沙漠研究 22-1, 65 -68 (2012 )"ICAL 1 / DT X"

Journal of Arid Land Studies－Refereed Paper－

Effects of Treated Wastewater Irrigation on Soil Salinity and Sodicity
at El Hajeb Region (Sfax-Tunisia)
Monem KALLEL*1), Nabil BELAID1), Tarek AYOUB2), Abdelmonem AYADI3) and Mohamed KSIBI1)
Abstract: The increasing scarcity of water in the world on one hand and the rapid population growth in urban areas on the otherrequire the use of appropriate water management practices. Treated wastewater reuse may be considered as an additional water
resource, especially important in arid and semi-arid countries. In arid regions such as near Sfax (Tunisia), treated wastewater
effluents (TWE) are often applied as agricultural irrigation. Irrigation TWE usually contains large amounts of fertilizing elements.
The objective of this study was to evaluate the impact of TWE irrigation on soil salinity and sodicity. In the city of Sfax, two sites
were selected with two soil types (fluvisol and calcisol) having been irrigated for 4 and 15 years, respectively. Irrigated andnon-irrigated study soils were analyzed for pH, nitrate and ammonia, electrical conductivity (ECs), exchangeable sodium percentage(ESP), sodium absorption ratio (SAR) and soil organic matter. The fluvisol, irrigated for only four years, is more affected by salinity
than the calcisol irrigated for 15 years. In the upper fluvisol layer irrigated by the treated wastewater, ECs reach 8 mS cm‑1 and ESP avalue of 15% while in all layers of the calcisol, ECs and ESP are lower and rarely exceed 4 mS cm‑1and 6% respectively. This result
is due to a combination of factors in the fluvisol treatment area including texture, cation exchange capacity, and irrigation procedureand crop management.
Key words: Calcisol, Exchangeable sodium, Fluvisol, Sodium absorption ratio, Wastewater reuse

1. Introduction
Tunisia extends from the Mediterranean coast in the Northto the Sahara desert in the South and its total surface area is 164,
150 km2. The Tunisian coastline extends over 1300 km.
Average annual rainfall is around 600 mm in the North, 300
mm in the Centre, and 150 mm in the South. The annual
evaporation varies between 1300 mm in the North to about
2500 mm in the South. The water resources are about 4.8
Milliard m3 of which 2.8 Milliard m3 are from surface waterand 2 Milliard m3 from groundwater. Continuing increases indemand of fresh domestic water by the urban sector haveindeed produced greater volumes of wastewater. Therefore,
in arid and semi-arid countries such as Tunisia which arefacing rising serious water shortage problems, reuse of urban
wastewater for non potable purpose, such as agriculture (Bahri,2002; Haruvy, 1997) has become an important concern.
Indeed, wastewater reuse for irrigation offers some attractive
environmental and socio-economic benefits including:
reduction of effluent disposal in receiving water bodies, supplyof nutrients as fertilizer and improvement in crop production
during the dry season (Yadav et al., 2002). This study
focuses on the impact of TWE irrigation on soil salinity andsodicity on a calcisol and a fluvisol which have been irrigatedfor 15 and 4 years respectively in the irrigated area of the arid
region of Sfax (Tunisia). At Sfax (second largest city inTunisia), signs of extremely low groundwater levels have beenregistered over the last three decades due to the increasing
number of wells used for crop irrigation (Bouri et al., 2008).
The Sfax treated wastewater has been reused for irrigation
since 1989. The irrigated area has been extended to an areaof 600 ha. This study is part of a research program evaluating
the impact of wastewater application on both soil and cropproperties. The goals of this study are to aid management ofcrop irrigation by wastewater, to reduce overexploitation of thelocal groundwater resources and to improve its recharge.

2. Materials and Methods
2.1. Study area
The irrigation area is located 10 km West of the town ofSfax (approximately one million inhabitants), next to thesewage treatment plant (Fig. 1) in crop fields which are
irrigated with TWE. The wastewater treatment plant receives
domestic effluents and industrial effluents. The region has anarid climate with mean monthly air temperatures ranging from
11.3 to 26.7°C, dry summers and annual rainfall of 200 mm
mostly occurring from October to December. An averageannual potential evaporation of 1200 mm, combined with the* Corresponding Author: monem.kallel@enis.rnu.tn
Route de Soukra Km 3,5 Po. Box 1173, 3038 Sfax, Tunisia
1) University of Sfax, Laboratory of Water, Energy and Environment, National School of Engineering of Sfax (ENIS)
3) École Nationale d’Ingénieurs de Sfax, Laboratoire de Radio-Analyses et Environnement (LRAE)

2) CRDA-Sfax

Fig. 1. Map of study area with location of the Sfax water treatment plant and of calcisol and fluvisol fields within the TWE irrigation perimeter.

low rainfall and high temperatures, make irrigation essential
for crop production. The present survey was carried out at
two selected sites chosen to represent both the soil typediversity and the variety of local agricultural practices andirrigation systems. Two main soil types occur in the study
area: calcisols and fluvisols (according to the FAO World
reference base for soil resources, 1998). The calcisols have ahomogeneous sandy to sandy loam texture, whereas thefluvisols present a clayey sand texture at the soil surface and afine sandy texture below 0.5 m depth. The calcisol field isused for production of successive winter and summer forage
crops in association with permanent harvesting of olives.
This kind of cropping system requires irrigation by open
surface furrows distributed every 24 m in-between each row ofolive trees. For the fluvisol field, irrigation is performed bydirect surface submersion for the summer crops and bysprinkler aspersion for the winter crops. The two study areas
also differ in their irrigation duration. The fluvisol study fieldhas been irrigated with TWE for only 4 years whereas thecalcisol has been subjected to TWE irrigation for 15 years. In
order to assess the effect of the wastewater, non irrigated fields
of both soil types (i.e. control) were selected near each irrigatedfield.
2.2 Sampling preparation and analyses
Treated effluents were sampled at the outlet of the Sfax
wastewater treatment plant at different times and conserved at–4°C before characterisation. Different parameters of effluent
samples were analyzed with standard methods (AFNOR,1997). Sampling was carried out with an Edelman-type
auger on summer crops plots only. At each sampling point,samples were taken from three layers: 0-30, 30-60 and 60-90
cm depth. After air-drying, the soil samples were sieved at 2mm. Soil pH was measured in a 2:5 soil:water slurry using aglass electrode. Determination of exchanged Ca2+ and Mg2+
was performed by Atomic Absorption Spectrophotometry(AAS) whereas exchanged Na+ and K+ concentrations weredetermined by Flame Atomic Emission Spectrometry (FAES).
Salinity of wastewater and soil was first estimated bymeasurements of the electrical conductivity (ECw and ECs forwater and soil respectively). The electrical conductivity ofthe soil samples (ECs) was determined on saturated paste
extracts of soils (U.S. Salinity Laboratory Staff, 1954). The
soil sodicity was also assessed by the exchangeable sodium
percentage (ESP). Potential risk of sodification of water andsoils has also been estimated by the sodium absorption ratio(SAR); SARs and SARw are respectively the sodium
absorption ratio for soil and water samples.

3. Results and Discussion
3.1. Treated wastewater characteristics
Characteristics of treated wastewater (TWE) used for
irrigation varied within and among the years of application(Table 1). The applied wastewater always remained alkalinewith an average pH of 7.7. It always presented a high level of
total dissolved solids (TDS) of 3.7 g L-1 and a high level ofsuspended matter (SS).
Levels of biochemical oxygen demand (BOD) andchemical oxygen demand (COD) indeed always largely
exceeded the Tunisian standards for wastewater reuse inirrigation (NT 106.03).
3.2. Factors controlling soil salinity and sodicity
Use of wastewater for irrigation has been recognized to be
hazardous with regard to soil salinity as well as soil sodicity.
In our case, the applied treated wastewater contains several
anionic species (chlorides, sulfates) that are mostly associatedwith sodium (Table 1). Soils are generally classified as saline
when they present an ECs of 4 mS cm-1 or more and areclassified as sodic when they present a SARs greater than 13 or
an ESP greater than 15% (Sumner, 1995). Using theseguidelines, the control fluvisol as well as the irrigated fluvisol(Fig. 3 b) are both saline, with ECs values exceeding the levelTable 1. Minimum and maximum values of water quality
parameters in the treated wastewater effluent (TWE)
generated by the Wastewater Treatment Plant of Sfax, as
characterized since 1984.
Parameter

Sfax effluent

Standards*

pH(7.1-8.7)
6.5 - 8.5
ECw mS/cm(4-7.7)
7
TDS g/L(3.56-5.13)
SS mg/L(29-275)
30
COD mg/L(123-700)
90
BOD5 mg/L(37-220)30(2.9-12.5)
Pt mg/L(0.35-50)
NO3- mg/L(903-2580)
2000
CL- mg/L(508-1950)
SO42- mg/L
(490-732)
HCO3- mg/L
(780-2100)
Na+ mg/L(17-105)
K+ mg/L(129-209)
Mg2+ mg/L(103-521)
Ca2+ mg/L(61-73)NH4+ mg/L
SARw(9.7-15.6)
EC: electric conductivity; TDS: total dissolved solids;
SS: suspended matter; COD: chemical oxygen demand;
BOD: biochemical oxygen demand;
SARw: sodium absorption ratio of water.
* Tunisian standards for wastewater reuse (NT 106.03)

of 4 mS cm-1, whereas the control calcisol is not saline. The
irrigated calcisol (Fig. 2) is of moderate salinity (Tedeschi andDell' Aquila, 2005). The calcisol is not sodic whether
irrigated for 15 years by the Sfax TWE or not. In contrast, thefluvisol irrigated for only 4-years is sodic with an ESP ranging
between 5 and 15% whereas the control fluvisol is not sodic(Fig. 3). It appears that irrigation by the same saline-sodic
Sfax wastewater impacted salinity and sodicity in different
manners, depending on the soil properties, the irrigation
procedure used and the type of crop management. The
studied calcisol and fluvisol mostly differ by their texture.
Soil texture strongly influences the soil permeability, the rate ofwater infiltration and the ability of soil particles to adsorb ordesorb chemical ions (exchange capacity) such as Na+ (Bauderet al., 2008). Consequently, a clay textured soil of relatively
high CEC, such as the studied fluvisol, presents the greatest
risk for binding the excess of sodium supplied by thewastewater. The fluvisol is also likely to better retain the salty
TWE. For this reason, the studied fluvisol samples presenthigher contents of exchangeable Na+. The sandy calcisol has
a good permeability and a low CEC (few exchange sites),retains less water and naturally loses water as well as soluble
salts from the root zone.

3.3. Factors controlling the extent of salt leaching

The ECs of the soil surface layers decreases with theleaching of salt by the TWE and by the autumn-spring rainfalls
and increases during the crop season in between periods ofirrigation, due to rise of saline water by evaporation or by rootuptake. In the present case, the main factors governing thesoil salinisation and sodification are the irrigation procedures
and choice of crop management. Rate of irrigation during thecrop season is regular enough to prevent the rise of salt fromthe deeper layers. Because crops only remove small amountsof salt (Nakayama and Bucks, 1986), vertical distribution ofsalt in soil is directly related to the water movements, which
are directly governed by the rate of irrigation. In the studied
calcisol, water movement is enhanced first by the goodpermeability due to the sandy to sandy loam texture andsecondly by the dismantlement of the deep calcareous crustthat has been performed in the irrigated fields only.
In addition, drainage of wastewater is favoured by thedensity of the permanent irrigation network and finally by theintensive rate of irrigation. Indeed, 1000 mm of treatedwastewater are applied by a furrow irrigation system during thesummer in the calcisol fields. This irrigation rate largely
exceeds the amount of water required for plant growth,enabling the leaching of salts. Conversely, the irrigation
procedure used in the fluvisol mostly led to the salinisation ofthe surface soil layer. Because of its clay texture, its higher
CEC and thus its higher water retention capacity, the fluvisol
retains more Na+ and is less permeable than the calcisol.

4. Conclusions
Irrigation by the Sfax saline-sodic treated wastewater(TWE) has significantly increased the soil salinisation andsodification of both studied soil types, particularly in the study
area of Sfax characterized by limited rainfall and highevaporation. Generally, wastewater irrigation management
aims at ensuring leaching of salts below the root systems(Mohammad Rusan et al., 2007). Here, this is not the caseeither for the calcisol or the fluvisoil, which both display
elevated SARs and ESP in the deepest soil layers. Irrigationby the Sfax TWE has affected the salinity, sodicity and soilorganic matter content of the two studied soils in different
manners, not only because of the different soil properties but
also due to the different crop management and irrigation
procedures. Soil CEC and buffer capacity as well as initialsoluble carbonate and salt contents are the main factors
controlling the extent of soil salinisation and salt leaching.
Organic matter addition, soil tillage and rate of irrigation are
the human factors determining the risk of further permanent
structural degradation of the soil.

Fig. 2. Calcisol properties in irrigated (IWC) and non irrigated (NIC= control) field. a) pH in soil-water extract. b) Electrical conductivity of extract
past (ECs). c) Exchangeable sodium percentage (ESP). d) NH4+ content in water extract. e) NO3- content in water extract. f) Soil organic mattercontent (SOM).

Fig. 3. Fluvisol properties in irrigated (IWF) and non irrigated (NIF= control) soil. a) pH in soil-water extract. b) Electrical conductivity of extract
past (ECs). c) Exchangeable sodium percentage (ESP). d) NH4+ content in water extract. e) NO3- content in water extract. f) Soil organic matter
content (SOM).

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