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Riparian zones are highly-dynamic transition zones between surface water (SW) and groundwater (GW) and function as key biogeochemical-reactors for solutes transitioning between both compartments. Infiltration of SW rich in dissolved oxygen (DO) into the riparian aquifer can supress removal processes of redox sensitive compounds like NO 3 − , a nutrient harmful for the aquatic ecosystem at high concentrations. Seasonal and short-term variations of temperature and hydrologic conditions can influence biogeochemical reaction rates and thus the prevailing redox conditions in the riparian zone. We combined GW tracer-tests and a 1-year high-frequency dataset of DO with data-driven simulations of DO consumption to assess the effects of seasonal and event-scale variations in temperature and transit-times on the reactive transport of DO. Damköhler numbers for DO consumption (DA DO) were used to characterize the system in terms of DO turnover potential. Our results suggest that seasonal and short-term variations in temperature are major controls for DO turnover and the resulting concentrations at our field site, while transit-times are of minor importance. Seasonal variations of temperature in GW lead to shifts from transport-limited (DA DO > 1) to reaction-limited conditions (DA DO < 1), while shortterm events were found to have minor impacts on the state of the system, only resulting in slightly less transport-limited conditions due to decreasing temperature and transit-times. The data-driven analyses show that assuming constant water temperature along a flowpath can lead to an over-or underestimation of reaction rates by a factor of 2-3 due to different infiltrating water temperature at the SW-GW interface, whereas the assumption of constant transit-times results in incorrect estimates of NO 3 − removal potential based on DA DO approach (40%-50% difference).

Nitrate pollution is a widespread problem in many catchments with intense agricultural activities. Denitrification is a redox process that removes nitrate from the aquatic system via its transformation to nitrogen gas. Denitrification is difficult to assess at larger scales since it depends on multiple factors, such as solute concentrations, temperature variations, and also the time that water resides in the subsurface, where reactions can take place. To evaluate how these factors can influence denitrification, we employed a coupled modeling approach representing the riparian zone of a 4 th order stream in central Germany. We found that temperature variations strongly regulate the process and that during the winter the aerobic (oxygen rich) zone around the stream expands, which further inhibits denitrification in the near stream groundwater. However, even in the winter denitrification occurs, but at larger distance from the stream where oxygen has been depleted sufficiently. With increasing temperature, the influence of other factors on denitrification and on the redox zonation around the stream decreases. Coupled numerical models can provide further insights into the occurrence and interrelations of the multiple processes controlling water quality patterns in river corridors.

Abstract:In the UNESP campus area in Rio Claro -SP, the water supply is stemmed solely from the local aquifer, consisting of sandy-silt Cenozoic sediments of the Rio Claro Formation. The mainly objective of this work was to expand the understanding of the Rio Claro Aquifer through the review of the conceptual model and construction of a numerical flow model, using the Finite Element Method as solution. The results obtained after the steady-state and transient simulations, show that the lithological and hydrous heterogeneity of the aquifer unit result in a lower average specific storage; show also that the local drainages act as discharge zones and the campus and neighbors area act as area of recharge. The hydraulic conductivity values vary from 10 -6 and 10 -4 m/s and the ratio recharge/precipitation was stipulated in 13%. For a hypothetical doubling the rate of exploitation of wells, as well as for the partial or total removal of recharge, it was noticed large change in sensitivity of the model and large impact on the aquifer.Keywords: Hydrological Modeling. Rio Claro Aquifer. Finite Elements. FeFlow. Resumo:No campus da UNESP em Rio Claro, SP, o abastecimento hídrico provém unicamente do aquífero local, constituído por sedimentos cenozoicos areno-siltosos da Formação Rio Claro. O principal objetivo deste trabalho foi a ampliação do entendimento do Aquífero Rio Claro, por meio da revisão do modelo conceitual e construção de modelo numérico de fluxo, utilizando o Método de Elementos Finitos como solução. Os resultados obtidos com as simulações em regime permanente e transiente mostram que a heterogeneidade litológica e hídrica da unidade aquífera resulta em baixo armazenamento específico médio; evidenciam também que as drenagens funcionam como zonas de descarga, e o território do câmpus e circunvizinhanças como área de recarga. A condutividade hidráulica varia entre 10 -6 m/s e 10 -4 m/s, e a razão recarga/precipitação foi estimada em 13%. Para uma hipotética duplicação na taxa de explotação dos poços, bem como para a supressão parcial ou total da recarga, notou-se grande alteração na sensibilidade do modelo e grande impacto no aquífero.Palavras-chave: Modelagem Hidrogeológica. Aquífero Rio Claro. Elementos Finitos. FeFlow.