RESUMEN
A comprehensive hydrogeological report was conducted to determine the origin, occurrence and processes affecting nitrogen in a Mediterranean coastal aquifer-lagoon system. Water levels, hydrochemical and isotopic data was gathered during a 4-year period in the La Pletera salt marsh area (NE Spain). They were collected from the alluvial aquifer, two natural lagoons and four other permanent lagoons excavated during a restoration process (in 2002 and 2016), two watercourses (the Ter River and the Ter Vell artificial channel), 21 wells (considering six of them for groundwater sampling) and the Mediterranean Sea. Potentiometric surveys were carried out seasonally, however twelve-monthly campaigns (from November 2014 to October 2015), and nine seasonal campaigns (from January 2016 to January 2018) were conducted for hydrochemical and environmental isotopes analyses. The evolution of the water table was analysed for each well, and potentiometric maps were plotted to determine the relationship between the aquifer and the lagoons, sea, watercourses, and groundwater flow. Hydrochemical data included physicochemical data measured in situ (temperature, pH, Eh, dissolved oxygen, and electrical conductivity), major and minor ions (HCO3-, CO32-, Cl-, SO42-, F-, Br-, Ca2+, Mg2+, Na+, and K+), and nutrients (NO2-, NO3-, NH4+, Total Nitrogen (TN), PO43-, and Total Phosphorus (TP)). Environmental isotopes included stable water isotopes (δ18O and δD), nitrate (δ15NNO3 and δ18ONO3) and sulphate isotopes (δ34SSO4 and δ18OSO4). Water isotopes were analysed for all campaigns, however, nitrate and sulphate isotopes water samples were only analysed in some particular surveys (November and December 2014; January, April, June, July and August 2015). Additionally, two more surveys for sulphate isotopes were conducted in April and October of 2016. The data generated through this research may be used as a starting point to analyse the evolution of these recently restored lagoons, and their future responses to global change. In addition, this dataset may be used to model the hydrological and hydrochemical behaviour of the aquifer.
RESUMEN
Coastal lagoons can act as sinks and sources of a wide range of substances, including nutrients, and pollutants. In these ecosystems, primary production is limited more by nitrogen than by phosphorus. For this reason, they are significantly vulnerable to nitrate pollution. In this study, a joined analysis of surface and ground water was conducted to determine the origin, occurrence and processes affecting nitrogen fate in a Mediterranean coastal aquifer-lagoon system. This included the analysis of water levels, as well as hydrochemical and isotopes data evolution for a 4-year period, which revealed two important insights of nitrogen cycling within the system. Firstly, we detected different origins for nitrate pollution (a mixture of sewage, manure and chemical fertilizers), as well as their nearly complete attenuation in the alluvial aquifer due to heterotrophic and autotrophic processes, favoured by the presence of organic matter and Fe-minerals in its sediments. Secondly, due to its rapid assimilation, inorganic nitrogen peaks in the lagoons were mainly detected after storm events. While nitrate peaks may be attributed to surface water runoff, ammonium peaks may be related to organic nutrient cycling. In contrast, we did not detect continuous and low N inputs, associated to groundwater flow. These results depict the need of a more integrated management strategy of these aquifer-lagoon systems.
RESUMEN
A particle size analyzer together with a video camera has been used to investigate the structure of the aggregates during the process of aggregation and breakup of particles in a shear flow induced by means of a Couette-flow system. Three different values for the shear rate have been used (25, 32, and 50 s-1). The initial volume fraction of the particles was phi0 = 5.0 x 10(-5). Different fractal dimensions (D3), D2, D1, and Dpf), the shape factor (xi2), and the aspect ratio (AR) have been used to characterize the structure of the aggregates. During the process of aggregation, no significant changes in the fractal dimensions, shape factor, and aspect ratio were found. The high values obtained for D3 (D3 = 2.2) and D2 (D2) = 1.98) show us the high compactness of the flocs. Also, values of the fractal dimension and shape factor have been found to not depend on the shear rate. Copyright 1998 Academic Press.
RESUMEN
An experimental study was carried out to investigate aggregation and breakup of particles in a Couette flow system with the inner cylinder rotating at constant speed. Experiments were conducted with monodisperse and polydisperse suspensions at different particle volume concentrations. Depending on both the shear stress and the particle concentration, three different regimes were found. In the first regime, when the concentration is less than a critical value, the final diameter of the aggregate is independent of concentration and depends only on the shear. When the concentration is higher, two new regimes are identified, with the final diameter of the aggregate depending on both shear stress and particle concentration. Also, the transition between these two regimes corresponds to the transition from laminar to turbulent flow. In addition, the final diameter of the aggregates in the turbulent flow regime is independent of the size of the primary particles and results to be is controlled by the Kolmogorov length scale. On the contrary, in the laminar flow regime the final diameter of the aggregates shows a light tendency to decrease as the diameter of the primary particles increases.
