The Ria Formosa wetland system is classified as a leaky coastal lagoon and covers
approximately 100 km2 of the South of Portugal, with roughly 50% being intertidal. Its
hinterland is set in an arid region and on a coastal plain subject to intensive agriculture since the
50’s. In spite of high exchange coefficients with the coastal ocean (50 and 75% at neap and
spring tides, respectively) and the annual total potential freshwater discharge from the hinterland
a fraction of the daily tidal prism, worrying signs of eutrophication have been detected during the
past couple of decades. These include fish- and clam-kill episodes, increased occurrence of
nuisance algal blooms and substitution of native sea grass communities by macroalgae.
Notwithstanding its critical importance for the evaluation of pollutant exposure period, the
literature includes a wide range of estimates for the ‘residence time’ of waters within the lagoon
(16 hours to 11 days, with an the apparent consensus falling within the 1-2 day interval) and this
point is a clear obstacle for a correct environmental risk assessment, including management of
the system. This lack of clarity is due in our view to two main factors: i) the lack of proper
physical definition of the term ‘residence time’, with its consequent misuse and misapplication in
context, a misconception that is unfortunately too common within the environmental community,
and different concepts in the application of transport time scales ii) the geomorphological and
hydraulic complexity of the system.. As part of ongoing research evaluating the role of
Submarine Groundwater Discharge (SGD) as a loading vector for nutrients (especially Nitrate)
into the lagoon, we use the radium quartet in combination with remote sensing and isotope
mixing models to develop and discuss a mixing timeline for the system, We conclude that the
average, whole-system residence time of waters within the lagoon is at least 4 days
