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Assessment of Shallow Groundwater Quality Using Water Quality Index and Human Risk Assessment in the Vogan-Attitogon Plateau, Southeastern (Togo)

Gbati Napo*, Kossitse Venyo Akpataku, Alfa-Sika Mande Seyf-Laye, Masamaéya D. T. Gnazou, Limam Moctar Bawa, Gbandi Djaneye-Boundjou

Laboratory of Applied Hydrology and Environment (Ex Laboratory of Water Chemistry), University of Lome, Lome, Togo

Abstract: The shallow Continental Terminal aquifer constitutes the primary water resource for drinking purpose in the coastal sedimentary basin of Togo, facing population growth, mining development, industrialization, and intensive agriculture. This study aims to assess groundwater quality and associated human health risks, to ensure the sustainable water supply in the basin. Major chemical parameters, and trace elements of forty-two samples of shallow groundwater and eight samples of surface water collected during march 2019 were analysed using AFNOR methods. The dataset obtained was used to evaluate drinking water quality and safety. The results showed that groundwater was slightly acid (4.83 < pH < 7.63, mean of 5,63) and fresh to brackish (82.4 < TDS < 6610 mg/L, mean of 652 mg/L). In contrast, surface water was circum-neutral (6.24 - 7.8, mean of 6,91), and fresh to very saline (134 < TDS < 33915 mg/L, mean of 9580 mg/L). For both groundwater and surface water, the cations and anions abundance order was respectively Na⁺ > Ca²⁺ > Mg²⁺ > K+ and Cl- > SO₄²⁻ > HCO₃⁻> NO₃⁻. Thus, water samples were predominantly Na - Cl water type (~88 %). The water quality index score based on major ions indicated that 93% of the groundwater samples were classified as excellent and good for drinking. The non-carcinogenic risk ranged from 3.887 to 17.650 for children, from 2.272 to 10.135 for adult females, and from 1.785 to 8.104 for adult males and was related principally to manganese, nitrate, arsenic, fluoride and cadmium. The carcinogenic risk ranged from 0.004 to 0.006 for children, 0.003 to 0.004 for adult females, and from 0.002 to 0.003 for adult males and was caused by arsenic and cadmium. The water quality is affected by seawater intrusion, evaporite dissolution and anthropogenic contamination. The consumption of groundwater may cause health effect on human.

Keywords: coastal sedimentary basin, groundwater, water quality index, human health risk assessment, trace elements, Togo

Received August 07, 2021; Revised September 10, 2021; Accepted September 22, 2021

Citation: Gbati Napo, Kossitse Venyo Akpataku, Alfa-Sika Mande Seyf-Laye, Masamaéya D. T. Gnazou, Limam Moctar Bawa, Gbandi Djaneye-Boundjou, Assessment of Shallow Groundwater Quality Using Water Quality Index and Human Risk Assessment in the Vogan-Attitogon Plateau, Southeastern (Togo). Journal of Environment Pollution and Human Health, 9 (2). 50-63. January 2021. doi:10.12691/jephh-9-2-4

1. Introduction

Groundwater is an essential freshwater source for drinking, domestic, irrigation, and industrial uses in many countries. Because of its potential resilience to climate variability in many areas of sub-Saharan Africa, groundwater plays a vital role in sustaining access to safe water in pursuit of the United Nations Sustainable Development Goal (SDG) 6 – water and sanitation for all by 2030 [1].

Coastal basins are more susceptible to natural and human-driven pressures because of their proximity to the sea, population growth, industrialization, and agricultural activities [2]. Consequently, shallow coastal aquifers in some developing countries, particularly in the Gulf of Guinea, face differing groundwater quality and deterioration challenges. Groundwater is highly vulnerable and subject to high pollution risks, particularly the shallow groundwater systems in Central and West Africa [3]. Seawater intrusion, mixing freshwater with saline lake and lagoon systems contribute to high groundwater salinity. The farming recharge, infiltration from unsanitary areas such as septic tanks and sewage contaminations are responsible for nutrient loads in shallow groundwater systems [4,5,6]. Although coastal sedimentary aquifers in Togo are less documented, the available studies on shallow aquifers focus mainly on the Southeastern part encompassing the capital Lome and its surrounding areas [5,7,8].

The Eastern part of the basin, mainly occupied by the Vogan-Attitogon plateau, has received little attention except for the rural water supply projects executed by the Directorate of Water Resources and its partners [8,9]. This part supports socio-economic activities, including fishing, agriculture, phosphate, and limestone mining [9,10].

The shallow Continental Terminal (CT) aquifer constitutes the primary water resource exploited by low-cost infrastructures such as wells and manual boreholes. The assessment of surface water quality based on trace element analyses has revealed contamination of anthropogenic origin through the intrusion of seawater loaded with phosphate effluents from the leaching of mining and agricultural soils [9]. Other studies showed that in some poultry farms in Togo's maritime region, groundwater consumed contains heavy metals such as arsenic, cadmium, chromium, copper, mercury, lead, nickel, and zinc [10]. However, the extent of contamination is not well known. The assessment of water intended for human consumption in the southern sedimentary basin is therefore essential to ensure population health and to fill the knowledge gap in shallow groundwater quality status in the whole basin. This global knowledge can help in defining hydro-zones and prioritizing groundwater management options.

Hydrochemical characterization with using conventional graphical methods (diagrams and plots) for classification, correlation, synthesis and illustration provide insight into groundwater quality. Water quality index (WQI) is other approach widely applied for evaluating and classifying water quality. This method makes transformation of water quality parameters into scores [11]. The use of Water quality index (WQI) scores with spatial analysis can help to appraise groundwater suitability for drinking purposes. Based on these tools, a study mapped WQI scores and derived water quality classes in the Gaza Strip (Palestine) and help to better understand of the groundwater quality spatial distribution [12]. Among the different water quality indices developed and used by several organizations and researchers to describe water resource suitability for different purposes, the drinking water quality index (DWQI) categorizes water resources intended for public consumption [11,13].

The human health risk assessment (HHRA) model is often used for evaluation of potentially harmful of water contaminants on adults and children health upon their exposure [13,14,15]. Chemical contaminant such as arsenic, cadmium, chrome are a major public health topic in the world. Although high concentrations of arsenic found in both surface and groundwater in some African countries, it has less interest [16]. In the maritime region (Togo), drinking water contains heavy metals such as arsenic, cadmium, chromium, copper mercury, lead, nickel, zinc [17].

This study aimed to assess groundwater hydrogeochemical characteristics, its suitability for drinking, and the health risks of selected trace elements (As, Cd, and Cr) on adults and children health of the Vogon-Attitogon plateau, Southeastern of Togo. This investigation is based on the determination of water physicochemical parameters and comparing them to WHO standards water quality, and comparing of health risk evaluated with the US Environment Protection Agency (US-EPA) guidelines for estimating the Human Health Risk (HHRA).

2. Materials and Methods

2.1. Study Area

Figure 1. Location map of the study area and groundwater sample points

The Vogan-Attitogon plateau in Togo's coastal sedimentary basin extends between 1°15'-1º45'N and 6º15'-6º35'E. The study area (3 700 km²) is drained by the Lake Togo, and the Mono river basins. Its lies between the Mono river in the East, the Lama depression in the north, the Haho river and the Togo Lake in the West, and the Atlantic Ocean in the South. The altitude varies between 20 and 70 m with an average of 40 m (Figure 1).

The climate is subequatorial Guinean, characterized by two rainy seasons (from march to july and from the end of september to the beginning of november) alternating with two dry seasons. The annual rainfall average is about 800 mm. Throughout the year, temperature varies from 24 to 36°C, the annual potential evapotranspiration (Turc's ETP) ranges from 1,450 to 1,600 mm [4]. All the rivers dry up in the dry season except the Mono river which has a perennial flow. The river valleys are wide, marshy and frequently flooded in the rainy season.

The geological map of the Vogan-Attitogon plateau is shown in Figure 2. The coastal basin of Togo is made of four major lithological formations depending on their constitution, from Maastrichtian to Quaternary. The Cretaceous formation are made of detrital rocks with clay or carbonate episodes and the Maastrichtian is formed by deposits dominated by a combination of detrital quartz and clay with organic matter. The Paleocene formation is characterized by a predominance of biochemical matter and Eocene deposits made of clay deposits with detrital quartz inputs. The Oligocene formation include nummulitic limestone and clay sand facies. The Continental Terminal is characterized by a continental mega-sequence, dated from the Mio-Pliocene to the Quaternary, and sandy clays [18].

Hydrogeologicaly, the coastal sedimentary basin is classified into four main aquifers. They are, from bottom to top of the stratigraphic series: a confined aquifer in Cretaceous sands, a confined aquifer in Eo-Paleocene sands and limestones, an unconfined sandy Continental Terminal aquifer, and an unconfined recent sandy aquifer [4]. The Continental Terminal aquifer is the most accessible and easily exploitable. Its productivity is estimated at 32,715 m³.d⁻¹. This aquifer contributes to more than 70% of the water supply in Lome. The depth and the thickness of the Continental Terminal generally increase from north to south. The depth ranges from 22 m near the Lama depression in the north to 97 m in the south of the study area [8].

Figure 2. Geological map of the study area

2.2. Sampling and Analytical Procedures

Groundwater samples were collected during the dry season (March 2019) from thirty-seven (37) boreholes and five (5) hand dug wells used for domestic and agricultural purposes, and eight (8) surface water samples. Cleaned polyethylene bottles were used for sampling. The polyethylene bottles were washed and rinsed several times using the water to be sampled, before collecting samples for physicochemical analyses. Bottles help prevent unpredictable changes in water characteristics.

Physicochemical parameters such as temperature, pH, TDS, and electrical conductivity (EC) were measured in the field at each sampling point using a HANNA portable equipment. Samples were kept at a temperature of +4°C and transported to the laboratory for testing. Major cations sodium (Na⁺), calcium (Ca²⁺), magnesium (Mg²⁺), potassium (K⁺), bicarbonate (HCO₃⁻), chloride (Cl⁻), sulfate (SO₄²⁻) and nitrate (NO₃⁻) were measured in the laboratory after filtration using a 0.45 µm size filter. Chemical analysis of major cations and anions was carried out using AFNOR standard methods. Ca²⁺, Mg²⁺, HCO₃⁻, carbonate (CO₃²⁻) were analyzed by titration. Na⁺, K⁺ were measured by flame photometry and nitrate (NO₃⁻), sulfate (SO₄²⁻), manganese (Mn²⁺), orthophosphate (PO₄²⁻), ammonium (NH₄⁺), fluoride (F⁻) and arsenic (As) with a UV spectrophotometer HACH LANGE DR 9000. Samples for determining trace elements such as cadmium (Cd²⁺), chromium (Cr⁺⁶) were acidified and an atomic absorption spectrometer was used.

The analytical accuracy was cross-checked by calculating ionic balance (IB) as follows:

[picture] | (1)

where and were expressed in meq/L. The calculated charge-balance error must be within an acceptable limit of ±10%.

2.3. Water Quality Index (WQI) Method

One of the most effective methods of communicating quality information is the water quality index. It shows great rigor in the selection of parameters and develops a common scale and assigning weights [12,13]. The WQI summarizes large amount of water parameters into a simple number, that express overall water quality, easily and more quickly understood than a set of results of large analyzed parameters.

The WQI method assigns weight to each parameter, and proceed to normalization of weight of each parameter and standardization of parameters. WQI is calculated by aggregation of the individual WQI scores. Weights of parameters (pH, TDS, Ca²⁺, Mg²⁺, Na⁺, K⁺, HCO₃⁻, SO₄²⁻, Cl⁻, NO₃⁻, Cr⁶⁺, Cd²⁺, As) ranging from one to five, based on the parameters importance for water quality assessment are presented in Table 1 [12,30]. The values of the weights were adopted from previous studies. Total dissolved solids and nitrate concentration considered as important indicators for assessing overall water quality and are given the maximum weight of five. A weight of three is assigned to pH because of its effect on heavy metals mobilization. Sodium, chloride, and sulfate were also assigned a weight of three according to their contribution to salinity and taste. A weight of two is given to calcium, magnesium, and bicarbonate according to their effect on water hardness. The minimum weight is given to potassium because it poses little risk to consumer health. Because of their effect on human health, a weight of three is given to cadmium, arsenic and chromium.

The normalized weight of each parameter is obtained as follows:

[picture] | (2)

wi is the normalized weight for parameter i, Wi is the assigned weight for parameter i, and n is the total number of parameters. Water quality observations are then standardized by dividing the values by their corresponding water quality standard:

[picture] | (3)

qi is the partial WQI score for parameter i, Ci is the observed concentration for parameter i, and Si is the water quality standard for parameter i. We apply the WHO guidelines to drinking water, as shown in Table 1.

Table 1. Physico-chemical parameters with the corresponding unit, WHO standard (Si), WQI weight (Wi), and normalized weight (wi)

The overall WQI score is obtained by summation of the scores of each parameter multiplied by their normalized weight:

[picture] | (4)

WQI score less than one indicates suitable water that can be used without any precaution. Higher values indicate unsuitable water quality. WQI scores are classified from < 0.5 to 3, corresponding respectively to excellent and unsuitable water for human consumption.

2.4. Human Health Risk Assessment (HHRA) Model

The human health risk assessment (HHRA) model is a method established the United States Environmental Protection Agency (US-EPA) for evaluation of potential harmful effects of contaminants on the health of children and adults through groundwater consumption [13]. Human body is exposed to harmful substances from groundwater through two main routes, orally drinking water and dermal contact [12]. In this study, the non-carcinogenic risks were assessed using parameters bellow: NH₄⁺, NO₃⁻, NO₂⁻, Mn²⁺, F⁻, As, Cr⁶⁺, and Cd²⁺. The non-carcinogenic risks via ingestion and dermal contact are determined as follows [13,14,15,19]:

[picture] | (5)

ADD: Average daily dose via ingestion (mg/kg day), C: concentration of pollutant in groundwater (mg/L).

The non-carcinogenic risk via dermal contact is expressed as:

[picture] | (6)

[picture] | (7)

[picture] | (8)

and : hazard quotient through oral and dermal exposure pathways, CDD: chronic daily dose via dermal contact (mg/kg day), DA: exposure dosage (mg/cm²), SA: skin surface area (cm²), and : reference dosage via oral and dermal contact (mg/kg day), and : gastrointestinal absorption factor. The meanings and index values of other parameters are shown in Table 2 and Table 3. Therefore, total non-carcinogenic risk HQtotal is calculated as follows:

[picture] | (9)

The allowable limit of non-carcinogenic recommended by the USEPA, is 1. The value of non-carcinogenic is higher than 1 indicates the non-carcinogenic risk of contaminant exceeding the allowable limit.

Chromium, Cadmium, and Arsenic can also induce carcinogenic risks for humans [13]. Chromium (VI) has been shown to cause stomach tumors in humans and animals exposed to chromium (VI) in drinking water [17]. Cadmium and its compounds are highly toxic, and exposure is known to cause cancer of the lung, prostate, kidney and pancreas. It has also been associated with cancers of the breast and bladder [22]. Prolonged ingestion of arsenic in drinking water is associated with an increased risk of bladder and skin cancer. Also, cancers of the lung, digestive tract, liver, kidney, and lymphatic and hematopoietic systems have been linked to arsenic exposure [23].

Table 2. Parameter values for health risk assessment through ingestion and dermal routes [20,21]

Table 3. Values of RfDoral, RfDdermal, and ABSgi for different ions [20]

In this study, the carcinogenic risks were assessed using Cr⁶⁺, Cd²⁺ and As as risk assessment parameters. The models for carcinogenic risk via ingestion and dermal contact are as follows [13]:

[picture] | (10)

[picture] | (11)

[picture] | (12)

[picture] | (13)

CR is the carcinogenic risk; SF is the slope factor for the carcinogenic contaminants (mg/kg day)⁻¹. The Slope factors SForal and SFdermal values for As, Cr⁶⁺, and Cd²⁺ are shown in Table 4.

Table 4. Slope factor of element [20]

3. Results and Discussion

3.1. Hydrochemical Characteristics

The statistical summary of hydrochemical parameters of groundwater samples is presented in Table 5.

The temperature of the groundwater varies from 29.4°C (P2) to 32.6°C (F6). The pH varying from 4.83 to 7.63 with a mean of 5.63 shows that the groundwater was acidic. The partial pressure of CO₂ ranging from 10⁻² to 10-0.097 atm was higher, and reflects probably soil layers CO₂ originated from biological activity or juvenile infiltration of precipitation. Low pH of groundwater samples may also results from the low content or lack of acid consuming materials such as carbonates minerals in the aquifer matrix. The shallow groundwater of the coastal aquifer of Benin, Ivory Coast, and Nigeria were also found acidic [2,6,24]. The industrialization of the coastal areas may also induce acidic rains which contribute to groundwater recharge.

The EC and TDS values ranged from 108 µS/cm (F4) to 8720 µS/cm (F19) and 102 to 6610 mg/L respectively. About 13% of samples have TDS higher than 1000 mg/L. The high value of the standard deviation of TDS (1165 mg/L) shows a widespread of groundwater mineralization and may indicate a range of factors affecting groundwater chemistry. The order of cation abundance was Na⁺ > Ca²⁺ > Mg²⁺ > K⁺ and the order of anion abundance was Cl⁻> SO₄²⁻ > HCO₃- > NO₃⁻. In surface water, TDS varies from 134 to 33915 mg/L. The order of ion abundance was Na⁺ > Ca²⁺ > Mg²⁺ > K⁺ for cations and Cl⁻> SO₄²⁻ > HCO₃⁻ > NO₃ ⁻for anions. Consequently, the Piper diagram (Figure 3) revealed the dominance of Na-Cl water type (77%) with accessory water types, Ca-Cl type (15%) and Ca-Mg-Cl type (8%). The Na-Cl water types are mainly located near the coast line, the Togo Lake, the Zowla Lake and the Mono river. The high salinity of water samples in the study area is found to be associated with the predominance of Cl⁻ and Na⁺ ions.

The predominance of Na- Cl water type was found in previous studies of the coastal shallow aquifers along the Gulf of Guinea and it was explained by the strong influence of coastal sea aerosol spray, atmospheric deposition, and saline water intrusion from seawater and lagoon systems [2,5,6]. In surface water, the Na-Cl water type was also found to be dominant (88%) with accessory Ca-Na-HCO₃ water type (12%). Consequently, the hydro system on the Vogan-Attitogon plateau may be considered as dominated by Na-Cl water types. However, Zio River waters in Togo's coastal sedimentary basin were characterized by the dominance of Ca-HCO₃ water type with Na-HCO₃ water type and a few Na-Cl water type [7]. Waters of the Zowla Lake were characterized in the dry season and rainy season by high TDS and chloride, indicating the dominance of Na-Cl water type [25].

Table 5. Statistical summary of chemical composition of groundwater and surface water

Figure 3. Piper classification diagram

3.2. Water Quality

According to analysis results the groundwater samples of the Vogan-Attitogon plateau were acidic, and 95% have pH under WHO standard (6.5-8.5). pH does not directly influence human health but determines most ions (Ca²⁺, Mg²⁺, HCO₃⁻). 29% of groundwater on the Vogan-Attitogon plateau have TDS higher than WHO standard (500 mg/L). High values of TDS in groundwater are generally not harmful to human beings but can affect taste and make water unsuitable and may cause gastro-intestinal irritation [39]. Samples F1, F6 and F19, have TDS over 2000 mg/L. The study shows that 17% and 7% of samples contain Cl⁻ and Na⁺ exceeding WHO suitable drinking water limits. Major ions such as Na⁺, Cl⁻ are 2-5 times higher than WHO standard for these samples. The potassium and sulfate levels are moderate and are all below the WHO standard except sample F19 for potassium. High concentrations of nitrate were found in samples F5, F13, F14, and P1. Also 7%, 24%, 69%, and 10% of groundwater sample content of calcium, iron, manganese, and nitrate exceed the guideline values for drinking fixed by the World Health Organization. A higher level of nitrate than WHO standard is undesirable in drinking water. Cadmium, arsenic, chromium, fluoride in almost all groundwater samples on the plateau were found to be under WHO standards. However, a high level of cadmium was reported in groundwater around the phosphorite treatment station at Kpeme [26]. The maximum level of cadmium was 24.74 µg/L. Fluoride was higher than the thresholds of 1.5 mg/L fixed by the World Health Organization. The fluoride present in the water is responsible for the discoloration of the teeth of many children and adults living in the phosphorite treatment area [27].

Table 6. WQI scores of samples

The Water Quality Index (WQI) score of samples calculated are shown in Table 6. WQI values in the study area range from 0.398 at F4 to 14.233 at F19. The CT groundwater of the Vogan-Attitogon plateau was classified using the water quality index (WQI) range as proposed by Horton (1965). This water quality classification shows that 71% of groundwater samples from CT on the Vogan-Attitogon plateau were classified as ‘’Excellent’’ and 21% as ‘’Good’’ (Table 7).

Table 7. WQI ranking and classification of groundwater

The spatial distribution of WQI is shown in Figure 4.

Figure 4. Maps of WQI

Samples with a high water quality index score are located in southern and eastern parts of the study area within fluvial formations. Studies generally show that shallow coastal aquifers, especially in the Gulf of Guinea, face various groundwater quality deterioration challenges, caused by seawater intrusion [2,5,6,28]. The mixing of freshwater with saline lake and lagoon systems contributes to high groundwater salinity [3]. Otherwise, the deterioration of shallow groundwater is due to anthropic factors such as recharge from farming, sewerage as well as septic contaminations [5]. The deterioration of coastal shallow groundwater quality appears as a worldwide problem. As opposed to shallow groundwater on the Vogan-Attitogon plateau, groundwater quality in the Gaza Trip was very poor and affected by many contaminant sources. The fraction of groundwater classified as "not good" jumped from about 30% to 55% between 2000 and 2010 [2930]. The high fraction of groundwater on the Vogan-Attitogon plateau classified as excellent and good could be related to the low level of urbanization in the study area.

3.3. Human Health Risk Assessment (HHRA)

In order to determine the health risk, the non-carcinogenic risk and the carcinogenic risk were calculated. The non-carcinogenic, carcinogenic health risk and human health risk has been calculated for different age groups in the study area.

3.3.1. Determination of Non-carcinogenic Risk

The values of non-carcinogenic risks for adults (males and females) and children through drinking water intake and dermal contact on the Vogan-Attitogon plateau are shown in Table 8.

The values of non-carcinogenic risk through oral exposure (HQoral) for males and females ranged from 0.571 to 3.732, and 0.727 to 4.749 with mean values of 0.963, and 1.226 respectively. The values of non-carcinogenic risk for children ranged from 1.244 to 8.127 with mean values of 2.098. The non-carcinogenic risk through dermal exposure (HQdermal) values were higher than HQoral and ranged from 1.214 to 4.373 for adult males, 1.545 to 5.565 for adult females and 2.643 to 9.523 for children with mean values of 1.622, 2.064 and 3.532, respectively. The Total non-carcinogenic risk (HQtotal) values varied between 3.887 to 17.650 for children with mean values were 5.629. The Total non-carcinogenic risk for adult females and males 2.272 to 10.135 and 1.785 to 8.104 respectively. The mean values were 3.290 and 2.585 respectively. All samples have HQtotal exceeding 1, the limits for non-carcinogenic risk for human health. This result suggests that samples from the CT aquifer of the Vogan-Attitogon plateau may induce non- carcinogenic risk to adult males, adult females and children. The HQtotal values show that children face to greater non- carcinogenic risk because their smaller body weights than adult females and males [13]. Adult females and children were exposed to contaminants than adult males.

The spatial distribution of non-carcinogenic risk among children, adult males and females in the study area is shown in Figure 5.

Table 8. Values of non-carcinogenic risks through drinking water consumption and dermal contact

Figure 5. Map of non-carcinogenic health risks for (a) adult males, (b) adult females and (c) children

Table 9. Extreme non-carcinogenic risks values (minimum and maximum) of contaminants in drinking water

The presence of Mn²⁺, NO₃⁻, F⁻, Cd²⁺ and As in CT groundwater would cause non-carcinogenic risk to adult males and females and children on the Vogan-Attitogon plateau, mainly at F19 and F39 (Mn²⁺), F3, F5, F13, F14, F15, F16, F23, F26, F28, F29, F36, P1 (NO₃⁻), F1 F2, F6, F11, F18, F19 and F22 (F⁻), F9, F10, P1 and P4 (Cd²⁺) and all samples are at non-carcinogenic risk caused by As. Table 9 shows the values of non-carcinogenic risk of different ions in drinking water in the study region.

The non-carcinogenic risk associated to each parameter (Table 9) shows that NH₄⁺, NO₂⁻ and Cr⁶⁺ can cause weake non-carcinogenic risk. But the ingestion of water may caused non-carcinogenic risk due to the presence of Mn²⁺, NO₃⁻, F⁻, As, and Cd²⁺. The order of non-carcinogenic risk for adults and children observed was NH₄⁺ < Cr⁶⁺< NO₂- < Cd²⁺ < F⁻ < As < NO₃⁻ < Mn²⁺. The health risk of children and females is much higher than for males. Groundwater contamination by Mn²⁺, NO₃⁻, F⁻, As, and Cd²⁺ is great cause for concern for drinking water supply in the study area. The previous studies in parts of the study area showed adult and children’s teeth discoloration caused by fluoride and cadmium [27]. Ingestion of nitrate in drinking water and dietary sources may result in cancer, congenital disabilities, infant methemoglobinemia or other adverse health effects [31]. Fluoride can accumulate in the body and it has been shown that continuous exposure causes damaging effects on body tissues, particularly the nervous system, in addition to dental and skeletal abnormalities [32,33]. Arsenic in drinking water was associated with increased incidents of skin cancers and cancers of the liver, bladder, respiratory and gastrointestinal tracts [23,34]. Exposure to cadmium causes renal tubular dysfunction, osteomalacia, osteoporosis, and spontaneous fractures. It has also been implicated in hypertensive disorders [35].

3.3.2. Determination of Carcinogenic Risk

Table 10 shows that the carcinogenic health risk of contaminants.

The carcinogenic health risk assessment showed that the carcinogenic risk through oral exposure (CRoral) is from 0.002 to 0.003, (mean of 0.002) for males, 0.002 to 0.003 (mean of 0.002) for females, and 0.004 to 0.006 (mean of 0.004) for children. The carcinogenic risk through dermal exposure (CRdermal) values were lower than CRoral with maximum values of 0.0004 for adult males, 0.0005 for adult females, and 0.0009 for children. The Total carcinogenic risk (CRtotal) values varied from 0.004 to 0.006 for children, 0.003 to 0.004 for adult females and 0.002 to 0.003 for adult males with means of 0.005; 0.003 and 0.002, respectively. Table 10 shows that the carcinogenic health risk classification order attributed to each element for age groups in the study area is As > Cd²⁺ > Cr⁺⁶.

The health risk of children is much higher than for males and females (Figure 6).

The carcinogenic health risk in the study area is mainly associated to arsenic and cadmium pollution. Arsenic, cadmium, and chromium content do not exceed the limits in drinking water (0.010 mg/L, 0.003 mg/L, 0.05 mg/L, respectively), but it induce a high carcinogenic risk for adults (males and females) and children in the study area. The ingestion of cadmium and arsenic is the chief reason of carcinogenic risks in the study area. The carcinogenic risks due to cadmium and arsenic was higher than the limit of carcinogenic risk of 10-5 fixed by US-EPA. The south and the North near mining sites at Kpeme and Tabligbo presents the high carcinogenic risk in this area. The atmospheric deposition of industrial dust containing Cd²⁺ and F⁻ may contribute to waterbody contamination [26] [27]. A study in Tabligbo confirmed the results of the present stud. Soils were affected by contamination of trace metal elements, including cadmium, lead, nickel, and copper [10].

Table 10. Extreme carcinogenic risks values (minimum and maximum) of contaminants in drinking water

Figure 6. Maps showing carcinogenic health risks for (a) adult males, (b) adult females and (c) children

3.4. Groundwater Salinity and Contamination

The quality of groundwater on the Vogan-Attitogon plateau was affected by diverse parameters. Correlation between certain elements were investigated to identify the Vogan-Attitogon plateau sources of CT groundwater pollution.

Scatter plots were used to appraise relationships between ions and the probably associated processes. The scatter plot Figure 7a shows a high correlation between Na⁺ and Cl⁻. The high values of Na⁺ and Cl⁻ are caused by seawater intrusion and by dissolution of evaporites in the alluvial formation. Salt dissolution in alluvium was also identified as the primary groundwater salinization source in the Shire River valley, Malawi [36]. The scatter plot of Na⁺/Cl⁻ vs Cl⁻ (Figure 7b) shows that the mineralization of groundwater is also influenced by evaporation. The median of Na⁺/Cl⁻ ratio of about 0.90 indicates seawater intrusion influence on groundwater chemical composition. Mixing of groundwater with seawater is also supported by the high correlation between (Na⁺+K⁺) and strong acids anions (Cl⁻+SO₄²⁻) (Figure 7d) [37].

The correlation between NO₃⁻ and Cl⁻ shows the source of chloride pollution. The scatter plot of NO₃⁻ vs Cl⁻ (Figure 7e) indicates that seawater intrusion was the origin of greater chloride and high TDS in groundwater [2]. The correlation between NO₃⁻/Cl⁻ and Cl⁻ shows the origin of nitrate pollution. NO₃⁻/Cl⁻ vs Cl⁻ scatter plot (Figure 7f) indicates the anthropogenic origin of nitrate in groundwater. Agricultural practices using fertilizers and contamination from human wastes are often the sources of nitrate in water in the study area [5,7]. High nitrate with low or increasing Cl⁻ suggests the impacts of urbanization and agrochemicals. A high concentration of nitrate was found in boreholes located in the center and the north of the Vogan-Attitogon plateau (F5, F13, F14, P1). However, the frequency of NO₃⁻ contamination is much lower than in the most urbanized areas around Lome, located in the western part of Togo's coastal sedimentary basin [5,38]. On the scatter plot of Ca²⁺/Mg²⁺ vs Cl⁻, samples with ratio Ca²⁺/Mg²⁺ greater than 1 (Figure 7c) suggests that seawater intrusion and other hydrochemical processes were controlling the mineralization of groundwater [28]. Iron and manganese contents are related to the geological environment and hydrogeochemical processes. The content of iron is influenced by precipitation, redox conditions and evaporation. Precipitations, redox conditions and evaporation have influence on the content of iron and manganese in groundwater. The pH of groundwater may cause pyrite dissolution which increases iron and manganese content. High concentration of iron and manganese in shallow groundwater on the Vogan-Attitogon plateau could be attributed to a higher intensity of evaporation and hydrochemical process of reduction [2]. Mining exploitation in the area contributes to groundwater degradation caused by trace elements pollution [10,26]. Figure 6 shows that higher carcinogenic health risks due to trace elements occurs in the south of the study area, around Kpeme, the site of phosphate treatment, and in the north, near Tabligbo, the site of clinker treatment.

Figure 7. Scatter plots of Na⁺ vs Cl⁻ (a); Na⁺/Cl⁻ vs Cl⁻ (b); Ca²⁺/Mg²⁺ vs Cl⁻ (c); (Na⁺+K⁺) vs (Cl⁻+SO₄²⁻) (d); NO₃⁻ vs Cl⁻ (e); NO₃⁻/Cl⁻ vs Cl- (f)

4. Conclusion

In this study, 42 samples of shallow groundwater from the Vogan-Attitogon plateau and 8 samples of surface water were analyzed to determine the physicochemical characteristics for assessing quality using WQI and the human health risk assessment model.

The ionic dominance pattern was in the order of Na⁺ > Ca²⁺ > Mg²⁺ > K⁺ for cations and Cl⁻> SO₄²⁻ > HCO₃⁻>NO₃⁻ for anions. Cl-Na water type was dominant, followed by accessory water types Ca-Cl and Ca-Mg-Cl. According to the water quality index (WQI), the groundwater samples classified as excellent and good for drinking account for 93% of all samples, while the samples unsuitable for drinking account for 5%. The poor and extremely poor water for human consumption was mainly located in the southern and eastern parts of the study area.

The assessment of non-carcinogenic risk varied from 3.887 to 17.650 for children, from 2.272 to 10.135 for adult females and from 1.785 to 8.104 for adult males with means of 5.629, 3.290 and 2.585 respectively. The non-carcinogenic risk for adults and children was observed in the order NH₄⁺< Cr⁶⁺ < NO₂⁻ < Cd²⁺ < F- < As < NO₃⁻ < Mn²⁺. The assessment of carcinogenic risk varied from 0.004 to 0.006 for children, from 0.003 to 0.004 for adult females and from 0.002 to 0.003 for adult males with means of 0.005, 0.003 and 0.002 respectively. The carcinogenic health risk in the study area is due mainly to contamination by arsenic and cadmium. Although arsenic, cadmium and chromium content do not exceed the WHO limits for drinking water, there is a high carcinogenic risk for different people in the study area. Groundwater quality is affected by seawater intrusion, evaporite dissolution, high nitrate content from farming recharge and trace elements related to the geological environment, hydrogeochemical processes and mining exploitation in the area. It is necessary to take action for sustainable groundwater quality management in the study area.

Acknowledgements

This study was carried out with the technical support of the Laboratory of Applied Hydrology and Environment (Ex Laboratory of Water Chemistry), University of Lomé (Togo), and the National Institute of Hygiene of Lome (Togo). Thanks for the collaboration between the Ministry in charge of water (Togo), the Laboratory of Applied Hydrology and Environment (University of Lomé, Togo), and the National Institute of Hygiene of Lome (Togo). The authors would like to thank Dr Gibrilla Abass and anonymous reviewers for their comments suggestions to improve the quality of the paper.

Funding

No funding was received for conducting this study.

Statement of Competing Interests

The authors declare they have no financial interests.

Ethical Approval and Informed Consent

Not applicable. The study does not involve human or animal subjects.

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