Study of historical groundwater level changes in two Belgian chalk aquifers in the context of climate change impacts

In Southern Belgium, 23% of abstracted groundwater volumes are from chalk aquifers which represent strategic resources for the region. Due to their specific nature, these chalk aquifers often exhibit singular behaviour and require specific analysis. The quantitative evolution of these groundwater resources is analysed for the Mons Basin and Hesbaye chalk aquifers as a function of past evolution, in the short and long terms. Groundwater level time series exhibit decreases when analysed over different periods. This is particularly visible for the Hesbaye chalk aquifer when comparing the 1960-1990 and 1990-2020 periods. Such decreases are associated to observed temperature increase and a precipitation decrease, inducing a decrease of aquifer recharge, and a probable increase of groundwater abstraction in the adjacent catchment. Past evolution is also discussed considering recent winter and summer drought events. The aquifers exhibit long delays in response to recharge events, particularly where the thickness of the partially saturated zone plays a crucial role in observed delays. Regarding future evolution, simulations of the impact of climate changes using medium-high emission scenarios indicate a probable decrease of the groundwater levels over the Hesbaye chalk aquifer.

Hydrogeochemical indications of regional flow in the Lower Greensand aquifer of the London Basin

The Lower Greensand (LGS) forms the second most important aquifer in the London Basin but, being largely absent beneath the city itself, has received much less attention than the ubiquitous overlying Chalk aquifer. While the general directions of groundwater flow in the Chalk are well established, there has been much less certainty about flow in the LGS owing to regionally sparse borehole information. This study focuses on two hitherto uncertain aspects of the confined aquifer: the sources of recharge to the west-central London Basin around Slough, and the fate of LGS water where the aquifer thins out on the flank of the London Platform in the Gravesend–Medway–Sheppey area on the southern side of the basin. The application of hydrogeochemical techniques including environmental isotopes indicates that recharge to the Slough area is derived from the northern LGS outcrop, probably supplemented by downward leakage from the Chalk, while upward leakage from the LGS in North Kent is mixing with Chalk water to the extent that some Chalk boreholes on the Isle of Sheppey are abstracting high proportions of water with an LGS fingerprint. In doing so, this study demonstrates the value of re-examining previously published data from a fresh perspective.Thematic collection: This article is part of the Hydrogeology of Sandstone collection available at: https://www.lyellcollection.org/cc/hydrogeology-of-sandstone

Fingerprinting sources of salinity in a coastal chalk aquifer in Denmark using trace elements

Salinity levels above the drinking water standard (>250 mg/l Cl–) are observed at shallow depth in a Maastrichtian chalk aquifer on the island of Falster, south-eastern Denmark. To understand the source of the salt, 63 samples from 12 individual, 1 m, screened intervals between 14 and 26 m b.s. were collected from 1 May to 4 June 2018. The samples were collected during a tracer test to estimate the dual porosity properties of the chalk and were analysed for a wide range of elements. Furthermore, samples from the Baltic Sea and from deeper saline aquifers in the area (40 and 85 m b.s.) were analysed for comparison. The geochemical data were analysed using an unsupervised machine-learning algorithm, self-organising maps, to fingerprint water sources. The water composition in the screened intervals at various stratigraphic levels has specific geochemical fingerprints that are maintained for the first days of pumping and are distinct amongst the different levels. This suggests an evolution in water composition because of reaction with the chalk. Water composition is distinct from both seawater from the nearby Baltic Sea and salty water from deeper levels of the reservoir. Thus, neither up-coning of salty water nor intrusion of seawater caused the elevated salinity levels in the area. The slightly saline composition of groundwater in the shallow aquifer (14–26 m b.s.) is more likely because of incomplete refreshing of the salty connate water in the chalk during the Pleistocene and Holocene. Furthermore, the geochemical fingerprint of salty water from the deeper aquifer at 40 m was similar to water from the Baltic Sea, suggesting a Baltic Sea source for salt in the aquifer at 40 m b.s., c. 100 m from the coast. Statistical analysis based on self-organising maps is an effective tool for interpreting a large number of variables to understand the compositional variation in an aquifer and a useful alternative to linear dimensionality-reduction methods such as principal component analysis. The approach using the multi-element analysis combined with the analysis of self-organising maps may be useful in future studies of groundwater quality.

A conceptual model of hydrogeological function and perchlorate transfer in the unconfined Champagne Chalk aquifer (NE France)

Groundwater of the unconfined Chalk aquifer in Champagne-Ardenne (NE France) is contaminated by perchlorate (ClO4−), a persistent water-soluble anion. The Chalk aquifer is a crucial water resource of the region, with complex hydraulic properties. The presence of ClO4− is of concern due to its potential adverse human health effects. In France, three sources of ClO4− contamination are suspected: industrial, military, and agricultural. Both a comprehensive understanding of hydrogeological characteristics of the aquifer and a sound knowledge of sources and behavior of ClO4− in groundwater are required to allow the sustainable use of this groundwater resource. From data acquired during hydrogeological, geochemical, isotopic (ClO4−) and groundwater age (CFCs and SF6) studies, and historical investigations in a study area located east of Reims, a conceptual model of Chalk aquifer function and ClO4− transfer has been established. High spatio-temporal heterogeneities in the unconfined Chalk aquifer are discussed. Different correlation between ClO4−, major ions and groundwater level fluctuations are shown and interpreted, highlighting main factors governing the Chalk groundwater geochemistry and ClO4− transfer mechanisms, including water level fluctuation, groundwater residence time, thickness of the unsaturated zone, superficial formations, distribution of fracture network, aquifer-river relationships, origin and location of ClO4− in soil and human activity.