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.

Introduction

The contamination of water resources in highly populated areas has become a global concern, limiting the resources’ availability. The presence of both inorganic and organic contaminants in water bodies may result in several health problems and in certain cases, lead to the entire loss of aquatic ecosystems (Bartzas et al., 2015). The impacts of surface water contamination are more noticeable in arid areas, as they continually experience rainfall shortages, high evaporation rates and in various cases (e.g. in Sub-Saharan Africa) lack adequate water storage facilities (Du Pisani, 2004). Pollution of water resources is one of the most clamant issues affecting our ecosystem and nearly all fresh water bodies are at risk of contamination due to human population expansion and developmental activities in and around them (Bordoloi and Baruah, 2014).

The quality of freshwater at any point on a landscape reflects the combined effects of many processes along water pathways (Wildi et al., 2010). Surface waters are most exposable to anthropogenic pollution, due to the accessibility for the disposal of wastewaters (Ali et al., 2016). Anthropogenic influences are known sources of water pollution and include urban, industrial and agricultural activities, (Snaddon, 1998). Population growth, followed by an increase in the number of settlements and the growth of industry, particularly along watersheds, that dump waste into surface waters, cause a decline in the quality of surface waters (Masere et al., 2012).

The decrease in water quality does not only prove to be expensive when it comes to treatment and possible reclamation, poor water quality is also detrimental to human health (Meybeck and Helmer, 1996). Surface water pollution has thus become a major concern across the world (Ali et al., 2016). Due to extensive anthropogenic inputs of nutrients, sediments and other water contaminants, the quality of surface water has deteriorated in many countries in the past few decades, (Mohammed A, 2014). New approaches towards achieving sustainable water resources management have thus been developed internationally (Sikder et al., 2015). Quality monitoring and pollution risk assessment of water resources have therefore become keynote procedures in the global agenda for environmental protection.

Water resource systems are subject to different impacts by anthropogenic pollution. Their intrinsic susceptibility to pollution may not allow them to resist pollution impacts brought about by the different types of contaminants that access the system (Diamantino et al., 2007). Quantifying water quality in a specific grade by using dominant parameters is therefore important, as this can explain the current extent of pollution with accuracy. The use of water contamination indices is therefore critical in the protection, assessment and restoration of water quality for catchment areas (Sikder et al., 2015). Vulnerability assessments and water quality indices are considered as key elements in the sound management of water resources (Sikder et al., 2015; Yan et al., 2015), as they can be used to simplify expressions of complex sets of pollution variables in the rivers, streams and lakes (Sikder et al., 2015).

Assessing the susceptibility of water resources to contamination is essential in that, it is not only a function of the intrinsic properties of the water flow system, but also of the proximity of contaminant sources and their particular characteristics that potentially increase the load of specific contaminants in aquifers (Bartzas et al., 2015). Pollution risk assessment is a tool which aids in identifying targets by recognizing hazards, assessing their severity and prioritizing them so as to find ways to tackle them (Liu et al., 2018).

WRASTIC is a pollution risk assessment method that was developed by the New Mexico Environmenttal Department Drinking Water Bureau (NMED/DWB) for the evaluation of watershed susceptibility to surface water contamination in any hydrogeologic setting based on major watershed characteristics and land uses (Report of New Mexico Water Utility Public Water System # 12345 New Mexico Environment Department -, 2004). The WRASTIC Index is the only method that offers a free location overall assessment of the risk status of a surface water body by grading up to seven different parameters which include human and animal wastewater, entertainment and recreational activities, agricultural and industrial activities, size of the area, road and transportation and vegetation density (Diamantino et al., 2007; Mirzaei et al., 2016). WRASTIC is very effective as a rating system for surface water bodies, as it determines the susceptibility of a water system to pollution. The method helps water resource planners and managers in assessing pollution vulnerability and identifying risk areas related to different pollution sources (Rud, 2019).

Sediments are important sinks for trace metals. They also act as a non-point sources of pollution and have the potential to release the sediment-bound metals and other pollutants to overlying waters, in turn adversely affecting aquatic organisms. Testing of both water and sediment samples within a waterbody is therefore essential in obtaining a more defined picture of its quality and health (Goher et al., 2014). The Pollution Load Index (PLI) technique is used to assess heavy metal contamination in a study areas sediment. The method takes into account the metal concentration in individual samples, the number of metals investigated and a contamination factor, giving an accurate overview of the sediment health in a water body. The contamination factor is the ratio of the metal concentration in comparison to the background value (value of the metal equal to the world surface rock average). The PLI, positively demonstrates simplicity as the results are easily interpreted, a PLI value greater than one (>1), is indicative of a polluted reservoir whereas a PLI less than 1 (<) indicates minimal pollution risk.

Namibia is considered to be the driest country in sub-Saharan Africa (De Bruine and Rukira, 1997). The average annual rainfall in Windhoek, the country's largest city, is estimated at approximately 360 mm/annum, and these are limited to convective showers during the rainy season which generally lasts from October to April (De Bruine and Rukira, 1997; Du Pisani, 2004). This is coupled with high evaporation rates averaging 3400 mm/annum, which result in fresh water resource deficits over most of the country (Du Pisani, 2004). Windhoek is reliant on three surface water dams, for approximately 70% of its potable water with the Goreangab Dam supplementing irrigation (Du Pisani, 2004; Lehmann, 2010). Due to the excessive pollutant emissions, the water quality of the Goreangab Dam has failed to meet the national standard criteria (Du Pisani, 2004; De Bruine and Rukira, 1997). Pollution is therefore a notable factor placing stress on the water scarcity issue experienced in the country (Lehmann, 2010).

In 2004 it was reported that the quality of the Goreangab Dam water had deteriorated to a point where the raw water design parameters were far exceeded, and that usage thereof would result in unsatisfactory final water quality specifications (Du Pisani, 2004). There are currently no scientific studies highlighting sources of pollution or the continued deterioration in water quality of the Dam. Moreover, there is no information on the vulnerability status of the reservoir (which is crucial for holistic monitoring, control and protection of the reservoir), correlation of the reservoirs water quality to surrounding anthropogenic activity or the risk they pose to the Dam; most studies focus on comparison of the physico-chemical parameters with water quality standards. This study therefore, focused on determining the susceptibility level of the Goreangab Dam to contamination by making use of the WRASTIC index and other pollution risk assessment methods, identifying, the possible sources and level of contamination and identify entry ways of contaminants into the catchment.