Biogeochemical processes in a freshwater–seawater mixing zone in permeable sediments along the coast of Southern Brazil

The southern portion of the Brazilian coast is dominated by coastal lagoons formed by sandy barrier spits with small inlets. This coastal configuration is a barrier to the surface flow of freshwater to the sea; thus, we suspect that a significant amount of freshwater flows through the permeable sands, beneath the barrier spits, where it mixes with seawater. We excavated an 18-m-deep well into the barrier spit which separates the Patos Lagoon from the South Atlantic. Using this well, we were able to sample interstitial waters from discrete layers, at 1-m intervals, which were analyzed for salinity, temperature, pH, nutrients (ammonium, nitrate, phosphate, and silicate), uranium, molybdenum, and barium. Similar analyses were made on surface water samples from the Patos Lagoon estuarine mixing zone.Results of well samples show a continuous increase in salinity with depth reaching 18 at the bottom. Ammonium and silicate are high, generally around 100 and 100–150 μM, respectively, throughout the subterranean profile. Phosphate shows a distinct maximum at about 6 m (ca. 25 μM), and nitrate is generally low in all well samples. Uranium and molybdenum exhibit a minimum in the well profile at about the same location where barium exhibits a maximum (greater than 2 μM). When results are compared to the surface lagoon–seawater mixing data, ammonium, phosphate, silicate, and barium in well samples of similar salinity show considerable enrichment, while a comparison of uranium and molybdenum data indicates significant depletion of these metals in most well samples.Based on these and other data, we deduce that the following processes are active: products of remineralization of organic detritus accumulated in lagoon sediments are advected through permeable sediments to the oceans; dissolution of biogenic solids and/or solid silicates mobilizes silicate; phosphate, uranium, and molybdenum are mobilized from phosphate-rich sediment layers; sulfate reducers remove uranium and perhaps molybdenum from solution throughout most of the well profile; barium is desorbed from solids in the subterranean mixing zone. These results demonstrate that freshwater discharged to the ocean through permeable sediments may have a significantly different composition than that discharged at the surface.