On October 20^th, a team from the Environmental Radioactivity Group went to the Mar Menor lagoon, Murcia, SE Spain, to do a week-long sampling campaign in collaboration with the Universidad Politécnica de Cartagena (UPCT). This sampling is framed within the OPAL (Origin and Pathways of Anthropogenic solutes into coastal Lagoons: groundwater, sediments, and episodic events) project (Opal Project), which aims at identifying and assessing the major pathways delivering nutrients, trace metals, and pollutants originated from anthropogenic activities to coastal Mediterranean lagoons connected to intensively used aquifers and their consequences on the lagoon geochemical cycles. This is already the 6^th time in the last 2 years we have been in the Mar Menor lagoon, and despite being one of the most polluted saltwater lagoons in Europe, we have grown fond of it.

The ecological degradation of Mar Menor started during the 20^th century with the industrialization of mining activities, tourism intensification, and intensive agricultural practices. However, the visible consequences have come to light only in the last years. Although the lagoon is far from recovering, the positive note is the rise of public awareness, and thanks to this, Mar Menor was recently recognized as a “personhood”, meaning that it now has its own rights.

Although this represents significant progress, it needs to be accompanied by a good recovery and management strategy, and for this, it is crucial to understand the functioning of this unique system. This is precisely what we are trying to do with our research, which is focused on the main transport pathways of nutrients and other dissolved compounds that travel from the adjacent land to the lagoon. In particular, we study groundwater that discharges into the lagoon, a process known as Submarine Groundwater Discharge (SGD).

This hydrological process can be studied using a wide range of techniques, from hydrogeological models to biogeochemical tracers, or even remote sensing techniques. The objective of this campaign was to focus on a few of these methods to compare them and assess their advantages and limitations. To achieve this, we selected three sites located along the lagoon coastline: San Pedro del Pinatar, Marina del Carmolí, and Punta Lengua de Vaca. In each one, we collected samples to assess Submarine Groundwater Discharge and the associated dissolved compounds with the following methods: seepage meters, radon mass balance, radium mass balance, and ²²⁴Ra/²²⁸Th disequilibrium.

Installation of seepage meters in Marina del Carmolí. (Credit: Valentí Rodellas).

In broad brushstrokes, seepage meters consist of benthic chambers that are placed in the seabed and isolate a portion of sediment-surface water interface. This allows to easily quantify the groundwater flow that seeps through the sediment and mixes with seawater, which is collected in a bag connected to the device.

Seepage meter and piezometers installed at different depths in San Pedro del Pinatar. (Credit: Valentí Rodellas).

Radium and radon are geochemical tracers of SGD. Since they are naturally present in the geological matrix, there is an enrichment of groundwater with Ra and Rn isotopes. As a result of this, when groundwater discharges in the lagoon, it also delivers these isotopes, constituting a proxy for SGD. To collect groundwater samples for radon and radium analysis, manual piezometers are placed at different depths as seen in the picture above.

Collecting porewater samples for radium in San Pedro del Pinatar. (Credit: Valentí Rodellas).

Lastly, ²²⁴Ra/²²⁸Th disequilibrium in coastal sediments indicates the presence of water transfer across sediment-water interface. This method essentially relies on the isotopic ratio: since ²²⁴Ra is produced by ²²⁸Th via radioactive decay, in a system with no water transfer between the sediment-water interface, the two isotopes will be in equilibrium. However, if a ²²⁴Ra deficit relative to ²²⁸Th is detected within the first cm of the sediment column, it will be indicative of water circulation through the sediments and exchange with the overlying seawater, as Ra is easily released to interstitial water while Th is particle reactive.

Collecting samples for radium in water (left), radon in water (middle), and radium and thorium in sediments (right). (Credit: Raquel González and Maria Muñoz).

Our fieldwork consisted in collecting sediment cores, water samples for radon and radium analysis, and estimating water volumes collected from the seepage meters. Once the samples were collected, it was time to go back to the UPCT labs where some were in charge of cutting the sediment cores, others of analyzing the radioactive isotopes with Radecc systems (Lucas cell-based alpha radiation detector), and others of preparing the material for the following day.

Cutting sediment cores for ²²⁴Ra/²²⁸Th disequilibrium analysis. (Credit: Valentí Rodellas).

Not everything was work though (albeit most of it), we came up with an extra-official name for the campaign: “Measuring Isotopes in the Lagoon with some Friends”, and we created an Asiatico ranking, a typical coffee drink from Cartagena. Although short, it was a very intensive campaign that left us exhausted but satisfied with the work done. The only regret we had is leaving 5 seepage meters behind in the UPCT because the van could not fit anything else. Luckily, we expect to be back by the end of this winter.

The OPAL team packed up and ready to return to Barcelona after a week of sampling.