Using environmental tracers to evaluate the preservation of palaeoclimate signals in aquifers of the London Basin, UK

• Published in: Journal of hydrology (Amsterdam) 2023-02, Vol.617, p.128972, Article 128972
• Main Authors: Darling, W. George, Gooddy, Daren C., Gulliver, Pauline L., Scott, Amy M., Ahearn, Sean P.
• Format: Article
• Language: English
• ISSN: 0022-1694; 1879-2707
• DOI: 10.1016/j.jhydrol.2022.128972

•14C ages from dissolved organic carbon (DOC) in 30 chalk and sandstone groundwaters.•DOC ages resolve problems with dissolved inorganic carbon (DIC) reservoir effects.•Sandstone noble gas recharge temperatures agree with climate changes back to 24 kyr.•CFCs indicate man-made contamination reaching over 40 km from chalk aquifer outcrop. The concept of aquifer basins as palaeoclimate archives has existed for some decades, yet few detailed studies comparing aquifer types have been carried out. To assess the potential of a particular aquifer as an archive, its hydrogeochemical characteristics must be thoroughly investigated, ideally in comparison to an adjacent aquifer which can be shown to substantially preserve its ice-age endowment at depth. The London Basin (UK) presents such an opportunity, containing two main aquifers of contrasting type: the Chalk, a fractured microporous limestone, and the Lower Greensand, a porous sandstone. Despite intensive exploitation of both, evidence for Devensian (late-glacial) water remains at depth, though this differs between aquifer type. To understand the reasons for this, a suite of environmental tracers has been applied. In addition to hydrochemistry, stable isotopes (δ18O, δ2H), carbon isotopes (δ13C-DIC, 14C-DIC) and noble gases (He, Ne, Ar, Kr and Xe), two tracers new to the basin (CFCs and 14C-DOC) have been used. In effect the Lower Greensand appears to be the ‘reference aquifer’, preserving recharge from prior to the Last Glacial Maximum (LGM), while the Chalk contains mixed water, with no remaining trace of the undiluted pre-LGM end member even at depth in remote parts of the confined basin. Whereas both aquifers had in the past given maximum 14C-DIC model ages ≥ 30kyr (the effective limit of that method), in the present study the use of 14C-DOC has reduced this to 23.4 kyr (Lower Greensand) and 17.2 kyr (Chalk). Similar contrasts in maximum stable isotope depletions (−8.2 ‰ and −7.8 ‰ δ18O) and noble-gas-derived recharge temperature minima (2.6° and 4.1 °C) were also observed. CFCs were found at all Chalk sites, with traces detectable even at 40 km from outcrop, so some climate signal degradation appears inevitable throughout the Chalk aquifer of the basin. A correlation between 14C activity and excess 4He suggests that deep saline water in the Lower Greensand could be ≥ 50kyr old. The use of 14C-DOC in particular appears to be key to understanding how reliable these individual aquifers are as palaeoarchives.