The term salinization may refer to several phenomena:
- Soil salinization refers to the precipitation of salts in the soil surface and root zone. These salts are highly soluble, so that the activity of water is greatly reduced, which hinders uptake by plants and causes significant losses in agricultural production. The problem is global in scale. Nearly 50% of irrigated lands in arid and semi-arid regions (FAO-UNESCO, 1992), 30% overall (Oldeman et al., 1991), suffer some degree of salinization problems.
- Water salinization refers to the increase in water salinity driven by evaporation, salt dissolution or mixing with saline water. Examples include lake salinization and aquifer salinization and, specifically in coastal aquifers, seawater intrusion.
Soil salinization is generally viewed as involving two processes (e.g., Fujimaki, 2006). First, salty water is brought to the surface by capillarity. Second, evaporation increases salinity and eventually causes salts to precipitate forming a crust, which may reduce further evaporation. Actually, the process is complicated by several factors (Gran et al., 2011a and b). The drop in water activity reduces evaporation. In fact, as activity equals the relative humidity of the air, a reduced activity implies that vapour will tend to flow towards salinized areas. Relative humidity is also affected by suction, large in the drying zone. Suction gradients cause an upwards vapour flux from wet to dry soil zones. Finally, energy drives the whole process. During summer, temperature decreases with depth, thus reducing vapour pressure below the evaporation front, which should cause a downwards flux of vapour, as observed in desert soils (Scanlon and Milly, 1996).
Figure 1: Evaporation from a soil column to simulate salinization (Gran et al., 2011b). Left, experimental set-up. Middle, conceptual model of liquid (dark blue) and vapor (light blue) water and solute (red) fluxes. Right, computed water fluxes . Water rises by capillarity to the evaporation front, a portion of the vapor diffuses upwards, leading to net evaporation. The rest diffuses downwards.
Thus, three competing mechanisms act during salinization: unsaturated flow of liquid, salinity and suction effects on water activity, and temperature driven vapour flux. Moreover, conventional retention and relative permeability curves, developed for agricultural soils, are not appropriate for dry and salty conditions. Because of these complexities, soil salinization is generally studied by means of water and salt balances, which may suffice for regional studies (e.g. Milzow et al., 2009), but does not acknowledge small scale processes. Understanding small scale processes is required for designing effective remediation strategies. As such, a large number of questions remained unanswered: how deep does salinization penetrate? Where does evaporation take place? Is salinization a surface process or can it occur at depth?
Soil salinization experiments
To answer these questions, we performed column evaporation tests (Gran et al., 2011b). We placed sand and silt columns with varying concentrations of epsomite (MgSO4·7H2O) and halite (NaCl) under a lamp, so that the soil surface radiation is similar to mid-latitudes summer radiation. The column weight was monitored and weight loss was attributed to evaporation. This allowed us to dismount the columns at varying degrees of saturation to measure temperature, salinity and water content versus depth. These variables are interesting, but do not suffice for a detailed characterization of actual processes, which requires detailed modelling.
Figure 2: Sensitivity analysis of simulated (and, when appropriate, measured) state variables to heat transfer during evaporation and formation of a saline crust on top of a column initially saturated with an epsomite diluted solution. Saturation, temperature and concentration are shown above. Vapor and liquid fluxes, and condensation (positive) or evaporation rates are shown below. (Gran et al., 2011a)
Soil salinization modelling
A coupled nonisothermal multiphase flow and reactive transport model was developed to study how thermal, suction and osmotic gradients interact during evaporation from a sandy soil. Under these very dry conditions; vapour fluxes become the main water flow mechanism. The model was manually calibrated with water content, temperature and concentration data from the evaporation experiment of Gran et al. (2011b). The retention curve and relative permeability functions were modified to simulate oven dry conditions. The model reproduces quite accurately experimental observations of varied nature (temperature, water content and salt concentration) (Figure 2). Most model parameters were either measured (retention curve) or derived from the literature (constitutive laws). Reliability of model parameters and the good qualitative fit between observations and model outputs supports the validity of the model, which prompted us to analyze and quantify the computed processes (Figure 2).
Results show that, from a mechanistic point of view, evaporation divides the soil into two markedly different zones. Above the evaporation front, the soil is dry, contains salts and water flux is restricted to vapour diffusion. Below the evaporation front, the soil is relatively wet (above irreducible water content), water flows upwards in liquid form, and downwards in vapour form, causing salinity to drop below the front. The mechanism displays positive feedbacks, as condensation will be most intense in areas of highest salinity, thus diluting saline water that may have infiltrated.
Evaporation causes vapour pressure to increase at the evaporation front, so that vapour flows upwards and downwards. Both fluxes occur throughout the experiment, but the relative importance of the downwards flux increases over time. In our model the downward flux is half that of the upward flux at the end of the experiment. As a result latent heat convection becomes a significant heat transport mechanism.
The evaporation front is very narrow, which contradicts earlier beliefs. Most evaporation concentrates in less than 1cm (Figure 2). Some evaporation occurs above the front, but condensation starts immediately below.
Lake salinization modelling
A natural extension of our work is the salinization of lakes, where the approach is similar to that of soils, except that the lake is usually viewed as a well-mixed system. Since salinization reduces water activity and thus the relative humidity at the water surface, evaporation is reduced in saline lakes. In cases when salinization causes the precipitation of hygroscopic salts, invariant points occur, so that the evaporation rate is effectively controlled by by the geochemistry of the system (Gamazo et al., 2011).
We have used the same approach to model the reservoir of Flix during clean-up operations.
Figure 3: Results of Gamazo et al. (2011) : (left) evolution of precipitated minerals and (right) zoom of dissolved species. Notice that water activity reproduces the data of Sánchez-Moral et al. (2002) and that wàter activity remains constant over sigificant periods of time (shaded regions in the right graph).
Further work and applications
We have continued working on salinization along several directions. On the one hand, we have used this understanding to gain insights on acid mine drainage generation mechanisms, which has led to the design of methods to control it (Acero et al., 2009; Bea et al., 2010a). In parallel, we have developed methods for accurate simulation of reactive transport under extremely dry and saline conditions (Bea et al., 2010b; Gamazo et al., 2012).
Implications for salt management are clear. Salinization occurs solely at the surface and may be controlled by reducing evaporation (e.g. by soil mulching) or by forcing it to occur far from the root zone (e.g. by drip irrigation). The important point, however, is that, accepting that salinization is an essentially surface process, suggests remediation methods.
Acero, P.; Ayora, C.; Carrera, J.; Saaltink, M.W.; Olivella, S., 2009, Multiphase flow and reactive transport model in vadose tailings, Applied Geochemistry, 24 (7), 1238-1250.
Bea, S.A.; Ayora, C.; Carrera, J.; Saaltink, M.W.; Dold, B, 2010, Geochemical and environmental controls on the genesis of soluble efflorescent salts in Coastal Mine Tailings Deposits: A discussion based on reactive transport modeling, Journal of Contaminant Hydrology, 111 (1-4), 65-82.
Bea, S.A.; Carrera, J.; Ayora, C.; Batlle, F., 2010, Modeling of concentrated aqueous solutions: Efficient implementation of Pitzer equations in geochemical and reactive transport models, Computers & Geosciences, 36 (4), 526-538.
FAO-UNESCO, June 1992, World Soil Res, Report 67 (2-7), Release 1.1, FAO-Rome
Fujimaki, H.; Shimano, T.; Inoue, M.; Nekane, K., 2006, Effect of a salt crust on evaporation from a bare saline soil, Vadose Zone J, 5, 1246-1256
Gamazo, P. ; Bea, S.A. ; Saaltink, MW. ; Carrera, J. ; Ayora, C., 2011, Modeling the interaccion between evaporation and chemical comModeling the interaction between evaporation and chemical composition in a natural saline system, Journal of Hydrology, 401 (3-4), 154-164, doi: 10.1016/j.jhydrol.2011.02.018
Gamazo, P. ; Saaltink, M. W. ; Carrera, J. ; Slooten, L. J. ; Bea, S., 2012, A consistent compositional formulation for multiphase reactive transport where chemistry affects hydrodynamics, Advances in Water Resources, 35, 83-93.
Gran, M., Carrera, J., Olivella, S., & Saaltink, M. W., 2011, Modeling evaporation processes in a saline soil from saturation to oven dry conditions. Hydrology and Earth System Sciences, 15(7), 2077-2089.
Gran, M.; Carrera, J.; Massana, J.; Saaltink, M. W.; Olivella, S.; Ayora, C.; Lloret, A., 2011, Dynamics of water vapor flux and water separation processes during evaporation from a salty dry soil, Journal of Hydrology, 396(3), 215-220.
Oldeman, L.R.; Hakkeling, R.T.A.; Sombroek, W.G., 1991, Int. Soil Ref. and Inf. Centre.
Scanlon, B. R.; Milly, P.C.D., 1994, Water and heat fluxes in desert soils 2, Numerical Simulations, Water Resour Res, 30(3), 721-733.