An analysis of ground water quality in a water stressed urban centre: a case of Gweru city, Zimbabwe

Groundwater has emerged as a vital resource for domestic water supply, providing 25–40% of the world's drinking water (Margat & van der Gun 2013). In urban areas, a significant increase in groundwater use by 63.1% of the population has been reported between 1990 and 2010 (WHO & UNICEF 2014). The increase in ground water use mostly emanates from third world countries where outbreaks of water related diseases are prevalent (Gleick 2003; Sorenson et al. 2011; Bain et al. 2012; Onda et al. 2012). Lately, urban water utilities have resorted to groundwater exploration, development and use for two reasons. Firstly, there is need to cope with increasing population, rapid urbanisation, increasing affluence, accelerated rates of industrialisation, increasing per capita water demand (Rao et al. 2004). Secondly, the cost for drilling boreholes and wells is relatively affordable to the municipalities and private, in-situ self-supply residents (Rao et al. 2004; Foster et al. 2010).

Although ground water use has increased in urban areas, there are still challenges associated with its use as potable water. In urban environments, ground water is subject to temporal and spatial quality deterioration mainly due to pollution (Van Der Hoven et al. 2005; Schot & Pieber 2012). Ground water pollution is a subset of complex interactions between different legal or illegal land uses with rainfall, runoff and ground water recharge processes (Pionke & Urban 1985; Coulter et al. 2004; Lerner & Harris 2009).

Increasing population, serviced by old low capacity sanitation and reticulation facilities results in underground sewer burst which directly load faecal pathogens (eg Escherichia coli), pharmaceuticals, disinfectants, detergents and other persistent micro-pollutants to the ground water. Urban agriculture, landfills and waste tips may input high levels of nitrates or nitrite, ammonium, pesticides and sulphides through runoff, infiltration and seepage (Wakida & Lerner 2005). Industrial waste contributes heavy metals, poly-aromatic hydrocarbons among other pollutants.

Common health effects of groundwater pollution by heavy metals include high blood pressure, digestive problems, kidney damage and mental retardation from mercury and diarrhoea, nausea and death from Arsenic (Jarup 2003). Nitrates and nitrites, although their effects on human health is disputed (Addiscott 1996; Addiscott & Benjamin 2004; Addiscott 2005; Almasri 2007), it is strongly argued to cause infant methemoglobinemia and gastric cancer (Bruning-Fann & Kaneene 1993). These potential health risks can be fatal considering that physical, chemical and biological parameters are not usually tested for during acute water shortages. Furthermore, privately owned boreholes and wells are not mandatorily tested and most groundwater users regards groundwater to be pure.

The high level risk associated with groundwater use, is in a way, an indicator of information deficiency on the quality of the resource itself as well as lack of baseline data to base the development of a groundwater framework on. Therefore, this research may be a starting point towards informing local level policy makers about the groundwater quality and implications of its use as potable water. This may ultimately lead to the development of groundwater governance structures to control how groundwater is exploited and used. This status quo in Gweru city applies to all the other cities in Zimbabwe and most of countries in southern Africa. The correlations of groundwater parameters developed in this research may aid in identifying sustainable groundwater treatment options.

This research seeks to analyse the borehole water quality in Gweru city and assess the suitability of the water for domestic use using the World Health Organisation (WHO) guidelines for drinking water and analyse correlations between groundwater quality parameters.

The research was carried out in Gweru the fifth largest City in Zimbabwe, based on population. It is located at 19.4511 °S, 29.8302 °E and 285 km south-west of the Capital City Harare. The city covers about 26 113Ha of the Sanyati catchment. It is at an altitude of 1,420 above mean sea level. Gweru city lies in Natural Region IV and receives an average rainfall of 600 mm bordered by a minimum of 400 and a maximum of 850 mm (Mugandani et al. 2012). The area receives most of its rainfall in the tropical summer (October to March) with maximum average temperature of 28.3 °C and the winter is fairly cold with minimum average temperature of 8.7 °C (Vincent & Thomas 1960).

The city of Gweru, like most of the cities in Zimbabwe, is experiencing rapid increases in population (Table 1); the population growth rate for 2002–2012 was 1.0 (ZimStat 2012). The population is housed in around 41,149 households in low, medium and high density residential areas. The population increase has surpassed the capacity of potable and wastewater treatment plants.

Potable water in the city is supplied from three dams: Gwenoro (31.4 × 10⁶ m³), White Waters (4.9 × 10⁶ m³) and Amapongokwe (37.6 × 10⁶ m³). Gwenhoro dam used to supply 93% of the city's water demand in 2002 and the supply capacity reduced by 10% in 2012. This is attributed to the change in catchment land uses, reduced rainfall amounts and increased groundwater abstraction in the upstream of the catchment including the city. There has been a decrease in the peak water levels in Gwenhoro dam (Table 2) which resulted in the decommissioning of the dam in October 2013. This further worsened the water scarcity situation and residents resorted to ground water for portable water. The city also tried to implement water rationing to cope with increasing demand and diminishing supply.

Zimbabwe's economy has been on the downward trend for the past two decades and performance of the city's water utilities has not been spared by the economic crisis. The city failed to repair breakdowns and to acquire chemicals for water treatment. The water scarcity situation resulted in a Cholera outbreak from October 2009 and lasted into March 2010. The outbreak claimed at least at least 4,000 lives.

In response to the 2009–2010 Cholera crisis, Non-Governmental Organisations (NGO), private property owners and the city council drilled at least 200 emergency boreholes. By the end of 2010 at least 400 public and private boreholes were serving the city residences. Each borehole in the high density suburbs services an average of 100 households and at most 20 households in the medium and low density suburbs. The boreholes were sunk to an average depth of 40 m.

A Pearson correlation test together with a two tailed significance test (p = 0.05) was carried out between water parameters. This was done to give preliminary options for ground water treatment and link the source and behaviour of the tested water parameters.

Turbidity values ranged from 0.013NTUs to 4.44NTUs. The highest turbidity value (4.44 NTUs) was recorded in Riverside residential area with the lowest (0.013 NTUs) recorded in Mkoba 11; a high density residential area. Borehole water pH ranged from 6.67 to 7.46 (Table 3). The lowest pH value was recorded in Mkoba 20 and the highest in Lundi Park residential area.

EC ranged from 56 to 336 μS cm⁻¹. The maximum and minimum EC values were recorded in Mambo (336 μS cm⁻¹) and Riverside (56.6 μS cm⁻¹), respectively. Total hardness ranged from 106 to 575 mgl⁻¹. Borehole water in Mkoba 11 and Riverside was classified as moderately hard (75–150 mg l⁻¹), while in Mkoba 19, 20 and Lundi Park was found to be hard water with total hardness ranges of 150–300 mg l⁻¹ and very hard water was found in Senga, Mambo, Mkoba 13 and Nashville boreholes (Table 4). Only 22% of the boreholes were within the WHO limit of water hardness. All the boreholes had water turbidity, pH and EC values within the WHO acceptable range.

Manganese, Sulphates, Potassium, Fluorides, Chlorides, calcium and Magnesium concentrations in all borehole locations were within the WHO limits. Only Iron concentrations were out of the recommended WHO limit in Mambo, Mkoba 20, Mkoba 19, Senga and Mkoba 13. Nitrates concentration ranged between 1.85 and 11 mg l⁻¹ the WHO limits were surpassed in Mkoba 13 (Table 5).

Total coliform load ranged from 14 CFU 100 ml⁻¹ to 167 CFU 100 ml⁻¹ and E. coli load ranged from 0 to 23 CFU 100 ml⁻¹. The highest faecal coliform load of 80 CFU 100 ml⁻¹ was recorded in Riverside and no E. coli was recorded in Nashville (Table 7). The risk posed by faecal coliform load in borehole water was low in 56% of the boreholes. Thirty-three percent of the boreholes were on intermediate risk and only 1 borehole (11%) was at no risk (Table 6). A substantial 67% of the boreholes were not conforming to the WHO standards of 0 E. coli in potable water.

Total hardness had a strong and significant correlation with turbidity, Calcium, Chlorides, Magnesium and conductivity (Table 8). Conductivity was significantly correlated to Fluoride, Calcium and magnesium. Chlorides were significantly correlated to Fluorides (r = 0.68), Iron was correlated to Calcium and Chlorides, Nitrates were significantly correlated to Sulphates (r = 0.78).

Total water hardness above recommended limits is stated as a major etiological factor causing cardiovascular disorders, diabetes, reproductive failure, neural diseases, and renal dysfunction (Subba Rao 2006; Sengupta 2013). In Gweru, seventy-eight percent of surveyed boreholes had hard or very hard water which is a health risk to the consumers. In addition, hard water use in bathing and washing requires higher volumes of soap to lather compared to soft water (World Health Organization 2011). The source of water hardness are naturally occurring rocks, felsites and porphyries; rich in magnesium, calcium, sodium and potassium. The metal ions Mg²⁺ and Ca²⁺ weathered from the rocks are responsible for water hardness. The ions concentrations were significantly high and correlated to water hardness (Table 8). These findings reinstate the notion that of the 1 in 8 people who lack safe drinking water a substantial percentage consume hard water (WHO & UNICEF 2012).

To reduce the risk associated with hard water, users should consider boiling the water as a primary precaution to get rid of temporary hardness. For secondary treatment options, there is need to further understand the nature of hardness; whether it is temporary or permanent hardness.

The iron guidelines provided for by WHO are not based on the health risk but on the aesthetics of drinking water. Higher concentrations, as recorded in 56% of the sampled boreholes, are responsible for bad taste, discolouring of water, bad smell, corrosivity (World Health Organization 2011). High concentrations of Iron could be related to the geomorphology and geology of the area; felsites rocks rich in iron especially the mafic rocks which can be fine grained thus high solubility and concentrations in ground water. Although there are high concentrations of iron in the drinking water the pH level recorded is relatively high (Table 3), suggestive of the presence of the non-ferrous form (Fe3⁺). Non-ferrous form is not bioavailable and is not easily absorbed in the gastrointestinal (GI) tract (Hallberg & Hulthén 2000; Hurrell & Egli 2010; Merrill et al. 2010; Merrill et al. 2012). However, low absorbance is not a cushion to the unknown effects of high iron concentrations because there is a potion that will be added by the dietary food, increasing the daily intake of Iron. In addition, once the non-ferrous form is in the acidic digestive system it can be reduced to the ferrous iron (Aster 2007).

The city council should consider reducing the iron concentrations using the following methods: First, an inexpensive biosand filtration at house hold or at borehole catchment level; this method is reported by Karakochuk et al. (2015) and Murphy et al. (2010) to successfully remove 98–99% and 99% of iron from groundwater, respectively. Second, water storage prior to use, that is, giving water settling time so as to increase likelihood of Fe²⁺ conversion to Fe³⁺ (Murphy et al. 2010).

Nitrates concentrations higher than the recommended limits cause infant methemoglobinemia (Bruning-Fann & Kaneene 1993). Only one borehole did not conform to the WHO limits and is located in one of the oldest high density suburbs in the city. Sewer leakages in the area is likely to be responsible for high nitrates levels recorded. The sewer system has outlived its design lifespan and is servicing more people than its design capacity. The borehole water is susceptible to nitrate loads associated with underground and aboveground seepage of waste water into the ground water. In addition, complex interactions of aquifers and groundwater movement may be responsible for import of nitrates from agricultural lands in neighbouring areas (Wakida & Lerner 2005). For example, Mkoba 13 is surrounded by areas under intensive urban agriculture. The residence practice rain fed agriculture on open stands that are yet to be built on or areas left for strategic reasons by the municipality. The average area under agriculture per urban farmer is 0.5 ha (Rakodi 1995). Of the urban farmers in surrounding areas, 65% are reported to over apply fertilisers to maximise yields (Sammie et al. 2014).

The fluoride concentrations in all the sampled boreholes were within the WHO limits. However, the concentrations were below 0.5 mgl⁻¹ which is reportedly the minimum required to protect people from dental problems (Ozsvath 2009). Generally, intake of 0.5–1.0 mgl⁻¹ is considered beneficial to human health; in the production and maintenance of health bones (Ayoob & Gupta 2006; Chilton et al. 2006; Mohapatra et al. 2009). In South Africa, the South Africa Dental Association recommends consumption of water with fluoride concentration in the range of 0.7–1.5 mgl⁻¹ to reduce level of tooth decay (SADA 2014). Concentrations of <0.5 mgl⁻¹ increases the risk of dental caries (Ozsvath 2009). In Gweru urban, only 22% of the boreholes had fluoride concentrations above the lower safe limit. This is risky given that Zimbabwe like other African countries has not set low and high empirically tested threshold levels on which the extremes of fluoride concentrations can be a problem or cause dental fluorosis (NRC 2006).

Ground water, in most cases, contains high concentrations of fluoride which causes dental and skeletal fluorosis with such cases recorded in China (Guo et al. 2007; Gao et al. 2013), Sri Lanka and Ethiopia (Rango et al. 2012) among others. In this study, the low concentration of fluorides can be attributed to two factors. First, the neutral to alkaline pH of the water which prevent dissolution of fluorides (Subba Rao 2006). Second, high calcium content which may lead to formation of CaF₂ (Subba Rao & John Devadas 2003). This is supported by the significant correlation between calcium and fluoride (p = 0.015).

Presents of coliform bacteria in water is used as an indicator of the general microbial conditions of the water and sub-groups may indicate the risks associated with use of such water. The risk posed to the Gweru residents by using ground water for portable purposes ranges from low to intermediate risk (Table 6). Most research done on water quality in Africa highlights the presence of coliform bacteria Total coliform bacteria indicates ‘All facultative anaerobic, gram-negative, non-spore-forming, oxidase-negative, rod-shaped bacteria that ferment lactose to acid and gas within 48 h at 35 °C or members of Enterobacteriaceae which are β-galactosidase positive’ (APHA 2012). They are common in the environment and are not explicitly harmful. Their presence however in drinking water indicates the probable occurrence of other pathogenic microbes (Cabral & Marques 2006). Faecal coliform is a subgroup of total coliform which is a thermo-tolerant bacteria that can grow at temperatures around 42 °C (Leclerc et al. 2001; APHA 2012).This subgroup is important in that most of the diseases causing bacteria e.g. vibrio cholerea and salmonella are spread in water contaminated with faecal matter (Grabow 1996).

Sixty-seven percent of the boreholes indicated presents of E. coli, which is consistently found in faeces of human beings and other animal (Tallon et al. 2005). E. coli is a subset of the thermotolerant coliforms with the enzyme β-glucuronidase and produce indole from tryptophan (Tallon et al. 2005). E. coli presents in sampled boreholes in Gweru is associated with pollution by sewage leaking into the groundwater. In addition, residents do their laundry including children nappies within 20 meter radius of the borehole. Safe laundry infrastructure was not included in the designs for ground water supply and can be a contributing factor to E. coli loads in the water. Although the presence of E. coli indicates faecal contamination and health risk, not all strains of E. coli cause diseases or pose a health risk, most E. coli are commensal bacteria of the GI tract (Salyers & Whitt 2002).

The research advances the need to carry out ground water quality tests in urban areas as a way of minimising the health risk associated use of groundwater. It was noted that 80% of the tested water quality parameters were within WHO guidelines for drinking water. Only Iron, nitrates, total hardness and E. coli load were above recommended limits set by WHO. Unless treated, it is very risky and not recommended for residents in Senga, Mkoba 20, Mambo, Mkoba 19, Riverside and Mkoba 13 to use borehole water for potable uses. It is therefore recommended that the city or households chlorinates the borehole water to reduce the health risk associated with E. coli. There exist strong and significant correlations between some water quality parameters, however, there are other parameters that were significantly correlated but with a weak correlation which suggest the possibility of their correlations to be strong if the analysis was carried out on short time/seasonal intervals.

Future research need to focus on heavy metal concentrations, temporal variability of ground water quality and seasonal correlations of water quality parameters.