Abstract
High evaporation rates in semi-arid to arid regions result in an increase in salinity that can exacerbate the effect of pollutants in water bodies.This study examines how groundwater drives pollution removal in wetlands, wells and springs within the Chaotic Subbetic Complexes (southern Spain). This evaporitic system, localy with a clear karstic functioning, is characterized by groundwater with a wide range of mineralization. Hydrochemical and multi-isotopic (δ34S, δ18O, δ15N, δ13C and δ2H) techniques were used to understand the geochemical processes leading to pollutant attenuation within the complexes. There, regional groundwater evolve from recharge/transition areas, with low salinity, to the discharge zone. Mineralization of groundwater depends on the dissolution of evaporitic deposits (gypsum, halite) of Keuper age, which increases salinity of the water drained by the outlet springs. δ15N and δ18O values of dissolved nitrate (NO3−) were used to estimate the relative contribution of N sources. NO3− is mainly derived from agricultural inputs (nitrate and urea fertilizers). Long-residence groundwater plays an important role in the biogeochemical evolution. Denitrification is responsible for NO3− removal in transitional zones and discharge springs. This process is promoted by the oxidation of organic carbon, derived from recharge areas and further transported to deeper zones of the aquifer. The findings of this study provide a new understanding of how hydrogeological functioning is connected to pollutant removal in an evaporitic karst system, where the scale of groundwater flows plays a key role in biogeochemical processes.
Highlights
Through inputs of agricultural and farming wastes, which include pesticides and drugs, and both organic and inorganic wastes from do mestic and industrial sources (e.g., Dodds, 2006; Destouni and Darracq, 2009; Bhagowati and Ahamad, 2019; Boyd, 2020), the excess in anthropogenic loading of nutrients derives in eutrophication of aquatic ecosystems, including wetlands (Lowe and Keenan, 1997; Smolders et al, 2010; Serrano et al, 2017)
Samples from Jarales and Brujuelo areas show a variable degree of mineralization, with a general trend to increase of electrical conductivity (EC) values from the highest to the lowest altitudes (Table 1, Fig. 1)
Similar drift was found in Brujuelo area: the lowest EC value was found in Ciruena wetland well (BP1; 4.2 mS/cm) and the highest in San Carlos spring (BS3; 196 mS/cm)
Summary
Through inputs of agricultural and farming wastes, which include pesticides and drugs, and both organic and inorganic wastes from do mestic and industrial sources (e.g., Dodds, 2006; Destouni and Darracq, 2009; Bhagowati and Ahamad, 2019; Boyd, 2020), the excess in anthropogenic loading of nutrients derives in eutrophication of aquatic ecosystems, including wetlands (Lowe and Keenan, 1997; Smolders et al, 2010; Serrano et al, 2017). The European Groundwater Directive (EC, 2006) considers nitrate (NO3− ) as priority substance due to its negative effects on health (Comly, 1945; Fraser and Chilvers, 1981; Ward et al, 2005) and on the eutrophication of surface water bodies (Dassenakis et al, 1998; Kraft and Stites, 2003). Removal of NO3− in wetlands depends on reduction-oxidation (redox) reactions which follow a sequence on the basis of thermody namic principles (Stumm and Morgan, 1981). It starts using oxygen (O2) as electron acceptor, followed by NO3− , manganese (Mn) and iron (Fe) oxides, sulfate (SO42-) and carbon dioxide (CO2). Dissimilatory nitrate reduction to ammonium (DNRA) involves the transformation of NO3− to ammonium (NH4+). Methanogenesis involves the biological decomposition of carbon biomass in the absence of oxygen
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