Processes influencing groundwater in the coastal aquifer of Troia Portugal

Groundwater fluctuation in coastal aquifers depends on a number of processes which interact with each other in a complex way. In this work, we analyzed the response of the groundwater’s quality and quantity indicators of Troia costal aquifer to several forcing factors. Troia peninsula is underlayed by a multi-layer aquifer consisting of an upper phreatic layer freshwater porous aquifer, a salt water sandy layer with interbeded clay lenses and a deeper semi-confined karst aquifer. This study focuses on the upper aquifer region (10m depth), where influences of oceanic and atmospheric drivers are expected to be strongest. Groundwater data was collected from a borehole located approx. 200m from the shoreline. Hourly records of the piezometric level, conductivity, and temperature data from the hydrological year 2006-2007 were related to data of barometric pressure, rainfall and tides using correlation and singular spectral analytical methods. All variables (precipitation, barometric pressure and tidal cycles) uniquely affect the groundwater’s level and quality with different magnitudes and time scales. Regarding the long-term and larger scales, precipitation seems to be the most influential factor, contributing to 46 % of the variability of the groundwater time series. This percentage of variabillity is due the seasonality of the water cycle, with 29% related to the semi-annual cycle and 17% related to the quarterly cycle. The barometric pressure seems to affect the groundwater in similar scales as the precipitation, however tidal cycles have a much smaller impact. The tidal data was modelled with WxTides software with an interval of 15 minutes. The cyclic patterns of semidiurnal and fortnightly tidal-induced sea level changes can clearly be observed in the records of the groundwater level throughout the entire time series. Tides and groundwater level present a maximum positive correlation coefficient of 0.58 in the month of August, when other forcing factors, such as precipitation, are the lowest. Groundwater level displays a 16-day time lag with the precipitation, a two-day time lag with the barometric pressure and a two-hour time lag with the modelled tides. The correlations and lags found in this study are being used as a basis for ongoing research on the complexity of groundwater level oscillations in littoral zones. The authors would like to acknowledge the financial support FCT through project UIDB/50019/2020 – IDL.