Rates of halite dissolution in natural brines: Dead Sea solutions as a case study

The rate of salt dissolution is an important factor in many natural processes including the formation of sinkholes along the western shores of the Dead Sea (DS). The formation of these sinkholes which appear during the last 20 years in ever increasing numbers (~ 7000), is attributed to salt dissolution in the subsurface, by saline groundwater. However the rate of salt dissolution in the subsurface, by DS brines diluted with groundwater, is not known and is the main purpose of this study. Whereas most previous experimental studies on halite dissolution rates were performed in pure NaCl solutions and in fresh water, this study focuses on natural DS brines. The chemical composition of the DS is complex and besides NaCl includes high concentrations of Br and of Mg, Ca, and K chlorides. The present study is a laboratory attempt to assess the dissolution rates of natural halite in dilutions of DS brines, 10% to 90% by volume, under stirring and under no stirring conditions. The dissolution rates have been estimated by first order kinetics in the stirred runs and by derivatives of power best fits in the unstirred runs. In quiescent conditions, at 10 min the dissolution rates exhibit a very close to linear, inverse relationship to the degree of dilution and ranged between 12.2 mg/cm2 min in 10% DS and 1.4 mg/cm2 min in 90% DS. With stirring, in dilutions of 50% DS, the dissolution rates were about twice larger than without stirring: at 10 min they were 13 mg/cm2 min vs. 5.8 mg/cm2 min, respectively. The dissolution rates in stirred runs of DS dilutions appear to be about threefold slower than those reported in pure NaCl solutions of similar salinity and in a quite similar experimental setup. This divergence emphasizes the importance of measuring halite dissolution rates in natural solutions. The measured dissolution rates, in both stirred and non stirred solutions, decrease exponentially as the degree of saturation vs. halite, DSH, becomes larger, i.e. as saturation is approached. The exponential decrease in non stirred solutions is much faster. The results of this study will enable future studies to determine the rate of sinkhole formation in the Dead Sea area and other locations where halite salt is dissolved.