The fate and accumulation of bromide during playa salt deposition: An example from Lake Frome, South Australia
An advectivn/diffusion model describing the distribution of Br - in porewaters underlying an accumulating deposit of halite on a playa lake can be used to predict the evolving composition of the accumulating salt growing as a result of evaporation. The Br/Cl ratio of the accumulating halite is proportional to the concentration or Br - at the surface of the brine body from which it is growing. The evolution of interstitial Br - and therefore the composition of the accumulating salt is a function of (1) the diffusion coefficient of Br - in the brine-saturated sediment, (2) the advective velocity of the interstitial water, (3) the initial concentration of Br - and Cl - at the onset of evaporation, and (4) the distribution coefficient of Br into the accumulating salt, all of which can be estimated for the sediments and brines of Lake Frome, South Australia. At Lake Frome, the model is used to estimate mean evaporation rates for the salt-covered surfaces which are found to be between 0.5 and 3.0 cm/y. These rates are lower than the rates estimated for the salt free surfaces of the lake by Allison and Barnes (1985) but are consistent with the expectation that groundwater input and therefore evaporation rates will be focused at the salt-free lake margins rather than at the salt-covered central regions of the playa and that the surface albedo of the salt-covered zones will reduce energy penetration to the sediment and thus evaporation.