Assessment of groundwater quality along cemeteries and associated potential health concerns in Dar es Salaam, Tanzania

Groundwater is the most dependable water source, serving over 50% of the global population in sustaining the livelihood of urban dwellers (Turajo et al. 2019) and continues to be the backbone of water and food security in various parts of the world (Najafzadeh et al. 2021).The overutilization of groundwater sources has also been held responsible for the variability and scarcity of surface water (Maghrebi et al. 2021; Najafzadeh et al. 2021). Though groundwater is considered to be the most resilient drinking water source in Africa, pollution resulting from poor waste management practices is a growing global challenge (Wang & Hao 2012). Such pollutions are attributed to rapid population growth, poor infrastructure and expansion of unplanned settlements, which affect public health and cause economic and social consequences (Żychowski & Bryndal 2015; Kandoli et al. 2019). Among the urban challenges affecting groundwater quality, especially in developing countries, include poor solid waste disposal practices, locations of dumpsites, onsite sanitation systems, urban agriculture and industrial effluents, yet little attention has been given to cemeteries as one of the possible source of groundwater pollution (Turajo et al. 2019; Maghrebi et al. 2021).

A cemetery is a place for institutional funeral practices, which has a special attention and significance for both the dead and the living (Guttman et al. 2012; Turajo et al. 2019). It is a place for depositing and transforming dead bodies and thus a historical memory of collectivity (Turajo et al. 2019; Egbimhaulu et al. 2020). Previous studies have indicated the potential of water resource pollution from cemeteries causing public health concerns (Zume 2011; Guttman et al. 2012; Turajo et al. 2019). Human corpses contain 50% water, 15% bones and about 35% organic substances (Kandoli et al. 2019). Thus, the decomposition of the dead bodies in cemeteries can leach organic, inorganic and trace elements including nitrogen, phosphorus, calcium and sodium into groundwater, causing negative effects to the environment and humans (Vaezihir & Mohammadi 2016; Trang & Luan 2018). Also, some toxic metals released from embalming and burial practices, varnishes, sealers, preservatives, metal handles and ornaments used on wooden coffins may be released into groundwater (Jonker & Olivier 2012; Idehen & Ezenwa 2019). It takes about 15–25 years for human corpse to decompose completely into skeletons (Kandoli et al. 2019).

Burial methods including individual or mass graves, depth of grave, material of coffin, soil properties, topography, geological formation, water table are among the key factors on the groundwater contamination (Kandoli et al. 2019; Turajo et al. 2019). After a period of time, materials and corpses would degrade under the surface and slowly leach into the groundwater systems (Vaezihir & Mohammadi 2016). Results from previous studies found cemeteries to be similar to municipal solid waste dumpsites and thus become a pollution source to groundwater causing danger to public health (Kandoli et al. 2019; Silva et al. 2019). Some studies indicate that some old cemeteries established at distant locations from cities are now part of the cities surrounded by residential buildings due to the population growth and encroachment of land especially in unplanned settlements (Oliveira et al. 2013; Egbimhaulu et al. 2020) and urban land development (Turajo et al. 2019). It is important when planning locations of cemeteries that adequate site investigation and risk assessment be undertaken (Turajo et al. 2019).

WHO has recommended that the distance between cemeteries and water sources used for portable water supply be at least 250 m and the burial site be at least 30 m away from any other water course or spring and at least 10 m from any field drain (Vaezihir & Mohammadi 2016). In major cities including Dar es Salaam, which is among the fastest-growing cities in the world, it is difficult, almost impossible, to obtain land dedicated for burial activities, due to high population and congestion of houses in unplanned settlements, and it is anticipated that cemetery sites may not be found and will become a global challenge (WHO 1998). Proper location of cemeteries is of great importance, if located in porous soil such as gravel and sand, movement of leachate is rapid and it will mix easily with groundwater under the site (WHO 1998). If the site has a steep hydrogeological gradient, which elevates the groundwater flow velocity, such distances may be greater than those recommended by WHO. In most cases, most of old cemeteries were established without carrying out hydrogeological investigation, which might affect water resources. Thus, continuous groundwater monitoring in areas serving potable water along cemeteries is of paramount importance in environmental studies.

In areas with a high water table, higher burial depth of human corpses can increase the adverse effects of cemeteries on the quality of groundwater (Kandoli et al. 2019). On the other hand, in contrast to Islamic culture, human corpse burial in non-Islamic culture is conducted with coffins. Coffins are made of metals including Fe, Cu, Ni, Pb and Zn (Silva et al. 2019; Turajo et al. 2019), which are used for decoration and durability, thus during decomposition of corpses and coffins, these metals are also released, causing soil contamination and ultimately pollution of groundwater (da Silva et al. 2019; Kandoli et al. 2019). If a cemetery is situated in a porous soil such as gravel or sand, the movement of leachate resulting from decomposed dead bodies and coffin seepage can be fast and blends with groundwater underneath the site (Turajo et al. 2019). Elevated concentration of heavy metals in groundwater to the levels above recommended standards for human consumption have been reported by Barros et al. 2008; Silva et al. 2018; Turajo et al. 2019.

Groundwater located near cemeteries may be polluted by decomposition of human corpses not because of any particular toxicity, but by elevating the levels of naturally occurring organic and inorganic substances to a level significant enough to make groundwater unsuitable for any use (WHO 1998). Elevated concentrations of nutrients, particularly nitrate compounds, have been reported by various scholars (Vaezihir & Mohammadi 2016; Nguyen & Luan 2018). Compared to the developed world, where environmental regulations are more stringent and burial practices tend to be highly regulated (Zume 2011), in developing countries, including Tanzania, environmental laws are somewhat ambiguous and not fully enforced. Previous studies done in the study area indicated that groundwater is prone to pollution resulting from anthropogenic activities, sea water intrusion, onsite sanitation systems, weathering of silicate minerals and dissolution of calcite and dolomite (Mtoni et al. 2013; Sappa et al. 2015). Boreholes along the coast had higher concentrations of Cl⁻ and Na⁺ than areas located further inland (Mtoni et al. 2013). The quality of groundwater in Dar es Salaam city is further altered due to hydrogeological formation, sand soil dominates most parts of the city with high water table. Most cemeteries in Dar es Salaam city are located in unplanned settlements where access to piped water supply is very limited, and hence groundwater is the most dependable water source in the study area. Unfortunately, no study has been done to investigate the impacts of cemeteries on groundwater quality, perhaps due to traditional spiritual and religious distress. Thus, assessing the implications of cemeteries located near boreholes on the quality of groundwater used for domestic purposes is a first step to raise environmental awareness among urban residents, urban planners and decision makers, hence the need of this study.

The study area is located in Dar es Salaam City, which is situated at 6 °48′ S, 39 °17′ E on a natural harbor of the coast Africa. It is Tanzania's largest city, the location of a major port and industrial centre, and the city is home to about 10% of the country's population, which is estimated to be about 60 million people (Rosen 2019). The case study involved cemeteries located within the city of Dar es Salaam in five different wards, namely Manzese, Mwananyamala, Sinza, Buguruni and Ukonga. These are public cemeteries, which have been in operation for some years, and buildings are located in the vicinity of cemeteries and because of unreliable water supply from the water supply authority, households have drilled private boreholes for domestic purposes including drinking and cooking. Selected cemeteries are located in unplanned settlements except in Sinza ward, which is a planned settlement. Residential houses are very close to all cemeteries and some boreholes were drilled between 6 and 40 m from the existing burial sites, with the cemeteries located in Buguruni ward are very close to boreholes (S1–S9) between 6 and 20 meters.

The geological formation of the study area consists of Quaternary and Neogene deposits (Mtoni et al. 2013). The lithological logs obtained from groundwater drilling reports showed that the study area is mainly characterized by alternating layers of sands, gravel and coral limestone of various degree of weathering, also reported from previous studies (Mtoni et al. 2013). Sand aquifers are the most important in the city for supplying groundwater, which is also prone to pollution due to its high permeability. The selected unplanned settlements are characterized by high population density, poor water supply infrastructures, poor road accessibility and poor waste management practices. Thus, limited access to water supplied by Dar es Salaam Water and Sewerage Authority (DAWASA) has caused most of residents in the study area to drill boreholes in their residential compounds. Some of boreholes in selected study area are in the vicinity of cemeteries making them prone to pollutions, hence the interest of this study.

Sampling of groundwater depended on the availability of boreholes located in the vicinity of cemeteries and willingness of borehole owners to participate. A preliminary survey was conducted in sampled wards to establish households with boreholes located within 40 meters of cemeteries. Sampled boreholes were those used for domestic purposes including drinking and cooking. GPS coordinates were recorded and a spatial distribution map was prepared as shown in Figure 1. A total of 23 boreholes were sampled, involving 6 boreholes from Manzese, 9 boreholes from Buguruni, 3 boreholes from Ukonga, 1 borehole from Mwananyamala, 3 boreholes from Sinza and 1 control borehole (S13), which is located away from cemeteries and no potential source of pollution nearby this borehole was observed. Before groundwater sampling, the boreholes' water was purged for two minutes to remove stagnant water in the casing/screen. Samples were then collected in 1 litre polyethylene bottles which were sterilized and rinsed using respective water samples prior to sampling. After groundwater sampling, all samples were kept in a cool box, packed with ice and transported to Ardhi University in the School of Environmental Science and Technology where all samples were kept in the refrigerator at 4 °C before laboratory analysis.

Physical parameters including temperature, total dissolved solids (TDS), EC and salinity were analyzed at the site using electrode meter, model HACH Sension 378 meter while pH was measured using direct measurement method, i.e. digital pH meter. Chemical parameters including, NH₃-N, NO₃-N, PO₄²⁻, SO₄²⁻ were analyzed using a UV-visible DR/4000 Spectrophotometer whereby Nessler, cadmium reduction, ascorbic acid and SulfaVer ^®4 methods, respectively were used as described in APHA (2017). The concentrations of ammonium and nitrate ions were obtained by using conversion method. All analytical works were conducted in the School of Environmental Science and Technology laboratory at Ardhi University, Tanzania.

Data analysis was carried out using descriptive analysis, which established means and standard deviations while Univariate and multivariate analysis were used to establish the correlation of parameters and significance difference between analyzed parameters. Multivariate data analysis was carried out using Paleontological Statistics (PAST) software package, version 3.08 developed by Palaentologia Electronica (Hammer et al. 2001) to evaluate the relationship between various physicochemical parameters, with statistical significance set at p < 0.05. Factor reduction using Principal Component Analysis (PCA) with Varimax rotation were used to reduce the large multi-dimensional dataset to a small number of new variables that accounted for at least 75% of the total variance. PCA is capable of identifying a subgroup of variables among a large group of variables which describes the main variability of the output in a multivariate regression (Najafzadeh et al. 2021). Relevant components were considered to be those with eigenvalues higher than 1. Hierarchical cluster analysis (HCA) using average linkage between groups was used to assess the similarities and differences between sampling sites and identify possible patterns in distributions of measured data. Cluster analysis and PCA were also used to assess the similarities among parameters analyzed. ANOVA-one factor was used to establish significance difference set at p < 0.05. Pearson correlation was also used to establish linear correlation among analyzed parameters.

Results of groundwater quality in terms of EC, TDS, salinity, SO₄²⁻, NO₃-N, NO₃⁻, NO₂-N, NO₂⁻, NH₃-N sampled alongside different cemeteries in Dar es Salaam city are summarized in Table 1 and compared with local and international drinking water quality standards. The quality of groundwater varies across sampling points. pH of groundwater along cemeteries ranged from 4.88 to 7.30 indicating slight acidic water especially in sampling point S4 with pH of 4.88 ± 0.18 and S13 with pH of 5.74 ± 0.15, while the rest of boreholes were nearly in the neutral condition of pH ranging from 6 to 7. A study done by Mtoni et al. (2013) showed that pH of groundwater in Dar es Salaam ranged from 5.7 to 8.7 while another study done by Mdoe & Bucheishaija (2014) indicated that pH of groundwater in selected unplanned settlements ranged from 4.5 to 7.7. The recommended pH Tanzania drinking water quality and WHO standard is 6.5–8.5. These results indicate that cemeteries have no implications on the pH of groundwater as revealed by similar study (Fineza et al. 2014). Acidic condition promotes solubility and mobility of metallic ions in water (Egbimhaulu et al. 2020). Findings of this study indicated that almost all water samples were saline as revealed by the concentration of salinity above 1‰ WHO recommended standard. High salinity can cause maintenance cost of water supply pipes (Egbimhaulu et al. 2020), caused by the damage from the saline water (Delgado et al. 2016).

Electrical conductivity (EC) ranged from 469.5 μS/cm to 2,852.33 μS/cm while TDS ranged from 317.03 mg/L to 1,543 mg/L indicating presence of dissolved ions in groundwater. About 16 boreholes equivalent to 69.6% of sampled boreholes had EC value above 1,500 μS/cm recommended WHO standards. A higher value of EC in groundwater indicates the quality of water to be saline and higher EC value is an indication of groundwater pollution (Fineza et al. 2014). The leachate from a decomposed body is characterized by high EC caused by salts containing phosphorus, nitrogen, Ca, Na, HCO₃⁻ and Cl⁻ (Żychowski & Bryndal 2015). Pearson correlation matrix between EC and TDS showed r = 0.94, indicating strong positive correlation as shown in Table 2 which is a typical relationship between EC and TDS.

The concentration of sulfate ranged from 15.1 mg/L to 195.5 mg/L, which is within the 400 mg/L Tanzania and 250 mg/L WHO recommended standards while the concentration of phosphate ranged from 0.61 mg/L to 6.68 mg/L whereby 9 boreholes equivalent to 39.1% had elevated concentration of phosphate greater than 2.2 mg/L recommended Tanzania drinking water quality standard. Phosphate and sulfate are considered to be among the components of human body. It is estimated that human body contains about 500 g of phosphorus (WHO 1998), thus the decay of a human corpse can introduce these pollutants into groundwater (Kandoli et al. 2019). Though the concentration of sulfate and phosphate are within permissible limits at some boreholes but the consequences of the cemetery to the groundwater should not be denied (Kandoli et al. 2019). Similar results were obtained in a study done by Fineza et al. (2014) which indicated slight concentration of phosphate and sulphate in boreholes located near burial sites.

The concentration of nitrate nitrogen ranged from 9.21 mg/L to 239.5 mg/L, so that 17 boreholes equivalent to 73.9% of sampled boreholes had elevated concentrations above the 50 mg/L WHO and Tanzania drinking water quality standards while the concentration of nitrate ion ranged from 40.76 mg/L to 1,058.5 mg/L and about 21 boreholes equivalent to 91.3% had elevated concentration of NO₃⁻ above 45 mg/L recommended Tanzania and WHO drinking water quality standards. Previous studies done along the coastal region including Dar es Salaam city revealed that the concentration of NO₃⁻ ranged from 0.04 mg/L to 435.4 mg/L (Mtoni et al. 2013) while another study done by Mdoe & Bucheishaija (2014) reported the concentration of NO₃-N ranged from 5 mg/L to 630 mg/L in squatter and non-squatter areas. The background nitrate concentration in Dar es Salaam is 10 mg/L (Elisante & Muzuka 2017). Thus, higher concentrations of nitrate nitrogen and nitrate ion in groundwater in the study area can be considered as evidence that cemeteries cause groundwater pollution that might lead to continuous pollution of the nearby water resources (Fineza et al. 2014; Vaezihir & Mohammadi 2016; Nguyen & Luan 2018). Also, previous studies done in the study area indicated that the majority of the boreholes had depth ranging from 40 m to 60 m, with average depth of 50 m (Mtoni et al. 2012), indicating that most of boreholes are drilled in a shallow aquifer that is prone to pollution. The current study involved boreholes with depth ranging from 45 m to 82 m (Table 1), with average depth of 61 m, which is not different from what previous studies reported, and regional groundwater flow is from west to east where the Indian Ocean is the discharge boundary (Mtoni et al. 2012, 2013).

Furthermore, the concentration of NO₂-N ranged from 0.01 mg/L to 2.17 mg/L, in which most of boreholes had nitrite nitrogen within recommended standards except for three boreholes that had a concentration above 0.5 mg/L recommended by WHO and Tanzania drinking water quality standards. The concentration of nitrite ion ranged from 0.02 mg/L to 7.12 mg/L whereby 7 boreholes equivalent to 30.4% had elevated concentrations above the 0.5 mg/L Tanzania and WHO recommended standard. Nitrate nitrogen (NO₃-N) is toxic to human beings due to its reduction to NO₂-N, which has higher affinity to hemoglobin (Sato et al. 2018). Also, the concentration of ammonia nitrogen ranged from 0.16 mg/L to 6.9 mg/L and about 11 boreholes equivalent to 47.8% of those sampled had an higher concentration of ammonia nitrogen than the 0.5 mg/L Tanzania recommended permissible limit in drinking water.

Elevated concentration of nitrogen compounds can be attributed to decomposition of the human body as nitrogen is among the main ingredients of human body proteins and some amino acids, so that the human body contains 1,800 g of nitrogen (WHO 1998). Thus nitrogen compounds can enter groundwater via leachate formed after decaying of the human body (Kandoli et al. 2019). These results coincide with results obtained from a similar study (Fineza et al. 2014), which also concluded that the presence of ammonia and nitrates can be considered as an evidence of pollution from cemeteries.

Results from the Pearson correlation coefficient matrix indicated that most water quality parameters had weak to moderate correlation signifying a non-significant relationship between the considered boreholes and analyzed parameters, which may be associated with various anthropogenic activities including the use of onsite sanitation systems in the nearby communities, poor solid waste disposal facilities and pollution from cemeteries (Turajo et al. 2019). A study done by Mtoni et al. (2013) along the coast region in Dar es Salaam showed that SO₄²⁻ and NO₃-N had weak correlation with r = 0.164 while findings from this study indicated a moderate correlation yielding r = 0.44 indicating the possible contribution of cemeteries to groundwater pollution sources. Similar observations have been made from related studies in which a weak correlation was associated with anthropogenic activities such as onsite sanitation system and agricultural activities (Kandoli et al. 2019). Electrical conductivity (EC) showed a strong correlation with TDS and salinity as well as NO₂-N with r = 0.94, 0.93 and 0.61 respectively as shown in Table 2. Groundwater contaminated with leachate from decomposed bodies contains elevated concentration of EC. Such leachate contains 60% water and 30% salts in the form of ions containing NO₃-N, PO₄³⁻, Ca, Na, HCO₃⁻ and Cl⁻ (Żychowski & Bryndal 2015), which possibly elevated the value of EC in sampled groundwater. Results from anova-single factor revealed that, there were significant differences between analyzed parameters in sampled boreholes, p < 0.05, which is in agreement with observations made that some boreholes had higher concentrations than recommended standards.

Results from PCA indicated that principal component 1 (PC-1) accounted for 96.5% of variance as shown in Table 3 which is explained by the values of electrical conductivity as summarized in Table 4. Principal component 2 is explained by the contribution of total dissolved solids indicating that groundwater located nearby cemeteries consists of dissolved ions, which contribute to significant variances in groundwater quality and health risks. Values of EC provide an assessment of total dissolved ions in water, which is contributed to by the presence of sodium, chlorides, sulfides and carbonates (Mjemah et al. 2012), the concentration of these ions in sampled groundwater are not covered by this study. However, previous studies in the case study revealed high concentration of various ions and groundwater were dominated by Na-Cl, Ca-HCO₃, Na-Ca-Cl and Na-HCO₃ water types (Mtoni et al. 2013). Thus, PC-1 indicates that groundwater used along cemeteries might have high concentrations of ions, which elevated the concentration of EC and accounts for 96.46% variances.

The results from hierarchical clustering using single linkage by applying similarity index of Euclidian indicated that initially two groups of groundwater samples were formed with S13 having a distinctive group as shown in Figures 2–4 indicating that a borehole used as a control is different from the rest of the boreholes. At about 450 distances, only three major clustering groups were formed with a pair of S18 and S20, S13 and a cluster of the remaining sampling points. At closer distance more clusters were formed, i.e. from 200 distances i.e. S3 and S4, S8 and S10, S15 and S16, S21 and S23 and some sampling points standing alone (S2, S12, S13, S14, S17) as shown in Figures 2 and 4. These clusters have characteristics in common, for example the value of electrical conductivity, concentration of sulfate, nitrate nitrogen and nitrite nitrogen are associated within those clusters signifying that the quality of boreholes across sampling locations are related (Figure 2) and possibly shares the most common source of pollution.

The concentration of organic and inorganic pollutants in groundwater may be increased by anthropogenic activities including cemeteries. The results of this study indicated that cemetery may have increased most of the analyzed physical-chemical parameters including EC, NO₃-N, NO₂⁻N, NH₄-N and PO₄³⁻, which were above WHO and TZS recommended standards in some boreholes. About 91.3% of 23 sampled boreholes had an elevated concentration of NO₃⁻ above permissible limits. Also, there was weak correlation among most of analyzed parameters except for TDS-EC and EC-salinity with r = 0.94 and 0.93 respectively, indicating potential multiple pollution sources which might be associated with the use of onsite sanitation systems in the study areas. The results of this study give an alarm that cemeteries might contribute to groundwater contamination; hence groundwater accessed in areas alongside cemeteries used as portable water sources should be continually monitored. Therefore, there is a need for future study to conduct research in the cemeteries by focusing on the elements that are from dead body, which are not associated with other sources such as formaldehyde, which is commonly used as embalming substances and pharmaceuticals. Also, this study recommends more studies on the impacts of cemeteries on the environment including establishing concentration of heavy metals, major ions and contributions of burial practices such as the use of coffins for non-Islamic practices, since coffins contribute to heavy metal pollutants along with decay of human corpses. Further study should establish the implications of the distance from cemeteries to boreholes as WHO standards recommend at least 250 meters from any water source used as potable water supply.

The author acknowledges support provided by Mr Robert Masanja and Andrew Ndomondo during field work, laboratory analysis provided by Mr Addo Ndimbo and Ramadhani Mbulume are also treasured. Conducive research and working environment provided by Ardhi University is highly appreciated.

The author is not affiliated with or involved with any organization or entity with any financial interest or non-financial interest in the subject matter or materials discussed in this paper.

This research received no specific grant from any funding agency in the public, commercial or non-profit organization.

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