Combined Use Of 3H/3He Apparent Age And On‐Site Helium Analysis To Identify Groundwater Flow Dynamics And Transport Of PCE

Abstract

<p><sup>3</sup>H and tritiogenic <sup>3</sup>He concentrations and their interpretation as <sup> 3</sup>H/<sup>3</sup>He apparent water ages have been proven to offer crucial insights on groundwater flow and transport processes. However, the analysis is expensive as well as labor‐ and time‐intensive. Recent developments of portable field‐operated gas equilibrium membrane inlet mass spectrometer (GE‐MIMS) systems provide however, a unique opportunity to measure relatively fast dissolved gas concentrations, such as <sup>4</sup>He, in groundwater systems with a high resolution at relatively low costs but they are not capable of providing an apparent age. However, <sup>4</sup>He accumulation rates are often obtained from <sup>3</sup>H/<sup>3</sup>He ages and it has been shown that non-atmospheric <sup>4</sup>He concentrations determined in the laboratory (e.g., by static (noble gas) mass spectrometry) and by field-based (GE-MIMS) methods closely agree. This agreement allowed to quantify the local (radiogenic) <sup>4</sup>He accumulation, e.g., we were able to establishing an inter‐relationship between <sup>3</sup>H/<sup>3</sup>He apparent groundwater ages and the non-atmospheric <sup>4</sup>He excess (e.g., calibrating the <sup>4</sup>He excess in terms of residence time).</p><p>We demonstrate that the <sup>4</sup>He excess concentrations derived from the GE‐MIMS system serve as adequate proxy for the experimentally demanding laboratory based analyses. The combined use of <sup>3</sup>H/<sup>3</sup>He lab‐ based ages and calibrated  <sup>4</sup>He ages opens new opportunities for site characterization due to the measurements facilitated by the GE‐MIMS.</p><p>For our urban and contaminated study site, we combine groundwater ages with hydrochemical data, water isotopes (δ<sup>18</sup>O and δ<sup>2</sup>H), and perchloroethylene (PCE) concentrations (1) to identify spatial inter‐aquifer mixing between artificially infiltrated surface water and groundwater originating from regional flow paths and (2) to explain the spatial differences in PCE contamination. Moreover, for some wells, we identify fault‐induced aquifer connectivity as a preferential flow path for the transport of older groundwater, leading to elevated PCE concentrations.</p>