Seasonal subsurface water quality variation of physiochemical and bacteriological characteristics in Kamutwa-Kigali, Rwanda

The quality of subsurface water determines whether it is suitable for drinking, irrigation, or industrial uses. Subsurface water is the largest supply of drinking water due to its extended retention time and the natural filtering effect of earth material (Zhu et al. 2020). In different countries, subsurface water serves as the primary source of water supply for the majority of their sectors. Due to the discharge of contaminants from various sources such as natural and anthropogenic activities, the subsurface water will be susceptible to contamination (Li et al. 2021).

The changes in climatic conditions, water residence time, aquifer materials, and soil inputs during water percolation or infiltration will also be the main causes of changes in subsurface water quality (Kumar et al. 2009). Poor water quality affects the well-being of humans and plant growth (Lin et al. 2022). One of the main causes of groundwater pollution, particularly in developing countries like Rwanda, is the use of uncontrolled fertilizers, manure in agriculture, and domestic pit latrines, where domestic sewage contributes to groundwater pollution (Kassegne & Leta 2020; Akhtar et al. 2021).

The water quality data of some existing usable groundwater resources are not available and there is no permanent or temporary monitoring of their quality changes due to the climate factors or other factors (Ntajal et al. 2022). Understanding the quality of subsurface water is essential for establishing its suitability for household, agricultural, and industrial use. Many aspects must be considered before discussing subsurface quality (El-Mostafa et al. 2014). Using the physicochemical and bacteriological metrics for assessing water quality gives an accurate impression of the contamination condition of the groundwater body (Traoré et al. 2023), which helps to assess the chemical status and pollution levels of the aquifer (Subba Rao et al. 2022).

Groundwater from boreholes, shallow wells, and springs is the most frequent source of drinking water in Rwanda's small towns and rural areas. In many sections of the country, rural populations use groundwater for residential and drinking purposes without strict water quality monitoring and treatment (Nayar & Patel 2021).

The main objective of this current study is to assess the quality of groundwater. For this purpose, physicochemical and bacteriological parameters have been assessed. Finally, the water quality index was calculated. Every year, more than 2 million people die worldwide from diarrheal infections, with poor sanitation and unsafe drinking water accounting for approximately 90% of deaths and disproportionately harming children (United Nations 2016). Poor drinking water quality causes over 50 different diseases, accounting for 80% of diseases and 50% of child mortality worldwide. However, water contamination causes diarrhea, skin ailments, malnutrition, and possibly cancer, among other problems that are associated with water pollution (Hardi et al. 2023).

Previously, no study has been done on the water quality status of the Kamutwa subsurface. Hence, it is paramount to evaluate the subsurface water quality based on climatic conditions by measuring the threshold levels to secure the Kamutwa people and the neighborhood by controlling the factors associated with pollution and taking sustainable measures to protect this water resource.

The study area was located in Rwanda, Nyarugenge district, Kigali city with a latitude of 1° 56′ 35″ S and longitude of 30° 4′ 45″ E near the wetland, where the water catchment area was not protected; and thus subjected to activities that can contaminate the aquifer. The area is characterized by floods during rainy seasons. The water is discharged by gravity and it is thought to be from recharge infiltration from a habitat hill. People around this site depend on this subsurface water for their daily activities without being aware of the level of pollution.

The study area experienced two rainy season regimes; March to May (MAM) locally known as the long rainy season, and September to November (SON) known as the short rainy season. Two dry seasons, June–August (JJA) locally known as the long dry season, and December–February (DJF) locally known as the short dry season on its annual cycle. Such terminologies are based on the amount of rainfall during the rainy season (Ilunga et al. 2004). Both seasons were considered in this study.

Samples were collected during rainy and dry seasons to understand how variations in climate might affect water quality.

The sampling process is scheduled from January to December 2022 on a seasonal basis. The raw water samples were obtained from a tap of subsurface water immediately at the site after allowing the water to flow for 5 min and sterilizing the tip of the tap with a lighter. On the sampling site, three types of samples (physicochemical and bacteriological) were collected in plastic bottles of 0.5 L capacity for laboratory analysis for each month. Three samples from each site were collected in sterilized plastic bottles and preserved properly to keep the bacteria alive until the bacteriological analysis. Bacteriological tests were performed within 6 h of collection to avoid the death or growth of organisms in the sample (Gebrewahd et al. 2020). An isothermal cooling box, pH meter, and conductivity meter were used for site measurements. Samples were preserved in the refrigerator at 4 °C in the laboratory to avoid any contamination. Blank and reagent solutions were used under the specified analysis. To test for coliform bacteria, samples were gathered in specific bottles within the laboratory.

The titration procedure relies on a calorimetric reaction. Initially, ammonium hydroxide–ammonium chloride buffer was used to raise the sample pH to above 10. Then, 0.1 g of the indicator dye was added as an indicator. When Ca²⁺ and Mg²⁺ are present, the pH reaches 10, and the indicator dye will make the sample turn red wine-colored. The sample is titrated against ethylene diamine tetra acetic acid (EDTA) until the color changes.

It is possible to quantify the separate ion concentrations (Ca²⁺ and Mg²⁺) that contribute to hardness by titration or other methods. The quantitative method of individual ions is a technique preferred by WHO (2017).

Alkalinity (carbonate (⁠⁠) and hydrogen carbonate (⁠⁠) ions were determined using a volumetric method (Omer et al. 2020). The amount of acid (such as sulfuric acid) required for the water sample to reach a pH of 4.2 is measured and reported as mg/L CaCO₃. All water sample alkaline chemicals have been consumed at this pH. Sulfuric acid that has been diluted (0.1 N or 0.02 N of H₂SO₄) is used as a titrant and a pH meter is used to determine the pH. 5 mg of CaCO₃ is contained in 1 mL of H₂SO₄, while 1 mL of 0.02 N H₂SO₄ has 1 mg of CaCO₃.

Measurement of electrical conductivity (EC) using a conductivity probe is a common method for the analysis of total dissolved solid (TDS) in water suppliers that detect the presence of ions in water samples (Xianhong et al. 2021). Depending on the type of water, conductivity measurements were multiplied by a factor to produce TDS values. High TDS concentration can also be measured gravimetrically, although this method excludes volatile organics.

Iron, zinc, and manganese levels in the water were measured by atomic absorption spectrometer using respective hollow cathode lamps (Anwar et al. 2023). Analysis of nitrate and sulfate was carried out using the HACH DR 5000 instrument by the procedures of the HACH manual.

It is determined by the presence of an organism that serves as a pollutant indicator, in this case, total and fecal coliforms were considered. The total coliform (TC) and fecal coliform (FC) colonies were counted after 24 h of incubation at 37 and 44 °C, respectively, using membrane filtration and membrane lauryl sulfate broth techniques. Tests were carried out with 100 mL of water aseptically filtered via a nitrocellulose filter. The filters were then placed on membrane lauryl sulfate broth for TC and FC. Yellow colonies in both TC and FC were counted using the colony counter (Montiel et al. 2023).

The mean values of all physiochemical and bacteriological parameters have been analyzed on a seasonal basis and compared with WHO standards as shown in Table 1.

Table 1

Seasonal mean values of physicochemical and bacteriological parameters and WHO standards

Parameters/Seasons .	pH .	EC .	TDS .	TH .	Fe2+ .	Alkalinity .	.	Mn2+ .	.	Zn2+ .	Turbidity .	TC .	FC .
.	.	μS /cm .	(mg/L) .	(mg/L) .	(mg/L) .	(mg/L) .	(mg/L) .	(mg/L) .	(mg/L) .	(mg/L) .	NTU .	(cfu/mL) .	(cfu/mL) .
Dry	6.53	750.41	377.58	78	0.05	53.16	4.44	0.14	9.94	0.127	0.515	0	0
Wet	6.77	605.66	304.16	54.08	0.03	56	9.85	0.19	24	0.125	0.915	0	0
WHO	6.5–8.5	NA	1,000	200	0.3	NA	50	0.4	250	0.3	5	0	0

Parameters/Seasons .	pH .	EC .	TDS .	TH .	Fe2+ .	Alkalinity .	.	Mn2+ .	.	Zn2+ .	Turbidity .	TC .	FC .
.	.	μS /cm .	(mg/L) .	(mg/L) .	(mg/L) .	(mg/L) .	(mg/L) .	(mg/L) .	(mg/L) .	(mg/L) .	NTU .	(cfu/mL) .	(cfu/mL) .
Dry	6.53	750.41	377.58	78	0.05	53.16	4.44	0.14	9.94	0.127	0.515	0	0
Wet	6.77	605.66	304.16	54.08	0.03	56	9.85	0.19	24	0.125	0.915	0	0
WHO	6.5–8.5	NA	1,000	200	0.3	NA	50	0.4	250	0.3	5	0	0

NA, not applicable.

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The lower value represents the acceptable /desirable limit and the higher value represents the permissible limit (WHO 2017).

From Table 1, the seasonal mean pH of Kamutwa subsurface water was generally found to be slightly acidic (below pH = 7.0) with pH ranging from 6.53 to 6.77 depending on dry and wet season, respectively. This water presents no contaminants (organic or inorganic) that can change the pH extreme values and mean average. Thus, the water presents a normal range value for domestic use within acceptable and the permissible limit in both seasons (WHO 2017).

The presence of suspended matter in water, such as clay, silts, finely organic and inorganic matter, and other microscopic organisms during the wet season, caused the highest turbidity. Rainfall can mobilize and transport sediments and pollutants and dissolve them in the water along with small clay mineral particles. Seasonal rainfall, on the other hand, can have an impact on the complex relationship between discharge and turbidity in urban watersheds (Rafiee 2020).

The highest mean TDS and EC values were measured in the dry season (JF and JJA) over the duration of the study. This might be as a result of the higher temperatures experienced during the dry season, which promoted weathering, desorption, ion exchange capacity, and dissolution processes. Additionally, water evaporated and ion concentrations rose throughout the dry season. High conductivity values also indicate large concentrations of dissolved inorganic matter in ionized form (Pandey et al. 2020).

From Table 1, the alkalinity of the Kamutwa subsurface was found in the range of 53.16 and 56 mg/L. During the wet season, pH was higher than in the dry season indicating higher acidity. The seasonal average hardness of the Kamutwa subsurface water was varying between 78 and 54.08 mg/L for dry and rainy seasons. The values were acceptable and within the permitted range in both seasons (WHO 2017).

The highest TH value was observed in the dry season (JF and JJA) throughout the period of study. This could be caused by the higher EC and TDS that occurred during the dry season. TH was raised during the dry season because of the high number of dissolved inorganic compounds in an ionized state and the concentration of salts dissolved in water (WHO 2017). The primary factors that contribute to water hardness are dissolved calcium and magnesium, which are predominantly the byproducts of dissolved dolomite and limestone from rock and soil components. Because of the high levels of TH in the groundwater, human beings may get kidney stones and heart diseases (Mohammad et al. 2015).

The level of heavy metals presented in a specific area varies according to the type of activities that are abundantly dominant in that region (Wang et al. 2021). Water frequently contains trace amounts of metals, which are typically safe for human health. Metals can dissolve in water naturally when it comes contact with rock or soil, but they can also dissolve in water as a result of contamination.

Bacteria are one among the main issues causing water contamination (Singh et al. 2019). The experiments performed on bacteriological analysis during both seasons generally indicated that the samples analyzed were free from indicators of FC and TC and complied with standard levels (WHO 2017). Some researchers indicated that subsurface water could be polluted with fecal materials due to inadequate protection, position and distance relative to latrine houses, unhygienic practices at the source, and poor environmental sanitation (Takal & Quaye-Ballard 2018).

The WQI approach was used to assess water contamination. The WQI values were 11.31 and 12.050 with respect to dry and wet seasons. Both seasons have shown excellent water quality, and are well fit for human consumption (Table 2).

Seasonal variability of the water quality characteristics of the Kamutwa subsurface has shown that during the rainy season almost all parameters increased. The pH values reveal that the subsurface water was slightly acidic in nature. The number of heavy metals (iron, manganese, and zinc), TH, alkalinity, and nutrients (nitrate and sulfate) were increased during the wet season compared to the dry season and the analysis indicated that all parameters analyzed in both seasons are within the permissible limit. The WQI has been calculated, indicating that water quality in the study area was ‘excellent’. No pollution has occurred even if the WQI is high during the wet season. The non-presence of high levels of contaminants may be due to the recharge area that is far from its discharge and other protective practices done around the source. Kamutwa subsurface water is good for domestic and drinking purposes but periodic monitoring and inspection of water quality should be conducted to look at different plausible pollution.

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

The authors declare there is no conflict.