Assessing land-ocean connectivity via submarine groundwater discharge (SGD) in the Ria Formosa Lagoon (Portugal): combining radon measurements and stable isotope hydrology

作者：Rocha Carlos^*; Veiga Pires Cristina; Scholten Jan; Knoeller Kay; Grocke Darren R; Carvalho Liliana; Anibal Jaime; Wilson Jean

来源：Hydrology and Earth System Sciences, 2016, 20(8): 3077-3098.

DOI：10.5194/hess-20-3077-2016

Natural radioactive tracer-based assessments of basin-scale submarine groundwater discharge (SGD) are well developed. However, SGD takes place in different modes and the flow and discharge mechanisms involved occur over a wide range of spatial and temporal scales. Quantifying SGD while discriminating its source functions therefore remains a major challenge. However, correctly identifying both the fluid source and composition is critical. When multiple sources of the tracer of interest are present, failure to adequately discriminate between them leads to inaccurate attribution and the resulting uncertainties will affect the reliability of SGD solute loading estimates. This lack of reliability then extends to the closure of local biogeochemical budgets, confusing measures aiming to mitigate pollution.
Here, we report a multi-tracer study to identify the sources of SGD, distinguish its component parts and elucidate the mechanisms of their dispersion throughout the Ria Formosa -a seasonally hypersaline lagoon in Portugal. We combine radon budgets that determine the total SGD (meteoric C recirculated seawater) in the system with stable isotopes in water (delta H-2, delta O-18), to specifically identify SGD source functions and characterize active hydrological pathways in the catchment. Using this approach, SGD in the Ria Formosa could be separated into two modes, a net meteoric water input and another involving no net water transfer, i. e., origi-nating in lagoon water re-circulated through permeable sediments. The former SGD mode is present occasionally on a multi-annual timescale, while the latter is a dominant feature of the system. In the absence of meteoric SGD inputs, seawater recirculation through beach sediments occurs at a rate of similar to 1.4 x 10(6) m(3) day(-1). This implies that the entire tidal-averaged volume of the lagoon is filtered through local sandy sediments within 100 days (similar to 3.5 times a year), driving an estimated nitrogen (N) load of similar to 350 TonNyr(-1) into the system as NO3-. Land-borne SGD could add a further similar to 61 TonNyr(-1) to the lagoon. The former source is autochthonous, continuous and responsible for a large fraction (59 %) of the estimated total N inputs into the system via non-point sources, while the latter is an occasional allochthonous source capable of driving new production in the system.

• 出版日期2016-8-1