The present study aims to assess the spatial and temporal variations of the hydrochemical characteristics of Mateur aquifer groundwaters, a crucial water resource in the northeast of Tunisia. The aquifer was subject to water quality deterioration due to salinization and nitrate contamination, and a new assessment of water quality was needed. For this purpose, 40 groundwater samples were collected during wet and dry seasons and analyzed for salinity, pH, T, O2, major cations and anions, and nutrient elements using standard methods and Water Quality Index (WQI). The results showed that most of the groundwater parameters were not within the permissible limits set by the World Health Organization in both seasons. The geochemical data were interpreted using WQI for drinking water. The spatial distribution maps of Water Quality Index showed that the highest quality was found, during both seasons, in the northwest and the southeast part of the aquifer, corresponding to the recharge zone, whereas the poor and very poor water quality was found in the outflow part of the aquifer. According to sodium adsorption ratio (SAR) and Na% values, most of the groundwater samples were not suitable for irrigation purposes and characterizing the eastern part of the aquifer and the outflow part of the aquifer, around the Ichkeul marshes.
Groundwater is becoming the major source of water supply for domestic, industrial, and agricultural sectors of many countries all over the world. It is estimated that approximately one-third of the world’s population uses groundwater for drinking [
An understanding of the spatial variation and processes affecting water quality is essential in sustaining usable water supplies under changing climate and local environmental pressures. Temporal changes of recharged water composition, hydrologic and human factors, may cause periodic changes in groundwater quality [
In such circumstances, the knowledge of temporal and spatial trends of water quality should help in the decision-making process, particularly in developing countries, where there are insufficient data [
In Tunisia, where the climate is dry over most of its territories, water resources are both scarce and unequally distributed through time and space, with a potential decrease, due to overexploitation, exploitation of nonrenewable deep aquifer, salinization, and pollution [
This paper highlights the spatial and temporal variations in groundwater quality in an alluvial agricultural plain in Mateur region and evaluates the suitability of groundwater for irrigation and drinking purposes for sustainable agriculture and basic human needs. For these purposes, we used an integrated Water Quality Index (WQI), the sodium adsorption ratio (SAR), the sodium percentage (% Na), and the permeability index (PI).
Mateur plain is located in Bizerte region, in the northeast of Tunisia (Figure
Location, geology [
Ombrothermic diagram.
Mateur plain covers different statigraphic units ranging from Triassic to Quaternary [
Hydrogeological cross sections: (a) Mateur and (b) Ras el Ain [
Piezometric maps of Mateur plain: (a) wet season and (b) dry season.
Water loss from the aquifer is through discharge to the lake Ichkeul, evaporation, and pumping for domestic and agricultural purposes. The water table generally lies within 8–10 m of land surface, facilitating additional groundwater loss via evapotranspiration. The alluvial aquifer has transmissivity values ranging from 2·10−5 to 20·10−2 m2/s.
According to Mori [
Mateur region land is mainly used for growing cereals and fodders. A large amount of synthetic fertilizers (DAP, ammonium, and super 45) are applied during the farming season (winter through spring) [
Details of fertilizer application for different crops in the study area.
Fertilizer type (kg·ha−1) | Cereals | Fodder | Legumes | Tree crops | ||||||
---|---|---|---|---|---|---|---|---|---|---|
Nov | Jan | Feb | March | Sept | Jan | Dec | Jan | Oct | Feb | |
DAP N-P2O5 | 150 | — | — | — | — | — | — | 200 | — | — |
Ammonium | — | 150 | 150 | 150 | — | 150 | — | 200 | — | 150 |
Super 45 P2O5 | — | — | — | — | 100 | — | 100 | — | 150 | — |
Groundwater samples were obtained from monitoring wells in the alluvial aquifers. A total of 70 samples were collected in wet and dry seasons in order to capture seasonal variability in water quality.
The piezometric level, pH, electrical conductivity (EC), dissolved oxygen, and temperature were measured in situ. Samples were kept at 4°C for their subsequent chemical analyses. Afterwards, the samples were filtered through a 0.45
The quality of chemical analysis was checked by making an ionic mass balance, accepting an error rate lower than 5%. Nitrate, nitrite, ammonium, and ortho-P were analyzed by colorimetric methods described by Rodier [
Statistical summary of hydrochemical parameters of the study area.
Unit | Wet season (May) | Dry season (October) | |||||
---|---|---|---|---|---|---|---|
Min | Max | SD |
Min | Max | SD |
||
Cl− | mg/L | 67.45 | 2406.90 | 524.66 | 106.50 | 4402.00 | 844.89 |
SO4−− | mg/L | 223.53 | 2483.48 | 497.45 | 221.89 | 3591.36 | 583.38 |
HCO3− | mg/L | 54.92 | 622.40 | 150.35 | 86.65 | 693.19 | 146.93 |
Ca++ | mg/L | 20.00 | 480.00 | 82.33 | 10.00 | 763.40 | 135.97 |
Na+ | mg/L | 49.00 | 1728.00 | 376.45 | 11.25 | 170.13 | 40.30 |
Mg++ | mg/L | 12.70 | 140.00 | 27.33 | 39.33 | 2576.00 | 491.23 |
K+ | mg/L | 0.78 | 101.60 | 17.84 | 0.00 | 169.00 | 29.88 |
T | °C | 16.20 | 26.80 | 2.16 | 18.20 | 26.70 | 1.98 |
Conductivity | mS/cm | 0.54 | 8.64 | 1.94 | 0.72 | 14.59 | 2.98 |
Salinity | g/L | 0.41 | 6.55 | 1.47 | 0.54 | 11.07 | 2.26 |
pH | 6.18 | 8.39 | 0.39 | 7.13 | 9.08 | 0.33 | |
O2 | mg/L | 1.20 | 7.60 | 1.36 | 0.60 | 5.20 | 1.09 |
NO−3 | mg/L | 0 | 273.15 | 79.76 | 0.72 | 278.6 | 64.99 |
NH4+ | mg/L | 0 | 6.63 | 1.08 | 0 | 0.2 | 0.03 |
NO2− | mg/L | 0 | 1.56 | 0.37 | 0 | 0.23 | 0.06 |
HPO4−− |
|
10 | 6.37 | 1.02 | 20 | 4.82 | 0.82 |
The pH values of Mateur alluvial aquifer water range from 7.18 to 8.39 during the wet season and between 7.02 and 8.11 throughout the dry season.
The spatial and temporal variations of pH are controlled by the quality and the infiltration rate of recharge water, the replenishing water rate and water-rock interaction in the aquifer. These pH values are all in the desirable limits set by the World Health Organization (WHO) [
The temperature values of groundwater varied from 18.9°C to 25.6°C during the wet season and from 18.2°C to 26.7°C during the dry season. The highest temperature values, for the same well or borehole, are recorded during dry season. The spatial variation of temperature is a function of the recharged water and of the infiltration transfer time, which in turn both depend on porosity, lithology, and thickness of the unsaturated zone. For the same sampling period, the spatial variation is marked by a decrease as the depth increases.
Water salinity of the Mateur aquifer varied from 0.1 to 4.5 g/L during the wet season and from 0.1 to 8.4 g/L throughout the dry season. The highest values were recorded during the dry season (October), whereas the lowest values were recorded during the wet season (April). The spatial distribution maps of salinity (Figures
Spatial distribution maps of salinity: (a) wet season and (b) dry season.
The concentration of chloride varied from 67 to 2406 mg/L during the wet season and from 49 to 4402 mg/L throughout the dry season. Sodium contents ranged between 49 and 1728 mg/L during wet season and between 39 and 2576 mg/L during dry season. Concentrations of chlorides and sodium show little seasonal variation. A slight decrease, more important in the most mineralized waters, can be explained by the dilution caused by rainwater infiltration during the wet season (April), whereas evaporation contributes to ion concentration increase throughout the dry season.
Cl shows a strong positive correlation with Na (Figure
Bivariate pot of (a) Na+ versus Cl− and (b) salinity versus Cl−.
The spatial distribution maps of Ca2+ levels show that the highest values characterize the northeastern and the outflow parts of the aquifer (Figures
Spatial distribution maps of calcium (Ca++): (a) wet season and (b) dry season.
Spatial distribution maps of sulfate (SO4−−): (a) wet season and (b) dry season.
A good positive correlation of Ca2+ versus HCO3− and SO42− (Figure
Geochemical relationship of (2 HCO3 + SO4) versus 2Ca.
Magnesium contents in Mateur aquifer ranged from 10 to 290 mg/L during the wet season and from 11 to 170 mg/L during the dry season. The lowest values are recorded throughout the wet season, while the highest values are recorded throughout the dry season.
A relationship of Ca versus Mg shows a positive correlation (
Bivariate pot of Ca2+ versus Mg2+.
The Piper diagram (Figure Mixed facies: Ca-Na-SO4-Cl and Ca-Na-HCO3-Cl Na-Cl facies: they characterize discharge zone and are well influenced by the leaching of salty deposits.
Piper diagram.
To assess the possible anthropogenic effects on groundwater quality, water samples were classified into three groups based on the NO3 concentration during the study period as (a) below 50 mg/L, (b) 50−150 mg/L,s and (c) above 150 mg/L. Figures
Spatial distribution maps of nitrate (NO3−): (a) wet season and (b) dry season.
Groundwater samples in the studied area are fairly oxygenated, so NO2 and NH4+ concentrations were almost absent, except few wells which have dramatically higher values. Nitrite contamination is indicative of local and recently generated contamination [
The absence of correlation between the different inorganic nitrogen compounds (NO3−, NO2−, and NH4+) and O2 ((Figures
Bivariate pot of NO2− versus NH4+ (a) and NO3− (b).
Bivariate pot of O2 versus NO3− (a) NH4+ (b) and NO2− (c).
The solubility of inorganic forms of phosphorus, mainly represented by orthophosphates, is pH dependent. The equilibrium reactions between orthophosphates are the following [
These reactions indicate that HPO42− is the dominant form within a pH range of 7.2 and 12.5. According to the pH values of groundwater in the study area, the dominant form of orthophosphates is HPO42− with contents between 10 and 482
Water Quality Index is one of the most effective tools to communicate information on the quality of water to the concerned citizens and policymakers. It is an important parameter for the assessment and management of groundwater. WQI is defined as a rating reflecting the composite influence of different water quality parameters [
To compute the WQI, three steps are followed [
Weight and relative weight of chemical parameters used for WQI computation.
|
|
| |
---|---|---|---|
Salinity | 1500 | 5 | 0.152 |
Bicarbonates | 1 | 0.030 | |
Chlorides | 400 | 5 | 0.152 |
Sulfates | 400 | 5 | 0.152 |
Nitrates | 50 | 5 | 0.152 |
Calcium | 300 | 3 | 0.091 |
Magnesium | 150 | 3 | 0.091 |
Sodium | 200 | 4 | 0.121 |
Potassium | 2 | 0.061 | |
Total |
Total |
To compute the WQI (
The computed WQI values are classified into five types, from “excellent water” to “water, unsuitable for drinking” (Table
Water Quality Index (WQI) ranges and percentage of samples during wet and dry seasons.
WQI range | Water quality | Wet season | Dry season |
---|---|---|---|
<50 | Excellent | 12.50 | 10 |
50–100 | Good | 40 | 45 |
100–200 | Poor | 35.50 | 35 |
200–300 | Very poor | 5 | 2.50 |
>300 | Unsuitable for domestic purposes | 7.50 | 7.50 |
The spatial distribution of Water Quality Index (Figure
Spatial distribution maps of WQI.
The long-term effect of irrigation water on physical and chemical properties of soil and crop productivity depends on the conductivity and sodium contents, dissolved inorganic carbon and alkaline earth elements (Ca and Mg) of irrigation water, and initial physical properties of soil [
In order to assess the irrigation suitability of Mateur aquifer groundwaters, the following parameters were used: sodium percentage (% Na), sodium adsorption ratio (SAR), and permeability index (PI).
The SAR values in the study area range between 4.08 and 13.9. All the sampling points on the US salinity diagram are shown in Figure
USSR diagram for the study area.
Irrigation quality of groundwater based on sodium percentage.
C2S1 (%) | C3S1 (%) | C4aS2 (%) | C4bS2 (%) | C4aS1 (%) | C4bS4 (%) | C4bS3 (%) | C3S4 (%) | |
---|---|---|---|---|---|---|---|---|
Wet season | 5 | 42 | 21 | 2 | — | 3 | — | — |
Dry season | 3 | 36 | 18 | 3 | 12 | — | — | 3 |
Spatial distribution maps of the irrigation suitability classes of the study area.
All groundwater samples on the Wilcox diagram are shown in Figure
Wilcox diagram for the study area.
Salinity and alkalinity hazards of irrigation water in US salinity diagram.
Excellent (%) | Good (%) | Permissible (%) | Doubtful (%) | Unsuitable (%) | |
---|---|---|---|---|---|
Wet season | 8 | 47 | 2 | 26 | 10 |
Dry season | 3 | 30 | 9 | 21 | 18 |
It is noteworthy that when the sodium content is high in irrigation water, this element tends to replace the Ca and Mg ions in the interlayer space of soil clays, reducing its permeability, and thus, causing poor internal drainage [
The permeability index (PI) of a water sample is computed from the following equation (
Permeability indices were plotted with the total ionic content of the groundwater samples on a Doneen’s chart [
In the study area, the PI ranged from 52.44 to 64.09 with an average of 56.13, Figure
A Doneen’s chart for the study area.
The objective of this study is to assess the seasonal variation in the physicochemical characteristics of water supply wells in the Mateur plain, Northern Tunisia. The WQI was applied to investigate the seasonal changes and the factors influencing groundwater hydrochemistry and hence its suitability for irrigation and domestic purposes. The investigation results suggest the following: (i) The highest quality was found, during both the wet and dry season, in the northwest and southeast part of the aquifer, corresponding to the recharge zone where 12.5% and 40% of groundwater samples fell into excellent to good categories, respectively. Toward the flow direction, groundwaters become poor to very poor and need treatment before consumption. (ii) Most of the groundwater samples fell in doubtful to unsuitable categories, characterizing the eastern part of the aquifer and the outflow part, around the Ichkeul marshes. These waters with high to very high salinity are unsuitable for irrigation purposes in ordinary conditions, an adequate drainage with low salinity waters and plants having good salt tolerance should be selected. (iii) The little seasonal change of groundwaters’ quality of Mateur aquifer is mainly related to dilution in the wet season, evaporation throughout the dry season, and agricultural activities.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that they have no conflicts of interest.
A table showing the subindex of