Heavy metals pollution in leachates and its impacts on quality of groundwater around Iringa municipal solid waste dumpsite.

doi:10.21203/rs.3.rs-1492095/v1

This paper presents the results on the heavy metals concentrations in leachates and on the quality of groundwater resources as impacted by leachates leaking from Iringa municipal solid waste dumpsite. The results on physicochemical properties such as pH, electrical conductivity (EC), and total dissolved solids (TDS) that were analyzed by using an electronic multiparameter analyser in both leachates and groundwater samples are also presented. Heavy metals (Fe, Pb, Cd, Cr, Cu, Ni, Mn, and Zn) were quantified using Atomic Absorption Spectrophotometer (AAS). Relatively higher electrical conductivity which ranged from 14090 ± 62.34 to 26700 ± 104.10 µS/cm and total dissolved solids (76930 ± 41.39–13360 ± 59.40 mg/L) than FAO irrigation water standard were recorded in all leachates. All leachates were identified to be alkaline in nature with pH ranging from 7.40 ± 0.02 to 9.10 ± 0.01; this signified older age of the Iringa municipal dumpsite which is at its methanogenic phase. Heavy metals analysis indicated no potential threat of Cd as its concentrations in all leachates were below the detection limit (< 0.01 mg/L). Meanwhile, the concentrations of Fe, Pb, Cr, Cu, Ni, Mn, and, Zn in all leachate samples were either above TBS guideline limits for wastewater reuse, or FAO standards for irrigation water. In all groundwater samples, total dissolved solids (446 ± 55.30–776 ± 52.34 mg/L) and pH (7.15 ± 0.02–7.6 ± 0.03) were within WHO acceptable range (pH = 6.5–8.5) for drinking water. Electrical conductivity (869 ± 89.64 to 1570 ± 105.50 µS/cm values) in three groundwater samples was above the WHO's desirable limit for drinking water. Whereas in some groundwater samples, the concentrations of Fe, Pb, Mn, and Zn exceeded WHO guideline limits for drinking water quality, implying that the water is unsuitable for human consumption without treatment. The results of Principal Components Analysis (PCA) in groundwater samples indicated that both geologic and anthropogenic processes could be the possible source of heavy metals pollution in the groundwater. However, the presence of heavy metals can be used as tracers of groundwater contamination in the study area. Moreover, water quality indices (WQI) for all groundwater samples ranged from 169.07 to 220.72 translating from poor quality (≤ 100 WQI ≤ 199.99) to very poor quality ((≤ 200 WQI ≤ 299.99). Therefore, the present study calls for the proper management of solid waste in the Iringa municipality to reduce further pollution of groundwater reservoirs through leachate percolation.

Solid waste

Leachate

Groundwater

Heavy metals

Iringa Municipal dumpsite

Despite other available options, the use of open dumpsite is the most adopted method for disposing of municipal solid waste, especially in low-income or developing countries (Saleh & Koller, 2019). The operation of open dumpsites is relatively economical, simple, convenient, and involve low technological barriers as compared to other solid waste disposal methods such as recycling, incineration, composting, and sanitary landfills (Agamuthu, 2013;). Globally, open dumpsites receive about 40% of the world’s waste generated and approximately four billion people in the world are served by open dumpsites (Mavropoulos & Newman, 2015. However, the challenge of using dumpsite as a way for disposing of solid waste is the formation of highly polluted leachates from disposed of solid waste (Maiti et al., 2016).

Leachate is a complex water-based solution that may constitute insoluble and dissolved substances such as inorganic and organic matters, and other xenobiotic organic compounds from physical and chemical, and biological (fermentative) processes (Bhalla et al., 2013; Peng, 2017). It is generated when rainwaters infiltrate through or out of waste deposits and dissolve the soluble components out of the solid waste materials. Leachates can also be produced during the dry season due to the co-disposal of liquid and sludge wastes with municipal solid wastes (Singh & Mittal, 2011). Leachates from solid waste dumpsites are normally highly polluted and potentially toxic. They are also known to contain a significant amount of heavy metals (Gamar et al., 2020) such as mercury (Hg), cadmium (Cd), arsenic (As), lead (Pb), chromium (Cr), nickel (Ni), and zinc (Zn). If not well confined, leachates laden with toxic heavy metals can migrate from open dumpsites to pollute nearby soil, groundwater and surface water bodies (Godwin & Oghenekohwiroro, 2016; Brennan et al., 2016). This may affect the quality of the environment and put human health at great risk of heavy metals exposure.

Tanzania is among the East African countries that face serious concerns and challenges on solid waste management (Nyampundu et al., 2020). Most municipal governments' capacities and facilities cannot sufficiently facilitate the management of rapid increasing solid wastes (Kalwani, 2010). Consequently, environmental, health and social concerns associated with improper solid disposal are prominent in many urban areas of the country (Yhdego & Kingu, 2016). The municipality of Iringa is no exception (Mkonda & He, 2017). It is one of the urban centres that have witnessed socio-economic and physical growth within the southern highlands zone of Tanzania (NBS, 2013). Recently, it is estimated that more than 48,180 metric tons of solid waste are generated per annum (equivalent to 132 metric tons per day). Nevertheless, the capacity of solid wastes collection with the Iringa municipality is only 30–54% in a day (IMC, 2017).

The Iringa municipality operates one official dumpsite located at a peri-urban ward of Kihesa-Kilolo which receives over 90% of solid waste. Since there are no properly engineered leachate confinement and treatment systems, and the dumpsite is situated on a moderately-slopping hill there is a possibility that leachates polluted with toxic metals may leak from the dumpsite and spread all over the soil surface and then enter the surface and groundwater bodies. Thus, there is a high chance for the Iringa municipal dumpsite to exert negative impacts on public health due to groundwater contamination by heavy metals. There is scant information regarding heavy metals contamination of groundwater around the Iringa municipal dumpsites in Tanzania. This underscored the need to assess the extent of heavy metals pollution in leachates from the Iringa municipal dumpsite and the suitability of groundwater use around the Iringa municipal dumpsite. The findings will elicit actions to stop further contamination of the groundwater resources and hence safeguard human health.

Chemicals and Reagents

The chemicals and reagents used were all of the analytical grades such as HNO₃ (65% w/w, 1.42 g/mL), HCl (37% w/w, 1.18 g/mL) (Laboratory equipment limited, Tanzania) and HClO₄ (70% w/w, 1.67 g/mL) 1000 mg/L stock standard solutions of heavy metals (Fe, Pb, Cr, Cd, Ni, Cu, Zn, and Mn) were purchased from Sigma-Aldrich Co. Ltd., UK. Deionized water from Geology Survey of Tanzania (GST). Certified reference materials (CRMs); NCS DC 273027(soil) and NCS CZ 27031 (Carrot) from China National Analysis Centre, China

Leachate Sampling

A sampling of leachates was conducted in Iringa municipal dumpsite which is an official dumping site of all solid wastes generated and collected within the urban centres of Iringa municipal (Fig 1). Leachate samples were collected during the wet season in March 2021 following heavy rains according to previously described procedures (APHA, 2005; Kanmani & Gandhimathi, 2013). Before sampling, all sampling equipment and plastic containers were washed with acid and rinsed with deionized water, and thereafter rinsed with sample liquids. Three (3) leachate samples were collected into 500 mL polyethene plastic bottles from each of the six (6) actual leachate streams appearing within the dumpsite. Therefore, a total of eighteen (18) samples of leachates were obtained and labelled accordingly as LS1, LS2, LS3, LS4, LS5, and LS6 to avoid confusion. In this study, some selected physicochemical parameters including electrical conductivity (EC), pH, and total dissolved solids (TDS) were analysed on-site by using one portion of each leachate sample. The remaining two portions of each leachate sample destined for heavy metals determinations were preserved by acidifying the samples to pH < 2 with concentrated 16.0 M HNO3 (65% w/w) (Victoria EPA, 2009). All samples were immediately transported in an ice-cool box to the Geology Survey of Tanzania (GST) laboratory, refrigerated at < 4 ^oC to stabilize heavy metals contents and then analysed within one week.

Groundwater Sampling

Groundwater samples were purposively collected during the wet season in March 2021. Therefore, groundwater sampling was undertaken in one borehole and three (3) residential wells that were close to the municipal dumpsite. Groundwater samples were collected in triplicate portions into 500 mL clean polyethene plastic bottles (Kwaya et al., 2020) from each groundwater source. So, a total of 12 samples were obtained and coded as GW1, GW2, GW2, GW3, and GW3 for Borehole, first well, second well, and third well, respectively. Coordinates as seen in Table 1, for each sampling point were recorded using a Geographic Positioning System (GPS). Likewise, one portion of each groundwater sample was used for on-site determination of pH, EC and TDS. The remaining two portions destined for heavy metals determinations were acid-preserved (pH < 2) to stabilize the metal contents, immediately transported in an ice-cool box to the GST laboratory, refrigerated at < 4 ^oC and then analysed within one week.

Table 1 Details of groundwater sampling

Sample code	Description	GPS location	Distance from dumpsite
GW 1	Borehole	7^o43^’20” S, 35^o43^’20” E	291.8 m
GW2	Open-well	7^o43^’22” S, 35^o45^’1” E	244 m
GW3	Open- well	7^o43^’21” S, 35^o43^’1” E	268.4 m
GW4	Open-well	7^o43^’18” S, 35^o43^’1” E	341.6 m

Preparation of leachate and groundwater samples

Samples of groundwater and leachate were prepared by the aqua regia acid digestion method according to the standard methods for the examination of water and wastewater (APHA, 2005) with modifications. The aqua regia was prepared by mixing 130 mL of concentrated 12 M HCl (37% w/w) with 120 mL of deionized water and then 150 mL of the resulting solution was added to 50 mL of 17 M HNO₃ (65% w/w) (Radojevic & Bashkin, 2006). Thereafter, 50 mL of each leachate sample and 20 mL of groundwater sample (pre-concentrated from 100 mL) were put into Pyrex Erlenmeyer flasks and pre-digested separately with 15 mL of aqua regia on a hot plate at 95 ^oC in a fume cupboard, until a clear solution was obtained. The digested mixture was then filtered through an acid-washed Whatman filter paper number 41 into a 50 mL standard volumetric flask and made to the volume by using deionized water before analysis by Atomic Absorption Spectrophotometer (AAS). All blanks were prepared from deionized water along with samples by using the same acid mixture and by following similar procedures used in digesting samples. Triplicate digestions were carried out for each type of sample to determine uncertainties in the procedures.

Sample analysis

In this study, an HI 2550 multiparameter analyser (HANNA Instruments Inc., USA) was used for on-site measurement and recording of pH, and EC in portions of the leachate and groundwater samples at 25^oC. The instrument was calibrated according to the manufacturer's instructions before it was used. Fe, Pb, Cr, Cd, Cu, Ni, Mn, Ni were analyzed in each digested samples solutions of leachates and groundwater using an Air/Acetylene-Varian SpectrAA-55B Flame Atomic Absorption Spectrophotometer (FAAS) (Varian Inc., Australia) available at GST laboratory, Tanzania. Before analysis, the instrument was calibrated using freshly prepared 1000 mg/L standard stock solution of each respective metal. The concentrations of each selected heavy metal in the digested sample was then determined directly using AAS. All samples were examined in triplicates to ascertain the reproducibility of the results. The detection limits of all selected heavy metals were 0.01 mg/L.

Calculation of groundwater quality indices (WQI)

Water quality indices (WQI) are tools used to determine water quality status. WQI helps to integrate all selected water quality parameters while comparing with the standards recommended by the government authorities to safeguard human health. Thus, WQI has been employed by many studies for such purposes (Khatita et al., 2017; Ozochukwa et al., 2019); El Baba et al., 2020). The relative weights (W_n) (Table 2) were calculated after assigning weights (w_n) to each of the eleven (11) selected parameters for study (Eq. 1). The weights used in this study were adopted from other studies (Guetaff et al., 2014; Ababou et al., 2015; Kumar & James, 2017; The weight values range from 1 to 5 and their assignment depend on the relative impacts of each parameter on the health and its importance in general water quality assessment (El Baba et al., 2020). Thus, the maximum weight of 5 was assigned to TDS, Cd, and Pb; the weight of 4 was given to Cr, and Ni; the weight of 3 has been assigned to parameters pH, EC, Fe, and Mn and; the parameters of Cu and Zn were assigned a weight of 2. The sub-index value (Q_n) were then obtained by dividing the value of the monitored parameter to the guideline limit value stipulated by the world health organization for drinking water quality; the resulting values were then multiplied by 100 (Eq. 2). The overall water quality index (WQI) was computed according to Eq. (3).

Where W_nis therelative weight; w_n is the weight and of the nth parameter; Q_n is the sub-index value of the n^th parameter; C_n is the determined value of the n^thparameter and S_n is the WHO standards for each parameter and; n is the number of parameters selected in this study.

The calculated WQI values were categorized as follows: Excellent quality (WQI < 50); Good quality (50 < WQI < 100); Poor quality (100 < WQI < 200); Very poor quality (200 < WQ < 300 and; Unsuitable for consumption (WQI > 300) (Abbas et al., 2013; Khatita et al., 2017).

Table 2 Input values used in the computation of water quality indices

Parameters	WHO Standard (S_n)	Assigned weight	Unit weight (W_n)
pH	8.5	3	0.076923
EC	1000	3	0.078947
TDS	500	5	0.131579
Fe	0.3	3	0.078947
Pb	0.01	5	0.131579
Cr	0.05	4	0.105263
Cd	0.03	5	0.131579
Cu	2	2	0.052632
Ni	0.07	4	0.105263
Mn	0.4	3	0.076923
Zn	5.0	2	0.052632
Total		39	1

Quality Assurance and Control of the analytical results

To ensure the accuracy and reliability of the results various quality assurance and control measures were employed in this study. In brief, all samples were carefully handled on sites and in the laboratory to avoid contamination. Polyethene containers were carefully sealed and capped, and coded with a permanent marker to avoid confusion. The accuracy and precision of AAS results were assessed through analysis of blanks and recoveries of certified reference material (CRM–Carrot: NCS ZC73031, National Analysis Center for Iron & Steel, Beijing, China). All blanks were below the detection limit (< 0.01 mg/L) which signified no blank correction needed. The obtained average percentage recoveries for three replicate analyses of NCS DC CZ27031 sample from all digestion procedures ranged between 92.8±1.26% and 99.8±8.3% and the relative standard deviations (% RSD) ranged between 5 and 11%. The obtained values were the acceptable control range of 80–120% for CRMs recoveries and ≤ 15% relative standard deviations for metal analysis (Addis & Abebaw, 2017; Akanchise et al., 2020). Moreover, all the solutions were prepared in deionized water and with analytical grade chemicals. The result of each parameter is reported as mean ± standard deviation (SD) of triplicate sample analysis.

Statistical analysis

The mean and standard deviations (SD) of the data from each triplicate analysis of a given sample were analysed by descriptive statistics. The mean concentration of heavy metals in different groundwater sources (GW1, GW2, GW3 and GW4) were compared using one-way ANOVA at 95% confidence level. Whereas, Principal Component Analysis (PCA) in Origin Pro 8.5.0 SRI software for window was used to analyse the origins of heavy metal pollutants in groundwater sources around the study area.

Physico-chemical properties and heavy metals concentrations in leachates

In this study, pH, electrical conductivity (EC), and total dissolved solids (TDS) were analysed along with heavy metals in six leachate samples. The results on pH, EC and TDS determinations are presented in Fig. 2 (a), (b), and (c), respectively. To assess the quality of leachates, the obtained results were compared against the regulatory limits established by the Tanzania Bureau of Standards (TBS) guideline limits for municipal and industrial wastewater reuse intended for agricultural irrigation (Tanzanian standards TZS 860:2005) (TBS, 2006) and standards for irrigation water stipulated by Food and Agricultural Organization (FAO, 1985). The standards are briefed in Table 3.

Table 3 Maximum permissible limits for municipal and industrial wastewater reuse (TBS, 2006) and irrigation water standards (FAO, 1985).

Agency	pH	EC	TDS	Cd	As	Cr	Cu	Ni	Fe	Pb	Mn	Zn
FAO (1985)	6.0–8.5	3000	2000	0.001	NA	0.1	0.2	NA	5.0	5.0	0.2	2.0
TBS (2006)	NA	NA	NA	0.1	0.2	1.0	2.0	NA	5.0	0.1	5.0	5.0

NA = not available, all units are in mg/L except pH which is unitless and EC which is in µS/cm

From Figure 2(a) it can be observed that pH values measured in leachates samples ranged from 8.40 to 9.10. The observed pH range in this study indicated alkaline behaviour of leachates which is typical of older landfills more than 10 years (Longe & Balogun, 2010; Kanmani & Gandhimathi, 2013). Out of six (6) leachate samples, four (4) samples (LS1, LS2, LS5 and LS6) had their pH values above FAO approved limit range (pH = 6.50 – 8.50) for irrigation water. The relatively higher pH levels leachates of this study signified that the Iringa municipal dumpsite is at its methanogenic phase due to older age > 16 years (Sawaya et al., 2021).

The study also revealed relatively higher EC values in all leachate samples ranging from 14090 to 26700 µS/cm at 25 ^oC (Fig. 2b). The Ec values were substantially higher than the FAO maximum permissible limit of 3000 µS/cm required for irrigation water. The relatively higher EC levels in leachates may be explained in terms of the presence of more dissolved inorganic cations such as such Na⁺, K⁺, Ca²⁺ and Mg²⁺some anions like Cl^-, and PO₄^2– (Naveen et al., 2018).

Meanwhile, TDS levels in leachate samples (Fig. 2c) were observed to vary from 6930 mg/L to 13360 mg/L at 25 ^oC. Three (3) leachate samples (LS4, LS5, and LS6) had the TDS values ranging between 10,000 and 100,000 mg/L indicating high saline behaviour) (Rabinove et al., 1958; Rusydi, 2018). The other leachate samples (LS1, LS2, and LS3) were identified to be moderately saline as their mean TDS values ranged between 3,000 and 10,000 mg/L. The difference in TDS values among leachate samples can be accounted for by the difference in storage time and composition of solid waste materials, and leaching conditions at different sampling points within the dumpsite. However, all leachate samples indicated TDS values far greater than the FAO (1985) maximum permissible limit of 2000 mg/L recommended for irrigation water. The relatively higher TDS levels indicate the presence of a large amount of dissolved inorganic content like Na⁺, K⁺, Cl^–, PO₄^2–, and heavy metals in dumpsite's leachates. Higher TDS values might be due to the leaching of soluble materials from solid waste disposed of in the dumpsite. The higher values of pH, EC, and TDS as observed in this study, advocate the need to confine and/or treat the leachates generated from Iringa municipal dumpsite before being discharged into the environment. The leaching of dissolved materials from dumpsite’s leachates into the underlying soils and water aquifer may have significant impacts on the physical and chemical properties of receiving soils and water bodies.

Moreover, the concentrations of selected heavy metal pollutants were quantified by AAS after digesting each sample three times by aqua regia (ratio 3:1 HNO₃/HCl). The concentrations (mean ± standard deviation, mg/L) of the selected heavy metals in six examined leachate samples are furnished in Table 4.

Table 4 Mean concentrations (mg/L) of heavy metals tested in leachates samples

Metals	LS1	LS2	LS3	LS4	LS5	LS6
Fe	625±15.90	347±10.10	524±13.50	442±6.70	251±21.40	72±3.91
Pb	0.59±0.10	0.56±0.06	0.45 ± 0.05	0.51 ± 0.02	0.40 ± 0.01	0.38±0.02
Cr	5.70±0.27	6.10±0.34	5.40 ±0.11	8.50±1.03	6.20±0.84	6.70±0.23
Cd	nd	nd	nd	nd	nd	nd
Cu	0.34±0.01	0.46±0.04	nd	nd	0.80±0.06	nd
Ni	0.39±0.05	0.43±0.06	0.21±0.02	0.30±0.01	0.36±0.09	0.48±0.10
Mn	17.20±1.51	11.60±1.80	9.40±0.73	19.41±2.30	12.90±1.93	14.40±1.34
Zn	8.40±0.71	6.40±0.23	8.80±0.63	6.20±0.43	5.60±0.64	26.10±3.04

nd = not detectable/below detection limit (<0.01 mg/L)

According to the results in Table 4, Fe was detected in abundant concentrations than any other heavy metal parameters measured in leachates samples. In fact, in our everyday life, steel objects are cheap and readily available and their disposal in the dumpsites also prevail. Thus, this could be a reason why the content of Fe in leachates was relatively higher than that of other metals. According to the results presented in Table 4, Cd was not detected (nd) in all leachate samples analysed for heavy metals. While Cu was detected in 75% of the leachate samples analysed. The lower levels of Cd < 0.01 mg/L in the leachates might be due to the lower amount of waste materials containing Cd disposed of in the dumpsite. Dilution of leachates by excess rainwater might also lead to low measurements of Cd as sampling was undertaken during the heavy rainy season in March 2021. Other heavy metals such as Fe, Zn, Mn, Cr, Ni, and Pb were detected in 100% of the examined leachate samples. The overall concentrations of heavy metals in leachate samples decreased in the order of Fe>Mn>Zn>Ni>Cr>Cu>Pb>.

From Table 4 it can also be observed that the mean concentrations of Cu in leachate samples varied from not detected to 0.80±0.03 mg/L. These values are comparatively lower than the maximum permissible limit of 2.0 mg/L set by TBS for wastewater reuse. However, when compared with FAO irrigation water standards, all the recorded mean concentrations values of Cu in leachate samples were slightly above the acceptable limit value of 0.2 mg/L for Cu. The mean levels of other metals including Fe (72±3.91–625±15.90 mg/L), Pb (0.38±0.02–059±0.10 mg/L), Cr (5.40±0.11–8.50±1.03 mg/L), Mn (9.40±0.73–17.20±1.51 mg/L) and Zn (5.60±0.43–26.10±3.04 mg/L) in all leachate samples analysed were above the TBS maximum permissible limit values of 5.0, 1.0, 1.0, 5.0, and 5.0 mg/L, respectively. In addition, the mean levels of Ni (0.21±0.02–0.48±0.10 mg/L) in leachate samples were found to exceed USEPA (2002) permissible limits of 0.2 mg/L authorized for discharging effluents/wastewater on surfaces. The presence of excess levels of different heavy metals in leachates signified the prevalence of indiscriminate disposal practices of wastes containing metals in the dumpsite. Field observation showed that the Iringa municipal dumpsite receives different kinds of solid waste ranging from rotten vegetables from markets, used up car batteries and dry cells, metal cans and scraps, leftover chemical pharmaceuticals, coloured plastic bags and bottles, worn-out car tyres, to demolition materials which upon decomposition contribute to different heavy metals concentrations in leachates.

The findings of this study were compared with that of other previous similar studies conducted in different places. A review of the literature showed that different studies have reported varying values of different heavy metals concentrations in leachates. For instance, in Ghana a study conducted by Boateng et al. (2019) reported the levels of Fe, Cr, and Zn in ranges of 10.885 –25.621, 0.701–1.918, and 1.083–1.722 mg/L, respectively in leachates produced from Oti landfill. A study conducted by Shemdoe (2010) reported mean concentrations of Pb (0.94 mg/L) and Cr (4.17 mg/L) in leachates emanating from Mtoni dumpsite in Tanzania. In addition, the mean values of Cu obtained in this study are within the range of 0.29 to 2.01 mg/L Cu reported by Hanson and Heiskala (2014) in leachates from former Vingunguti dumpsite in Tanzania. The observed similarity in lower Cu concentrations measured in leachates from Iringa municipal dumpsite and that from former Vingunguti dumpsite in Dar es Salaam might be due to the common source and a small amount of Cu-containing materials in waste. The content of Cu in the dumpsites' leachates is associated with indiscriminate disposal of electronic wastes such as circuits boards, electronic chips, and scraps of computer components in the dumpsites (Kiddee et al., 2013). Cu-containing waste materials are needed by some industries for recycling into new useful products. Thus, in Tanzania Cu-containing materials are highly scavenged by some people for income before and/or after their disposal in dumpsites. This may be the reason for the observed lower Cu concentration in the tested leachates. The variation dumpsites’ leachates composition with regards to heavy metals concentrations is attributed to a great difference in the levels of urbanization and industrialization to the places where dumpsites are found. For instance, the number of industries in Iringa municipal is relatively lower than in Dar es Salaam city, thus, the diversity and amount of solid waste material generated and disposed of differ appreciably. Moreover, dumpsites’ age and climatic conditions of the area; have a significant effect on the composition of leachates produced from different dumpsites.

Generally, the incidence of toxic heavy metals such as Pb and Cr in leachates presents major environmental and public health concerns. The presence of these heavy metals may elicit toxic effects even in low doses of exposure. If the generated leachates are not well controlled, they will continue to migrate through the underlying soil layers; this may lead to further groundwater contamination. Additionally, the leaching of heavy metals from dumpsite’s leachates is also likely to contaminate the underlying agricultural soils around the dumpsite making it unsuitable for agriculture. Consequently, important food crops cultivated in these contaminated soils may also uptake heavy metals and accumulate up to toxic levels; the situation is likely to hamper food security and jeopardize the health and life of the people.

Physico-chemical properties s and heavy metals concentrations in groundwater

Solid waste dumpsites may pollute environmental resources including agricultural soil, surface and groundwater bodies. Contamination of groundwater with various dissolved suspended organic and inorganic substances such as heavy metals that are present in leachate is a major threat to public health and other living organisms. Groundwater contamination by leachates could potentially affect the physical, chemical and biological properties of water making it unsuitable for drinking and other domestic uses. In this regard, three physicochemical properties (pH, EC, and TDS) were assessed in groundwater samples collected from four wells that were within the vicinity of Iringa municipal solid waste dumpsite. The results on physicochemical properties and heavy metals determinations in groundwater samples are presented in Fig. 3 ((a) (b), and (c)).

It was noted that groundwater samples from all wells were free from colours and odours. The measurement of pH is important concerning water quality because the solubility, bioavailability, and toxicity of chemical constituents are influenced by pH. Most heavy metals pollutants tend to be very toxic at lower pH levels due to an increase in solubility. Based on the findings of this study, pH values in groundwater samples from four wells varied from 7.15±0.02 to 7.60±0.03 (Fig. 3a), which indicated neutral to moderate alkaline behaviour of groundwater in the study area. All groundwater samples from four wells had pH values lower than the WHO guideline limit range (pH= 6.5–8.5) prescribed for drinking or potable water. The observed results imply that pH was not a problem to the quality of groundwater resources around the study area. The results also suggested little soluble foreign materials entering the groundwater resource as a result of leachates percolation or flying ashes from the dumpsite that could potentially upset the pH. The observed pH values of this study were also within the pH range of 7.12–8.1 measured in groundwater near the Onderstepoort Landfill in Pretoria, South Africa (Tshibalo, 2017). Another similar study undertaken in India by Maiti et al. (2016) reported pH values in the range of 7.2–7.4 in groundwater near Dhapa closed dumpsite.

The mean values of EC in groundwater samples from all groundwater wells ranged from 869±89.64 to 1570±105.50 µS/cm (Fig. 3b). The maximum EC value was recorded in GW3. According to results in Fig. 3 (b), EC values of all groundwater samples except in GW2 exceeded the maximum permissible limit value of 1000 μS/cm as per WHO drinking water quality recommendations (WHO, 2004). EC is an indirect measurement of TDS in solutions, thus, higher EC values in groundwater imply high concentrations of anions and cations in groundwater that might have leached from dumpsite’s leachates. The higher EC levels in water can significantly exacerbate the aesthetic properties such as taste, odour, colour turbidity, salinity, hardness, softness, and temperature in water. The relatively high EC levels in irrigation water may also affect nutrients uptake by plants. Comparatively, Saheed et al. (2020) reported lower values of EC (225–586 µS/cm) than those of this study in groundwater near a municipal dumpsite in Ibadan Metropolis, Nigeria

Additionally, the mean concentrations of TDS in all groundwater samples varied from 446±55.30 to 776±52.34 mg/L (Fig. 3c). The maximum TDS values were registered in GW3, however, TDS levels in all groundwater samples were relatively lower than the levels recorded in leachate samples (6930±41.39–13360±59.40 mg/L). The lower TDS values in groundwater samples also agreed to the lower EC measurements. A similar study in Nigeria by Akinbile and Yusoff (2011) reported TDS values in the range of 18–432 mg/L in three borehole water near the dumpsite of Ondo state. TDS values < 500 mg/L indicate that the water is suitable for drinking, while water with TDS values > 1000 mg/L is considered unacceptable for human consumption (USEPA, 2018). In this study, the levels of TDS in GW1 (566±20.35 mg/L), GW3 (776±52.34 mg/L), and GW4 (665±40.41 mg/L) were above 500 mg/L which is a desirable limit for drinking water recommended by WHO (2011). For health reasons, the appropriate limit of TDS in water for human consumption is 500–1000 mg/L (Rusydi, 2018). Thus, all TDS values are within 500–1000 mg/L indicated that the groundwater around the dumpsite is safe for human consumption as potable water. Lower TDS values also implied less impact on groundwater contamination by leachate migration from Iringa municipal dumpsite through groundwater aquifers. However, TDS only measures the solids in water; it does not give any indication of whether those solids are beneficial or harmful. Thus, TDS alone is not a sufficient parameter for assessing the quality of water. It has been reported that elevated levels of TDS can alter the ionic composition and intensify the toxicity of water (Sasikaran et al., 2012). Moreover, the presence of high levels of TDS in water makes it unpotable and unobjectionable to consumers due to salty taste, odours and colours. Therefore, from a public health standpoint, the relatively high TDS values in drinking water may also lead to gastrointestinal problems such as laxatives and constipation (Sasikaran et al., 2012; Pande, 2015). The high TDS concentrations may also affect persons with kidney and heart diseases (Meride & Ayenew, 2016).

On the other hand, the percolation of dumpsite leachates into the underlying soil can significantly contaminate the receiving groundwater around the dumpsite. In this regard, groundwater samples were analysed along with leachates for different heavy metals concentrations (Fe, Pb, Cr, Cd, Cu, Ni, Mn, and Zn) by using AAS. The results on heavy metals concentrations in four (4) groundwater sources (GW1, GW3, GW4, and GW4) are presented in Table 5.

Table 5 Mean concentrations (mean ± standard deviation, mg/L) of heavy metals in groundwater samples

Metals	GW1	GW2	GW3	GW4
Fe	0.34±0.01	0.48 ± 0.06	0.50±0.04	0.38±0.02
Pb	nd	0.10±0.04	0.10±0.01	0.20±0.00
Cr	nd	nd	nd	nd
Cd	nd	nd	nd	nd
Cu	nd	nd	nd	nd
Ni	0.10±0.07	0.20±0.03	0.13±0.40	nd
Mn	7.40±0.59	5.70±0.80	7.70±0.65	5.50±0.14
Zn	8.10±0.45	5.60±0.17	1.60±0.85	18.40±1.05

nd = not detectable or below detection limit (<0.01 mg/L).

The results in Table 5, indicated that the contents of Cr, Cd, and Cu were not detected in any groundwater samples. However, high concentrations of Cr and Cu were observed in leachate samples (Table 4). The lower concentrations of these heavy metals in the groundwater samples may be accounted for small quantities of solid waste materials containing Cu, Cd, and Cr disposed of in the dumpsite. Ni and Pb metals were detected by 75% of the groundwater samples tested for selected heavy metals. Other metals including Fe, Mn, and Zn were detected in all groundwater wells studied. The distribution of the detected heavy metals appeared in different descending orders as follows: Zn >Mn>Fe>Ni> Pb for GW1, Mn>Zn>Fe>Ni>Pb for GW2 and GW3, Zn>Mn>Fe>Pb>Ni for GW4. One-way analysis of variance (ANOVA) at 95% confidence level indicated that there were statistical differences in the concentration means of different heavy metals in groundwater samples (p < 0.05). The distribution The maximum permissible limits (MPLs) of heavy metals suggested by the World Health Organization (WHO, 2017) as briefed in Table 2 were used for assessing the drinking suitability of groundwater in the study area.

The mean concentrations of Fe (0.34±0.01 to 0.50±0.04 mg/L.) in all groundwater samples were appreciably higher than the WHO guideline limit of 0.3 mg/L set for drinking water. According to WHO (2011) recommendations, Fe concentrations above 0.3 mg/L may upset the aesthetic features of water and its potability. Excess Fe concentrations also raise the turbidity of water which encourage the growth of ferrous bacteria that speed up the rusting process of metal which come in contact with water. Boateng et al. (2019) also reported a higher concentration of Fe in the range of 0.732–2.292 mg/L in groundwater around Oti landfill in Kumasi, Ghana. Additional amounts of Fe in leachates and its overtime release into underlying groundwater may be due to the co-disposal of steel scraps that prevail in the dumpsite.

The mean values of Pb concentrations in groundwater samples except for GW1 were greater than the WHO guideline limits of 0.01 mg/L required for drinking. Excess Pb concentrations in water samples implied that the groundwater resources in the study area are unsuitable for drinking and other domestic use unless measures are taken to bring Pb levels down. It has been reported that exposure to the high concentration of Pb through drinking water can cause brain damage and mental retardation in children, abortion, birth abnormalities, allergies, chronic renal diseases and hypertension (Engwa et al., 2018; Obasi & Akudinobi, 2020). Groundwater contamination by Pb in the study area may be due to Pb-containing solid waste such as Pb based batteries, coloured plastics, and discarded paint containers that are indiscriminately disposed of in the dumpsite. However, historical Pb depositions from combustions of leaded-gasoline in automobiles might have contributed significantly to Pb contamination in groundwater as the result of the study area's proximity to the Iringa-Dodoma highway.

Ni was observed in the groundwater samples with concentration values ranging from not detected to 0.20 mg/L were not in line with the WHO guideline limit of 0.07 mg/L in all groundwater samples except for GW4. Excess Ni levels in groundwater samples indicated potential health risks to the consumers. Increased exposure to Ni containing compounds can cause carcinogenic health effects to both humans and animals (European Commission, 2009). The contents of Ni in the dumpsite is associated with indiscriminate disposal of waste materials containing Ni such as Ni-plated materials, Ni coloured products, ceramics, and Ni-Cd batteries in the dumpsite. In comparison with other previous studies, the measured values of Ni contents in this study were below the range of 0.257–0.357 mg/L of Ni measured in water from the borehole as reported in a similar study conducted in Egypt (Abd El-Salam & Abu-Zuid, 2014).

In addition, Mn registered concentrations in groundwater samples ranging from 5.50±0.14 to 7.70±0.6.5 mg/L. Accordingly, the results indicated that all groundwater contained relatively higher levels of Mn than the maximum permissible safe limit values of 0.4 mg/L as stipulated by the WHO guidelines for drinking water quality. Thus, groundwater resources in the study area may be unsafe for human consumption owing to excess Mn concentrations. Other similarly studies such as that of Abd El-Salam and Abu-Zuid (2014) in Egypt reported higher levels of Mn ranging from 0.257 to 0.357 mg/L of Mn in monitoring borehole water near Alexandria's solid waste sanitary landfill. Uncontrolled disposal of Mn-based batteries is one of the probable sources of Mn contamination in the groundwater resources in the study area.

Further findings show that the contents of Zn as determined in groundwater samples varied from 1.60±0.85 to 18.40±1.05 mg/L. Yet, this range exceeded the WHO permissible safe limit value of 3.0 mg/L, required for drinking water. Such excess release of Zn concentrations in groundwater may be due to leakage of Zn- laden leachates as the result of co-disposal disposal of solid waste containing Zn metals. However, random dumping of galvanized objects and zinc scraps from spent dry cells near the residential area may also result in the additional release of Zn in the groundwater resources around the study area. The element of Zn is essentially important in the human body when present within the guideline limit. However, human exposure to excess Zn concentration through drinking water can cause adverse health effects. Some of the adverse health effects associated with exposure to elevated levels of Zn through drinking water include diarrhoea, vomiting, vascular shocks, and metal fume fever. The values of Zn concentrations in this study were comparatively higher than the ranges of 0.90–1.3 mg/L of Zn in groundwater samples as reported in similar studies conducted in Nigeria by Olorunfemi et al. (2018).

The occurrence of elevated levels of some heavy metals in groundwater samples suggested appreciable impacts of heavy metals discharge into groundwater systems through seepage of leachates from the dumpsite. Consequently, the groundwater resources around the Iringa municipal dumpsite may be unsuitable for drinking and other domestic uses without treatment. When water contaminated with heavy metals is used for drinking it can lead to serious health effects. For instance, human exposure to Pb through drinking water can lead to negative health effects such as cancer, brain damage, renal dysfunctions, endocrine and reproductive disorders (Obasi & Akudinobi, 2020).

Principal components analysis of heavy metals in groundwater

Meanwhile, the principal component analysis (PCA) was used to identify the origins of physicochemical and heavy metals pollutants in groundwater in the study area using Window software. Before analysis heavy metals which recorded concentration values below the detection limit in all groundwater were omitted in the analysis. The first and the second principal components (PC1 and PC2, respectively) had Eigenvalues greater than 1 accounting for 88.61% of the total explained variance. Table 6, indicates that the factor loadings of Fe, Pb, Ni, and Mn were 0.3259, 0.4903, and 0.3931, respectively, for PC2. The factor loadings in PC2 were 0.6476 and 0.5269 for Fe and Pb, respectively, showing that all stated heavy metals with no similarity in loadings. Therefore, this indicates that Fe, Pb, Ni, and Mn, might have emanated from quite different sources. For instance, Pb and Mn releases may be associated with the uncontrolled disposal of Pb and Zn based batteries. The lack of association in loadings between variables signified that there is no clear demarcation for the origins of heavy metals pollutants in groundwater resources around the study area. Thus, heavy metal contaminants in groundwater could have emanated from different sources around the study area including seepage of leachates from the municipal dumpsite. As stated before, historical vehicular emissions might have contributed to groundwater contamination by Pb. This is due to study area proximity to the highway. The presence of heavy metals in groundwater can be used as tracers for the exact origin of groundwater contamination around the Iringa municipal dumpsite in further studies.

Table 6 Principal Component analysis results

Metals	Coefficients of PC1	Coefficients of PC2
Fe	0.3259	0.6476
Pb	-0.4079	0.5269
Ni	0.4903	0.2872
Mn	0.3931	-0.4630
Zn	-0.5767	-0.0782
Eigenvalue	2.94879	1.4819
Variability (%)	58.98	29.64
Cumulative	58.98	88.61

Assessment of water quality status around the study area

The overall water quality indices (WQI) were derived from eleven (11) selected parameters which included pH, EC, Fe, Pb, Cr, Cd, Cu, Ni, Mn, and Zn to assess the drinking suitability of the groundwater resources of the study area. The computed values for WQI are furnished in Table 7.

Table 7 Water quality indices (WQI) of groundwater samples

Sampling Site	WQI (%)	Water quality status
GW1	203.17	Very poor
GW2	182.26	Poor
GW3	220.72	Very poor
GW4	169.07	Poor

The WQI values ranged from 169.07 to 220.72 translating from poor quality (≤ 100 WQI ≤ 199.99) to very poor quality ((≤ 200 WQI ≤ 299.99). GW2 and GW4 scored WQI values of 182.26 and 169.07, respectively, showing poor quality. GW1 scored WQI value of 220.72 indicating poor quality. The maximum WQI was obtained in GW3 (220.72); the observed value implied very poor quality of groundwater for drinking purposes. Generally, the water quality assessment results suggest that the groundwater resources of study are polluted and thus unfit for human consumption without treatment. The deterioration of the groundwater resources was attributed to the presence of relatively higher concentrations of heavy metals and other physicochemical pollutants. This may be a result of waste disposal at the Iringa municipal dumpsite.

The aim of this study was to assess the concentration of heavy metals, pH, EC, and TDS in leachates and groundwater samples collected around the Iringa municipal solid waste dumpsite as well as to evaluate the drinking suitability of groundwater. The results indicated that pH, EC, and TDS in all leachate samples analysed were above the acceptable limits set by FAO. The results also showed that there is a potential health risk mostly associated with excess heavy metals contamination generated by the Iringa municipal dumpsite. It was observed that approximately 88% of the heavy metals analysed in leachates their concentrations exceeded the TBS guideline limits for municipal and industrial wastewater reuse. This implied that the seepage of those polluted leachates into the underlying environment may pose adverse health effects to the residents whose lives depend on the resources around the dumpsite. Although, pH, EC, and TDS was not a problem with regard to quality of groundwater, the concentrations of Fe, Pb, Ni, Mn and Zn in some groundwater samples were above the WHO guideline limits for drinking water quality were These results stress that the groundwater resources around the Iringa municipal dumpsite are unsafe for human consumption. This calls for stringent measures to bring the levels down to their normal safe limits. Otherwise, in over long-time consumption of these groundwater without treatment may lead to carcinogenic and non-carcinogenic health effects to adults and children population. Thus, the findings of this study give a scientific basis that provokes policy-making and deliberate efforts to reduce pollution in the surrounding environment of Iringa municipal dumpsite, Tanzania, and perhaps other parts of the world that aspire to sustainable waste management to minimize environmental and human health hazards. We recommend that the local municipal should relocate the dumpsite at least 500 m away from the localities that have human activities or residential developments. The measures will help to safeguard people from the adverse effects of exposure to heavy metals and other noxious contaminants associated with the mismanagement of solid waste.

Ethical Approval

Not applicable

Consent to participate

Not applicable

Consent to publish

The authors declare that this manuscript is original, has not been published before and is not currently being considered for publication elsewhere. We confirm that the order of authors listed in the manuscript has been agreed by all of us. Also all the authors have agree to submitt this manuscript for publication consideration .

Authors Contributions

The study conceptualization was done by V.S and C.F. Sample collection, preparation and analysis were performed by V.S. The first draft of the manuscript was written by V.S. The authors C.F and F.K commented on previous versions of the manuscript. All authors read and approved the final manuscript.

Funding

The authors would like to thank the Mbeya University of Science and Technology for financial support. Thanks to Mkwawa University College of Education and Geology survey of Tanzania for material support.

Competing Interests

The authors have no relevant financial or non-financial interests to disclose related to the work submitted for publication.

Availability of data and materials

The data that support the findings of this study are available from the correspondence author (V.S) upon reasonable request.

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Heavy metals pollution in leachates and its impacts on quality of groundwater around Iringa municipal solid waste dumpsite.

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Abstract

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Introduction

Materials And Methods

Results And Discussion

Conclusion

Declarations

References

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