Hydrochemistry of surficial brines in Hutt lagoon, Western Australia

Major elemental compositions in surficial waters collected systematically, since 1981, from selected monitoring sites along the landward and seaward margins of the evaporitic Hutt Lagoon, in Western Australia, indicate a predictable annual concentration cycle imposed by strong seasonally-defined climatic factors. The amount of surface water in the lagoon is determined by a balance between influxes and evaporative reflux of solution. Influxes are from the sea, direct precipitation, subsurface input of fresh-brackish groundwaters from the lagoon-marginal aquifers, and evaporative pumping of the concentrated brines interstitial/intrastratal to lagoonal evaporitic units. This balance between influx/reflux budgets, together with the topographic relief in the lagoon, basically control the surface-water circulation system in such a way that the lagoon changes from an essentially playa environment in the dry seasons to one which accomodates large ponds several decimeters in depth. Comparison of the chemistry of the surficial solutions in adjacent ponds indicate that the ionic concentrations in individual samples are greatly influenced by (1) the physiographic setting of the pond (2) the influx-reflux relationship and (3) the types and the extent of selective ionic segregation due to processes including the common-ion effect, adsorption onto clayey flocculates and variation in P_CO2 conditions. Variation in crystal habits, fabrics and structural features of contemporary gypsite deposits can be directly related to seasonal changes in hydrogeochemical conditions at depositional sites. Therefore, characterization of cyclic changes in hydrologic settings should form an integral part of the identification of formative processes, and hence, interpretation of palaeoenvironmental conditions.