Developing a contamination susceptibility index for the Goreangab Dam in Namibia

Abstract

The convenience and quality of life provided by dams is highly dependent on the quality of the retained water. Human intervention plays an important role in defining the quality of the retained water as expanding human populations have a large impact on the surrounding environment and the, quality of impounded water. The types and extent of human activities taking place in and around the dam will generally have an effect on the unique physical and chemical properties of water within the reservoir, thereby affecting the biodiversity and related functions thereof.Therefore, the need for comprehensive water quality assessment and reporting tools including contamination susceptibility indices cannot be overemphasized. The study thus aimed at developing a contamination susceptibility index for the Goreangab dam by evaluating the effects of anthropogenic activities on surface water quality through the analysis of the physico-chemical properties of the water column and sediments, an aquatic invertebrate evaluation and pollution risk assessment, using the Pollution Load and the wastewater presence, recreational impact, agricultural impact, size of the watershed, transportation avenues, industrial impact and vegetative ground cover, (WRASTIC) indices.Analytical experimental studies were used as references to methods employed in this research. Water, aquatic invertebrates and sediment samples were collected from 6 sampling sites, selected on the basis of their exposure to anthropogenic activity. The quality parameters investigated included dissolved oxygen, pH, total dissolved solids, chemical oxygen demand, temperature, electric conductivity and select heavy metals Pb, As, Fe, Zn, and Hg. Water and sediment samples were analysed using the Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) and X-ray fluorescence (XRF) methods respectively. These were compared to the allowable limits stipulated in the Namibian Water Act (Act No. 54 of 1956). With the exception of TDS, Electric Conductivity and COD all other physical parameters recorded levels below the national set standard.Metal composition in the water column were all below the allowable limits, decreasing in the order Fe > Zn > Hg > Cd > As > Pb. The sampling stations upstream recorded higher average concentrations of metals in comparison to the downstream areas.The Pollution Load Index (PLI) results indicated deteriorating quality of soil sediments for all sampling stations, with higher deterioration upstream as these areas were privy to sewage and wastewater effluent. The aquatic invertebrate inventory and identification results categorised upstream Goreangab dam as a seriously modified habitat with very poor water quality whereas the downstream areas were found to be moderately modified. The average heavy metal concentrations were found to be below allowable limits for most of the sampling sites. Based on bio-monitoring and soil analyses results, there was indication of poor and deteriorating water quality at all sampling sites.The WRASTIC index indicated that the watershed was at high risk of contamination with a score of 51 and also identified five (5) of the sampling stations as risky areas serving as entry points for pollutants into the dam. The WRASTIC score is subject to a 3-year waver, indicating that the dam will be at a higher risk sooner as activity around the watershed continues to increase. The overall study results suggest that anthropogenic activity is a major factor in the contamination of the watershed and contributes greatly to its vulnerability. Regular screening of the Goreangab dam for sources of pollution will need to take place, along with continuous quality monitoring and assessment for the successful protection and restoration of the dam.