Using 81Kr and Noble Gases to Characterize and Date Groundwater and Brines in the Baltic Artesian Basin on the One-Million-Year Timescale

Analyses for $^{81}$Kr and noble gases on groundwater from the deepest aquifer system of the Baltic Artesian Basin (BAB) were performed to determine groundwater ages and uncover the flow dynamics of the system on a timescale of several hundred thousand years. We find that the system is controlled by mixing of three distinct water masses: Interglacial or recent meteoric water (\delta^{18}\text{O} \approx -10.4\unicode{x2030}) with a poorly evolved chemical and noble gas signature, glacial meltwater (\delta^{18}\text{O} \leq -18\unicode{x2030}) with elevated noble gas concentrations, and an old, high-salinity brine component (\delta^{18}\text{O} \geq -4.5\unicode{x2030}, \geq 90 \text{g Cl}^{-}/\text{L}) with strongly depleted atmospheric noble gas concentrations. The $^{81}$Kr measurements are interpreted within this mixing framework to estimate the age of the end-members. Deconvoluted $^{81}$Kr ages range from 300 ka to 1.3 Ma for interglacial or recent meteoric water and glacial meltwater. For the brine component, ages exceed the dating range of the ATTA 3 instrument of 1.3 Ma. The radiogenic noble gas components $^{4}$He* and $^{40}$Ar* are less conclusive but also support an age of > 1 Ma for the brine. Based on the chemical and noble gas concentrations and the dating results, we conclude that the brine originates from evaporated seawater that has been modified by later water-rock interaction. As the obtained tracer ages cover several glacial cycles, we discuss the impact of the glacial cycles on flow patterns in the studied aquifer system.Comment: Accepted for publication in Geochimica et Cosmochimica Act

Eesti Kambrium-Vendi põhjaveeladestiku vete keemiline ja isotoopne areng

Väitekirja elektroonilisest versioonist puuduvad publiktsioonid.The thesis studies the formation of chemical and isotope composition of groundwater in the Cambrian-Vendian aquifer system over the last 27,000 years. In North-Estonia, the Cambrian-Vendian aquifer system grounwaters are characterised by low 14C activities (1.4 – 6.8 pmc) and considerably more negative δ18OH2O values (-22 to -18‰ VSMOW) compared to local precipitation (-10 to -12‰VSMOW). The phenomenon has been explained by the origin of these waters from glacial meltwater covering Estonian territory from 27,000 to 10,000 radiocarbon years ago. At the same time, 14C activities in the groundwater suggest that the isolation age from the atmosphere is 35,000 to 15,000 radiocarbon years. This discrepancy can be explained by the fact that through geochemical processes 14C datings seem to be older than they actually are. In order to find the actual residence time of the groundwater, profound knowledge of geochemical processes influencing chemical and isotope composition of the groundwater is needed. Chemical composition of the Cambrian-Vendian groundwater is the result of a long-term interaction between the water and its surrounding rocks. The mixing model used in this thesis confirms mixing of fresh glacial water with relict mineral water. Organic material found in Cambrian clays has released carbon, which has participated in both sedimentary and dissolution processes, thus influencing the carbon cycle in Cambrian-Vendian aquifer system. The carbon, through bacterial oxidation and dolomite dissolution processes, has affected the 14C considerably. During the research, it also became clear that both crystalline basement rocks and cation exchange processes have had an important geochemical effect on 14C age calculations. The correction calculations made to 14C age, based on the given geochemical model of carbon, yield the age of 25,000 to 12,000 radiocarbon years. This corresponds to the period when the Estonian territory was covered by a continental glacier. Western waters proved to have a younger age, which most likely suggests that, during the existence of the glacier, there was a more intense water exchange in the western part of the aquifer system.Käesolev doktoritöö uurib Kambrium-Vendi veeladestiku põhjavete keemilise ja isotoopkoostise kujunemist viimase 27 000 aasta jooksul. Põhja-Eestis on Kambrium-Vendi veeladestiku põhjavetele iseloomulikud madalad 14C aktiivsused (1.4 – 6.8 pmc) ja märksa negatiivsemad δ18OH2O väärtused (-22 kuni -18‰ VSMOW) võrreldes kohalike sademetega (-10 kuni -12‰VSMOW). Seda nähtust on seletatud Kambrium-Vendi põhjavete pärinemisega liustiku sulavetest, mis kattis Eesti territooriumit 27000 kuni 10 000 radiosüsiniku aastat tagasi. Samas annavad 14C aktiivsused Kambrium-Vendi veeladestiku lahustunud karbonaatides põhjavee isoleerumisajaks atmosfäärist 35 000 kuni 15000 radiosüsiniku aastat. Selle põhjuseks on asjaolu, et läbi geokeemiliste protsesside muutuvad 14C dateeringud põhjaveest näivalt vanemaks. Et arvutada tegelik põhjavee resideerumisaeg, on vaja põhjalikke teadmisi geokeemilistest protsessidest, mis on põhjavee keemilist- ja isotoopkoostist mõjutanud. Kambrium-Vendi põhjavee keemiline koostis on kujunenud pikajalise vee ja ümbritsevate kivimite vastastikuse mõju toimel. Töös kasutatud segunemismudel kinnitas mageda liustikulise ja reliktse mineraalvee segunemist. Süsinikuringet on Kambrium-Vendi põhjaveeladestikus oluliselt mõjutanud Kambriumi savides leviv orgaaniline materjal, mille lagunemisel vabanenud süsinik on osalenud nii settimis- kui lahustumisprotsessides. See süsinik on oluliselt mõjutanud näivat 14C vanust läbi bakteriaalsete oksüdeerimisprotsesside, aga ka kaudselt, dolomiitse tsemendi lahustamisega. Samuti selgus töö käigus aluspõhja kivimite ja katioonvahetusprotsesside oluline geokeemiline mõju 14C vanuse arvutustele. Esitatud süsiniku geokeemilise mudeli alusel tehtud 14C vanuse parandusarvutused andsid Kambrium-Vendi veeladestiku põhjavete vanuseks 25 000 kuni 15 000 radiosüsinikuaastat, mis vastab ajavahemikule, mil Eesti teritoorium oli kaetud mandriliustikuga. Vanuseliselt osutusid nooremaks läänepoolsed veed, mis ilmselt viitab põhjaveeladestiku lääneosa aktiivsemale veevahetusele liustiku eksisteerimise jooksul

Geochemical Processes Controlling Ionic Composition of Water in the Catchments of Lakes Saana and Saanalampi in the Kilpisjärvi Area of North Scandinavia

The study focuses on chemical composition of stream and subsurface water in the catchments of two small arctic alpine lakes in the Kilpisjärvi area (northwest Finland). Differences and changes in chemical components of both water types are followed in order to detect spatial variability and impact of environmental factors. To achieve this, ion compositions of subsurface water and streams were measured at 12 sites in the catchments of Lakes Saana and Saanalampi during four years (2008–2010, and again in 2017). In the Lake Saanalampi catchment, the salinity of stream water (7.0 to 12.7 μS·cm−1) corresponded to that of snow. In the catchment of Lake Saana, however, the conductivity in stream water was much higher (40 to 220 μS·cm−1), connected mainly to the increase of SO42− and less with Mg2+ and Ca2+ contents, especially in the western part of the Saana catchment. These results demonstrate that arctic conditions do not preclude intense chemical weathering where conditions are favourable. Although chemical composition of the soil fluid does not match the geochemical signal from the local soil, rock composition, especially the presence of pyrite, is the main controller of chemical weathering rates of the rocks on the area. This supports earlier views that the character of precipitation mostly controls water chemistry of local lakes in the Kilpisjärvi area

Intrusion of Saline Water into a Coastal Aquifer Containing Palaeogroundwater in the Viimsi Peninsula in Estonia

The Viimsi peninsula is located north-east of Tallinn, capital of Estonia. The Cambrian-Vendian (Cm-V) aquifer system is a sole source of drinking water in the area. Historically, the groundwater exploitation has led to freshening of groundwater in the peninsula, but in recent years an increase in chloride concentrations and enrichment in δ18O values has been detected, but in recent years hydrochemical parameters indicate an increasing influence of a saline water source. The exact origin of this saline water has remained unclear. The aim of the current study is to elucidate whether the increase in Cl− concentrations is related to seawater intrusion or to the infiltration of saline water from the underlying crystalline basement. To identify the source of salinity, chemical composition of the groundwater and the isotope tracers (e.g., δ18O and radium isotopes) were studied in the Viimsi peninsula in the period from 1987 to 2018. Our results show that chemical composition of Cm-V groundwater in the peninsula is clearly controlled by three-component mixing between glacial palaeogroundwater, saline water from the underling crystalline basement and modern meteoric water. The concentrations of Ra are also significantly affected by the mixing, but the spatial variation of radium isotopes (226Ra and 228Ra) suggests the widespread occurrence of the U in the surrounding sedimentary sequence. Our hypothesis is that, in addition to U originating from the crystalline basement, some U could be associated with secondary U deposits in sedimentary rocks. The formation of these secondary U deposits could be related to glacial meltwater intrusion in the Pleistocene. Although the results suggest that the infiltration of saline groundwater from the underlying crystalline basement as the main source of salinity in the study area, the risk of seawater intrusion in the future cannot be ruled out. It needs to be highlighted that the present groundwater monitoring networks may not be precise enough to detect the potential seawater intrusion and subsequent changes in water quality of the Cm-V aquifer system in the Viimsi peninsula

Redox zonation and organic matter oxidation in palaeogroundwater of glacial origin from the Baltic Artesian Basin

Ordovician-Cambrian aquifer system (O-Cm) in the northern part of the Baltic Artesian Basin (BAB), Estonia, is part of a unique groundwater reservoir where groundwater originating from glacial meltwater recharge from the Scandinavian Ice Sheet is preserved. The distribution of redox zones in the anoxic O-Cm aquifer system is unusual. Strongly reducing conditions are found near the modern recharge area characterized by low concentrations of sulphate (< 5 mg.L-1) and the presence of CH4 (up to 3.26 vol%). The concentrations of SO42-increase and concentrations of CH4 decrease farther down the groundwater flow path. Sulphate in fresh glacial palaeogroundwater originates probably from pyrite oxidation while brackish waters have gained their sulphate through mixing with relict saline formation waters residing in the deeper parts of the aquifer system. Stable isotopic composition of sulphate, especially relations between delta O-18(SO4)- delta O-18(water) (Delta O-18(SO4-) (H2O) from + 20.5 to + 31.1 parts per thousand) and delta S-34(SO4)-delta S-34(H2S) (Delta S-34(SO4-) (H2S) value of + 47.9 parts per thousand) support a widespread occurrence of bacterial sulphate reduction in fresh glacial palaeogroundwater. We propose, that the observed unusual redox zonation is a manifestation of two different flow systems in the O-Cm aquifer system: 1) the topographically driven flow system which drives the infiltration of waters through the overlying carbonate formation in the modern recharge area; 2) the relict flow system farther down the groundwater flow path which developed as a response to large hydraulic gradients imposed by the Scandinavian Ice Sheet in Pleistocene. Thus, the strongly reducing conditions surrounding the modern recharge area may show the extent to which post-glacial recharge has influenced the aquifer system. O-Cm aquifer system is an example of an aquifer that has not reached a near-equilibrium state with respect to present day flow conditions and still exhibits hydrogeochemical patterns established under the influence of a continental ice sheet in Pleistocene

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

We review our current understanding of groundwater flow history in the northern part of the Baltic Artesian Basin (BAB) from the end of the Late Pleistocene to current conditions based on the hydrogeological studies carried out in 2012â2020 by the Department of Geology, Tallinn University of Technology and its partners. Hydrogeochemical data and various numerical models are combined in order to understand the link between glaciations and groundwater flow. The results of our earlier research and published literature on groundwater flow history in the BAB are also taken into account. The reconstruction of groundwater flow history is based on the database of the isotopic, chemical and dissolved gas composition of groundwater. The database contains data on 1155 groundwater samples collected during 1974â2017. We find that groundwater in the BAB is controlled by the mixing of three distinct water masses: interglacial/modern meteoric water (δ18O â â11â°), glacial meltwater (δ18O ⤠â18â°) and an older syngenetic end-member (δ18O â¥â4.5â°). The numerical modelling has suggested that the preservation of meltwater in the northern part of the BAB is controlled by confining layers and the proximity to the outcrop areas of aquifers. Aquifers containing groundwater of glacial origin are in a transient state with respect to modern topographically-driven groundwater flow conditions. The most important topics for future research that can address gaps in our current knowledge are also reviewed

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

The Ordovician-Cambrian aquifer system in the northern Baltic Artesian Basin contains glacial palaeogroundwater that originates from the Scandinavian Ice Sheet that covered the study area in the Pleistocene. Previously, no absolute dating of this palaeogroundwater has been attempted. In this multi-tracer study, we use H-3, C-14, He-4 and stable isotopes of water to constrain the age distribution of groundwater. We apply the geochemical modelling approach developed by van der Kemp et al. (2000) and Blaser et al. (2010) to calculate the theoretical composition of recharge waters in three hypothetical conditions: modern, glacial and interstadial for( 14)C model age calculations. In the second phase of the geochemical modelling, the calculated recharge water compositions are used to calculate the C-14 model ages using a series of inverse models developed with NETPATH. The calculated C-14 model ages show that the groundwater in the aquifer system originates from three different climatic periods: (1) the post-glacial period; (2) the Late Glacial Maximum (LGM) and (3) the pre-LGM period. A larger pre-LGM component seems to be present in the southern and north-eastern parts of the aquifer system where the radiogenic He-4 concentrations are higher (from similar to 3.0.10(-5) to 5.5.10(-4) cc.g(-1)) and the stable isotopic composition of water is heavier (delta O-18 from - 13.5 parts per thousand to -17.3 parts per thousand). Glacial palaeogroundwater from the north-western part of the aquifer system is younger and has C-14 model ages that coincide with the end of the LGM period. It is also characterized by lower radiogenic( 4)He concentrations (similar to 2.0.10(-5) cc.g(-1)) and lighter stable isotopic composition (delta O-18 from -17.7 to - 22.4 parts per thousand). Relations between radiogenic He-4 and C-14 model ages and between radiogenic He-4 and Cl(- )concentration show that groundwater in the aquifer system does not have a single well-defined age. Rather, the groundwater age distribution has been influenced by mixing between waters originating from end-members with strongly differing ages. Overall the results suggest, that in the shallower northern part of the aquifer system, significant changes in groundwater composition can be brought about by glacial meltwater intrusion during a single glaciation. However, multiple cycles of glacial advance and retreat are needed to transport glacial meltwater to the deeper parts of the aquifer system