Assessment of phosphorus sediment–water exchange through water and mass budget in Passaúna Reservoir (Paraná State, Brazil)

Water and mass balances were used to assess phosphorus sediment–water exchange over the period 2010–2012 in Passauna Reservoir, Parana State, Brazil. The reservoir is relatively small and is mainly used for drinking water abstraction. Its present mesotrophic condition is threatened by increasing point and diffuse pollution loads, through population growth and agricultural and industrial expansions. The monthly water budget terms over 2010–2012 indicate a dominant, though gradually decreasing input via the different rivers and tributaries and an increasing contribution through groundwater inflows. Unaccounted-for terms in the water budget were estimated at ca. 10 %. Annual phosphorus budgets were used to derive the net P sediment–water exchange terms. These amounted to about 30 % of the annual P inlet, as a P accumulation into the sediment of the reservoir; no seasonal trends were observed. Considering the main geochemical characteristics of the basin, much of the sediment phosphorus binding will take place by Al–Mn–Fe minerals and oxyhydroxides. In this respect, possible large reductions in the water oxygen contents might likely trigger P releases from the sediment, which require further water management to mitigate impact.