Groundwater quality assessment and its suitability for agricultural purposes in the Behbahan Plain, SW Iran

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.

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.

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.

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.

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 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).

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)).

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).

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).

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.

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.

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).

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.

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.

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.

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).

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.

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).

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).

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).

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.

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