Our story is one of the Ria Formosa Lagoon, Faro, Portugal, one of the largest lagoons in Europe and a very important ecosystem. It hosts a vast amount of dynamic life, from microorganisms to plants and, from invertebrate to higher animals. Specifically, the marsh grassland and abundant aquatic life provide ideal conditions for migrating birds. Moreover, the flourishing sea grass meadow breeds many kinds of mollusk and fish, which sustains the ecosystem and aquaculture.
During the 1970s, the local vegetation was progressively substituted with olive trees, tomato and citrus fruit farming. This led to an increased application of fertiliser on the land, which was often overloaded. As groundwater flowed through this region, it became contaminated with high concentrations of nutrients, including nitrate, nitrite, ammonium and phosphate, which were then transported into the lagoon. This had a significant effect on the ecosystem. In the 1980s, it was noticed that algae blooms were occurring more and more often, and the water was becoming less clear. This was a very important issue due to the large community depending on the lagoon for their livelihoods, including domestic use, tourism, and aquaculture.
Researchers began to question what had led to this nutrient imbalance in the ecosystem. Initially, surface water input was blamed. However, Ria Formosa has a semiarid climate and is only connected to one river. This could not be the main cause of the water deterioration as the surface water input is quite limited. The only viable conclusion was that groundwater was the pollution source.
Submarine groundwater discharge (SGD) is any and all flow of water underground from land to sea. The flow is due to a hydraulic gradient, which transports the water from land to sea, primarily at low tide, and may force seawater into the aquifer at high tide, causing seawater intrusion. It occurs at nearly every point of the Earth’s subsurface. As it is transported it has the potential to become heavily contaminated with nutrients, heavy metals, organic pollution and more. This is primarily due to the slow water movement and the lack of photochemical reactions once it goes underground.
Determining the source, flow and quantity of groundwater discharging into the lagoon is exceptionally difficult. Direct measurements, using seepage meters, may lead to large uncertainties. Natural tracers, such as radon (Rn), hydrological stable isotopes (H2 and O18) and radium (Ra) are now used to quantify groundwater with mass balance calculations in mixing models. Tracers, such as these are ideal as they are more enriched in groundwater relative to seawater, are conservative and can be measured at low concentrations.
We, the Biogeochemistry Research Group in TCD Geography, aim to precisely quantify the levels of nutrients, dissolved organic matter and dissolved inorganic carbon being discharged into the lagoon as a result of groundwater. In order to fulfil these objectives, we must distinguish the SGD derived nutrients from other inputs, such as a waste-water treatment plant located in the lagoon discharging treated water and diffusion through recirculation of brackish water via a sandy beach within the lagoon.
Water Discharging into the Ria Formosa Lagoon