Groundwater Characteristics and Quality in the Coastal Zone of Lomé, Togo

Abstract

The unprecedented development of coastal cities in West Africa is marked by anarchic urbanization accompanied by ineffective environmental management, leading to water pollution. This study is conducted in the southern districts of Lomé, Togo, an area built on sandbars where inappropriate attitudes, behaviors, and inadequate hygiene and sanitation practices prevail. The objective of this study is to characterize the quality of groundwater in the study area. Bacteriological and physicochemical analyses were carried out on 11 wells in 10 districts in the southern districts during the four seasons of the year. The analysis shows that the groundwater is polluted in all seasons. Nitrate concentrations exceed 50 mg/L in 65% of the samples, while chloride levels surpassed 250 mg/L in 18% of the cases. Regardless of the season, the dominant facies is sodium chloride and potassium chloride. In all districts, the analysis of microbiological parameters including total germs (30 °C, 100/mL), total coliforms (30 °C, 0/mL), Escherichia coli (44 °C, 2/250 mL), fecal streptococci (0/100 mL), and anaerobic sulfite reducers (44 °C, 2/20 mL) reveals values exceeding the European Union standards (2007). Groundwater contamination is facilitated by the sandy nature of the soil, which increases its vulnerability to various pollutants. Togo continues to experience cholera outbreaks, aggravated by poor sanitation infrastructure and limited vaccination coverage. Public health efforts are directed toward improving sanitation and raising awareness about waterborne and non-communicable diseases.

1. Introduction

Developing countries in general, and African cities in particular, are experiencing unprecedented urbanization, characterized by a lack of planning for the most essential sanitation services [1]. The situation is even more serious in coastal cities, characterized by rapid population growth and weak environmental governance. In this context, poor waste management (solid, liquid, excreta) is a threat to groundwater quality [2], yet it is valued by a variety of programs, plans, and projects. Water is essential to human dignity, as it is vital for survival, hygiene, food production, education, and economic development [3,4,5].

Sustainable Development Goal 6 (SDG 6) aims to ensure universal access to water and sanitation. It focuses on the availability and sustainable management of water resources, promoting water security for all [6]. Achieving SDG6 will require ensuring access to adequate and equitable sanitation and hygiene for all and ending open defecation while paying attention to the needs of women, girls, and vulnerable people. However, when people reside in vulnerable environments, their activities ultimately degrade the quality of the water resources available to them [7,8]. Achieving the Sustainable Development Goals (SDGs) on water, sanitation, and hygiene in Africa requires a dramatic acceleration in the current rates of progress, according to a UNICEF/WHO special report. Major inequalities persist within countries, particularly between urban and rural areas, between regions, and between the richest and the poorest. In urban areas, two out of five people lack access to safe drinking water, two out of three lack access to safe sanitation, and half the population lacks basic hygiene services. As a result, countries and cities on the African continent are facing major water-related problems in addition to floods, droughts, and concomitant pollution problems [9].

In Togo, urban sustainability issues are included in the National Development Plan, sustainable development agendas, and the government roadmap 2021–2025, which aims to increase the share of households with access to drinking water and sanitation. The observation is that in the city of Togo (Lomé), which is home to 27% of the country’s population [10], the population still has difficulty accessing clean water. The water supply is provided by a failing water supply network that is struggling to keep up with urban sprawl. Even if the water network is available in the lower town of Lomé, not all households are connected to it due to the high cost of the subscription, thereby excluding a significant portion of the population from this service. Thus, the rate of access to drinking water in Lomé in 2022 was 69% according to the ministry in charge of water. Faced with this situation, the population resorts to groundwater from traditional wells, especially since in this sector of the city, the water is at a shallow depth and is very accessible to the population. The practices of autonomous sanitation not in accordance with standard construction standards, poor management of solid waste at the household level, and poorly respected hygiene rules contribute to the degradation of water quality. Along the West African coast, groundwater is subject to multiple threats stemming from both human activities and climate change. A regional study conducted along the Benin coast utilizing hydrogeochemistry and stable water isotopes revealed that the upper, more heterogeneous aquifer contains a variety of water types, whereas the lower aquifer is predominantly characterized by a sodium chloride (Na-Cl) water type. The upper aquifer exhibited numerous signs of contamination, primarily due to anthropogenic activities and saltwater intrusion from adjacent lakes and lagoons. In contrast, the deeper aquifer showed a more pronounced geogenic influence [11]. Building on this context, the present study aims to assess the microbiological and physicochemical quality of groundwater in the lower town of Lomé by analyzing key water quality parameters. The findings reveal that the water is consistently of a sodium chloride to potassium type throughout the year. From a bacteriological standpoint, none of the groundwater samples met drinking water standards. Similarly, several physicochemical parameters exceeded the guideline values established in [12]. These results underscore the need for sustainable groundwater preservation strategies, such as solar disinfection, the application of Moringa-based purification methods, and continuous public awareness campaigns promoting best practices in groundwater use.

2. Materials and Methods

2.1. Study Area

Lomé, the capital city of Togo, is located on a low-lying coastal plain predominantly composed of sandbanks. The city extends northward over a series of Terre de Barre plateaus, which date back to the Neogene period. These plateaus are characterized by the presence of closed depressions, topographic features believed to result from sediment compaction processes. According to [13], this compaction affects extensive clay lenses that are interlayered with both fine and coarse detrital deposits. This plateau area is called the “Upper Town”, while the area of low topographic cordons is called the “Lower Town”. These two topographies are separated by a coastal re-entrant sheltering a lagoon (Figure 1).

The irregular surface is marked by numerous circular depressions (100 to 400 m), with altitudes of 10 and 35 m. The slopes are very steep but are generally gentle towards the lagoon (1%). These natural depressions or those dug out in the absence of a drainage network serve as a receptacle for runoff water in certain districts.

The coastal plain (from west to east) is made up of well-differentiated sand ridges corresponding to poorly developed soils on sandy alluvium with altitudes of 6 to 7 m. Of fluvio-marine origin, the internal ridges are made up of reworked sediments of 90% fine to medium sand, with shell debris. The external ridges are a succession of sand spits 2 to 3 km wide with an average height of 5 m and are composed of 98–100% coarse sand [13].

From a hydrogeological point of view, the coastal sedimentary basin contains a multi-layer aquifer with high groundwater potential and productivity. Three aquifers follow one another at depth below the Quaternary water table [14,15,16]. The Continental Terminal, the most exploited aquifer in the coastal sedimentary basin, is an unconfined water table with static levels varying between 10 and 40 m. The Paleocene is a succession of limestones and sands with a thickness of 2 to 30 m, whose static level is between 30 m west of Lomé and 90 m north of the city. Finally, the Maastrichtian is an alternation of more or less thick sandy and clayey layers. The most productive levels are often located at a depth of more than 50 m in the north. Further south of the city, they are found at more than 200 m.

The city of Lomé enjoys a humid tropical climate with four seasons, two rainy seasons alternating with two dry seasons: a long dry season from November to mid-March; a long rainy season from mid-March to mid-July; a short dry season from mid-July to the end of August; and a short rainy Season from September to October. The average monthly temperatures range from 26.3 °C in August to 29.7 °C in March during the long dry season (Figure 2).

The city of Lomé is home to nearly 27% of the national population (8,095,000 inhabitants [10]), 30% of whom live in the Lower Town, which includes its original core, the urban center, shops, and the seaport area.

2.2. Site Selection and Sampling

Groundwater characterization was carried out through physicochemical and bacteriological analyses of water samples collected from 10 districts, selected to ensure a representative spatial distribution of the southern part of Lomé [17,18]. In total, 88 water samples were analyzed at a rate of 22 samples per season over the 4 seasons, or 8 samples per district, except in one district where 16 samples were taken due to the proximity of two wells, one watertight and the other non-watertight.

These analyses made it possible to determine the quality of the water based on the absence or presence of microorganisms and high values of physicochemical elements during the four seasons of the year (long rainy season (LRS), July, short dry season (SDS), August, short rainy season (SRS), October, and long dry season (LDS), January).

2.3. Method of Collection

Some sterilized 500 mL bottles were used for microbiological analyses and 1500 mL plastic bottles for physicochemical analyses. The well water is collected using buckets found on the edges of the wells, and it is in the same condition as that of the users. Once drawn, the water is poured directly into sterilized glass bottles; in contrast, plastic bottles are rinsed two to three times prior to filling to ensure a homogeneous sampling environment. The glass and plastic bottles were then closed and placed in a cooler containing cold accumulators to maintain the sample at a temperature of approximately 4 °C. The samples were transported to the Laboratory of Microbiology and Quality Control of Foodstuffs (LAMICODA) and the Laboratory of Water Chemistry (LCE) of the University of Lomé for analysis. In addition, our (04) bacteriological parameters were selected to identify fecal contamination [19].

2.4. Parameters Analyzed

The physicochemical analyses focused on temperature, pH, electrical conductivity, turbidity, chlorides, nitrates, and nitrites, which were represented and compared to a World Health Organization (WHO) standard or the European Union (EU) when applicable. The main cations, such as calcium (Ca²⁺), magnesium (Mg²⁺), sodium (Na⁺), and potassium (K⁺), and the main anions, bicarbonate (HCO₃²⁻), sulfate (SO₄²⁻), chloride (Cl⁻), and carbonate (CO₃²⁻), were analyzed in the from of a Pipper diagram [20] in order to identify water types and the geochemical processes that affect them. Diagram software, Version 9.20 was used for this purpose to display the hydrochemical structure of water. The selected parameters are shown in

Table 1. Physicochemical and bacteriological parameters studied.

Parameters	Role
Bacteriological parameters
Total germs (30 °C)	Also known as total aerobic mesophilic flora germs, they provide information on the general wholesomeness (hygienic conditions of preparation or production) of a product, water, etc.
Total coliforms (30 °C) (44 °C)	Total coliforms are considered test germs for fecal contamination.
Escherichia coli (44 °C)	E. coli is of intestinal origin and may also be of fecal origin or originate from soil or plants. Its presence in well water is considered an indication of pollution.
Fecal streptococci (37 °C)	Fecal streptococci provide confirmation of the nature of fecal pollution.
Anaerobic sulfite reducer	Used as an indicator of previous faecal pollution.
Physicochemical parameters
Temperature	Temperature regulates the maximum concentration of dissolved oxygen in water and influences the rate of chemical and biological reactions.
pH	Corresponds to the concentration of H+, the hydrogen potential, and determines whether the water is acidic or basic.
Electrical conductivity	The ability to conduct electricity. It is used to assess the quantity of mineral substances dissolved in ionic form.
Turbidity	Water disorder and poor light penetration, affecting photosynthesis.
Calcium (Ca2+) Magnesium (Mg2+) Sodium (Na+) Potassium (K+)	Cations are among the major ions in water: calcium, magnesium, sodium, and potassium. These are the major components that determine the hardness of water, particularly calcium and magnesium. They are linked to the rock formations crossed (limestone, dolomite).
Nitrates (NO3−) Chlorides (Cl−) Sulfates (SO42−)	These are the anions that make up the major ions in water. A high chloride and sulfate content intensifies the corrosiveness of water. Chlorides express the quantity of the chloride ion (Cl−) present in the water and help to determine the overall salinity of the water.
Carbonates (CO32−) Bicarbonates (HCO3)	These parameters play a role in stabilizing the pH.
Nitrate/nitrite (NO2) Ammonium (NH4+)	Nitrogenous matter represents the sum of organic and ammoniacal nitrogen; its presence indicates nitrogenous mineral pollution.
Oxidizable materials (KMnO4) Orthophosphates (PO43−)	Used to estimate total organic pollution.

.

The search for pathogenic germs (fecal coliforms, E. coli, total coliforms, fecal streptococci, sulfite-reducing anaerobic bacteria) makes it possible to highlight pollution or contamination of fecal origin [21,22], etc. To prove this origin, the method of [23], the ratio (R) between total coliforms and fecal streptococci, makes it possible to state whether it is one of the following:

• Animal origin (R < 0.7);
• Human R > 4;
• Mixed when 0.7 < R < 1;
• Uncertain origin when 1 < R < 2;
• Mixed predominantly human 2 < R < 4.

3. Results

3.1. Physicochemical Characteristics of Groundwater in the Lower Town of Lomé

It is observed that the average water temperature varies in the lower town of Lomé from 27.4 °C to 28.13 °C (

Table 2. Some physicochemical parameters of well water in the lower town of Lomé.

	T °C	pH	Cond. Elec. 25 °C—µs/cm	Dissolved Solids—mg/L	Nitrates (NO3)—mg/L	Nitrites (NO2−)—mg/L	Chloride (Cl−)—mg/L
Nyekonakpoè	27.60	7.20	2532.50	1919.50	33.40	0.30	354.15
Kodjoviakopé	27.70	7.40	994.00	753.50	93.30	0.60	133.15
Hanoukopé	28.10	7.10	2197.50	1740.80	14.30	0.10	499.35
Wetrivikondji	28.00	7.10	829.00	671.30	107.00	0.30	90.88
Béniglato	27.90	7.40	1137.30	861.50	27.10	1.80	153.93
Amoutivé	27.50	7.20	1144.30	867.50	70.60	1.90	115.13
Gbényédji 1	27.50	7.20	1792.00	1358.50	99.20	3.20	219.93
Gbényédji 2	27.40	7.20	1160.80	880.00	104.30	2.00	157.18
Akodessewa Kpota	27.60	7.20	2384.00	1807.30	191.40	1.30	193.95
Ablogame	27.60	7.20	1553.50	1177.80	140.70	1.40	203.45
Souza Nétimé	27.60	7.20	1153.80	874.50	127.90	1.50	113.85
Mean	27.68	7.22	1534.43	1173.84	91.75	1.31	203.18
SD	±0.22	±0.11	±600.71	±459.28	±53.02	±0.93	±121.93
Min	27.40	7.10	829.00	671.30	14.30	0.10	90.88
Max	28.10	7.40	2532.50	1919.50	191.40	3.20	499.35

). The highest values are recorded in Hanoukopé, while the lowest are recorded in Gbényédji 2. Fresh water is more appreciated as drinking water, but biological production depends strongly on temperature, which affects its physicochemical properties, in particular the solubility of gases and the speed of chemical reactions [24].

The average pH varies from 7.10 in Hanoukopé and Wétrivikondji to 7.40 in Béniglato (Table 2). The pH of drinking water usually varies from 6.5 to 8.5, but it is characterized as neutral with a value of 7 [24], as is the case for the wells in the lower town of Lomé. The pH depends on the origin of the water, the geological nature of the substrate, and the watershed it crosses [24]. It conditions the physicochemical balance between water, dissolved gases, carbonates, and bicarbonates, which confers favorable development to aquatic life.

Regarding electrical conductivity, the recorded averages range from 829 to 2532.5 µs/cm, from Wetrivikondji to Nyékonakpoè (Table 2). The water samples exceed the accepted conductivity standard of 1000 µs/cm, which guarantees good quality and a pleasant taste for drinking water [25]. Conductivity expresses the mineralization of the water and represents the quantity of dissolved salts, and, therefore, its capacity to conduct current. It gives an idea of the geological substrate crossed.

Chloride averages range from 90.88 mg/L in Wétirvikondji to 499.35 mg/L in Hanoukopé, with an overall average of 203.18 mg/L (Table 2). In 81.8% of cases, the chloride level is below the WHO standard of 250 mg/L.

As for dissolved solids, their concentrations vary from 671.3 to 1919.5 mg/L, always from Wetrivikondji to Nyékonakpoè. All water samples have dissolved solid values significantly higher than the standard, which is set at 300 mg/L [25] (Table 2). Total dissolved solids estimate the total residue remaining after evaporation of a water sample that has been filtered to remove suspended solids larger than 1 mm [26].

The nitrate standard set at 50 mg/L is exceeded in 60% of neighborhoods, with values ranging from 14.3 in Hanoukopé to 191.4 in Akodessewa Kpota. Nitrite values, meanwhile, range from 0.1 in Hanoukopé to 3.20 mg/L in Gbenyegbji2, with a standard of 3 mg/L (Table 2).

All other anions and cations in the water samples are shown in the Piper diagram. Anions are bicarbonate (HCO₃²⁻), sulfate (SO₄²⁻), chloride (Cl⁻), and carbonate (CO₃²⁻), while cations are represented by calcium (Ca²⁺), magnesium (Mg²⁺), sodium (Na⁺), and potassium (K⁺). Figure 3 shows that, whatever the season, the dominant facies is sodium and potassium chloride, which are very pronounced during the dry season, but during the rainy season, the grouping is less compact.

3.2. Bacteriological Characteristics of Groundwater in the Lower Town of Lomé

Microbiological analyses of total germs, total coliforms, Escherichia coli, fecal streptococci, and sulfite-reducing anaerobes give values well above [12] (

Table 3. Microbiological analyses of well water from the coastal zone of Lomé.

Quartier	Total Germs (30 °C) 100/mL	Total Coliforms (30 °C) 0/mL	Escherichia coli (44 °C) 2/250 mL	Fecal Streptococcus 0/100 mL	Sulfite-Reducing Anaerobes (44 °C) 2/20 mL
Nyekonakpoè, LRS	1.66	1	1	1	1
Nyekonakpoè, SDS	2800	22	2	1	1
Nyekonakpoè, SRS	3700	6	1	4	1
Nyekonakpoè, LDS	8500	3	1	2	12
Kodjoviakopé, LRS	2.94	72	6	5	4
Kodjoviakopé, SDS	4300	59	1	118	1
Kodjoviakopé, SRS	3200	210	1	3	2
Kodjoviakopé, LDS	4100	7	1	12	1
Hanoukopé, LRS	39,000	1000	112	41	<1
Hanoukopé, SDS	16,000	110	<1	42	6
Hanoukopé, SRS	1400	66	<1	11	<1
Hanoukopé, LDS	3600	18	1	2	4
Wetrivikondji, LRS	15,000	84	1	1	1
Wetrivikondji, SDS	1800	61	1	1	1
Wetrivikondji, SRS	330	11	1	1	1
Wetrivikondji, LDS	3000	30	1	38	6
Béniglato, LRS	740	14	<1	<1	2
Béniglato, SDS	12,000	22	<1	1	10
Béniglato, SRS	900	5	1	2	2
Béniglato, LDS	14,000	6	<1	2	6
Amoutivé, LRS	5000	17	1	1	1
Amoutivé, SDS	200,000	180	1	2	4
Amoutivé, SRS	2300	60	1	18	1
Amoutivé, LDS	7700	52	1	1	50
Gbényedji 1, LRS	36,000	2	1	8	2
Gbényedji 1, SDS	120,000	430	1	4	74
Gbényedji 1, SRS	3600	37	1	9	8
Gbényedji 1, LDS	14,000	240	1	4	20
Gbényedji 2, LRS	4000	5	<1	<1	<1
Gbényedji 2, SDS	30,000	80	<1	2	12
Gbényedji 2, SRS	50,000	260	<1	<1	<1
Gbényedji 2, LDS	5500	38	<1	51	22
Akodessewa Kpota, LRS	1930	42	13	3	6
Akodessewa Kpota, SDS	1900	83	5	20	2
Akodessewa Kpota, SRS	1000	100	68	21	1
Akodessewa Kpota, LDS	4000	16	1	23	10
Ablogamé, LRS	5000	15	1	1	18
Ablogamé, SDS	2100	100	1	1	2
Ablogamé, SRS	2700	42	1	1	10
Ablogamé, LDS	9600	40	1	4	4
Souza-Nétimé, LRS	5000	7	1	3	2
Souza-Nétimé, SDS	7500	39	1	12	2
Souza-Nétimé, SRS	2900	31	1	1	1
Souza-Nétimé, LDS	6700	95	1	1	1
Min	1.66	1	1	1	1
Mean	15,063.7	86,8	6,7	11,7	7.9
Max	200,000	1000	112	118	74
SD	±34,859.9	±163.8	±21.6	±21.3	±14.0
Standard OMS	100/mL	0/mL	2/250 mL	0/100 mL	2/20 mL

).

The values from the bacteriological analysis of the well waters of the lower town show large disparities from one point to another for the different parameters, in particular for the total germs, whose value is 34,859.9. For the total coliforms, the dispersion is also important, with a value of 163.8. The Escherichia coli and total streptocopes drop to 21.6 and 21.3. Finally, for sulfite-reducing anaerobes, the dispersion is 14.

Figure 4 illustrates the seasonal distribution of total germs in well water across the 10 districts of the study area. During the long dry season (November–March), none of the wells complied with the 100 CFU/100 mL standard for total germs. Concentrations ranged from 3000 to 14,000 CFU/100 mL, with the highest values recorded in the Béniglato and Gbenyiédji 1 districts. In the long rainy season (March–July), only two wells located in Nyékonakpoè and Kodjoviakopé demonstrated concentrations below the standard, with values of 1.66 and 2.94 CFU/100 mL, respectively. However, extreme values reached up to 39,000 CFU/100 mL in Hanoukopé. During the short dry season (July–September), all districts exceeded the acceptable threshold, with values ranging from 1800 CFU/100 mL in Wetrivikondji to a peak of 200,000 CFU/100 mL in Amoutivé. A similar pattern was observed during the short rainy season (September–November), with counts ranging from 330 CFU/100 mL in Wetrivikondji to 50,000 CFU/100 mL in Gbényédji 2. Across all seasons, 95% of wells were found to be contaminated with total germs, indicating widespread microbiological pollution. Only two wells met the standard during the long rainy season, highlighting the severity and persistence of microbial contamination in the area.

Figure 5 outlines the seasonal variation in total coliform concentrations across wells in the 10 districts based on the standard of 0 CFU/mL. During the long dry season (November–March), concentrations ranged from 3 CFU/mL in Nyékonakpoè to 240 CFU/mL in Gbényédji 1. In the long rainy season (March–July), levels varied from trace amounts (<1 CFU/mL in Nyékonakpoè) to a maximum of 1000 CFU/mL in Hanoukopé. During the short dry season (July–September), concentrations ranged from 22 CFU/mL in Nyékonakpoè to 430 CFU/mL in Gbényédji 1. In the short rainy season (September–November), values ranged between 5 CFU/mL in Béniglato and 240 CFU/mL in Gbényédji 2. Across all four seasons, none of the wells complied with the standard of 0 CFU/mL for total coliforms.

Figure 6 illustrates the concentration of Escherichia coli in wells across the 10 districts of the lower town of Lomé. During the long dry season (November–March), all wells recorded E. coli levels below the acceptable standard of 2 CFU/250 mL. In contrast, during the long rainy season (March–July), three wells exceeded this standard: Kodjoviakopé (6 CFU/250 mL), Hanoukopé (112 CFU/250 mL), and Akodesséwa Kpota (13 CFU/250 mL). In the short dry season (July–September), only the Akodesséwa Kpota well exceeded the standard, with a value of 5 CFU/250 mL. Similarly, during the short rainy season (September–November), Akodesséwa Kpota again exceeded the threshold, recording 68 CFU/250 mL. Overall, 11% of the wells showed coliform contamination.

The fecal streptococcus analysis indicates that all wells are contaminated, exceeding the permissible standard of 0 CFU/100 mL. During the long dry season, concentrations ranged from 1 CFU in Souza Nétimé to 51 CFU in Gbényedji 2. In the long rainy season, values ranged from 1 CFU in Nyékonakpoè and Ablogamé to 41 CFU in Hanoukopé. Throughout the short dry season, levels ranged from 1 CFU in several districts to a peak of 118 CFU in Kodjoviakopé. In the short rainy season, concentrations varied from trace levels to 68 CFU in Akodessewa Kpota (Figure 7). Overall, 47% of the wells exceeded the standard, indicating fecal streptococcal contamination.

Figure 8 represents seasonal variation and spatial distribution of anaerobic sulfite-reducing bacteria (Figure 7) at 44 °C in groundwater wells across the coastal zone of Lomé during the four annual seasons across the 10 districts. The data indicate partial compliance with water quality standards throughout the year. During the long dry season, only two wells (Kodjoviakopé and Souza Nétimé) met the recommended standard, while others exceeded the value, with the highest concentration reaching 50 CFU/20 mL in Amoutivé. In the long rainy season, five wells complied with the standard, but contamination was still observed, with a notable peak of 18 CFU/20 mL in Ablogamé. In the short dry season, three wells remained below the threshold, and another three met the standard, while Gbényédji 1 recorded an elevated concentration of 78 CFU/20 mL. During the short rainy season, only three wells, Hanoukopé (4 CFU/20 mL), Gbényédji 1 (8 CFU/20 mL), and Ablogamé (10 CFU/20 mL), surpassed the standard. Overall, 43% of the wells were found to be contaminated by anaerobic sulfite-reducing bacteria.

The application of the [23] method for searching pathogenic germs shows that 73% of well water samples have pollution of human origin, 18% of animal origin, 6.8% of uncertain origin, and, finally, in 2% of cases, it is mixed (

Table 4. Origin of contamination in wells, depending on the season.

Season

LRS

SDS

SRS

LDS

LRS

SDS

SRS

LDS

LRS

SDS

SRS

LDS

Quartier

Nyekonakpoè

Nyekonakpoè

Nyekonakpoè

Nyekonakpoè

Kodjoviakopé

Kodjoviakopé

Kodjoviakopé

Kodjoviakopé

Hanoukopé

Hanoukopé

Hanoukopé

Hanoukopé

R = CT/SF

Season

LRS

SDS

SRS

LDS

LRS

SDS

SRS

LDS

LRS

SDS

SRS

LDS

Quartier

Wetrivikondji

Wetrivikondji

Wetrivikondji

Wetrivikondji

Béniglato

Béniglato

Béniglato

Béniglato

Amoutivé

Amoutivé

Amoutivé

Amoutivé

R = CT/SF

Season

LRS

SDS

SRS

LDS

LRS

SDS

SRS

LDS

LRS

SDS

SRS

LDS

Quartier

Gbényedji 1

Gbényedji

Gbényedji 1

Gbényedji 1

Gbényedji 2

Gbényedji 2

Gbényedji 2

Gbényedji 2

Akodessewa

Akodessewa

Akodessewa

Akodessewa

R = CT/SF

Season

LRS

SDS

SRS

LDS

LRS

SDS

SRS

LDS

Quartier

Ablogamé

Ablogamé

Ablogamé

Ablogamé

Souza-Nétimé

Souza-Nétimé

Souza-Nétimé

Souza-Nétimé

R = CT/SF

Uncertain origin

human contamination

animal contamination

mixed origin

).

The origin of groundwater contamination varies across districts and seasons in the lower town of Lomé.

-

The Nyékonakpoè well exhibits human-origin pollution during the short dry season and contamination of uncertain origin in other seasons.

-

Kodjoviakopé shows animal-derived contamination during the short and long dry seasons and animal contamination in the rainy season.

-

Hanoukopé, Amoutivé, Ablogamé, and Souza Nétimé wells are consistently polluted by human sources year round.

-

In Wetrivikondji, contamination is human related, except in the long dry season, when it is of mixed origin.

-

The Béniglato and Gbényédji 1 wells are affected by animal-origin contamination during the long rainy season and human-origin contamination during other periods.

-

In Gbényédji 2, pollution is primarily of animal origin year round, except during the short dry season.

-

Akodessewa Kpota is characterized by human-origin contamination throughout the year, except in the long dry season, when the source is animals.

In all cases, pollution of anthropogenic origin predominates, hence the need to find ways to reduce this pollution.

4. Discussion

This study aims to analyze the physicochemical and bacteriological characteristics of well water in the lower town of Lomé. The findings reveal microbial contamination across all 11 wells located in 10 districts, observed consistently over the four seasons of the year. It should be remembered that the lower town of Lomé is one of the first areas of occupation of the capital by the population. Traditional wells are present in all the concessions due to the accessibility of water in the ground at less than 1 m. The required distance between wells, toilets, and sumps, according to WHO standards of 15 m, is not always respected, which exposes the wells to contamination. Micropollutants migrate both vertically and horizontally through the subsurface toward groundwater, a process facilitated by the region’s sandy soils [27], which are characterized by high porosity and unconsolidated formations. These conditions enhance the infiltration of leachates into the aquifer system.

The temperature of the water samples (25.5°C and 28.5 °C) is within the most favorable ranges for the development of bacteria. The same temperatures were observed in Guinea-Bissau [28]. The impact of high temperature also depends on the level of eutrophication, as this increase in temperature leads to a decrease in the saturation concentration of dissolved oxygen in the water, thus favoring the appearance of bacteria [29,30]. Temperature is also decisive for the solubility of gases, the dissociation of dissolved salts, and the determination of pH [31], which here varies between 6.83 and 7.73 and has an alkaline tendency. As [32,33] have already pointed out, high water temperatures constitute a culture medium for microorganisms and, therefore, constitute favorable conditions for water pollution in tropical environments.

The sodium–potassium chloride hydrochemical facies of the analyzed waters reveals both natural mineralization related to the geological nature of the aquifer and organic mineralization related to hygiene and sanitation practices [25]. In addition, the water tables are exposed to pollution, saline intrusion, which explains the high chloride levels and conductivity (664 to 2990 µs/cm). However, Refs. [11,23] have shown that salinization may be related to the intrusion of lagoon water in the area.

Microbiological analyses have shown that the water from the wells is of insufficient quality compared to WHO and EU criteria (2007), indicating fecal contamination. This soil context is conducive to groundwater contamination. This is the case in Ouagadougou, where a study of 32 borehole water samples [34] reveals the contamination of water points near latrines, garbage, etc. These microbiological results corroborate those of [28,34,35,36,37,38]. Similar results were found in Meknes, Morocco [24], for total coliforms and fecal streptococci. The same is true in Abengourou, Ivory Coast, where [39] shows that 28% of wells are contaminated. Ref. [34] also reports the presence of bacteria such as E. coli, anaerobic sulfite reducers, and fecal coliforms. This fecal contamination is further supported by the methodology outlined in [23], which identifies human-origin pollution in the majority of wells. However, the relatively low concentration of E. coli observed in the current study contrasts with the findings of [34], suggesting potential variations in contamination sources or temporal factors. However, Ref. [38] showed less pollution in the boreholes located on the plateau in Lomé. It shows a significant difference (p < 0.05) between the water from the wells and that of the boreholes for all germs. These are located on sandy clay soil, which makes it difficult to drill large-radius wells. The boreholes are dug to an average depth of 30 m [38].

Microbiological contamination by pathogens such as E. coli, anaerobic sulfite-reducing bacteria, and fecal coliforms is a significant health risk, often leading to diseases like cholera. This has been evident in Lomé during flooding periods, particularly in 2017, 2023, and 2024, when hospitals in Lomé’s sanitary districts reported 3, 21, and 23 cases, respectively. Flooding exacerbates groundwater contamination by increasing infiltration and percolation in the soil, which, over time, worsens pollution. During these periods of heightened risk, awareness campaigns are conducted to educate the public on preventive behaviors to avoid diarrheal diseases. These include regular hand washing with soap and water, proper food protection, thorough washing of fruits and vegetables before consumption, and the disinfection of rainwater, cistern water, river water, marsh water, and pond water. In addition to these measures, Ref. [40] recommends simple water decontamination techniques, such as solar disinfection, where bottles of polluted water are exposed to sunlight for several hours, leading to the inactivation of microbes. Filtration through sand and gravel columns can remove up to 97% of E. coli. Ref. [41] demonstrated the effectiveness of water treatment using Moringa oleifera in Ghana, which resulted in a 90.99% reduction in fecal coliform concentrations in surface water. While these methods have been successfully implemented in certain areas, regular updates and monitoring campaigns are essential. These efforts should involve partnerships with NGOs in the water sector to ensure continuous monitoring and quality assurance, particularly in the poorest households.

5. Conclusions

The lower town of Lomé, corresponding to the oldest settlement area of the city, is built on a substratum of sandy ridges and is experiencing unprecedented population growth. These sands are the seat of a water table that is heavily exploited by the population for various needs. Hygiene and sanitation practices that are not very respectful of the environment contribute to water pollution. The infiltration of wastewater into sandy soils, combined with poor sanitation, contributes to the degradation of water quality. The bacteriological and physicochemical analyses carried out on 88 samples during all four seasons of the year are not satisfactory. The bacteriological and physicochemical analysis of well water samples shows that water pollution is of varied origin. The bacteriological parameters show quite disparate values from one well to another, but with a predominance of pollution of human origin, followed by that of animal origin. The bacteriological pollution parameters exceed the standards required for drinking water by the WHO. The same is true for the physicochemical parameters. Their high concentration indicates a varied pollution that remains fairly identical, both in the dry season and in the rainy season, hence the sodium and potassium chloride chemical facies observed. The chloride levels exceed the WHO and EU standards (250 mg/L). For the four microbiological parameters (total germs (30 °C), 100/mL, total coliforms (30 °C), 0/mL, Escherichia coli (44 °C), 2/250 mL; fecal streptococci, 0/100 mL; and anaerobic sulfite reducer (44 °C), 2/20 mL), none of the samples meet the guideline values. In view of this observation, continued awareness raising among the population about the pollution of water tables and their protection is required, as is the establishment of a continuous filtration system for well water in order to provide the population with better quality water. Solar depollution and sand filtration are simple and effective methods for improving water quality. However, to reduce contamination, it is crucial to monitor and ensure compliance with standards for installing sanitation facilities in homes. Additionally, the establishment of a national standard for wastewater discharge into the environment is strongly encouraged.