Groundwater–surface water interactions in a freshwater lagoon vulnerable to anthropogenic pressures (Pateira de Fermentelos, Portugal)

Groundwater–surface water interactions are investigated in the Pateira de Fermentelos lagoon, a shallow freshwater body with important aquatic and terrestrial ecosystems. The monthly monitoring of water levels proves that the lagoon drains phreatic aquifers which in turn contribute to recharge the underlying semi-confined aquifers. Besides rainwater, four types of natural waters contribute for the water balance and water quality in the lagoon: (1) the Cértima River with a Ca–HCO3 water type and circum neutral pH; (2) the Cretaceous aquifers with a Na–Cl–HCO3 water type and acidic pH; (3) the Triassic aquifers with a mixed water type, and slightly alkaline pH; and, (4) under certain conditions, the Águeda River with a diluted Na–Cl water type. Calculation of the water budget of the lagoon was developed with a lumped parameter model earlier proposed. The results of the lumped parameter model show that subsurface lateral flow is an important contributor to surface water bodies when compared to aquifer discharge. Albeit the lagoon drains impacted aquifers with high nitrate content, surface water bodies revealed low nitrate content throughout most of the 1-year monitoring period. The low nitrate content in the lagoon reflects a natural attenuation capacity that ranges between 8% and 13% which is mainly attributed to biogeochemical processes occurring in the hyporheic and riparian zones associated to the lagoon.