Reaction of subsurface coastal aquifers to climate and land use changes in Greece: modelling of groundwater refreshening patterns under natural recharge conditions

Abstract

This paper studies the multicomponent ion exchange process and freshening time under natural recharge conditions for three coastal aquifers in Greece. Due to over-pumping and the dry years of 1980–1990 decline in groundwater quality has been observed in most of the Greek coastal aquifers. This decline is caused by a lack of reliable water resource management, water abstraction from great depths, and seawater intrusion resulting in a rise of the fresh/salt water interface (salinisation process) due to a negative water balance. The reverse phenomenon, which should lead to groundwater freshening, is a long process. The freshening process shows chromatographic patterns that are due to chemical reactions such as calcite dissolution and cation exchange, and simultaneously occurring transport and dispersion processes. Using the geochemical simulation codes PHREEQE and PHREEQM (Parkhurst et al., US Geol. Surv. Water Resour. Invest., 80–96 (1980) 210; Appelo and Postma, Geochemistry, Groundwater and Pollution (1994)), these patterns were analysed and the above-mentioned processes were simulated for carefully selected aquifers in Peloponnesus and Crete (Greece). Aquifers of the Quaternary basin of Glafkos in Peloponnesus, the Neogene formations in Gouves, Crete, and the carbonate aquifer of Malia, Crete, were examined as representative examples of Greek coastal aquifer salinisation. The results show that when pumping was discontinued, the time required for freshening under natural conditions of the former two aquifers is long and varies between 8000 and 10,000 years. The Malia aquifer on the other hand, has a freshening time of 15 years. Freshening time was shown to depend mainly on cation exchange capacities and the recharge rate of the aquifers.