<p>Groundwater dating and travel time distributions are important tools and data for assessment of the vulnerability of water supply wells towards pollution from the surface. Here we present selected results from more than 30 water supply and monitoring wells from major Danish water companies. The wells were recently sampled and investigated using multiple environmental tracers including <sup>85</sup>Kr, <sup>39</sup>Ar, <sup>3</sup>H/<sup>3</sup>He, <sup>14</sup>C, SF<sub>6</sub>, CFCs and noble gases and different groundwater modeling techniques. The results demonstrate the value of groundwater dating and travel time estimations for the assessment of the history and fate of contaminants in the subsurface. This information is crucial for the assessment of the efficiency of measures to mitigate pollution of groundwater by harmful substances such as pesticides, nitrate and a large range of emerging contaminants. We demonstrate how groundwater ages and travel time distributions can be used to assess the vulnerability or susceptibility of water supply wells towards pollution, and how level specific sampling in long well screens can provide additional important information for assessment of the vulnerability of deep and shallow parts of a water supply well. Potential applications of the estimated travel time distributions include 1) improved management of well fields 2) development of pumping strategies and well screens minimizing the risk of pollution of drinking water wells, and 3) assessment of the adequacy of regulations established by authorities to protect valuable groundwater resources against pollution. &#160;&#160;</p>
<p>Lake Kivu, located on the border of Rwanda and the Democratic Republic of Congo, is a very peculiar lake in several aspects. The meromictic lake shows a vertical stratification dominated by high salt concentrations of up to 6 &#8240; resulting in a very thick monimolimnion of 420 m (max depth ~492 m). This extremely large non mixing part of the lake functions as a reservoir for very high concentrations of volcanogenic gases like methane and carbon dioxide (up to 20 and 100 mmol/l respectively) resulting in a growing hazard for millions of local residents. Our aim of this study is to get insights into the hydrological dynamics, solute transport and the lakes mixing behavior utilizing radiometric dating with <sup>39</sup>Ar.</p> <p>The noble gas isotope <sup>39</sup>Ar (t<sub>1/2</sub> = 269 a) covers a unique time span for studying the dynamics of aquatic and glacial systems of the last millennium. Although this tracer has been acknowledged for decades, studies so far are limited by its low abundance, little radioactivity and hence huge required sample sizes (~1000 L water). Until today environmental routine measurements are mainly confined to groundwater reservoirs, where nearly unlimited sampling is possible. The application of techniques from atomic physics using a magneto optical atom trap (MOT) solves the problem by reducing sample volume requirements by several orders of magnitude. The problem of the very low isotopic abundance of 10<sup>-16</sup> is resolved by resonant multi-photon scattering of <sup>39</sup>Ar in the MOT. This technique named Argon Trap Trace Analysis with its very low minimal sample size of 0.5 cm&#179;STP pure argon enables easy sample handling in the field as well as common sampling procedures like Niskin bottles for aquatic systems, drill core sampling for glacial systems or as in the case of Lake Kivu spray chamber gas sampling in remote places. It is thus a door opener for new geophysical research fields that were excluded from radio-argon dating so far.</p> <p>Here we present our most recent results of sampling campaigns in 2018 and 2019 using samples of about 25 &#8211; 40 L gas-water mixtures corresponding to 0.5 &#8211; 10 cm&#179;STP pure argon showing surprisingly high ages for the lake water.</p>
<p>Timescales of ventilation of the Arctic Ocean are still only poorly known. The commonly used tracers for ocean ventilation studies like CFCs and SF<sub>6</sub> are limited to young water masses that are either close to the surface or in highly ventilated deep waters. The radioisotope <sup>39</sup>Ar with its half-life of 269 years covers time scales of 50 to 1000 years, perfectly suitable to investigate ventilation timescales of deep and intermediate water masses within the Arctic Ocean. The new measurement technique called Argon Trap Trace Analysis (ArTTA) only requires samples sizes of a few liters of ocean water, instead of the previous low-level counting method, which required about 1000 liters of water. The benefit for ocean studies is evident, much more samples can be taken during one cruise if ArTTA is applied. This enables a better resolution of the water column in great depths at the desired sampling location in the Arctic Ocean. Combined with the additional data of the CFC-12 and SF<sub>6</sub> measurements, ventilation timescales of the complete water column from surface to bottom are obtained by constraining transit time distributions via this multi-tracer approach.</p><p>Another focus of this study is the saturation of all gaseous transient tracers. It is determined by surface conditions as well as interior mixing processes. Measurements of stable noble gas isotopes (He, Ne, Ar, Kr, Xe) are used to determine possible saturation anomalies that arise during air bubble dissolution, rapid cooling and subduction, or ice formation and subsequent interior mixing of water masses. These saturation distortions for different boundary conditions are of key importance to correct the input function for gas tracers in the Arctic Ocean and hence to constrain the ventilation timescales. The uncertainty of the age distributions will be reduced, and ocean circulation models can be improved.</p><p>This contribution presents first stable and radioactive noble gas results of the project Ventilation and Anthropogenic Carbon in the Arctic Ocean (VACAO), which is part of the Synoptic Arctic Survey carried out in summer 2021 on the Swedish icebreaker Oden.</p>
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