Salt redistribution during freezing of saline sand columns with applications to subsea permafrost

Abstract

Laboratory experiments were designed to investigate salt redistribution during the freezing of saline sand columns and to obtain information on salt movement in saturated sands and reconstituted subsea permafrost samples. The results of these experiments were combined with results from field investigations of subsea permafrost at Prudhoe Bay, Alaska to develop an improved understanding of salt redistribution during freezing and the movement of salt in the seabed sediments. These processes can produce soil solution salinities in the sediments greater than about 50 ppt. Comparison of spring and fall salinity profiles indicate salt movement with velocities of at least 2 m/year. Laboratory freezing (downward) tests of saline sand columns show significant salt redistribution at growth rates between 0.1 and 2 cm/day. Salt movement was observed with velocities of at least 2 cm/day. Salt movement in the unfrozen soil solution in partially frozen sand appears to be the result of gravity drainage. Freezing upward produced no significant salt redistribution. Salt fingering experiments showed that salt fingers could move with velocities of several cm/hr and suggest that it may be a major mechanism for rapid salt movement in subsea permafrost. Fingers (freshwater) at a thawing fresh ice boundary overlain by thawed saline soil solution displayed similar rapid movement behavior. Laboratory measurements of the hydraulic conductivity, K, of subsea permafrost samples yielded values that were 10$\sp2$ to 10$\sp3$ times greater than previously reported in-situ measurements. While it is difficult to apply the laboratory results to subsea permafrost under field conditions, these greater values for K and the large salt fingering velocities suggest that gravity-driven convection, in the form of salt fingering, should be considered as a primary mechanism for rapid salt transport in subsea permafrost.