A multidisciplinary approach for assessing natural attenuation of pollutants in a highly saline lake-aquifer system: the case of pétrola lake, Spain
Valiente Parra, Nicolás
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Saline lakes are subject to numerous environmental impacts closely related to human activities, foremost pollution. These lakes are mostly located in arid and semi-arid regions associated with endorheic basins. These features, combined with the low precipitation and high evaporation rates typical for these regions, lead to the accumulation and biomagnification of pollutants. Saline lakes are polluted mainly through inputs of agricultural wastewater along with organic and inorganic wastes from domestic and industrial sources. This excessive anthropogenic loading of nutrients results in eutrophication of these ecosystems. Aquatic interfaces are critical sites for nutrient turnover and biodiversity. Here, microbially mediated redox processes exert an important control on water quality. Two main interfaces are present in saline lakes: (1) the sediment-water interface; (2) the freshwater-saltwater interface. While the former is the most important one for microbial turnover, the latter plays also a key role in pollution attenuation based on solute transport by density-driven flows. In this context, Pétrola Lake is one of the most representative saline wetlands in the Castilla-La Mancha region (central Spain). The lake is disturbed by agricultural activities and urban wastewater discharges that enter the waterbody directly. These practices degrade inland waters by introducing pollutants into the system (e.g. nitrate). The aim of this thesis was to assess the natural attenuation of pollutants in the Pétrola lake-aquifer system, with particular emphasis on nitrogen and sulfur recycling at both interfaces. Because reliable measurements are essential to study nitrate removal, a specific method for nitrate determination in hypersaline waters was also investigated. Additionally, the present thesis presents a risk assessment study of heavy metal pollution in the lake-aquifer system. For such purposes, a multidisciplinary approach was used comprising hydrochemical, multi-isotopic, geophysical, and molecular techniques. The study provided the first evidence for the coexistence of denitrification, DNRA, and anammox in a hypersaline lake. At the freshwater-saltwater interface, heterotrophic denitrification was promoted by the transport of solutes from Pétrola Lake to the underlying aquifer by a density-driven flow. Simultaneously, sulfur was recycled due to the influence of this density-driven flow together with sulfate-reduction processes. To determine nitrate under hypersaline conditions, a fast and cost-effective method using low sample volumes (500 µL) was described. Finally, the study on heavy metal pollution showed a noticeable concentration of lead in the sediments from Pétrola Lake related to anthropogenic sources. Since Pétrola Lake is a typical hypersaline aquatic ecosystem, the findings of this study can provide a better understanding of how these ecosystems can act as reactive zones for urban and agricultural pollution.