Pleistocene and Holocene groundwaters in the freshening Ledo-Paniselian aquifer in Flanders, Belgium
K. Walraevens, M. Van Camp, J. Lermytte, W. J. M. Van der Kemp, H. H. Loosli, 2001. "Pleistocene and Holocene groundwaters in the freshening Ledo-Paniselian aquifer in Flanders, Belgium", Palaeowaters in Coastal Europe: Evolution of Groundwater since the Late Pleistocene, W. M. Edmunds, C. J. Milne
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The Ledo-Paniselian aquifer presents a case study of evolution of fresh groundwater from sea water under the changing piezometric and climatic conditions of the Pleistocene and Holocene. Hydrogeochemical, isotopic, experimental and hydrodynamic results are used in the interpretation. The distribution of groundwater types in the Ledo-Paniselian aquifer is determined by two end members: fresh Ca-HCO3 recharge water and sea water-saturated sediments. Hydrogeochemical modelling supports the view that mixing of the end members and cation exchange are the main processes; calcite dissolution is also important. Cation exchange consists, in the first place, of desorption of the adsorbed marine cations (Na+, K+ and Mg2+) in exchange for the freshwater cation Ca2+.
Groundwater δ18O is around the value of modem precipitation in the area (−6.5‰) for the samples with higher radiocarbon contents; it is < −7.0‰ for the groundwater containing the lowest radiocarbon levels. An overlapping transition zone exists between both groups. δ13C becomes heavier for the samples containing the lowest radiocarbon levels, indicating chemical dilution.
Pore waters from the Bartonian clay show preferential flow paths. Faster flow paths are more strongly leached, leading to low total dissolved solids (TDS), low sulphate concentrations and low Mg2+/Ca2+ ratios; the slower pathways still contain gypsum, increasing the sulphate concentrations and TDS, and Mg2+/Ca2+ ratios are higher because they were less reduced by cation exchange resulting from freshening.
Four methods for determining cation exchange capacity (CEC) and adsorbed cations are compared: the NH4OAc method, two BaCl2 methods (one in unbuffered and the other in buffered conditions) and a new NaCl/NH4Cl method. Reasonable CEC values are obtained with the NH4OAc method. Comparing the measured equivalent fractions of the adsorbed cations with those calculated from the pore solutions, using the computer programme PHREEQC, it can be concluded that the NaCl/NH4Cl method produces the best results. The proton exchange capacity of decalcified sand from the Ledo-Paniselian aquifer was determined to be c. 1−1.5 meq/100 g in the pH range 5–8.5.
A hydrodynamic model is developed to explain the evolution of groundwater and for evaluating the effects of pumping at both local and regional scales. Model calculations show that the observed freshwater-saltwater distribution is not the result of the present freshwater flow conditions but the result of different flow regimes during the ice ages when sea levels were much lower. Occurrence of a permafrost layer during cold periods could have had a dramatic impact on the groundwater flow system by, at least temporarily, decreasing the recharge of the aquifers. The existence of the Saalian ice sheet in The Netherlands could have influenced the flow in the deeper Eocene-Oligocene aquifers. The high pressures that existed under the ice sheet could have reversed the flow direction from north to south.
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Palaeowaters in Coastal Europe contains 17 contributions from an international array of authors. They discuss the history of groundwater evolution during the late Pleistocene in the coastal areas of Europe from the Baltic region to the Iberian peninsula and the Canary Islands. Geochemical and geophysical techniques for evaluating palaeowaters are reviewed. The focus of the book is on changes in the hydrogeological regime during the Quaternary and their impacts on groundwater movement and chemistry in European coastal aquifers.
The work summarized in the papers was carried out by a partnership of European scientists under the auspices of the PALAEAUX project, an EC initiative. Researchers from the fields of hydrogeology, geochemistry, isotope hydrology and Quaternary studies attempted to reconstruct the most probable movement of groundwater in the study area over the past 100 000 years and its response to climatic events of global significance during the last glacial cycle. The results of this work, summarized in this volume, allow a better understanding of the water resources found at and near the coastlines of northern and western Europe. During times of lowered sea level, it appears that groundwaters were replenished to depths greater than occur at the present day. These pristine freshwater reserves are an irreplaceable asset. Their location at coastlines where populations and water demands are high and often seasonal means that they need careful management to avoid over-exploitation or contamination. The inevitable conflicts that this resource management creates are discussed.
Palaeowaters in Coastal Europe: evolution of groundwater since the late Pleistocene will be of interest to Quarternary scientists, hydrogeologists, marine scientists engaged in coastal research and those involved in environmental science and the management of groundwater assests.