Sabkha hydrology (capillary evaporation)

The hydrology of an evaporitic saline mudflat and its landward surrounds (dry mudflat or sandflat) is controlled by the position of the watertable in the saline sediments (Figure). Pores below the watertable are filled by water of variable salinity and constitute the phreatic or saturated zone where the water volume saturation is total (equals 1 or 100%). The phreatic zone is the region where gravity-driven groundwaters seep down the potentiometric slope toward discharge zones (forced-convection) and dense free-convecting brines, which are created by the evaporation of free surface waters, sink into underlying strata (brine reflux). Above the watertable is the vadose zone, where pores are filled by a combination of soil air and varying levels of water and brine, water volume saturation is less than one.

The vadose zone in dry mudflats or sandflats contains an uppermost interval known as the soil moisture zone where the water content can vary considerably over short time frames. Light rains typically penetrate the soil moisture zone but are not sufficient to saturate the whole vadose zone. Pore water returns to the atmosphere, via evapotranspiration, without ever replenishing the watertable. Fluctuating evapotranspiration levels in this zone vary widely between storms, reflected in variable metabolic activities of the soil biota, variable mean moisture contents, and varying O2 and CO2 levels. Pedogenic carbonates and more soluble salts are typically precipitated as surface crusts and near-surface cements as a result of these fluctuations. With the next heavy rain, the less soluble carbonate precipitates remain, while any more soluble salts tend to be flushed back into the subsurface, through a now saturated vadose profile, to re-enter the phreatic water mass.

The process of flushing surface waters to the phreatic zone is known as infiltration. In a homogeneous medium, it requires the whole vadose profile to be water-saturated. In an inhomogeneous medium, such as a karstic or a fractured and fissured evaporitic landscape, replenishment to the watertable can be more focused. Once the more soluble ions are back in the phreatic mass, they continue to flow as dissolved load down the potentiometric gradient and ultimately into the discharge depression where, driven by solar concentration, emerging waters once again can precipitate salts in sabkhas, pans, brine lakes and seaways.

The lower part of the vadose zone beneath a dry mudflat is made up of the capillary fringe (aka tension-saturated or wetting zone). In a saline mudflat or sabkha, the capillary fringe extends to the landsurface. The diameter of pore throats in the capillary zone controls the thickness of the capillary zone; the smaller the throat, the thicker the capillary zone. It is a few mm thick in sands and metres thick in fine clays. Pores in the capillary fringe are not entirely filled with water until its lowermost regions, there 100% water/brine saturation defines the top of the phreatic zone and the position of the watertable, hence the characteristic flat landsurface in all sabkhas. Water film and gas phases coexist in the interstitial pore space of much of the capillary zone. The relative humidity of the gas phase (saturated vapour pressure) is, however, equal to the thermodynamic activity of water, and so no evaporative concentration can occur in the capillary zone unless it is very close to the landsurface. Loss of pore water from the capillary fringe in a mudflat typically only occurs in its uppermost few centimetres. 

Hydrological zonation in the sabkha mudflat and its landward surrounds

Continental sabkha near El Maraqi, Siwa oasis, Egypt (seepage outflow at nothern edge of the Western Desert erg (sand sea)

The flat surface (top of capillary fringe) of the marine margin Abu Dhabi sabkha

This is one of several simple principles of soil physics associated with capillary fringes in groundwater discharge zones (sabkhas) that at times have been ignored in sedimentological models of sabkha deposition. First, if the capillary fringe is relatively thick, say 0.5 - 1.0 m, then host sediments must (by definition) have small pore throats and so must have low permeabilities. A sabkha, by definition, is a zone characterised by the growth of displacive and replacive capillary salts. Salinity of pore water in this capillary fringe is elevated (indicated by the preservation of gypsum, anhydrite, halite etc.). This low permeability means that high rates of transient short-term rapid lateral or vertical flow through a sabkha capillary fringe is impossible. When a sheet flood covers the 0.5-1.0 m thick saline capillary fringe of a sabkha with a sheet of fresher surface water (rainwater or seawater) this water does not begin to infiltrate until its density exceeds that of water in the underlying capillary pores. Then, even when densities do exceed that of the underlying waters, the low permeability of the narrow pore throats means the rate of recharge is slow. To accumulate salts in the capillary fringe, the overall solute-carrying water flow vector over time must be upward, not downward. It also means that almost all of the water in occasional fresher water sheets covering a sabkha is lost to the atmosphere. Low permeabilities mean a sabkha is not capable of maintaining longterm downward reflux flow of water or solute to the phreatic mass. This is true in all evaporitic mudflats. This notion of per ascendum rather than per descendum hydrologies is correctly accommodated in water flow models for most continental sabkhas and mudflats. This was less so in some sedimentological models of modern marine margin sabkhas, or at least this was so until the last decade when detailed hydrological data sets were collected, and coastal sabkha hydrologies quantified, rather than assumed (see Warren, 2016; Chapters 2 & 3).

Nodular and enterolithic anhydrite with obvious truncation surface preserved in middle of evaporite unit, Al Du'yybaya sabkha, Abu Dhabi, UAE

Pressure ridges in the halite crust (lowest point) of the Ras Qaryah sabkha, East Coast, Saudi Arabia

Widespread displacive calcium sulphate growth in the capillary zone sediments raises the sabkha surface out of the capillary zone to form a widespread erosional surface. As the rising sediments enter the vadose zone, they dry and are blown away to leave behind the characteristic erosion surface of a sabkha sequence, covered in places by a deflation veneer. This erosion surface, especially where it truncates enterolithic or ridge anhydrite is one of the most useful indicators of the sabkha hydrology. Like other Stokes surfaces, it defines the top of the capillary fringe and is driven by a precipitation/deflation process known as hydraulic pumping or hydraulic jack-up

Its presence in an ancient sabkha clearly distinguishes this style of truncated nodular anhydrite layering as penecontemporaneous and cannot be the result of burial conversion of gypsum to nodular anhydrite.In times of drought much of the middle and upper sabkha is covered by a pock-marked crust of halite, crosscut by pressure ridges and petees. This crust contains a range of efflorescent bittern and nitrate salts, but is dissolved and the soluble products flushed seaward with the next intense rainstorm or sheet flood. It is an ephemeral crust with a decoupled hydrology much like salt crusts in Australian salt lakes, as described in Warren 2016, Chapter 2.

Capillary evaporation in the sabkha precipitates nodules and crystals of gypsum and anhydrite in the capillary zone. This increasing volume lifts the upper part of the sediment column out of the zone of capillary water into the vadose zone where it deflates to leave behind the characteristic erosional surface that caps modern and ancient sabkhas and is a type of Stokes surface (after Warren, 1991).