Control mechanisms for dissolved phosphorus and arsenic in a shallow lake

This study investigated P and As sediment remobilisation in Lake Yangebup, a shallow lake with an overlying floc layer that covers the consolidated sediment. This floc is frequently resuspended into the water column, a process that was postulated to produce high P and As lakewater concentrations. Rate investigations using deionised water showed that P and As remobilisation reached steady state after 20 h in the consolidated sediment and within 1 h for the floc. Floc resuspension in lakewater showed no net release of either P and As, indicating that the floc was in constant equilibrium with the water column. A protocol to distinguish between desorption and dissolution was applied to both sediments and the response of remobilisation to varying slurry density and As addition measured. For the consolidated sediment, the concentration of Fe(II), P and As were unaffected above a slurry density ∼30 g L −1 and added arsenate (10–100 μg L −1) did not significantly change As and P remobilisation. It is shown that these results do not fit an adsorption/desorption equilibrium formulation for P and As remobilisation. Instead, the evidence suggests that the solubility of a thin, non-stoichiometric FeP x FeAs y oxyhydroxide surface coating determined the remobilisation process. Data scatter lead to some uncertainty in the floc results but suggest that dissolved P is controlled by dissolution, while dissolved As is controlled by adsorption/desorption. The results conclusively show that P and As remobilisation was lower from the floc than from the consolidated sediment and that the removal of the floc would not lower P and As lakewater concentrations. Implications of these results for the management of As in Lake Yangebup are outlined.