Application of environmental isotope methods in assessing groundwater dynamics of an intensively exploited coastal aquifer in Portugal

As a result of intense exploitation over the last thirty years, a significant decrease in the piezometric levels of the Aveiro confined aquifer, located on the northwest coast of Portugal, has been reported. Hydrochemical and environmental isotope techniques were applied to evaluate the effects of this intense pumping. Prolonged over-exploitation of the confined aquifer may lead to deterioration of water quality in the system as a result of mixing with highly polluted shallow groundwaters and/or due to seawater intrusion. An insight into the dynamics of the confined aquifer was gained by investigation of the distribution of 14C content in the total dissolved inorganic carbon (TDCI) along the general direction of groundwater flow. Between 7 and 12 km from the recharge area the 14C content decreases almost linearly with the increasing distance, indicating an apparent flow velocity of around 1 m/a. However, between 12 and 19 km the concentration of 14C remains almost constant, indicating the much higher apparent flow velocity (around 5 m/a) resulting from intense flushing of the aquifer during the last glacial period. This flushing was induced by the decrease in the seal level by about 100 m during this period, and the corresponding higher hydraulic gradient in the aquifer. Chemical and isotopic evidence points to a good isolation of the deeper aquifer; all waters collected from the deep aquifer are tritium free. The changes in hydrochemistry of groundwater along the flow paths can be accounted for by water/rock interaction processes. An enrichment in 18O and in 2H downgradient from the recharge area can be observed along the major flow paths, with more positive δ values for higher apparent 14C ages. This apparent enrichment in heavy isotopes observed for Aveiro palaeowaters probably reflects the fact that the global ocean was enriched in both 18O and 2H during the glacial period. (author). 13 refs, 5 figs