Impact of ambient temperature on evaporation from surface-exposed fractures

Surface-exposed fractures (SEFs) form a unique link between the atmosphere and the deep vadose zone. Quantifying evaporation and salt-accumulation rates within these SEFs is essential for understanding processes leading to groundwater salinization and contamination via these fractures. In this study, evaporation from SEFs (ESEFs) was quantified, mainly as a function of ambient atmospheric temperature, by using large-scale laboratory experiments and measuring evaporation under controlled conditions. In addition, ESEF was theoretically quantified based on the physical processes that govern it. The theoretical model was used to analyze ESEF rates as a function of ambient temperature, temperature gradient, fracture-aperture, and matrix pore size. ESEF was experimentally found to increase as the ambient temperature decreased. Measured evaporation rates were between about 110 and 260 g d(-1) per square meter of fracture surface, for temperature differences between rock-bottom and the atmosphere of 0 degrees and 13 degrees C, respectively. Comparing these values with model results suggests that convection is the driving process for enhanced evaporation at low ambient temperatures. Finally, we show that ESEF rates decrease as a result of salt precipitation. During a similar to 9-month period, with an imposed temperature difference of 13 degrees C, ESEF decreased from similar to 260 to similar to 95 g d(-1) m(-2) due to salt accumulation near and on the fracture surfaces. Evaporation rates began decreasing after about 100 g m(-2) of salt had precipitated and decreased to less than 50% of the initial rate after 160 g m(-2) of salt had precipitated. We thus show that not only temperature, but also salt precipitation, largely affect ESEF rates.