Dating of glacial palaeogroundwater in the Ordovician-Cambrian aquifer system, northern Baltic Artesian Basin

Global Isotope Hydrogeology―Review

“…Mixing of shallow and deep groundwaters has been identified using tritium and radiocarbon data (e.g., Chen et al, ; Eissa, ; Sikdar & Sahu, ), and other regional‐scale studies explicitly identify admixtures of modern and fossil groundwater in well water samples (Dassi et al, ; Sharif et al, ; Stadler et al, ). The processes that mix modern and fossil waters may include pumping of wells with long screens spanning depths both shallow (containing modern water) and deep (containing fossil water; e.g., Zuber et al, ); drawdown of modern waters to deeper parts of the aquifer by pumping (e.g., Cheng et al, ; Kingsbury et al, ; Samborska et al, ; Taufiq et al, ); pumping‐induced upwelling of fossil waters to shallower depths (Celle‐Jeanton et al, ; Sadek & El‐Samie, ); preferential flows of modern water to deep depths; hydrodynamic dispersion during groundwater flow leading to samples containing a wide range of groundwater ages (e.g., Pärn et al, ; Weissmann et al, ); or quick flows from the land surface to deep depths along the well annulus. Regardless of the processes that cause modern groundwater to co‐occur with fossil groundwater in well waters, the implication for water quality management is clear: because precipitation that fell within the past ~60 years commonly co‐occurs with fossil well waters, many well waters comprised mostly of fossil groundwater remain vulnerable to pollutants derived from modern‐era land uses (Jasechko, Perrone, et al, ).…”

Late Pleistocene and Holocene groundwater flow history in the Baltic Artesian Basin: a synthesis of numerical models and hydrogeochemical data

“…Our previous work on the Cambrian-Vendian aquifer system (CVAS), an important source of public water supply in northern Estonia, has provided compelling evidence that the aquifer has been strongly impacted by glaciations and recharged by glacial meltwater. The evidence can be summarized as follows: the groundwater has the lightest known oxygen isotopic composition in Europe (δ 18 O values as low as -23‰; Raidla et al 2019a), unexpectedly high gas concentrations, absence of 3 H and a low radiocarbon concentration (Vaikmäe et al 2001a;Raidla et al 2012;Vallner & Porman 2016;Pärn et al 2019;Vallner et al 2020). Our first noble gas analyses suggest that palaeorecharge took place at temperatures around the freezing point (Vaikmäe et al 2001a).…”

“…It has been shown that in the northern part of the BAB, the spatial and vertical distribution of glacial palaeowater in shallower aquifer systems overlying the CVAS is wider than previously thought (Pärn et al 2016(Pärn et al , 2018(Pärn et al , 2019Pärn 2018). The occurrence of glacial palaeogroundwater is especially wide in the O-Cm aquifer system with fresh groundwater in the northeastern part of this aquifer system having δ 18 O values as low as -22.4‰ (Pärn et al 2016;Fig.…”

“…The results of the studies on the origin, geochemical evolution and age of groundwater in the aquifer systems of the northern BAB have been published in recent decades in a number of papers and dissertations (Mokrik 1997;Vaikmäe et al 2001aVaikmäe et al , 2001bVaikmäe et al , 2008Karro et al 2004;Marandi 2007;Raidla et al 2009Raidla et al , 2012Raidla et al , 2014Raidla et al , 2016Raidla et al , 2019aRaidla et al , 2019bRaidla 2010;Pärn et al 2016Pärn et al , 2018Pärn et al , 2019Gerber et al 2017;Suursoo et al 2017;Pärn 2018;Vallner et al 2020). However, several recent studies on the palaeowaters in the BAB have employed other meth odologies besides geochemistry and 14 C dating, which have been traditionally used for that purpose.…”

“…McIntosh et al (2012 have shown that shallow biogenic methane in the Illinois Basin (USA) was stimulated by subglacial recharge. Taking the current understanding of groundwater ages in the northern part of the Baltic Artesian Basin (BAB) into account and considering the isotope data and the results of the noble gas analyses, it is likely that palaeorecharge of the CVAS occurred mainly during the last glaciation (Vaikmäe et al 2001a;Raidla et al 2009Raidla et al , 2012Pärn et al 2016Pärn et al , 2019Vallner et al 2020). If these groundwater resources originate from the last glaciation, an insight into their distribution and current dynamics which are important for sustainable groundwater development can only be gained through a detailed analysis of the groundwater flow history in the basin.…”