Spatial and temporal patterns in water quality analysis near effluent dump-sites in Harare, Zimbabwe

Water quality deterioration is a critical concern in urban and peri-urban areas. This crisis is often coined a governance challenge, as city councils fail to provide safe and adequate potable water (Mwedzi & Bere 2021). Africa has some of the highest water risk in the world with more than 400 million individuals lacking access to portable drinking water (Kanyangarara et al. 2021). Inadequate infrastructure, poor financing, incompetence, corruption, poor coordination and lack of participation are often cited as the factors contributing to this crisis. Urbanization further exacerbates water quality deterioration. The expansion of impervious surfaces reduces groundwater recharge while increasing surface runoff, which often carries pollutants into surface water bodies. Lake Chivero, the main drinking water source for Harare, Zimbabwe is polluted from various agricultural and industrial activities within the catchment and has been reported to be eutrophic since, recently (December 2024), the lake's cyanobacterial blooms were topical and were reported to have led to the death of wildlife in the Lake Chivero Recreational Park. This degradation of surface water diminishes its suitability as a drinking water source. Consequently, waterborne diseases have become a major concern in developing countries, accounting for approximately 70% of all diseases and causing an estimated three million deaths annually (Matsa et al. 2021). According to the World Health Organization (WHO 2011), more than 12,000 deaths that occur in Zimbabwe per year are attributed to water, sanitation and hygiene-related diseases or injuries.

In Zimbabwe, the failure of local boards to remedy these problems has led to increased water rationing problems. Nairizi (2017) indicated that water shortages have been experienced even when normal to above-normal rainy seasons would have been experienced. Water shedding has increased, impacting most suburbs, with 150 ML a day being shed from the normal 320. Consequently, there have been recurring reports of waterborne disease outbreaks in some of Harare high-density suburbs (Tanyanyiwa & Mutungamiri 2011; Chirisa et al. 2015). Residents have therefore resorted to underground water for their sustenance. Some residents from low-income suburbs have also resorted to shallow and unprotected wells, which pose public health risks. Furthermore, some of these high-density suburbs, namely Glen View, Budiriro, Kuwadzana, Kambuzuma, and Hatcliffe, are located near waste effluent disposal farm sites and farming areas. This proximity raises concerns about heavily polluted underground water supplies. Furthermore, construction of increased Blair toilets and breakage of wastewater pipes have added to the chances of underground water pollution, posing risks to public health and ecosystems.

Since the 1990s, Harare's low-income suburbs have experienced recurring outbreaks of cholera and typhoid (Nyaruwata 2020). Recurring typhoid spikes have been reported in 2010, 2011, 2012, 2016, and 2017 (Muti et al. 2014). A study by Takavada et al. (2022) in Harare's Glen View and Budiriro found that typhoid cases during the first week of the 2011 outbreak were clustered around specific boreholes. In September 2018, the Ministry of Health and Child Care of Zimbabwe declared a cholera outbreak and notified WHO that it was a result of underground water pollution. The most affected suburbs in Harare were Glen View and Budiriro.

Given the alarming rate of Harare's rapid urbanization, intermittent water supply, proliferation of dump sites, and inadequate waste management, ground water contamination poses significant public health risks. This study intends to establish the influence of season and location on contamination of high-density underground water sources in areas near effluent disposal farm sites in Harare suburbs. Answering the above-outlined objective will help in understanding the factors influencing water contamination and inform effective strategies for mitigating these risks and protecting public health. Furthermore, the findings guide policymakers, regulators, and stakeholders to develop evidence-based interventions and improve water quality management in Harare's suburbs.

We hypothesize that (1) there is spatial variation in the physical, chemical, and biological characteristics of underground water in boreholes near the city of Harare dump sites, (2) underground water parameters in boreholes near the city of Harare dump sites differ from the set WHO standards, and (3) season and location lead to variation in underground water contaminants in areas near effluent disposal farm sites in Harare high-density suburbs.

To investigate the relationship between water quality parameters and seasonal factors, data collection was conducted in two seasons, wet (February 2021) and dry (August 2021). Sampling was carried out at boreholes strategically located within the study area to ensure the representation of diverse environmental and site-specific conditions. At each borehole, on-site measurements were taken for water quality parameters using a handheld YSI Pro-plus Multi-Parameter Water Quality Meter (Xylem Inc, USA). These included conductivity, temperature, total alkalinity, turbidity, fluoride, phosphates, nitrates, free ammonia (NH₃), chlorides, and pH.

Water samples, 1 L per borehole, were collected following standard procedures designed to maintain sample integrity and prevent contamination. The borehole was running for 1 min before obtaining a sample, allowing the water to run so as to allow temperatures in the water stream to be equal from inside the borehole to the water coming out of the nozzle. The spouts of the borehole were cleaned and sterilized before sampling with 70% methanol to prevent contamination of samples. The sampling bottles were rinsed with the sample water prior to filling. Nitric acid was used to stabilize the samples at pH < 2. Samples were transported to the laboratory in pre-sterilized, sealed containers and stored at 4 °C until analysis.

Concentrations of calcium (Ca²⁺), iron (Fe²⁺), and manganese (Mn²⁺) were determined with a flame atomic absorption spectrophotometer (Varian Australia Pty Ltd, Victoria, Australia). For microbiological analysis, 100 μL of the sample was inoculated into sterilized plates with selective media, including thiosulfate-citrate-bile-sucrose agar for Vibrio cholerae, Salmonella-Shigella agar for Shigella, and eosin methylene blue agar for Escherichia coli. The plates were incubated at 37 °C for 24 h. Colonies with characteristic morphology on each selective medium were further identified using Gram staining, where bacteria were heat-fixed onto microscope slides, stained with crystal violet, iodine, and safranin, and observed under a light microscope at ×1000 magnification.

Data analysis was conducted using the R-statistical package, Canoco5, and Past 4.01 (Hammer et al. 2001). First, the Shapiro–Wilk test in Past 4.01 was used to assess the normality of all physio-chemical and biological parameters. Results showed that only six physio-chemical variables (pH, total alkalinity, conductivity, Cl, total hardness (TH), and Ca) met the normality assumptions. One-way Analysis of Variance (ANOVA) was then applied in R-Statistical software to determine if these parameters differed across distance, season, and area.

For the remaining eight physio-chemical and three biological parameters that did not meet normality assumptions, the Kruskal–Wallis test was employed to examine differences across distance from effluent disposal farm sites, season (dry and wet), and area. Additionally, the Wilcoxon rank-sum test (Mann–Whitney U test) was used to compare the means of physio-chemical and biological parameters with WHO standards.

To explore similarities in physio-chemical parameters across different distance categories, seasons, and areas, principal component analysis (PCA) was performed using Euclidean distances in Canoco version 5.

The values of environmental and biological variables recorded in six suburbs in Harare (Budiriro, Mabvuku, Hatcliffe, Kambuzuma, Kuwadzana, and Glenview) are summarized in Table 1. Notably, water quality parameters such as Cl, Fe, phosphates, nitrates, and total coliform levels exhibited significant seasonal variations (t-test, p < 0.05). Chloride and phosphates were higher in the dry season, while iron, nitrates and total coliforms were higher in the wet season. In contrast, Ca, conductivity, total alkalinity, TH, pH, Mn, turbidity, fluoride, Free NH₃, and temperature remained relatively consistent across seasons, showing no significant differences (p > 0.5). Notably, biological variables (plate count and total coliform) differed across seasonality, in which they were both higher in the wet season (t-test, p < 0.05). Furthermore, no Shigella or V. cholerae were detected at any of the sampling sites across all seasons. However, E. coli was detected at specific sites in Budiriro in the wet season and Kuwadzana in the dry season only.

The values of environmental and biological variables recorded in six locations in Harare (Budiriro, Mabvuku, Hatcliffe, Kambuzuma, Kuwadzana, and Glenview) are compared to the WHO standards. Unfortunately, only seven of the parameters of this study had the standards, as shown in Table 2. Notably, turbidity and Fe significantly exceeded the WHO standards (Table 2, t-test, p < 0.05). In contrast, water Cl, nitrates, fluoride, Mn, and free NH₃ were significantly lower than the WHO standards (p < 0.05). Furthermore, total coliform significantly exceeded the WHO standards (mean = 0.487, t-test, p = 0.021), as illustrated in Table 2.

This study reported seasonal variation in underground water quality with deteriorating water quality observed in the dry season compared to the wet season. However, iron, total nitrogen, and total coliforms were higher in the wet season. The high chloride and phosphates in the dry season were expected as the dry season often results in lower water levels, which can concentrate pollutants and lead to higher contamination risks (Isiuku & Enyoh 2020). On the other hand, iron levels in underground water are higher in the wet season due to increased reduction. During this period, the water table rises, and the increased water flow leads to the mobilization of iron-rich sediments and soils. As a result, iron oxides are reduced, releasing soluble iron into the water, thereby increasing iron concentrations in groundwater.

We observed higher amounts of nitrates in the wet season as expected. The elevated nitrate levels during the wet season can be attributed to the widespread use of nitrogen-based fertilizers in Zimbabwe's agricultural sector. These fertilizers, including ammonium nitrate, urea, and sodium nitrate, are commonly applied during the agricultural season (Nadarajan & Sukumaran 2021). As the rain falls, these nitrates are leached into the soil and eventually transported into groundwater, resulting in higher concentrations. The difference in seasonal concentrations of nitrates and phosphates can be explained by their different environmental behaviours. Unlike nitrates, phosphates tend to be more strongly adsorbed to soil particles and sediments, making them less mobile in water (Meng et al. 2014). However, during the dry season, phosphates can be released from soil particles and sediments through desorption processes, leading to higher concentrations in groundwater (Ebrahimi & Ojani 2024).

These results underscore the need for holistic management approaches that consider specific seasonal characteristics that influence water quality parameters. For example, during the rainy season, excessive sediment runoff can increase turbidity and nutrient levels, negatively impacting water quality (Gumbo 2011). Consequently, management strategies need to be tailored to address these seasonal variations. This might involve implementing targeted pollution control measures during specific times of the year.

Higher total coliforms observed in the wet season are consistent with other studies (Wuta et al. 2016; Murei et al. 2024). This is attributed to several causes including contamination due to higher infiltration rates, flooding of boreholes, and increased septic tank leakage. Such seasonal changes underscore the critical need for regular monitoring and effective management of Harare water supplies, as fluctuations in water quality can have significant implications for public health (Moyo et al. 2024). Seasonal changes in water quality parameters in Harare demand adaptive management strategies aligned with Zimbabwe's environmental regulations, particularly the Environmental Management Act (Chapter 20:27) and the Water Act (Chapter 20:24). These Acts provide a legal framework for addressing challenges such as pollution during the wet season and water scarcity in the dry season. Tailored approaches include runoff control, water conservation, and catchment protection, supported by regular monitoring and enforcement. By integrating these measures with existing policies, Harare can ensure sustainable water management throughout the year

The observation of E. coli in Budiriro and Kuwadzana is particularly concerning as this can indicate a possible presence of multiple harmful pathogenic microorganisms associated with faecal pollution which cause gastrointestinal distress. The presence of E. coli in areas previously affected by cholera and typhoid outbreaks (Chiniko 2019; Robert et al. 2021; Cheung 2024) highlights an urgent need for intervention to mitigate potential public health risks. The WHO recommends that drinking water must not contain faecal contamination or E. coli.

Notably, total coliforms, turbidity, and iron in this study exceeded the WHO standards. The combination of high total coliform levels and turbidity is particularly concerning, as it substantially heightens the risk of waterborne illnesses such as diarrhea, cholera, and dysentery, which are significant public health threats in many areas (WHO 2011; Matsa et al. 2021). High turbidity in drinking water can provide a conducive environment for the proliferation of microbial pathogens (Ricolfi et al. 2020). Elevated iron levels, although not a frequent cause of outbreaks, can also trigger gastrointestinal illnesses and lead to tissue damage over time (Hoko 2005). Excessive turbidity and iron not only pose health risks but can also create significant aesthetic issues concerning water quality, such as foul taste, unpleasant odour, and discoloration, rendering the water unpalatable for consumption (Karanfil et al. 2003). This finding underlines the urgent need for interventions to improve water quality in Harare, particularly regarding the management of turbidity and coliform levels, to safeguard public health and enhance the overall safety of potable water (Ndunguru & Hoko 2016).

Interestingly, distance from the effluent disposal farms did not emerge as a critical factor, with only a few sites responding haphazardly to the distance category variable. The PCA analysis revealed interesting results, where stronger correlations were found between water quality and biological parameters and specific sites, rather than with seasonality or distance from effluent disposal farms. This suggests that site-specific characteristics, such as local geology, soil type, and land use, play a more significant role in determining water quality. Past studies have demonstrated that E. coli is frequently detected in neighbourhoods such as Budiriro, Kuwadzana, and Glenview, where poor waste management practices, inadequate sanitation facilities, and close proximity to open drains contribute to water quality deterioration (Mhondoro et al. 2019; Takawira & Mbanga 2023). For instance, a study by Cheung (2024) identified elevated levels of E. coli in the Budiriro area, attributing this to the high population density and lack of proper sewage disposal systems.

This study reinforces ongoing environmental research (Dalu et al. 2011; Mugadza et al. 2021; Takawira & Mbanga 2023) indicating that E. coli levels in some hotspots have remained persistently high over time, revealing entrenched contamination. Thus, there is a need for a more comprehensive and coordinated approach involving government, civil society, and community members to combat water quality issues effectively. By adopting a site-specific approach, water quality can be more effectively managed and improved, ensuring safe and reliable drinking water for Harare's residents. Regular monitoring and proper waste management strategies are also crucial to prevent effluent disposal from impacting groundwater quality and public health (Hoko 2005).

In conclusion, underground water quality in Harare suburbs deviates significantly from WHO Standards, posing a significant public health concern. Although V. cholerae and Shigella were not detected in this study, the intermittent presence of E. coli in the Budiriro and Kuwadzana suburbs is alarming and warrants attention. This research highlights the critical role of seasonal fluctuations and site-specific characteristics in shaping water quality, underscoring the need for tailored approaches to underground water management. By acknowledging these factors, effective strategies to mitigate waterborne health risks and ensure a safer, more sustainable water supply can be ensured.

The authors would like to thank the City of Harare Health Department for permission to collect this data.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.