Effects of salinity on open soil evaporation processes

1. Effects of salinity on open soil evaporation processes M.Gran(1), J.Massana(1), J.Carrera(1), M.W.Saaltink(1), S.Olivella(1), C.Ayora(2), A.Lloret(1) (1) Dept. of Geotechnical Engineering and Geosciences, School of Civil Engineering, Universitat Politècnica de Catalunya (UPC), Barcelona. (2) Institute of Earth Sciences Jaume Almera, Spanish Research Council (CSIC), Barcelona, Spain.

2. MOTIVATION • Water flow near the surface during evaporation: • Liquid phase (upwards) • Vapor (only upwards?) • Affected by temperature gradients • Precipitation of minerals affects: • Soil properties (diffusivity) • Density • Osmotic effects • Solute mass transport affected by: • Evaporation (increase concentration?) • Advection (upwards) • Dispersion (downwards?)

3. OBJECTIVE • Perform column evaporation experiments in order to evaluate: • Time evolution of evaporation • Spatial distribution of water and salt • Water and solute mass fluxes

4. INDEX • Materials & methods • Column • Procedure • Column dismantlement • Results • Evaporation rate • Water content profile • Salt concentration profile • Temperature profile • Conclusions • Model

5. Materials & methods: Column • Size: h = 25 cm, Φ = 14’4 cm • Solid fill: sand & silt • methacrylate surrounded by heat insulator • Salts: • halite (NaCl) • model to seawater • 7g/kg and 20g/kg • epsomite(MgSO4·7H2O) • model to mine water • 14g/kg and 40g/kg

6. Procedure • Evaporation under a 175w lamp • Control the degree of saturation by weighs • Continue until 32% saturation:68% of initial water has evaporated

7. Column dismantlement • Take soil samples • Measure weight of moist and dry sample • Dissolve sample to measure electrical conductivity • Calculate(recall that sulphate minerals are hydrated) • Water content • Precipitate • Dissolved salts content

8. INDEX • Materials & methods • Column • Procedure • Column dismantlement • Results • Evaporation rate • Water content profile • Salt concentration profile • Temperature profile • Conclusions • Model

9. Evaporation rate: RESULTS • Slower for: Silt(initially) Sand (finally)

10. Evaporation rate • Slower for: High concentration

11. Evaporation rate • Slower for:  Epsomite

12. Water content profile ►Liquid ►Total water • Smooth for silts • Dry front for sand at 5cm depth

13. Salt concentration profile SAND SILT • Similar behavior for halite & epsomite • Different profile for sand & silt, but common in: • Brine at the top • Lower than initial below evaporation front

14. Salt concentration profile • High concentration at top (evaporation) • Down to evaporation front? (3’5cm) • BUT, notice minimum • AND, why concentration increases again downwards?

15. Temperature profile Heat sink? Evaporation at 7cm depth? Two different slopes show a sudden temperature decrease due by evaporation

16. Comparative • Notice that: • Temperature slope change & minim are coincident • BUT, not with evaporation front from water content!

17. Zoom Evap front from water vol cont Evap front from conc Evap front from temp Then,where is the evaporation front?

18. CONCEPTUAL MODEL Precipitation zone Evap zone Vapor Dispersion diffusion Liquid Advection Solute Water

19. CONCLUSIONS Role of osmotic effects • Water contentdistribution & evaporation rate depends on: • Material • Salt • Initial concentration • Salt content distribution depends on: • Material • Water recirculates beneath the evaporation front (downwards vapor & upwards liquid)

20. MODEL • Preliminary modeling with the program RETACO • Modification of Van Genuchten retention curve to reproduce the behavior of soil in oven-dry conditions Lab results Model results

21. THANKS !!!