Effects of land use and land cover on water quality of River Namatala and its tributaries in Kyoga Basin, Eastern Uganda

Abstract

Although water quality issues are gaining increasing attention globally, research is inconclusive on how land use affects water quality in dynamic tropical catchments. This is more apparent in many catchments in Uganda undergoing land use transformations with implications on water quality. This study was conducted to assess the effect of land use on water quality in Namatala sub-catchment on mountain Elgon, with rapid urbanization. Three objectives for the study were formulated; (i) to characterize the existing land use types and their implications on water quality in Namatala sub catchment, (ii) to assess the effect of land use on the nutrient enrichment and microbial inputs in catchment rivers (iii) to assess the level of heavy metals concentrations in Namatala river associated with specific land use practices. A cross-sectional design was adopted for the study. Land use characterization was determined using satellite imagery data which was used to classify land use. Random Forest algorithm that follows supervised classification were used to generate distinct land use classes and Landsat 8 panchromatic band (15m pixel resolution) was applied to fuse with selected multispectral bands (30m pixel resolution), thereby improving the spatial detail of the final RGB composite. Water quality assessment was premised on collection of water samples at 30 strategic locations (with point source challenges identified during the preliminary assessment) along river Namatala. The samples were analyzed to assess the levels of nutrients in the river water (phosphorus, ammonium and nitrates). Additional tests were conducted to assess the concentrations of heavy metals such as manganese (Mn), copper (Cu), lead (Pb), zinc (Zn), aluminium (Al), cobalt (Co), arsenic (As), iron (Fe), nickel (Ni) and chromium (Cr) in the river water. The study results revealed that the sub catchment along River Namatala is dominated by shrubland (30%), followed by tree cover (28%), cropland (20%), grassland (17%), built-up areas (4%) and wetlands (1%). Nitrates (mg/l) were falling within a range of 0.21-20 with a mean of 2.0. Phosphates ranged between .06-2.2 mg/l with a mean of 0.25. Ammonium had a range of 0.003-0.86 mg/l with a mean of 0.07. With a significant variation (p<0.05), phosphorus concentrations in the river water along forested sites were higher than those in built-up and shrub areas. The mean phosphate content was higher in built-up areas. Low levels of ammonium in river water were found in sites dominated by tree cover, built-up, and shrubs. On the other hand, the highest ammonium levels were found in wetlands and cropland sites. The mean concentrations of heavy metals in catchment river water varied. Manganese averaged 34.0 mg/l, copper (1.57 mg/l), lead (0.18µg/l), zinc (5.21 µg/l), aluminium (10.99 µg/l), cobalt (0.20 µg/l), arsenic (0.34 µg/l), iron (99.7 µg/l), nickel (0.50 µg/l) and chromium (0.22 µg/l) and were within the permissible drinking water thresholds recommended by the World Health Organization (2011) with exception of except Vanadium (2.90 µg/l). Land use had a significant effect on water quality with water samples collected from cultivated areas and settlements exhibiting higher concentrations of nutrients and heavy metals. Mitigating water quality changes in Namatala river and its tributaries will require preservation and conservation of the natural ecosystems through regulation of farming activities and settlements.