The Ribe Formation in western Denmark — Holocene and Pleistocene groundwaters in a coastal Miocene sand aquifer

The Ribe Formation is a regionally extensive Miocene sand aquifer that is present in western Denmark at depths ranging from 100 to 300 m below ground surface. Groundwater chemistry and isotope data collected from more than 40 wells show that the Ribe Formation mainly contains high quality Cabi-carbonate type groundwater of Holocene age (100–10 000 abp). Pleistocene age groundwaters, identified on the basis of stable isotopes, noble gases and corrected14C values, are present below the island of Rømø, in discharge areas near the coast, and in hydraulically isolated inland areas. The groundwater age distribution in the Ribe Formation was successfully simulated with a numerical groundwater flow model and particle tracking only when the14C content in groundwater was corrected for both geochemical reactions and diffusion. The results indicate that geochemical and physical processes significantly influence the14C content of groundwater and that the correction factors required for the two processes are of the same magnitude. Flow modelling results indicate that Pleistocene groundwaters were emplaced at depth within the Ribe Fromation under low base-level conditions that prevailed throughout the late Pleistocene — near the coast these waters are essentially isolated from the present flow system, and Pleistocene freshwater may be present offshore. Seismic surveys show that conditions offshore are favourable for the presence of Pleistocene freshwater within the Ribe Formation and other aquifers.

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.

Palaeowaters in European coastal aquifers — the goals and main conclusions of the PALAEAUX project

The PALAEAUX project has brought together up-to-date geochemical, isotopic and hydrogeological information on coastal groundwaters across Europe in a transect from the Baltic to the Canary Islands. These data have been interpreted in relation to past climatic and environmental conditions, as well as extending and challenging concepts about the evolution of groundwater near the present day coastlines. Groundwater movement beyond the present coastline as well as emplacement on shore to greater depths (up to 500 m) than allowed by the present-day flow regime has occurred, hence offshore freshwater reserves are inferred in some coastal areas. The main attributes of palaeowaters, in terms of water quality, are their high bacterial purity, total mineralization that is often less than that of modern waters and being demonstrably free of anthropogenic chemicals. However, in the Mediterranean coastal areas, lower recharge leads to higher salinity conditions in both palaeo- and modern waters.

Freshwater of high quality originating from different climatic conditions to the present day, when the sea level was much lower, is found at depth beneath the present-day coastline in several countries. Recharge is shown to have been more or less continuous during the past 100 ka, even beneath the ice, as demonstrated by groundwaters from Estonia, having δO values of c. −22%o. However, elsewhere (UK and Belgium) an age gap can be recognized indicating that no recharge took place at the time of the last glacial maximum. Devensian recharge temperatures (soil air temperatures) were some 6°C colder across Europe than at the present day.

The development of aquifers in Europe during the past 50–100 a, by abstraction from boreholes, has generally disturbed flow systems that have evolved over varying geological timescales, especially those derived from the Late Pleistocene and Holocene. Hydrogeophysical logging has demonstrated time and quality stratified aquifers resulting in mixed waters being produced on pumping. A range of specific indicators, including 3H, 3H/3He, 85Kr, chlorofluoro-carbons and pollutants, have been used to recognize the extent to which waters from the modern (industrial) era have penetrated into the aquifers, often replacing the natural palaeogroundwaters.

In the coastal regions, many problems for management are identified, including issues relating to quantity and quality of water, seasonal demand, pollution risks and ecosystem damage, requiring a new look at legislation.

Palaeogroundwater of glacial origin in the Cambrian-Vendian aquifer of northern Estonia

A strongly depleted stable isotope composition, absence of 3H and a low radiocarbon concentration are the main indicators of glacial origin of groundwater in the Cambrian-Vendian aquifer in northern Estonia. It is concluded from noble gas analyses that palaeorecharge occurred at temperatures c. 0°C. In some wells unexpectedly high gas concentrations have been found. Excess air, up to c. 50 %, is common but two-five times oversaturation is very unusual, requiring special processes and explanations, e.g. oversaturation may indicate recharge under highpressure conditions, perhaps by subglacial meltwater recharge through the aquifers. Analyses of the gas composition in some groundwater samples also showed a rather high concentration of CH4, indicating the influence of biogenic reactions in the subsurface that could cause the rather negative δ13C values. Results of δ13C analyses in two CH4 samples also show that the CH4 is most likely of a biogenic origin. Based on the isotope data, the results of noble gas analyses, and considering the palaeoclimatic and palaeoenvironmental situation in Estonia during the late Weichselian time, it is concluded that palaeorecharge of Cambrian-Vendian aquifer most probably occurred during the last glaciation, probably by subglacial drainage through the tunnel valleys.