The production of phosphoric acid by the Tunisian Chemical Group, in Sfax, Tunisia, led to the degradation of the groundwater quality of the Sfax-Agareb aquifer mainly by the phosphogypsum leachates infiltration. Spatiotemporal monitoring of the quality of groundwater was carried out by performing bimonthly sampling between October 2013 and October 2014. Samples culled in the current study were subject to physicochemical parameters measurements and analysis of the major elements, orthophosphates, fluorine, trace metals, and stable isotopes (18O, 2H). The obtained results show that the phosphogypsum leachates infiltration has a major effect on the downstream part of the aquifer, where the highest values of conductivity,
Groundwater pollution proves to be potentially threatening as it puts at jeopardy the hygienic integrity of a huge water reserve. The intensification of industrial activities, as well as the diversification of the storage modes of by-products production, makes groundwater resources vulnerable and can be considered as the main factors responsible for groundwater pollution. The groundwater quality is equally important as its quantity to the suitability of water for various purposes [
In Tunisia, climatic constraints, with a moderate rainfall contribution, which are unequally distributed in the space and irregular in time, as well as a strong evaporation power, make water resources limited. Socioeconomic development and the spread of industrialization have led to pressuring the resources and increasing the demand. Thus, the water supply, estimated at 472 m3/inhabitant in 2010, will decrease to 315 m3/inhabitant in 20 years [
Urban, industrial, and tourist centers have been developing along the eastern coast of Tunisia. Several industries are located in Sfax area, especially those of the Tunisian Chemical Group (TCG) for the enrichment and transformation of natural phosphate. The TCG activity focuses primarily on the chemical fertilizers and phosphoric acid production, by the fluorapatite transformation. This production model generates gaseous emissions, liquid discharges, and huge amounts of phosphogypsum (PG), issued from the treatment of phosphate rock with sulfuric acid [
The study area is situated in the coast of Sfax, where the TCG plant, a discharge of domestic waste, and a station of wastewater treatment are located (Figure
Location map of the study area showing piezometers and reference well sampled for groundwater analysis.
Geological map of Sfax [
Hydrogeological cross-section along transect AB, as indicated in Figure
Groundwater samples were culled bimonthly from ten piezometers in the phreatic aquifer of Sfax-Agareb, between October 2013 and October 2014 (Figure
Chemical composition of groundwater samples
Months | Unit | Oct 13 | Jan 14 | Mar 14 | May 14 | Aug 14 | Oct 14 | ||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Parameters | Min | Max | Min | Max | Min | Max | Min | Max | Min | Max | Min | Max | |
|
°C | 12.1 | 13.9 | 10.1 | 12.8 | 11 | 11.7 | 11.10 | 12.20 | 12.2 | 13.2 | 12.00 | 13.1 |
pH | mg/L | 5.18 | 7.74 | 5.75 | 8.1 | 5.58 | 7.95 | 5.56 | 7.85 | 5.2 | 7.79 | 5.50 | 7.76 |
O2dissolved | mg/L | 0.9 | 2.15 | 0.9 | 4.2 | 0.8 | 3.5 | 0.80 | 3.12 | 0.7 | 2.9 | 0.50 | 2.8 |
EC |
|
2100 | 33200 | 1590 | 33000 | 1650 | 32600 | 1680 | 34300 | 1800 | 34700 | 1900 | 34900 |
K+ | mg/L | 2.7 | 22.70 | 2.25 | 19 | 2.15 | 19.6 | 2.60 | 19.50 | 2.5 | 21.2 | 2.7 | 23.68 |
Na+ | mg/L | 111.5 | 6965 | 139.4 | 6760 | 142 | 6300 | 115.16 | 6124 | 198 | 6600 | 195 | 6515 |
Ca2+ | mg/L | 59 | 986 | 49 | 904 | 50 | 851 | 54.36 | 1075 | 51 | 1080 | 58 | 1020 |
Mg2+ | mg/L | 37 | 945 | 22.5 | 920 | 24 | 940 | 29.75 | 1085 | 25 | 1050 | 35 | 1080 |
Cl- | mg/L | 110.6 | 5480 | 104.3 | 5396 | 101 | 5372.5 | 151 | 5475 | 135.5 | 5810 | 112 | 5632.5 |
|
mg/L | 256.4 | 9143.9 | 195.2 | 9661.5 | 271 | 9533.9 | 199.20 | 8533.9 | 245 | 9313.9 | 255 | 9973 |
|
mg/L | 215 | 4396.1 | 201.5 | 4295.7 | 204 | 4352.9 | 261.25 | 4543.7 | 210 | 4857.3 | 210 | 4948.5 |
Ortho-P | mg/L | 2.15 | 67.3 | 2.5 | 81.7 | 2.7 | 71.00 | 1.90 | 136.65 | 1.5 | 92 | 1.25 | 114.95 |
|
mg/L | 0.6 | 26 | 0.6 | 27 | 0.50 | 19.00 | 0.50 | 29.00 | 0.5 | 17 | 0.6 | 19 |
Al | mg/L | 0.02 | 1.8 | 0.03 | 1.57 | 0.03 | 0.90 | 0.06 | 0.98 | 0.02 | 0.96 | 0.03 | 0.97 |
Zn | mg/L | 0.03 | 1.46 | 0.02 | 1.41 | 0.02 | 1.44 | 0.03 | 1.47 | 0.04 | 1.51 | 0.03 | 1.05 |
The application of multivariate statistical analysis offers a clearer understanding of water quality and enables comparison of the different water samples [
On global scale, 15% of the PG production is recycled while large quantities are stored in the factories vicinity, which are disposed mostly in big piles. They are located in coastal areas with phosphoric acid plants nearby, both as dry or wet staking and without treatment [
Chemical composition of PG leachate in May 2014.
Parameters | Leachate PG |
|
---|---|---|
EC | 22700 | |
pH | 1.3 | 6.5–8.5 |
F | 3500 | 3 |
|
6730 | 0.05 |
|
1357 | 500 |
|
3240 | 600 |
|
198 | 200 |
Cl- | 1560 | 600 |
Na+ | 2000 | 500 |
K+ | 139 | 50 |
Cr | 1.2 | 0.5 |
Cd | 0.8 | 0.005 |
Zn | 4 | 5 |
Al | 4 | 5 |
Cu | 0.5 | 0.5 |
Fe | 6 | 1 |
Units in mg/L except pH and EC (
Groundwater temperature tends to vary from 10.1 to 13.9°C. The spatiotemporal variation of this parameter did not show any audible alternation during the whole period of testing (Figure
Distribution of temperature (a), pH (b), dissolved O2 (c), and EC (d) in groundwater of Sfax-Agareb aquifer.
Concentrations of
Understanding of the water-rock interaction and the associated reactions in the aquifer is essential to identify the variability of Calcite: Gypsum:
In order to calculate activities and ion activity product, the computer code PHREEQC [
The results show that the groundwater of the upstream zone of the phreatic aquifer (P1, P2, P3, P4, P5, and SP3) is undersaturated compared to gypsum (Figure
Equilibrium diagram of gypsum.
Equilibrium diagram of calcite.
The sodium and chloride concentrations varied from 111.5 to 6965 mg/L and from 101 to 5810 mg/L, respectively. The Group I: Na+ and Cl− contents are less than 50 mmol/L and 30 mmol/L, respectively. It concerns water of the upstream part of the study area (P1, P2, P3, and P4). Group II: Na+ and Cl− concentrations range from 50 to 120 mmol/L and from 30 to 100 mmol/L, respectively. This group encompasses water of the downstream zone, near the evaporation ponds (SP1, SP2, SP3, and SP5). Group III: Na+ and Cl− contents are greater than 250 mmol/L and 150 mmol/L, respectively. It includes water of SP4 piezometer, affected by the wastewater and the infiltration of PG leachate relatively rich in Na+ and Cl− (Table
Plot of Na+ against Cl−.
The enrichment in Na+ and Cl−, from the upstream to the downstream, would be related to water-reservoir rock interaction, saline water infiltration from the evaporation ponds, and marine intrusion. During the study period, potassium concentrations are almost homogeneous at the same sampling location, with values ranging from 2.15 to 23.68 mg/L. The concentrations of Mg2+, which fluctuate between 22.5 and 1085 mg/L, are positively correlated to
Plot of Ca2+ against Mg2+.
Ortho-P and fluorine concentrations fluctuate between 1.5 and 136.65 mg/L and between 0.5 and 29 mg/L, respectively. Groundwater sampled near the PG discharge site showed high concentration levels of Ortho-P, which are larger in terms of extent and impact than those taken from the upstream part (Figure
Distribution of orthophosphates (a) and fluorine (b) in Sfax-Agareb groundwater.
Plot of Ortho-P against fluoride.
The source of trace metals in the groundwater could be geogenic, but high concentrations above the permissible limit of drinking water standards raise the suspicions of industrial contamination sources [
To discuss the relationships between the physicochemical parameters, major elements, Ortho-P, and fluorine in groundwater samples, PCA was used to distinguish the contributions of the natural and anthropogenic processes to the Sfax-Agareb aquifer, in the site of TCG.
Two independent factors were extracted, which explain 83.77% of the total variance. The first factor (F1) presents 72.34% of the total inertia. It is defined by EC and the
Summary of the PCA results including the loadings and the eigenvalues.
Variables | Axe F1 | Axe F2 |
---|---|---|
Na+ | 0.9902 | 0.0400 |
Cl- | 0.9537 | 0.2568 |
K+ | 0.9646 | −0.0319 |
Ca2+ | 0.9162 | 0.2195 |
|
0.7810 | −0.3565 |
Mg2+ | 0.8930 | 0.3142 |
|
0.8782 | 0.4375 |
|
0.1665 | −0.3803 |
pH | −0.7253 | 0.6316 |
|
0.8026 | −0.3231 |
F- | 0.8387 | 0.4741 |
Dissolved O2 | −0.8304 | 0.3433 |
EC | 0.9894 | 0.1067 |
Al | 0.7383 | −0.4214 |
Zn | 0.9474 | 0.0343 |
Eigenvalues | 10.8513 | 1.7147 |
% variance explained | 72.3422 | 11.4312 |
% cumulative variance | 72.3422 | 83.7734 |
Spatial distribution of the variables and individuals in the axes system F.
The PCA confirm the different geochemical correlation and classification of the Sfax-Agareb groundwater, in TCG site, into two types: groundwater samples collected from the downstream part, which are influenced by the anthropogenic processes, largely controlled by the PG leachate percolation and the seawater intrusion. Samples collected from the upstream part of the study area were principally controlled by the natural recharge, with no evidence of high anthropogenic impacts.
The stable isotope composition of water in the study area varies from −5.97 Group I is generated by the most enriched water in isotopes: SP1, SP2, SP4, and SP5 in the downstream part of the water table. Group II involves water characterized by the lowest levels of
Isotopic and chloride composition of selected groundwater samples in October 2014.
|
|
Cl- (mg/L) | |
---|---|---|---|
P1 | −5.97 | −36.01 | 151 |
P2 | −5.88 | −35.98 | 356 |
P3 | −5.64 | −35.37 | 581 |
P4 | −5.61 | −35.22 | 776 |
P5 | −5.57 | −32.18 | 1739.5 |
SP1 | −3.58 | −26.80 | 2627 |
SP2 | −4.12 | −27.20 | 2130 |
SP3 | −5.50 | −34.47 | 745.5 |
SP4 | −3.65 | −26.10 | 5475 |
SP5 | −4.10 | −26.88 | 2094.5 |
Sfax rainwater | −4.60 | 10 | |
Seawater | 0.00 | 19000 |
The
Cl−-
The total dissolved solids of group I water, which are characterized by higher contents of Cl− and
The geochemical study of the major elements, Ortho-P,
The authors declare that they have no conflicts of interest.