Impact of hyporheic zones on nutrient dynamics

Riverine sediments play a fundamental role within the fluvial system, since they represent potential removal zones of stream-borne pollutants and, in particular, nutrients derived by anthropogenic activities. The region of sediments where the exchange and mixing of surface and subsurface waters occurs is the hyporheic zone. This region is also a place of intense biogeochemical activity, influencing both the flora and the fauna living in the fluvial environment. In the last decades several works were focused either on the water exchanges or the biochemical reactions in the hyporheic zone but just few considered the interactions of both hydraulic and biochemical processes. In this thesis the reactive transport of oxygen and the most common water-borne nutrients (i.e., dissolved organic carbon, nitrate and ammonium) in a duned streambed is investigated. In particular, a numerical model is employed to simulate the flow field, the biogeochemical reactions and the solute spatial distribution in the hyporheic zone. Sensitivity analyses are also performed to study the influence of different hydrological and chemical properties of the system on the net solute fluxes across the streambed. Finally, the effect of sediment heterogeneity on substance reaction rates and, specifically, on nitrate source/sink role played by the sediments is also analyzed for a rippled streambed.