Accumulation of selected nutrients and heavy metals in the Khubelu river catchment, Mokhotlong, Lesotho

Abstract

Water is essential to all life forms. It is a fundamental enabler of socio- economic development and essential for maintenance of ecological integrity. A good quality water is therefore, a necessity to achieve all these sustainable pillars, thus calls for global, continental, regional as well as national communities to protect water resources against pollution. Freshwater sources are naturally scarce and not evenly distributed across the globe hence pollution and drought further reduce freshwater availability. Presence of contaminants in freshwater systems is therefore, undesirable and necessitates all concerned and affected parties to act decisively to minimise the impacts. Close monitoring and studies on aquatic environmental pollutants are key more especially in areas like Khubelu River Catchment (KRC) where diamond mining and construction of large storage dams occur in close proximity. This study, therefore, seeks to determine levels of selected nutrients and heavy metals in the KRC, identify point and non-point sources of pollution as well as to evaluate possible impacts on the environment. These objectives were achieved through use of various analytical methods. The flow injection analysis was used in analysing for ammonia, ion chromatography for nitrates and nitrites analysis and inductively coupled plasma optical emission spectrophotometry for selected heavy metals. The results indicated the concentration of nitrates/nitrites as N in the range from 0.13 – 167 mg/L i and 0 – 67.8 mg/L during dry and wet seasons respectively. Ammonia as N ranged from 0.17 – 1.89 mg/L and 0.15 – 0.68 mg/L in the dry and wet season respectively. Copper levels ranged from 0.039 – 0.219 mg/L and 0.011 – 0.029 in the dry and wet season respectively. Cadmium, arsenic and mercury were not detected in both sampling seasons whereas lead and chromium were only detected during dry season. The levels of lead ranged from 0 – 0.020 mg/L and that of chromium were found to lie between 0 and 0.046 mg/L. Based on point and non-point sources, all the heavy metals detected were found to be non-point with the geology of an area supporting their natural occurrence. The nitrates and nitrites on the other hand proved to be mainly point sources because they were highest at Patising stream which carries some effluent from Letseng diamond slime dams. The downstream concentration levels confirm these as they are higher than the upstream levels. The presence of these nutrients upstream also confirms some contribution by land use activities such as animal droppings and crop farming as well as organic fertilisers thus minor component can be attributed to non-point sources. Ammonia is also distributed across all the sapling locations but highest at Patising still confirming the influence by the mine. Its presence in the upstream also confirms some contribution from non-point sources and natural processes of ammonia cycle. The results obtained in the present study indicated that there was a definite pollution in the Khubelu River Catchment with respect to HMs and nutrients studied. Land use activities around the catchment were the route source of this pollution particularly nutrients because heavy metals proved to occur naturally within the KRC and the geology of the area supported that. These HMs (Pb, Cu & Cr) and nutrients (NO3-, NO2- & NH3) can have serious health implications for both human beings and biota. It is therefore recommended that appropriate joint monitoring programme by the Departments of Environment and Water Affairs be done in order to minimise the possible impacts. Detailed river health study that will investigate all drivers and response factors be done to evaluate all possible migration routes of these HMs and their impacts. Another study on determination of HMs in fish tissues such as liver and kidney might be helpful to investigate possible bioaccumulation effects of these HMs. Public health study on Patising community to investigate residents’ health status with respect to the pollution in the catchment might be of importance. It might also worth investigating how the Patising stream might have impacted on the livestock of the community residing in the area because my observation and through communication with an area chief is that, the stream is no longer used for portable water instead the community uses water from the upstream confluence of Khubelu River and Patising.