Market gardening is a pillar of food security in urban Africa, providing fresh produce rich in vitamins and minerals [1]. Its success depends closely on the availability of quality irrigation water, as each crop has specific water needs, which vary according to climatic conditions [2]. In coastal areas, access to fresh water is often limited and subject to climatic and geographical constraints. Several West African countries, such as Benin and Senegal, are already facing salt stress that affects their horticultural production [3][4]. In Togo, the coastal zone is a strategic space for market gardening: it supplies the urban markets of Lomé, supplies the sub-region and sometimes opens up export prospects [5]. However, this coastal position exposes water resources and soils to strong environmental pressures. The proximity of the sea and the low altitude of the coastline favour saline intrusion, a phenomenon aggravated by the permeability of sandy soils, coastal erosion and the rise in sea level, estimated at around 12 cm in 2025 and projected to exceed 80 cm by 2100 according to IPCC climate scenarios. The accumulation of dissolved salts (Na⁺, Cl⁻, HCO 3 − ) in groundwater compromises soil fertility and directly threatens sensitive crops such as tomatoes, peppers and African eggplants [6]-[9]. The impacts on agricultural yields are well documented: salinity reduces plant water absorption, causes ionic imbalances and limits root development. Yield losses of up to 46% were observed for tomatoes irrigated with highly saline water (EC = 8.6 dS/m) compared to moderately saline water (3.0 dS/m), accompanied by a decrease in aerial growth and fruit size [10][11]. In addition to these pressures, the coastline is eroding by up to 10 to 15 m per year in some areas, reducing the available agricultural space [12]. The Togolese Institute for Agricultural Research (ITRA) already estimates that a third of the soils in the maritime region have worrying salinity levels, forcing producers to adapt their practices or abandon certain plots. In response to these challenges, initiatives such as the IFAD-supported RESADE project are promoting adapted agricultural techniques and the introduction of salt-tolerant varieties to strengthen farmers’ resilience [13]. At the same time, electrical conductivity measurements carried out on irrigation water on the Togolese coast reveal a high spatio-temporal variability, without however making it possible to identify the exact nature of the salts involved [14], hence the need for more in-depth chemical analyses [15]. Thus, salinization appears to be a major issue for the sustainability of Togolese coastal market gardening, with direct socio-economic repercussions on food security and agricultural incomes [16][17].

The objective of this study is to characterize the physicochemical quality of irrigation water in the Togolese coastal zone using three complementary approaches: an overall assessment based on key indicators such as salinity, pH, dissolved solids and major ions, an analysis using Richards and Wilcox diagrams, and a characterization by production area by districts. The challenge is twofold: to assess the compatibility of this water with market gardening needs and to propose strategies for sustainable management, in order to preserve both agricultural productivity and the quality of water resources.

The heterogeneity of the groundwater resources used by market gardeners suggests a generalized exploitation of different hydrogeological units. The shallow sandy water table characteristic of this coastal zone remains shallow and the water tables are particularly vulnerable to anthropogenic and environmental influences [26][27]. The significant difference between the depths of the water tables on the different sites studied confirms this variability in access to water resources. Thus, the spatial distribution of boreholes reveals two contrasting areas in terms of proximity to the coast: an area further from the sea (Ablogamé, Baguida) and an area closer to the sea (Agodékè, Kpogan, Agbavi).

Indeed, [28] explains that proximity to the sea can be a risk factor for the quality of exploited groundwater, particularly in connection with saline intrusion. This intrusion is a well-documented phenomenon in coastal areas where shallow water tables are directly influenced by sea level rise, tides, and excessive pumping [29].

In areas such as Kpogan or Agbavi, located in the immediate vicinity of the sea, the overexploitation of water tables could theoretically unbalance the natural hydraulic gradient and promote the upwelling of saline water. However, the results indicate that, in the current context, the waters analysed remain in compliance with WHO and FAO standards and do not show signs of marked salinization. This situation contrasts with other coastal regions of West Africa where cases of progressive salinization have been reported, directly affecting agricultural practices [30][31]. Thus, although the risk exists, it is not yet evident in the area studied.

In short, the depth of the water tables and their geographical location in relation to the sea play a decisive role in vulnerability to salinization. A differentiated and preventive management of water resources, taking into account this spatial configuration, is necessary to ensure the sustainability of market gardening activities in this region.

The variation in the temperature of water resources, although small in appearance, is statistically very significant and seems to be directly related to the geographical position of the sites in relation to the coast. The lowest temperatures were recorded in Ablogamé and Agbavi, located inland, while the highest temperatures were measured in Agodékè and Kpogan, closer to the sea. This spatial distribution obtained on the Togolese coast suggests a thermal effect of maritime proximity, probably due to a warmer and humid coastal microclimate, the influence of warmed runoff, and denser urbanization in coastal areas that can amplify the urban heat island effect [32][33]. In addition, the thermal conductivity of soils and the depth of water tables could play a role in the variability observed [34].

Even moderate groundwater warming can affect the physicochemical and microbiological quality of water, with implications for public health and the sustainable management of water resources in a context of increasing population pressure and climate change [35][36]. In the context of market gardening, the temperature of the irrigation water is also a significant parameter for plant growth. Temperatures that are too low or too high can induce physiological stresses in plants, affect nutrient solubility and slow down soil biological processes [37].

The temperatures observed in this study, which are compatible with the requirements of vegetable crops, therefore indicate that water resources on the Togolese coast are not a limiting factor in terms of thermal conditions. Unlike surface water, groundwater does not experience sudden increases in daytime temperature, which limits potential thermal shocks during irrigation.

Measurements of the electrical conductivity (EC) of irrigation water in the market gardening sites studied reveal significant variations between zones. EC, a key indicator of water salinity, directly influences the availability of nutrients to plants and their ability to absorb water [38][39].

The maximum value obtained at Ablogamé could be explained by anthropogenic factors such as increasing urbanization, domestic discharges or the uncontrolled use of agricultural inputs. As this site is further from the sea, the salinity seems to be more of anthropogenic origin than the result of marine saline intrusion. Conversely, the low levels observed at Agbavi and Kpogan, located on the seashore, indicate almost absent or negligible salinization, although these areas remain vulnerable to future intrusion in the event of overexploitation of the water table [40].

The notable internal variability recorded in Ablogamé, Baguida and Kpogan could result from a mixture of several water supply sources (groundwater, reclaimed water, etc.), but also from uneven borehole management or local differences in geology and land use [41]. This heterogeneity poses a particular challenge for water quality management in urban agriculture, where practices are often informal and poorly regulated. The intermediate values observed in Agodéké and Baguida indicate a transitional situation, possibly linked to average pressure on the resource, both in terms of exploitation and exposure to sources of pollution.

According to [42], human activity, particularly intensive irrigation and agricultural practices, can have a major impact on groundwater salinity. Excessive exploitation of water tables in dense urban or agricultural areas can lead to an increase in conductivity, unlike natural saline intrusion, which occurs more slowly and locally.

Referring to the [43] classification, electrical conductivities between 0.750 mS/cm and 2.250 mS/cm correspond to slightly saline waters. The waters used by market gardeners on the Togolese coast therefore fall broadly into this category. However, this water is generally suitable for the majority of crops, provided that a reasoned management of irrigation is implemented. This reflects an overall favourable situation, in contrast to other coastal regions heavily affected by salinisation.

Thus, in Morocco, in the Chaouia region, the electrical conductivities of groundwater near the shore reach values greater than 3 mS/cm [44]. Similarly, in the Dakar region, [45] revealed elevated EC levels, suggesting marked saline intrusion. On the Beninese coast, [46] reported values ranging from 0.7 to 2 mS/cm, reflecting mild to moderate salinity. Comparatively, the situation on the Togolese coast appears to be much more favourable, as salinity is not yet a limiting factor for irrigation.

This relative stability could be attributed to several factors, including natural hydrostatic pressure that maintains a separation between fresh and salt water [47]. Freshwater recharge by rainfall and rivers also plays a key role in preserving the hydrogeological balance and limits the risk of saline intrusion.

In short, although local variations exist, the electrical conductivity of groundwater on the Togolese coast remains generally compatible with the requirements of market gardening. These results underline the positive potential of the resource, while recalling the need to put in place preventive management mechanisms in order to limit possible future degradation.

The results indicate that the groundwater used is generally soft, with concentrations of total dissolved salts (TDS ≤ 1000 mg/L) in perfect correlation with electrical conductivity. This situation, observed in Agbavi, Kpogan, Ablogamé, Baguida and Agodéké, reflects the combination of environmental, anthropogenic and seasonal factors influencing the composition of groundwater [48][49].

In Ablogamé, the higher concentrations reflect slightly more mineralized water, probably related to the use of well water or surface water tables rich in soluble salts. Conversely, the lower levels recorded in Agbavi and Kpogan reflect good quality water resources, from less mineralized springs and therefore better suited to market gardening.

In the geographical context of the Togolese coast, TDS concentrations could be expected to lead to progressive soil salinization, with a negative impact on plant growth. Indeed, the accumulation of dissolved salts in the soil can increase electrical conductivity and reduce water availability for roots, due to reverse osmosis phenomena [50]. However, the results of this study show the opposite: the measured values remain within a range favourable to the majority of crops, confirming the good suitability of the groundwater of the Togolese coast for irrigation.

These observations are consistent with those reported in other regions where, despite the proximity of the sea, the water tables remain relatively spared from salinization thanks to efficient freshwater recharge and favorable hydro-geological conditions [45][47]. This situation is a major asset for the sustainability of coastal market gardening, while stressing the importance of regular monitoring in order to prevent any risk of adverse developments.

The pH of irrigation water is an essential parameter for assessing its quality. According to FAO guidelines, the recommended pH range for irrigation is between 6.5 and 8.4. Thus, the values measured in this study (7.18 to 7.50) are comfortably within this range, indicating irrigation water of acceptable quality [51]. A slightly basic pH is generally favourable for the majority of vegetable crops, as it optimises the availability of essential nutrients. For example, a pH between 6.5 and 7.5 is often considered optimal for the availability of micronutrients such as iron, manganese, and zinc.

Although the measured pH values are within the acceptable range, it is important to note that variations in pH can affect the operation of irrigation systems. For example, a high pH can lead to the formation of limescale deposits in pipes, while a low pH can accelerate the corrosion of equipment. However, the values measured in this study do not suggest such immediate risks [52].

With regard to the chemical quality of water resources on the Togolese coast, the low variability of ions such as calcium, magnesium, ammonium, nitrites and sulphates, as well as slightly basic pH values, suggests a water table that is little affected by major anthropogenic intrusions, such as domestic discharges or agricultural effluents rich in nitrogenous matter. With respect to nitrates, it should be noted that they result from the natural process of nitrification of ammonium by the action of bacteria of the genus Nitrosomonas and Nitrobacter [53]. However, the literature indicates that the presence of nitrates in drinking water is very often attributable to human activities: the use of mineral fertilizers and manure, associated with intensive farming, as well as animal manure or deficient septic systems, are important sources [54]. The risk of contamination is all the more marked when the soil covering the water table is sandy and the water table is shallow, conditions that favour the infiltration and rapid migration of nitrates.

However, the coastal area studied corresponds precisely to an area of intensive market gardening, where the use of chemical inputs and organic composts (from poultry droppings and cattle manure) is common. It is characterized by a low, sandy coast, subject to strong erosion to the east of the main jetty of the port of Lomé, with a pronounced departure of sediments [19]. Sandy soils [20] accentuate this vulnerability. These contextual elements suggest that, although the concentrations measured in this study do not indicate marked pollution, the area remains at increased risk of nitrate contamination due to its soil characteristics and agricultural uses.

At the international level, high nitrate levels in groundwater are a widely documented problem, particularly in relation to agricultural practices, and concern many OECD countries [55]. In fact, the most readily available forms of nitrogen are nitrate and ammonium, but nitrate ( NO 3 − -N) is most commonly found in irrigation water. Ammonia nitrogen rarely exceeds 1 mg/L, except when ammonia fertilizers or wastewater are added to the water resource. The concentration in most surface and groundwater is generally less than 5 mg/L of NO 3 − -N, but some specific groundwater may contain amounts exceeding 50 mg/L. Drainage water from the area below the root system frequently has higher nitrogen levels due to deep fertilizer leaching [56]. Reducing groundwater contamination by nitrates requires the implementation of adapted agricultural practices focused on improved nitrogen management in the field. Among the most relevant measures is targeted nitrogen fertilization, which involves adjusting fertilizer inputs to the actual needs of crops and their growth cycle. This approach, based on nitrogen balance assessment and the use of monitoring tools (sensors, soil analyses, fertility mapping), minimizes excess nitrogen that could be leached into groundwater [56]. Splitting applications, avoiding fertilization before heavy rainfall, and covering the soil between crop cycles are complementary practices that enhance the effectiveness of this strategy.

The values observed in the study area are therefore part of a more global dynamic, with no direct link to specific local anthropization, but the environmental and agricultural context of the Togolese coast calls for particular vigilance. Finally, potassium levels appear to be associated more with the ecological characteristics of the water table than with anthropogenic inputs, which confirms the good general chemical quality of the groundwater analysed. These findings suggest that in the absence of marked urbanization, groundwater quality could be significantly better [57].

The high potassium concentrations observed can be explained by both natural sources and anthropogenic inputs. On the one hand, the release of K⁺ can result from the weathering of potassium-rich minerals (feldspars, micas) in aquifer formations, and from leaching facilitated by favorable hydrological and textural conditions (example, high infiltration, irrigation, sandy soils). For example, one study shows that irrigating a sandy soil resulted in K losses of up to 387 kg∙ha⁻¹ to the aquifer [58]. Another study, in a semi-arid basin, identified the use of potassium fertilizers (KCl) as a significant source of K in the groundwater [59]. On the other hand, the use of soluble potassium fertilizers can directly feed the water table by percolation: a soil column experiment recorded significant K leaks after the addition of KCl [60].

Unlike chloride, sodium, potassium, bicarbonate and nitrate ions, the higher concentrations of these ions at Ablogamé could be attributed to several factors: the proximity of the sea and the possible overexploitation of the water table can promote an upwelling of saline water, as suggested by the high levels of chlorides and sodium. However, it is important to emphasize that the concentrations measured are still much lower than those of seawater, whose electrical conductivity is around 50 mS/cm and the total dissolved solids (TDS) content is about 35,000 mg/L [48][61]. On the contrary, the water tables studied have EC and TDS values compatible with fresh water, confirming that no major salinization is observed despite the immediate proximity of the Atlantic Ocean. A second factor would be the intensive use of fertilisers: nitrate and potassium concentrations can result from a more intensive use of mineral fertilisers, which are often rich in these elements. The market gardeners of Ablogamé could have more intensive cultivation practices than in other areas. Another factor would also be increasing urbanization: as Ablogamé is on the outskirts of Lomé, it is likely that urbanization and soil sealing will modify the local hydrogeochemical regime. These observations are in line with the results of [62], which found higher concentrations of dissolved ions in peri-urban areas of Togo, due to increasing anthropogenic pressures.

The relatively stable concentrations of calcium, magnesium, sulphates and ammonium reflect a homogeneous chemical composition of the water table exploited for irrigation. This constancy, independent of geographical areas, suggests a common underground supply or one that is little influenced by anthropogenic inputs. Similar results have been observed in coastal water tables in West Africa, particularly in Benin, where concentrations of these ions have remained stable at comparable market gardening sites [63]. These levels are consistent with good water quality for irrigation, in accordance with the thresholds set by the Food and Agriculture Organization of the United Nations (FAO), which considers calcium and magnesium concentrations below 20 meq/L to be non-limiting for irrigation [43].

The pH indicates a slight basicity, which is favourable to the bioavailability of nutrients in the soil for the majority of vegetable crops [38]. In addition, the low nitrite concentration is a reliable indicator of water with low levels of organic matter or domestic discharge, which is essential for the safety of irrigated vegetable crops.

The concentrations of certain elements such as sodium, potassium, chlorides, bicarbonates and nitrates show a high spatial variability, especially with significantly higher concentrations at Ablogamé. This situation suggests a marked anthropogenic influence in Ablogamé, probably linked to the intensive use of nitrogen fertilizers, uncontrolled irrigation practices, and saline intrusions linked to the overexploitation of the water table [62]. It could also result from domestic or industrial effluents (salt-rich wastewater), as the site is located in the immediate vicinity of an industrial zone in Lomé. These concentrations raise potential risks of soil salinization (particularly by sodium and chlorides), as well as nutritional imbalances (excess nitrates and bicarbonates) that can affect crop growth and quality [38][64][65].

Variations in the chemical composition of groundwater cannot be understood solely from the perspective of anthropogenic inputs; geological factors play an equally crucial role. Indeed, the nature of the aquifer formations (sands, clays, sandstones, marine deposits) directly influences mineral weathering reactions, basal mineralization, and ion exchange within the aquifer. For example, in the Atlantic Coastal Plain (United States), the distribution of hydrochemical facies shifts from a Ca-Mg-HCO₃ type in well-drained areas to a Na-Cl type towards the coast, linked to mineral dissolution, cation exchange, and seawater intrusion. Similarly, in a coastal alluvial plain in China, the transition from a Ca-HCO₃ hydrotype to a highly mineralized Na-Cl hydrotype towards the sea has been attributed to evaporite dissolution, concentrated evaporation, and seawater infiltration [66]. These studies illustrate that the concentrations of major ions (Ca²⁺, Mg²⁺, Na⁺, Cl⁻, HCO 3 − , SO 4 2 − ) result from a combination of natural processes (weathering, dissolution, cation exchange, marine intrusion) and not solely from agricultural or urban impacts. Indeed, the study area is characterized by Quaternary deposits consisting primarily of marine and coastal sands, forming poorly consolidated sandbars—an environment particularly permeable to infiltration. These formations therefore offer strong infiltration pathways for groundwater, which would explain why ion exchange reactions, the dissolution of sedimentary minerals and the coastal marine influence would significantly modulate the levels of major ions (Ca²⁺, Mg²⁺, Na⁺, Cl⁻, HCO 3 − , SO 4 2 − ) in these aquifers.

The assessment of the quality of irrigation water on the Togolese coast, through Richards and Wilcox’s diagrams, highlights convergent and generally positive perceptions of the suitability of these waters for agricultural use.

The Richards diagram [24], based on electrical conductivity (EC) and sodium adsorption ratio (SAR), helps to assess both the level of total salinity and the risk of soil sodization. More than 90% of the water analysed reflects a moderate risk of salinity, suitable for crop growth and the preservation of soil structure in the long term [38]. These classes require relatively flexible management, even for less salt-tolerant species, contrary to the recommendations of [67], which recommend drainage, organic amendments, and gypsum to limit sodium accumulation. A study of the Mnasra plain water table (Morocco) obtained similar results, with a majority of samples classified as “good to acceptable” according to SAR and %Na [68].

The Wilcox diagram [25], based on CE and %Na, offers another perspective focused on immediate ionic toxicity. Here, 86.67% of the samples are rated as “excellent to good” quality, while only 3.33% are rated as poor. This optimistic view is probably related to the lack of consideration of SAR, which allows for the assessment of long-term effects on soil structure [69]. In an example in Ethiopia (Koga area), the waters were classified as C1S1 to C2S2 (low to moderate salinity and sodium), thus largely suitable for irrigation [70].

These two approaches are therefore complementary. Richards is more conservative and better suited to a sustainable vision, while Wilcox is pragmatic, relevant in short-cycle or immediate-use systems. Other studies confirm this complementarity: the joint analysis of these diagrams in the Senegal Valley (Lower Delta) has made it possible to map the potential impact on soil degradation by salinization and sodization [71].

Ultimately, the waters of the Togolese coast are a resource that is generally favourable to irrigation and offer significant potential to support market gardening. Their integrated and proactive management including regular monitoring of physicochemical parameters, the choice of adapted crops and the implementation of resilient cultural practices (localized irrigation, organic amendments, rotations, etc.) can further enhance their agronomic value. Some areas such as Ablogamé, where ionic variability is more marked (Na⁺, K⁺, NO 3 − , Cl⁻, HCO 3 − ), appear to be strategic spaces for developing innovative water and soil management solutions. The special attention paid to these sites is therefore not only a constraint, but an opportunity to improve soil fertility and consolidate the sustainability of market gardening production in the medium and long term.

The study highlights, through an overall assessment, a favorable physico-chemical quality of irrigation water along the Togolese coast, characterized by low salinity and a composition well adapted to the needs of the crops. Analysis using the Richards and Wilcox diagrams confirms this suitability, while also pointing out some limitations related to dissolved salt content and nutrient imbalances. Finally, characterization by production area reveals significant spatial variability, which is more closely related to geological and environmental conditions and local agricultural practices than to direct saline intrusion.

While this water quality is an asset for the sustainability of market gardening, the high concentrations of nitrates reflect the impact of anthropogenic inputs and underscore the need for rational fertilizer management. In this context, continuous monitoring and a localized approach to groundwater management appear essential to anticipate changes and prevent degradation. Preserving these resources requires appropriate agricultural practices, integrated water and soil management, and strategies to protect the fragile boundary between freshwater and saline water, especially in the face of increasing demographic and climatic pressures.