Pleistocene and Holocene groundwaters in the freshening Ledo-Paniselian aquifer in Flanders, Belgium

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 δO is around the value of modern 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 NHOAc 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

Isotopic methods and their hydrogeochemical context in the investigation of palaeowaters

Isotope and geochemical techniques are the primary way in which the residence time, recharge conditions and subsequent evolution of palaeowaters can be determined. Isotopic species and noble gas concentrations are used as residence time and palaeoclimate indicators. Among the former, 14C is pre-eminent in late Quaternary studies because of an age range which covers the Pleistocene-Holocene transition. However, its use is constrained by frequent difficulties in determining the dilution of dissolved 14C due to water-rock interaction. A combination of 14C data with 226Ra and 4He results may be useful for Holocene waters but they can also be used to validate the carbon systematics assumed for 14C dating. For waters beyond the range of 14C dating, 81Kr, 36Cl, 4He and chemical tracers can be applied. Stable isotope ratios and noble gas concentrations primarily reflect climatic conditions at the time of recharge. While the noble gases provide absolute values for recharge temperatures, stable isotopes are only relative indicators that vary regionally. The PALAEAUX programme has examined these aspects in some detail by looking at the δ18O shift between Pleistocene and Holocene waters on the European scale, and by calculating δ18O/ΔT ratios from δ18O v. recharge temperature plots for aquifers at different distances from the Atlantic Ocean. Indications are that the more positive δ18O value of ocean water during the Pleistocene dominates in the more westerly European countries over the negative δ18O shift during cooler conditions. There are also indications that air-mass circulation during the Pleistocene was similar to the present day. The evolution of a palaeowater can best be studied by measuring chemical tracers; this is possible in freshwater aquifers, where a clear trend of geochemical reactions is observed, and in freshening marine aquifers. Chemical and isotopic tracers can also be used to study the movement of the front between palaeowater and younger components that must be identified in coastal aquifers to guarantee a sustainable water use

Development of an AMS method to study oceanic circulation characteristics using cosmogenic 39Ar

Initial experiments at the ATLAS facility [Nucl. Instr. and Meth. B 92 (1994) 241] resulted in a clear detection of cosmogenic 39Ar signal at the natural level. The present paper summarizes the recent developments of 39Ar AMS measurements at ATLAS: the use of an electron cyclotron resonance (ECR) positive ion source equipped with a special quartz liner to reduce 39K background, the development of a gas handling system for small volume argon samples, the acceleration of 39Ar8+ ions to 232 MeV, and the final separation of 39Ar from 39K in a gas-filled spectrograph. The first successful AMS measurements of 39Ar in ocean water samples from the Southern Atlantic ventilation experiment (SAVE) are reported