Abstract

Groundwater is the main agricultural water resource in arid and semi-arid regions, so, preserving it is extremely important. In this study, groundwater quality was assessed for irrigation, using the principal chemical and physical quality parameters from 30 wells in the study area. Groundwater classification on the basis of electrical conductivity reveals that more than 85% of the samples taken fall into the ‘unsuitable’ and ‘doubtful’ classes. On the basis of Richards's classification, 67% of the samples are unsuitable for irrigation. Most, however, appear suitable for irrigation based on their sodium adsorption ratio, %Na, permeability index, magnesium adsorption ratio, Kelly's ratio and residual sodium carbonate water quality indices. The concentration of boron varies between 0 and 2 mg/l, within the FAO's (Food and Agriculture Organization of the United Nations) acceptable standard range. About 60% of the wells are not suitable for drip irrigation because of the water's potential for clogging. There will be no water infiltration problem, if groundwater in the study area is used for irrigation. With respect to the SO42− and Cl concentrations, 53% and 13% of the samples collected are unsuitable for irrigation, respectively. Less than 10% of the samples have ‘severe’ constraints restricting their use for irrigation with respect to nitrogen. For sprinkler irrigation, however, the groundwater is subject to ‘severe’ restrictions. Geochemical investigations indicate that the water chemistry is affected by processes including evaporation, water-rock interactions and human activities.

INTRODUCTION

Where surface water resources are limited, groundwater is used extensively in agricultural activities, especially in arid regions. Groundwater quality is a function of physical and chemical parameters, controlled dominantly by local geochemistry and anthropogenic activities (Singh et al. 2011). Geochemical processes in groundwater, and reactions between groundwater and local minerals during groundwater movement affect water quality. Excess concentrations of dissolved ions in water affect the physicochemical properties of soils, decreasing fertility and damaging soil structure (Ravikumar et al. 2011; Esmaeili & Moore 2012).

Among environmental pollutants, heavy metals are particularly important because of their toxic properties, ability to accumulate, and high durability in living things. Introducing toxic elements to the soil through anthropogenic activities causes soil and, consequently, groundwater pollution. Polluting agricultural soil with heavy metals introduces them into the food chain through sorption by plants. Although some metal species are required in negligible amounts for plant growth, their concentration, even in slight excess of acceptable levels, may be dangerous for plant and animal life (Kabata-Pendias 2011; Jaishankar et al. 2014). Irrigation water quality standards have been established on the basis of three factors: (1) the total concentration of dissolved salts, which affects crops through osmosis; (2) the concentration of particular species, like boron, which may be toxic to plants or have undesirable effects on crop quality; and (3) the concentrations of cations such as Na+, that destroy the soil structure and decrease its permeability (Karanth 1987).

The most important activity on the Behbahan Plain is agriculture, and most of the water used is groundwater. Studying the groundwater's chemical composition can identify the causes of water quality changes and the extent of pollution. The hydrochemistry and suitability of groundwater for potable use on the plain have been evaluated by Ehya & Marbouti (2016). In this study, groundwater quality has been assessed in the aquifer beneath the plain with respect to agricultural use.

Study area

The study area is in southeastern Khuzestan Province, SW Iran, between latitudes 30 31′ and 31 00′ N, and longitudes 50 00′ to 50 30′ E (Figure 1). The Behbahan alluvial plain has a mean elevation of 313 m above sea level and is part of the Jarrahi River watershed, covering 1,320 km2 in the southern foothills of the Zagros Mountain Chain. Climatologically, the area is arid to semi-arid, with average annual precipitation of 336 mm, relative humidity of 46%, and pan evaporation of 3,401 mm. The maximum temperature can be 50 °C in July and August, while the minimum is below 0 °C in January and February. The hot season lasts seven months, from April to October.

Figure 1

Maps of the study area and the sample well locations (modified from Ehya & Marbouti 2016).

Figure 1

Maps of the study area and the sample well locations (modified from Ehya & Marbouti 2016).

The study area is surrounded by limestones, evaporites, marls, sandstones and conglomerates with ages ranging from Upper Cretaceous to Recent. The young and permeable alluvial sediments arise from erosion of the surrounding geological formations and comprise the Behbahan Plain aquifer. Coarse-grained sediments are dominant in the northern part of the study area, while silty and clayey sediments are distributed elsewhere (Figure 2(a)). In the central and southern areas, calcareous sediments, marls and gypsum are found, in addition to clayey and silty sediments. The maximum thickness of alluvium–130 m – is found in the central parts of the plain, while 20 to 50 m are found toward the margins. The principal direction of groundwater flow is from the northern and eastern margins toward the western side of the plain (Figure 2(b)). The groundwater flow trend does not change much between the wet and dry seasons, because of the continuous and abundant recharge from the Bangestan and Khaviz anticlines, north and northwest of the plain.

Figure 2

(a) Map of sediment type distributions in the Behbahan Plain aquifer (adapted from Shahsavari et al. 2009); (b) A map showing the direction of groundwater flow in the Behbahan Plain. Contours (meters) represent the depth to the water table (adapted from Ehya & Marbouti 2016).

Figure 2

(a) Map of sediment type distributions in the Behbahan Plain aquifer (adapted from Shahsavari et al. 2009); (b) A map showing the direction of groundwater flow in the Behbahan Plain. Contours (meters) represent the depth to the water table (adapted from Ehya & Marbouti 2016).

Data source and methods

Groundwater samples from 30 wells, used predominantly for agricultural purposes, in the Behbahan Plain, were analyzed by Ehya & Marbouti (2016) and the results are presented in Table 1. The data are used here to evaluate the groundwater quality for irrigation purposes. Because of this, various indicators including the sodium percentage (%Na), sodium adsorption ratio (SAR), residual sodium carbonate (RSC), permeability index (PI), magnesium adsorption ratio (MAR) and Kelly's ratio (KR) have been calculated. The spatial distribution of the groundwater quality indices has been produced using ArcGIS 9.2 software, based on the Inverse Distance Weighted (IDW) interpolation method.

Table 1

Statistical summary of hydrochemical parameters for groundwater in the Behbahan Plain (adapted from Ehya & Marbouti 2016)

Variable Range Maximum acceptable (Ayers & Westcot 1994
T°C 23–30 – 
pH 6.55–7.71 8.5 
EC (μS/cm) 497–12,190 3,000 
TDS (mg/l) 331–8,570 2,000 
Na+ (mg/l) 17.48–1,909 919 
K+ (mg/l) 2.34–62.4 
Ca2+ (mg/l) 68–769 400 
Mg2+ (mg/l) 16.2–447.6 60 
Cl (mg/l) 13.68–405.65 1,063 
HCO3 (mg/l) 170.8–405.65 610 
CO32− (mg/l) 0.0 
SO42− (mg/l) 95.52–383.84 960 
NO3 (mg/l) 1.4–111.4 10 
B (mg/l) 0.0–2.01 
Variable Range Maximum acceptable (Ayers & Westcot 1994
T°C 23–30 – 
pH 6.55–7.71 8.5 
EC (μS/cm) 497–12,190 3,000 
TDS (mg/l) 331–8,570 2,000 
Na+ (mg/l) 17.48–1,909 919 
K+ (mg/l) 2.34–62.4 
Ca2+ (mg/l) 68–769 400 
Mg2+ (mg/l) 16.2–447.6 60 
Cl (mg/l) 13.68–405.65 1,063 
HCO3 (mg/l) 170.8–405.65 610 
CO32− (mg/l) 0.0 
SO42− (mg/l) 95.52–383.84 960 
NO3 (mg/l) 1.4–111.4 10 
B (mg/l) 0.0–2.01 

RESULTS AND DISCUSSION

Temperature and pH

The groundwater temperature is between 23 and 30 °C. Its pH is in the range 6.55 to 7.71, indicating that the groundwater is slightly acidic to alkaline (Table 1). The pH values are within the normal range for irrigation water (Ayers & Westcot 1994), and the lowest are found in the urban areas (Ehya & Marbouti 2016).

Salinity hazard

Salinity hazard is considered important in classification of irrigation water, because the high electrical conductivity (EC) results in saline soils (Esmaeili & Moore 2012). In natural waters, the dissolved solids originate from rock and soil weathering. The salinity hazard was determined using the EC values for this study (Table 1), as they range from 497 to 12,190 μS/cm. Table 2 shows groundwater classifications based on EC values (Scofield 1936). According to this classification, 43% of the groundwaters sampled are ‘unsuitable’ for irrigation, 43% are ‘doubtful’, and only 14% are suitable for irrigation. The latter are from wells in the north and northwest of the study area. In northwest of the plain, the presence of gravel and coarse-grained sand horizons (Figure 2(a)) facilitates groundwater flow, decreasing the contact time between the water and the aquifer, thus limiting the dissolution of carbonate and evaporitic minerals. The highest EC values are found in the urban areas (Ehya & Marbouti 2016).

Table 2

Classification of groundwater based on EC values (Scofield 1936)

EC (μS/cm) Water Class No. of samples Samples (%) 
<250 Excellent – – 
250–750 Good 
750–2,000 Permissible 
2,000–3,000 Doubtful 13 43 
>3,000 Unsuitable 13 43 
EC (μS/cm) Water Class No. of samples Samples (%) 
<250 Excellent – – 
250–750 Good 
750–2,000 Permissible 
2,000–3,000 Doubtful 13 43 
>3,000 Unsuitable 13 43 

Sodium percentage (%Na)

Sodium is important in evaluating water quality for irrigation as high concentrations lead to alkaline soils. Sodium destroys the soil structure, decreasing its permeability, hardening it, and inhibiting plant growth (Todd 1980). The sodium percentage (%Na), also known as soluble sodium percentage (SSP), is calculated using Equation (1), where all ion concentrations are in meq/l (Todd 1980):  
formula
(1)
The values of %Na range from 7.3 to 56.3. Scofield (1936) classified waters using %Na values (Table 3), and, using that classification, 73% of the groundwaters are excellent or good for irrigation. The remaining 27% are acceptable for irrigation, but extensive use would increase the amount of Na in the soil. In the southwestern and central parts of the plain the soil is clayey (Figure 2(a)). Sodium replaces Ca+2 and Mg+2 in the clay mineral lattices, with adverse effects on soil drainage, permeability, and aeration, and on water circulation. So, soils with high levels of Na are hardened and crops do not get enough water (Tijani 1994; Singh et al. 2014). The highest %Na values are found in the northern and northeastern parts of the Behbahan Plain, and the lowest in central and southeastern areas (Figure 3(a)).
Table 3

Classification of groundwater based on %Na (Scofield 1936)

% Na Water class No. of sample Samples (%) 
<20 Excellent 30 
20–40 Good 13 43 
40–60 Permissible 27 
60–80 Doubtful – – 
>80 Unsuitable – – 
% Na Water class No. of sample Samples (%) 
<20 Excellent 30 
20–40 Good 13 43 
40–60 Permissible 27 
60–80 Doubtful – – 
>80 Unsuitable – – 
Figure 3

Distribution of %Na, RSC, PI, MAR, KR and SAR values over the Behbahan Plain.

Figure 3

Distribution of %Na, RSC, PI, MAR, KR and SAR values over the Behbahan Plain.

Residual sodium carbonate (RSC)

An important factor affecting water's suitability for irrigation is the concentrations of bicarbonate and carbonate. Waters with high concentrations of these components have high pH values and irrigating with them renders the soil infertile because of sodium carbonate deposition (Eaton 1950). As RSC increases, so does the potential sodium hazard, because much of the Ca and some of the Mg ions are precipitated. Such salts are concentrated when the soil dries out (Glover 1996). The RSC level is calculated using Equation (2), in which all ion concentrations are in meq/l (Eaton 1950):  
formula
(2)

Irrigation water is classified regarding RSC, as presented in Table 4 (Eaton 1950). The RSC values in groundwater samples from the study area range from − 1.5 to −131.5 meq/l. On that basis, the waters are safe for irrigation with respect to the RSC index. RSC values decrease from the north towards the south of the plain (Figure 3(b)).

Table 4

Classification of groundwater based on RSC values (Eaton 1950)

RSC Water Class No. of samples Samples (%) 
<1.25 Suitable 30 100 
1.25–2.50 Marginal 
>2.50 Unsuitable 
RSC Water Class No. of samples Samples (%) 
<1.25 Suitable 30 100 
1.25–2.50 Marginal 
>2.50 Unsuitable 

Permeability index (PI)

Long-term irrigation affects soil permeability in alluvial regions. It is changed by Na+, Mg2+, Ca2+ and HCO3 ions (Singh et al. 2014), and the permeability index (PI) is a good parameter for this purpose. PI is calculated using Equation (3), where all ion concentrations are in meq/l (Doneen 1964):  
formula
(3)
The PI values for groundwater samples from the study area range between 13 and 56.9, and PI values are plotted versus the groundwaters' total ionic content on a Doneen chart (Figure 4). The chart shows three water classes: (I) PI is low and the water is suitable for irrigation; (II) PI is higher than in class I, but the waters are commonly acceptable; and (III) PI is high and the water cannot be used for irrigation. Nearly 93% of the waters sampled are in ‘Class I’, with two in ‘Class II’, and the lowest and highest PI values are seen in the northwestern and southern regions of the plain, respectively. Increasing PI values have been found in other regions of the plain, suggesting that their soil is being affected by long-term irrigation (Figure 3(c)).
Figure 4

Permeability index classification of irrigation waters for soils of medium permeability (after Doneen).

Figure 4

Permeability index classification of irrigation waters for soils of medium permeability (after Doneen).

Magnesium adsorption ratio (MAR)

High concentrations of Mg2+ in water have negative effects on soil quality, because the soil becomes alkaline, which reduces crop yields (Kumar et al. 2007). If the MAR exceeds 50, irrigation is not practical. The MAR is calculated from Equation (4), in which all ion concentrations are in meq/l (Paliwal 1972):  
formula
(4)
MAR values from the study area fall in the range 18.4 to 76.9. The MAR classification of groundwater is given in Table 5 (Paliwal 1972) and, as can be seen, only 7% of samples have MAR values exceeding 50. Thus 93% of the waters sampled are suitable for irrigation on the basis of this index. In the urban areas in the south, the proportion of Mg is considerably higher (Figure 3(d)).
Table 5

Classification of groundwater based on MAR values (Paliwal 1972)

MAR Water Class No. of samples Samples (%) 
<50 Suitable 28 93 
>50 Unsuitable 
MAR Water Class No. of samples Samples (%) 
<50 Suitable 28 93 
>50 Unsuitable 

Kelly's ratio

Kelly (1940) evaluated the effect of sodium on irrigation water quality in terms of another ratio (KR), which can be calculated using Equation (5), where all ion concentrations are in meq/l (Kelly 1940):  
formula
(5)
KR values from the study area range from 0.72 to 1.26. Table 6 shows the classification of groundwater based on KR values (Kelly 1940). If KR exceeds 1, the Na concentration is too high; waters with KR below 1 are suitable for irrigation. In the study area, KR exceeds 1 for one sample, the others, however, are all acceptable for irrigation (Table 6). KR is at its highest in the plain's northern and northeastern areas, with the lowest values in the central and southern parts (Figure 3(e)).
Table 6

Classification of groundwater based on KR values (Kelly 1940)

KR Water Class No. of samples Samples (%) 
<1 Suitable 29 96.7 
>1 Unsuitable 3.3 
KR Water Class No. of samples Samples (%) 
<1 Suitable 29 96.7 
>1 Unsuitable 3.3 

Heavy metals

Not all trace elements are toxic and many are essential for plant growth – e.g., Fe, Mn, Mo and Zn – in small quantities. However, excessive quantities of heavy metals can cause hazardous accumulations in plant tissue and inhibit growth. Most heavy metals are readily fixed and accumulate in soils, and since this is largely irreversible, repeated applications of excess heavy metals contaminate soils, and may render them unproductive or their products unusable (Ayers & Westcot 1985). The concentrations of some heavy elements in the samples are given in Table 7. The Se concentration in all samples and that of Mn in two samples exceeds the maximum recommended by FAO (Ayers & Westcot 1994). The concentration of Cr in well No. 6, in the north of the study area, also exceeds the FAO's irrigation standard. It is thought that urban and industrial sewage entering the groundwater probably led to these concentration increases (Singh et al. 2014). The concentrations of Fe, Zn, Co, As, Cd and Pb in the groundwater from Behbahan Plain are at the FAO's irrigation limit (Ayers & Westcot 1994).

Table 7

Basic statistical parameters for the heavy metals (adapted from Ehya & Marbouti 2016)

Heavy metals Range (mg/l) Maximum allowable (mg/l; Ayers & Westcot 1994
Fe 0.0–0.90 
Mn 0.0–0.51 0.2 
Zn 0.0–1.56 
Co 0.02–0.05 0.05 
Cd 0.0–0.02 0.01 
Cr 0.02–0.13 0.1 
Pb 0.0–0.84 
As 0.0–0.09 0.1 
Se 0.1–0.49 0.02 
Heavy metals Range (mg/l) Maximum allowable (mg/l; Ayers & Westcot 1994
Fe 0.0–0.90 
Mn 0.0–0.51 0.2 
Zn 0.0–1.56 
Co 0.02–0.05 0.05 
Cd 0.0–0.02 0.01 
Cr 0.02–0.13 0.1 
Pb 0.0–0.84 
As 0.0–0.09 0.1 
Se 0.1–0.49 0.02 

Boron

Boron (B) is a metalloid and its behavior is between those of metals and non-metals. It is found as calcium and sodium borates in soil, and its presence is essential for plant growth. However, high boron concentrations can poison plants (Ayers & Westcot 1985). The boron concentration in samples from the study area was in the range 0 to 2 mg/l (Table 1). The FAO's boron classification for irrigation waters is presented in Table 8. About 94% of the samples have B concentrations below 0.7 mg/l, meaning that they are excellent for irrigation in this respect. The concentration in the other samples was also below 2 mg/l, so they are suitable for irrigation. The highest boron concentrations are found in the south and southwest of the study area (Ehya & Marbouti 2016).

Table 8

Classification of groundwater based on Boron concentrations (Ayers & Westcot 1985)

Boron (mg/l) Water Class No. of samples Samples (%) 
<0.7 None 28 94 
0.7–3 Slight to moderate 
>3 Severe – – 
Boron (mg/l) Water Class No. of samples Samples (%) 
<0.7 None 28 94 
0.7–3 Slight to moderate 
>3 Severe – – 

Assessing potential for water infiltration problems

Problems arise when irrigation water enters the soil too slowly during normal irrigation to replenish it with water needed by the crop before the next irrigation (Ayers & Westcot 1985). Infiltration is affected by both the SAR and EC. The SAR suggests the extent to which irrigation water tends to be involved in cation-exchange reactions in the soil. Sodium replacing adsorbed calcium and magnesium causes damage to the soil structure due to the dispersion of clay particles. This in turn reduces the number of pores in the soil, making the soil more compact and impervious (Bouderbala 2015) – the pores are responsible for soil aeration and drainage. The SAR index is calculated using Equation (6) (Karanth 1987) with the ion concentrations expressed in meq/l:  
formula
(6)
Irrigation waters with low EC values have similar effects on the soil to those with high SARs, so, low EC and/or high SAR can both lead to breakdown of soil aggregates, sealing the soil and forming crusts (Ayers & Westcot 1985). Soil texture itself can also affect the severity of water infiltration problems. At any given EC and SAR levels, infiltration problems are worse in soils with higher clay contents. The type of clay is also important (Santos et al. 2003).

On the Behbahan Plain, the groundwater SAR values range from 0.4 to 12.1. The SARs are lowest in the northwest, and increase rapidly in the urban areas and southern segment of the plain (Figure 3(f)). Infiltration problems may occur only in relation to two samples from the study area – Table 9 –, their severity depending on local soil characteristics.

Table 9

Combined effect of irrigation water SAR and EC on the likelihood of water infiltration problems (Ayers & Westcot 1985)

  Risk of water infiltration problems
 
Low Slight to moderate High 
SAR EC (dS/m or mmhos/cm)   
0–3 above 0.7 0.7–0.2 below 0.2 
No. of samples 18 
3–6 above 1.2 1.2–0.3 below 0.3 
No. of samples 
6–12 above 1.9 1.9–0.5 below 0.5 
No. of samples 
12–20 above 2.9 2.9–1.3 below 1.3 
No. of samples 
20–40 above 5.0 5.0–2.9 below 2.9 
No. of samples 
  Risk of water infiltration problems
 
Low Slight to moderate High 
SAR EC (dS/m or mmhos/cm)   
0–3 above 0.7 0.7–0.2 below 0.2 
No. of samples 18 
3–6 above 1.2 1.2–0.3 below 0.3 
No. of samples 
6–12 above 1.9 1.9–0.5 below 0.5 
No. of samples 
12–20 above 2.9 2.9–1.3 below 1.3 
No. of samples 
20–40 above 5.0 5.0–2.9 below 2.9 
No. of samples 

Groundwater in the study area has also been classified using a Richards (1954) diagram, which is based on EC and SAR. In this classification (Figure 5), 6% of samples fall in the ‘C2S1’ class with moderate salinity and low alkalinity, and are suitable for agricultural irrigation. Some 21% fall in the ‘C3S1’ class, and 6% in ‘C3S2’, representing high salinity with low alkalinity and high salinity with moderate alkalinity, respectively. ‘C3S1’ and ‘C3S2’ waters are suitable for irrigation with appropriate treatment. A further 27% of the waters belong in the ‘C4S1’ class (very high salinity with low alkalinity), and 40% in ‘C4S2’ (very high salinity with moderate alkalinity), neither of which is suitable for irrigation.

Figure 5

Richards (1954) diagram for groundwater samples.

Figure 5

Richards (1954) diagram for groundwater samples.

Sodium toxicity

Plant roots absorb sodium ions for transport to the leaves, while many species also absorb sodium directly through their leaves from water on them – e.g., rainfall or sprinkled irrigation water. The sodium accumulation can reach toxic concentrations after an extended period, typically many days or weeks (Ayers & Westcot 1985), but not all crops are equally sensitive; toxicity symptoms may appear on almost any crop if the sodium concentrations are high enough. The permissible SAR levels for surface and sprinkling irrigation waters, with respect to sodium toxicity are given in Table 10. Twenty samples have ‘low’ levels of restriction on use for either surface or sprinkled irrigation, according to their SARs, while the others are subject to ‘slight to moderate’ restriction. Only one water, from the urban zone, would be subject to ‘severe’ restrictions on surface irrigation use (Table 10).

Table 10

Permissible SAR levels for surface and sprinkler irrigation waters (Ayers & Westcot 1985)

SAR Degree of restriction on use
 
None Slight to moderate Severe 
Surface irrigation <3 3–9 >9 
No. of samples 20 
Sprinkler irrigation <3 >3  
No. of samples 20 10  
SAR Degree of restriction on use
 
None Slight to moderate Severe 
Surface irrigation <3 3–9 >9 
No. of samples 20 
Sprinkler irrigation <3 >3  
No. of samples 20 10  

Potential for clogging in drip irrigation systems

In arid and semi-arid areas, drip irrigation is widely used to save water, and emitter clogging is a major problem. Such systems are designed to deliver water to plants at very low rates, so the water must pass through very small openings (emitters). Water quality has a great influence on clogging potential (Ayers & Westcot 1985). The major chemical contributors to clogging are set out in Table 11. With respect to pH, 33 and 67%, respectively, of the samples can be considered as having either ‘none’ or ‘slight to moderate’ potential to cause clogging. In relation to TDS, however, 6% of samples have ‘no’ potential for clogging, for 37% it is ‘slight to moderate’, and for 57% ‘severe’. The latter wells, with water unsuitable for drip irrigation, are in the central, southern and southwestern parts of the plain. Those with ‘slight to moderate’ clogging potential are in the north and northeast, while the best water for drip irrigation is found in the northwest of the plain.

Table 11

Influence of water quality on the potential for clogging in drip irrigation systems (Ayers & Westcot 1985)

Potential problem Units Degree of restriction on use
 
None Slight to moderate Severe 
pH  <7.0 7.0–8.0 >8.0 
No. of samples 10 20 
Samples (%) 33 67 
Dissolved solids mg/l <500 500–2,000 >2,000 
No. of samples 11 17 
Samples (%) 37 57 
Manganese mg/l <0.1 0.1–1.5 >1.5 
No. of samples 27 
Samples (%) 90 
Iron mg/l <0.1 0.1–1.5 >1.5 
No. of samples 29 
Samples (%) 97 
Potential problem Units Degree of restriction on use
 
None Slight to moderate Severe 
pH  <7.0 7.0–8.0 >8.0 
No. of samples 10 20 
Samples (%) 33 67 
Dissolved solids mg/l <500 500–2,000 >2,000 
No. of samples 11 17 
Samples (%) 37 57 
Manganese mg/l <0.1 0.1–1.5 >1.5 
No. of samples 27 
Samples (%) 90 
Iron mg/l <0.1 0.1–1.5 >1.5 
No. of samples 29 
Samples (%) 97 

In terms of iron concentration, none of the water samples have ‘severe’ potential for drip irrigation clogging. Equally, with respect to manganese, only one well, in the southwest, has ‘severe’ clogging potential.

Chloride and sulfate concentrations

All natural waters contain some chloride (Cl), which, as chlorides are commonly soluble, contributes to the total salt content (salinity) of soils. Chloride is essential for plant growth (Karaivazoglou et al. 2005). Nevertheless, the commonest form of toxicity arises from chloride in the irrigation waters (Ayers & Westcot 1985). Chloride is neither adsorbed nor fixed by soils, but moves readily with water, and is taken up by the crop, accumulating in the leaves. Irrigation waters with high chloride concentrations also reduce the availability of phosphorus to plants, inhibiting plant growth.

Sulfate (SO4) is abundant in most natural waters and readily soluble in most forms. It has no characteristic action on the soil, other than contributing to the total salt content, while, in irrigation water, it has fertility benefits. Sulfate toxicity is rarely a problem, except at very high concentrations when it may interfere with nutrient uptake (Raghupathi & Ganeshamurthy 2013). As with chloride, high sulfate ion concentrations reduce phosphorus availability to plants. As Table 12 shows, 53 and 13%, respectively, of the groundwaters are unsuitable for irrigation because of their SO42− and Cl concentrations.

Table 12

Permissible concentrations of chloride and sulfate for irrigation waters (Scofield 1936)

Class Cl (mg/l) SO42− (mg/l) Samples (%) of (ClSamples (%) of SO42− 
Very good <4 <4 20 
Good 4–7 4–7 20 – 
Usable 7–12 7–12 17 10 
Usable with caution 12–20 12–20 30 30 
Harmful >20 >20 13 53 
Class Cl (mg/l) SO42− (mg/l) Samples (%) of (ClSamples (%) of SO42− 
Very good <4 <4 20 
Good 4–7 4–7 20 – 
Usable 7–12 7–12 17 10 
Usable with caution 12–20 12–20 30 30 
Harmful >20 >20 13 53 

Nitrogen

Nitrogen (N) is an important plant nutrient, with deficiency typically reducing crop yields (Bauder et al. 2014). The common sources are natural soil nitrogen or added fertilizers, but nitrogen in irrigation water has much the same effect (Ayers & Westcot 1985). Excess nitrogen will cause problems – e.g., by over-stimulation of growth, delayed maturity or poor quality. Nitrogen (NO3-N) concentrations above 5 mg/l may affect sensitive crops, but most are relatively unaffected below about 30 mg/l (Ayers & Westcot 1985). In the study area, the nitrogen content of 25 samples, mainly from the urban zones, would lead to ‘slight to moderate’ (18) and ‘severe’ (7) restriction on use as irrigation water (Table 13). As large areas of the plain are under intense agricultural use, and ammonium nitrate- and potassium-bearing fertilizers are used, it is possible that nitrogen has been brought into groundwater through fertilizer dissolution (Ehya & Marbouti 2016).

Table 13

Permissible nitrogen (NO3-N) concentrations for irrigation water (Ayers & Westcot 1985)

 Degree of restriction on use
 
 None Slight to moderate Severe 
Nitrogen (NO3-N) (mg/l) < 5 5–30 > 30 
No. of samples 18 
 Degree of restriction on use
 
 None Slight to moderate Severe 
Nitrogen (NO3-N) (mg/l) < 5 5–30 > 30 
No. of samples 18 

Bicarbonate

When water with high concentrations of bicarbonate, calcium and sulfate is used in sprinkling irrigation, the problem of white stain formation on leaves or fruits may occur. No toxicity is involved, but the spots are of aesthetic concern when flowers, vegetables or fruit are sold on the fresh market. For fruits like apples and pears, expensive treatment (acid washing) is required before marketing (Ayers & Westcot 1985). The bicarbonate concentration in all samples exceeds 8.5 mg/l (Tables 1 and 14), indicating that the groundwater is ‘severely’ restricted in potential use for sprinkler irrigation. The high concentration of bicarbonate is probably due to the abundance of marls and lime-based rocks in the area. Bicarbonate concentrations increase towards city centers, because of the infiltration of sewage enriched with organic matter – as organic materials decompose, the carbon dioxide produced yields bicarbonate ion in the water.

Table 14

Permissible bicarbonate (HCO3) concentrations for irrigation water (sprinklers only; Ayers & Westcot 1985)

 Degree of restriction on use
 
 None Slight to moderate Severe 
Bicarbonate (HCO3) (mg/l) <1.5 1.5–8.5 >8.5 
No. of samples 30 
 Degree of restriction on use
 
 None Slight to moderate Severe 
Bicarbonate (HCO3) (mg/l) <1.5 1.5–8.5 >8.5 
No. of samples 30 

Salt sources in groundwater

Many factors including insufficient recharge, water-rock interactions (e.g. evaporite dissolution), groundwater evaporation, and influx of saline waters may cause groundwater salinization (Richter & Kreitler 1993; Vengosh 2003). Irrigation with low quality water will always result in the concentration of some soluble minerals in the soil and repeated irrigation over a long period will introduce a considerable amount of salt. Salts affect the soil's physical and chemical properties, and the resulting physicochemical changes will affect, in turn, the growth of plants (Ayres et al. 1993).

In order to identify the mechanisms causing groundwater salinity in arid and semi-arid regions, Na/Cl versus EC scatter plots are usually used (Esmaeili & Moore 2012). Figure 6(a) is a diagram of this type for groundwater samples from the Behbahan Plain. Where there is a horizontal linear trend, this suggests that evaporation and transpiration are responsible for increasing the sodium concentration. The amount of chlorine, however, is probably increased by anthropogenic activities such as agricultural reflux waters (Esmaeili & Moore 2012). Figure 6(b) shows the scatter plot of Cl versus Na+, with many points near the freshwater evaporation line, indicating that evaporation is a common groundwater phenomenon in the study area. Long-term evaporation raises the TDS content of groundwater (Brindha et al. 2014). Variations in groundwater TDS may also be related to its use on the surface and/or to contamination (Ellaway et al. 1999; Gillardet et al. 1999; Jalali 2009).

Figure 6

(a) Relationship between EC and Na+/Cl ratio in groundwater; (b) plot of Na+ versus Cl in groundwater.

Figure 6

(a) Relationship between EC and Na+/Cl ratio in groundwater; (b) plot of Na+ versus Cl in groundwater.

The concentrations of ions like SO42−, Na+ and Cl are usually increased in groundwater by the use of agricultural fertilizers, animal excreta, or by urban and industrial effluents. These sources could all cause TDS concentration changes in the groundwater in various part of the study area. The highest concentrations of SO42−and Cl are found in urban and countryside areas, respectively (Ehya & Marbouti 2016), indicating that water chemistry in the study area is affected by anthropogenic activity.

Figure 7(a) and 7(b) show that with increasing total dissolved ions (TDI), the sum of the concentrations of all cations and anions, the concentrations of calcium and sulfate increase linearly in the study area. A linear relationship also exists between the sulfate and calcium concentrations (Figure 7(c)), indicating gypsum dissolution. The low HCO3/SO4 ratio in the groundwater – below 1 – and the high sulfate concentration also confirm the influence of gypsum dissolution on the change in groundwater quality (Ehya & Firouzeh Moghadam 2017).

Figure 7

Physicochemical parameter diagrams for the study area's groundwater.

Figure 7

Physicochemical parameter diagrams for the study area's groundwater.

Since sodium and chloride versus TDI also have linear trends (Figure 7(d) and 7(e)), the salinity of groundwater could also arise from halite dissolution. The concentration of bicarbonate versus TDI varies little (Figure 7(f)), indicating that some carbonate dissolution also occurs in the aquifer. The linear relationship between magnesium concentration and TDI (Figure 7(g)) indicates a possible magnesium origin from dolomite dissolution, as the Mg/(Ca + Mg) ratio is less than 0.6 in all samples (Ehya & Firouzeh Moghadam 2017).

Summary and conclusions

The quality of groundwater in the Behbahan Plain has been assessed with respect to its suitability for irrigation. Apart from wells in the northwest, more than 85% of the waters sampled are ‘unsuitable’ or ‘doubtful’, based on EC values. Using Richards' classification, 67% of samples are of ‘C4S1’ or ‘C4S2’ class, and unsuitable for irrigation. The remaining 33% under this classification scheme fall in the ‘C2S1’, ‘C3S1’ and ‘C3S2’ classes, and are suitable. However, nearly all groundwaters in the study area are excellent or good, and suitable for irrigation, under the SAR, PI, %Na, MAR, KR and RSC indices.

The boron concentration is within the FAO's guidance levels for irrigation water.

About 60% of the groundwaters are not suitable for use in drip irrigation systems, because of their potential for clogging. The majority of groundwaters, however, have no potential for causing water infiltration problems. Based on the SO42− and Cl concentrations, 53 and 13%, respectively, of the groundwaters are unsuitable for irrigation. On the other hand, less than 10% of groundwaters in the area have ‘severe’ restrictions for irrigation use because of their nitrogen content. The study area's groundwater is ‘severely’ restricted in its potential use for sprinkler irrigation.

Taken together, groundwater in the central and southern parts of the Behbahan Plain, which is close to residential and agricultural areas, does not have the appropriate quality for irrigation, while outside these areas, especially to the northwest, water quality increases.

Scatter diagrams have been used to show that the groundwater chemistry is controlled predominantly by evaporation and water-rock interactions. Some portion of the SO42−, Na+ and Cl ions are thought to have been introduced into the groundwater from agricultural fertilizers, animal excreta, and urban and industrial sewage sources, causing changes in groundwater TDS concentrations – in other words, the groundwater chemistry is also affected by anthropogenic activities. The source of groundwater salinity could be dissolution of gypsum, halite and possibly carbonates, according to scatter plots of bicarbonate, Ca2+, Na+ and Cl versus TDI. Dolomite weathering is the source of Mg in the groundwater.

ACKNOWLEDGEMENTS

This study was financially supported by the second author when pursuing a Master of Science degree from the Behbahan Islamic Azad University.

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