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.

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?)

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

1. Materials & methods  Column  Procedure  Column dismantlement 2. Results  Evaporation rate  Water content profile  Salt concentration profile  Temperature profile 3. Conclusions 4. Model INDEX

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 (MgSO 4 · 7H 2 O) model to mine water 14g/kg and 40g/kg

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

Materials & methods Column dismantlement 1. Take soil samples 2. Measure weight of moist and dry sample 3. Dissolve sample to measure electrical conductivity 4. Calculate (recall that sulphate minerals are hydrated)  Water content  Precipitate  Dissolved salts content

Materials & methods 1. Materials & methods  Column  Procedure  Column dismantlement 2. Results  Evaporation rate  Water content profile  Salt concentration profile  Temperature profile 3. Conclusions 4. Model INDEX

RESULTS Slower for: Silt (initially) Sand (finally) Evaporation rate:

Results Slower for: High concentration Evaporation rate

Results Slower for: Epsomite Evaporation rate

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

Results Salt concentration profile Similar behavior for halite & epsomite Different profile for sand & silt, but common in:  Brine at the top  Lower than initial below evaporation front SANDSILT

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

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

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

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

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

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

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

THANKS !!!