<p>Mar Menor (SE, Spain) is one of the largest coastal lagoons in the Mediterranean basin. Its ecological and economic significance has led to its inclusion in several protection programmes, both on a national and international level. In the last decades, this semiconfined habitat has been under high anthropogenic pressure from agricultural, mining and tourism activities, which have resulted in significant changes, such as eutrophic events and their cascading ecological effects. Previous research suggests that this degradation is linked to the introduction of nutrients and contaminants to this ecosystem, which are accumulated in the sediments of the lagoon. In this work, sediment cores from key locations of the Mar Menor were collected in order to estimate the amount of accumulated chemical compounds, such as metals and organic compounds. The results of this study are used to reconstruct the historical record of contaminants, which can fuel future contamination episodes in the lagoon.  </p><p>  </p>

<p>The Mar Menor coastal lagoon (Spain) is a critical ecological and socioeconomic ecosystem and the first in Europe to be granted rights of personhood. However, pollution from past and present activities such as mining, agriculture, urbanization, and tourism threatens its health and ecological stability. Previous research has shown the importance of metal contamination in the lagoon and its link to nearby mining activities, but little consideration has been given to historical changes in this industry and in other potential metal sources. In this work, metal concentrations have been analyzed in 12 sediment cores dated with <sup>210</sup>Pb, allowing the reconstruction of the recent (last ~150 years) metal contamination in the lagoon. The main metal sources have been identified by using multivariate statistical methods. Metal contamination from mining activities (point-source pollution) peaked in the mid-20th century, whereas nonpoint-source metal contamination reached its highest level in more recent decades. Despite the current decrease in metal deposition trends, concentrations in surface sediments still exceed sediment quality and ecotoxicological thresholds in areas close to former mining sites. Therefore, they need to be considered in future management strategies, which should also include the evaluation of sources and processes that are still supplying them to the lagoon.</p> <p> </p>

<p>The use of radiotracer techniques has been a fundamental tool for characterizing fluxes of solutes and water flows into the coastal ocean driven by submarine groundwater discharge (SGD). Indeed, the scientific interest in the use of radionuclides as tracers of SGD started developing in the late 90s when high activities of Ra isotopes and <sup>222</sup>Rn in the coastal ocean were associated with groundwater inputs. Since then, the number of articles published about SGD has considerably grown and the technical improvements in radiotracer methods have often been accompanied by concurrent scientific advances in the understanding of the process. Although current research in SGD is conducted through multiple techniques (direct measurements, hydrological, geophysical, and geochemical techniques), the use of tracers such as Ra isotopes and <sup>222</sup>Rn continues to be the most used and widespread method. Therefore SGD estimates are likely to be highly dependent on the methodological biases associated with radiotracer techniques. The aim of this study is to evaluate the main biases and assumptions relative to the use of Ra isotopes and <sup>222</sup>Rn in SGD studies through a meta-analysis of the published academic literature. The results of this work highlight that a significant number of SGD studies using radionuclides as tracers are based on erroneous assumptions or inaccurate calculations leading to unreliable SGD quantifications, thus preventing its use for comparison with other studies or extrapolating from local to regional-global scale. These results also emphasize that the SGD community should seek comparison, reproducibility, and multiapproach studies that help to understand the complexity of SGD in multiple sites and bridge the gap between different quantification methods.</p>

<p><span lang="EN-US">Mar Menor, on the northern Mediterranean coast of Spain, is a coastal lagoon with a surface of 135 km<sup>2</sup>, surrounded by agriculture fields and several towns, that support an important touristic activity. In addition, mining has been an historical activity on the region. In order to elucidate the input of metals supplied by Submarine Groundwater Discharge (SGD) to the lagoon and the spatial and temporal distribution of potentially toxic elements in this coastal ecosystem, the concentrations of dissolved metals (V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As and Cd) were measured in surficial water courses (“ramblas”), groundwaters (including piezometers and boreholes) and lagoon waters. Samples were collected in two sampling campaigns (July and November, 2021), representing contrasting climatic seasons. These results were combined with the estimation of SGD fluxes to the coastal lagoon, allowing evaluating the fluxes of the studied pollutants.  Occurrence of these pollutants in water from the lagoon has been previously registered, but none of those works has studied at the same time, and under a holistic point of view, all the possible aquatic pathways of dissolved metals, including the role of SGD as a conveyor of these potentially toxic elements. </span></p> <p>Keywords: Metal release, Coastal pollution, Land-Sea interaction, Submarine Groundwater Discharge.</p>

<p>Submarine Groundwater Discharge (SGD) is recognized as a major source of water and solutes to the coastal ocean, and it is particularly relevant in arid or semi-arid zones. SGD is generally defined as the flow of groundwater from continental margins to the coastal ocean, including thus both fresh groundwater from aquifer recharge and seawater recirculation through the coastal aquifer. Due to its high heterogeneity both in space and time, SGD is difficult to detect and quantify. As a consequence, numerous methods to study SGD have been developed over the last decades. These approaches mainly include hydrogeological approaches, geophysical techniques, direct seepage measurements, and the use of geochemical tracers. Each method presents its challenges, limitations, and advantages and each one works on different spatial and temporal scales, thus targeting different components of SGD. Therefore, comparing SGD studies with estimates derived from different methods is often complex and misleading if the characteristics and assumptions of each quantification technique are not taken into account. This highlights the need to conduct studies comparing SGD derived from different methods, not only to obtain more accurate SGD estimates but also to obtain instrumental information on the characteristics of the estimated fluxes. To this aim, a combined use of different approaches to estimate SGD was applied in a Mediterranean coastal lagoon (Mar Menor, Spain), including direct measurements with seepage meters, radium isotopes, and radon mass balance, <sup>224</sup>Ra/<sup>228</sup>Th disequilibrium in coastal sediments, radon vertical profiles in porewater sediments, and hydrologic modeling. Mar Menor is Europe's biggest saline coastal lagoon, and it is connected to a highly anthropized quaternary aquifer. In this coastal system, SGD is likely playing a major role in the eutrophication of the lagoon. However, despite the economic and biological importance of this lagoon, data about this system is still incomplete, and mostly only hydrological modeling has been performed.</p><p> </p><p><strong>Keywords: </strong>Submarine Groundwater Discharge, radioactive tracers, seepage meters, porewater exchange, hydrological modeling.</p>