tannery effluent discharge on the Nabajjuzi Wetland Ecosystem

Abstract

A study on the impact of Tannery effluent discharge on water quality and macroinvertebrates of the Nabajjuzi Wetland was carried out in order to provide sufficient data for its effective management. Water samples and macroinvertebrates were collected at one upstream site and three downstream sites relative to tannery effluent discharge. No macroinvertebrates were found at first and last downstream sites and only a few macroinvertebrates (a total of 12 individuals) from only one order i.e Hemiptera were collected at the mid-point of the downstream sites. At the upstream site, more macroinvertebrates (a total of 132 individuals from 4 taxa) some of which were associated with clean water quality were collected. All study sites had zero %EPT and %CHIR. This distribution of macroinvertebrates can be associated with the very low DO at all study sites (1.32±0.09mg/L – 1.67±0.20mg/L) and increasing concentrations of BOD5 (7.37±2.47mg/L – 199.75±44.01mg/L), COD(44.15±10.35mg/L – 1,192.30±220.60mg/L), TSS(57.00±23.07mg/L – 1699.9±578.32mg/L), Cl- (56.17±50.37 mg/L – 1,454.60±331.94mg/L), EC (47.42±2.14μS/cm – 3087.10±612.89μS/cm) and Cr(0.30±0.06mg/L – 0.86±0.17mg/L) from upstream to downstream sites respectively. This was in line with findings under classification of water quality based on Hilsenhoff (1988) which rated water quality at the upstream site as fair with relatively significant organic pollution, and that of the mid downstream site as poor with very significant organic pollution. This is also in line with results of the Kruskal-Wallis test which showed that the concentrations of EC, COD and TCU, Chlorides at all downstream sites, and BOD5 and TSS at the first and second downstream sites and Sulphates at the first and last downstream sites as significantly different (Kruskal-Wallis Test, p< 0.05) relative to that at the upstream site. Although Hierarchical Agglomerative Cluster Analysis identified three different classes (class 1 containing the first and last downstream sites, class 2 containing the mid downstream site and class 3 containing the upstream site), regression analysis showed strong association between classes containing downstream sites while the associations between downstream and upstream sites were weak. It was concluded that tannery effluent discharge has a direct impact on both the physico-chemical quality and macroinvertebrate characteristics along the Nabajjuzi Wetland which is significant. It was recommended that quality of effluent discharge should be reduced to concentrations in line with the National effluent discharge standard.