Desalinated water. Aspects to take into account

The importance of agricultural water use on a global scale is 70%, while in arid and semi-arid regions with highly technified agriculture, figures of over 85% are reached, as is the case in southeast Spain (PHDS, 2022). The intensification of water scarcity represents a risk for the role of irrigated agriculture in global food security in the medium and long term, associated with the impossibility of meeting irrigation water demands by using conventional water resources in the future. Therefore, new solutions are needed to maintain or improve sustainable agricultural production, including new or alternative water sources, innovative strategies for water conservation, or more efficient and productive irrigation systems, in line with the guidelines of the European Green Pact. 

Potential advantages and disadvantages of desalinated seawater  

In general, the main advantage of AMD is its status as an inexhaustible water resource not subject to climatic variations, as well as its low salinity, with an electrical conductivity of around 0.5 dS m-1, which allows it to compensate for the high salinity of other water sources. However, the use and acceptance of AMD for agricultural irrigation is limited mainly by production costs and a unique physico-chemical composition. The agricultural application of AMD is more costly due to the high price of water compared to conventional sources and requires more specialised fertigation programmes and the management of specific agronomic risks, such as boron phytotoxicity or soil alkalinisation. The main advantages and disadvantages of using AMD for irrigation are summarised in Table 1. 

Table 1. Summary of advantages and disadvantages of AMD.

Phytotoxicity problems and risk of soil alkalinisation

This is because about 55% and 31% of the dissolved salt content in seawater is due to these ions, which continue to predominate in proportion after the reverse osmosis process. Furthermore, the concentration of B presented is very high in the AMD, which is due to two factors: the high concentration of this element in seawater (4.5 - 6 mg/L) compared to conventional water (generally very close to 0 mg/L in surface water and up to 1.5 mg/L in groundwater) and the high permeability of RO membranes to the passage of B in neutral and acidic media (Yermiyahu et al, 2007; Raveh and Ben-Gal, 2016) compared to all other ions. In fact, while the single-stage separation efficiency of reverse osmosis membranes is higher than 98% for Na, Mg, Ca, K, Cl and SO4 ions, the efficiency for B is only 71% (Martinez, 2009).  

Na concentrations in the IDAMs of southeastern Spain vary between 76 and 115 mg/L, therefore, these values in AMD should not cause phytotoxicity problems in crops. However, high Na concentrations can damage some physical properties of the soil by the dispersion of clays which can lead to: (i) structural collapse of soil micro-aggregates, (ii) decrease of permeability and (iii) decrease of the soil's organic matter. hydraulic conductivity (iii) more susceptible to erosion, (iv) soil compactionand (v) oxygen depletion in the soil due to reduced aeration (Mandal et al., 2008; Muyen et al., 2011). 

To reduce soil sodicity irrigation it is important that the divalent cations Ca and Mg are present in adequate concentrations in the water. Therefore, the RAS, which relates the balance in the concentration of Na to Ca and Mg, must be evaluated in the irrigation water in order to know the risks of sodicity in the medium and long term. 

Boron in AMD 

Boron, when naturally present in water, is found to form an equilibrium between boric acid [B(OH)3] and borate ion [B(OH)4-], in which the boric acid form prevails. This is because the dissociation constant of boron is pKa= 9.15 , which makes it a very weak acid. At higher pH, from pH 10 onwards, the dominant species becomes the metaborate anion [B(OH)4-].  

The concentration of boron in seawater is around 4 mg/L. This water, when desalinated by reverse osmosis, reduces its boron concentration to 0.8 mg/L - 1.5 mg/L, depending on the conditions and methods used in desalination. In order for the boron to be effectively rejected in the membranes, it is essential to avoid boron in the form of boric acid, since the absence of charge and acidic hydrogens makes it capable of forming hydrogen bonds with the active groups of the membranes. (María Fernanda Chillón Arias, 2009) 

In general, there is a narrow range between boron deficiency concentrations 0.5 mg/L in woody crops such as citrus and (1-4 mg/L) in horticultural crops.  

Surface irrigation waters typically have boron concentrations below 0.1 mg/L, which means that additional fertiliser inputs (via micronutrient complexes) are often required to ensure optimal crop development. However, non-conventional water resources, such as reclaimed water and desalinated seawater, are distinguished by significantly higher boron concentrations than surface water. The concentration of B in the AMD produced in the IDAMs of south-eastern Spain varies between 0.56 mg/L and 0.92 mg/L, values that are higher than the maximum tolerances indicated for woody crops such as citrus and could therefore cause toxicity problems and yield reductions.  

In case the end use of AMD is agricultural irrigation, the control of boron (B) concentration in the product water can also be the subject of a specific post-treatment. B in irrigation water can be reduced to a level below the toxicity threshold of sensitive crops with reverse osmosis and cation exchange resins to remove boron from AMD at plot scale (Imbernon-Mulero et al., 2022).  

The concentration of B in desalinated seawater produced in Spain has traditionally been limited to 1 mg/L, which was the maximum value set out in Royal Decree 140/2003, which established the sanitary criteria for the quality of water for human consumption. This value has recently been raised in Royal Decree 3/2023 to 1.5 mg/L, except when the total origin of the water is transitional or coastal water and the potabilisation treatment is desalination, in which case a maximum value of 2.4 mg/L will be applied.  

These concentrations largely satisfy crop needs, but may induce phytotoxicity, especially in more susceptible crops such as citrus. The most common symptoms of B toxicity are burnt edges on older leaves, yellowing of leaf tips and accelerated decay, which can even lead to plant death (Martinez-Alvarez et al., 2017).  

The first stages of boron toxicity usually appear as a leaf tip. yellowing or mottling. In severe cases, gum spots appear on the lower surfaces of the leaves with premature leaf drop. Severe symptoms may include dieback of twigs. 

Establishing boron (B) tolerance limits in irrigation water for different crops is a complex task that requires meticulous experimental work. Several factors, such as crop variety, soil characteristics, water chemistry, climatic conditions and irrigation management practices, have a significant influence on crop response to B and thus on the results obtained (Grattan et al., 2015). 

Table 2 illustrates the varying sensitivity of crops to B. It shows that the most sensitive crops are woody crops, especially citrus and stone fruit trees, while crops with short vegetative cycles, such as vegetables and grasses, are less affected. 

However, when interpreting Table 2, it is essential to consider that the values refer to the maximum concentration of B tolerated in the saturation extract of the soil, without any reduction in yield or vegetative growth. These values are generally higher than those found in irrigation water, estimated to be between 1.4 and 1.9 times higher under moderately drained conditions (around 25%) (Jame et al., 1982).  

As we can see, the whole genus of citrus and stone fruit trees are especially sensitive and vulnerable to irrigation with water containing high concentrations of boron, as is the case of AMD.

Boron tolerance of different crops.