Combined method of 3H/3He apparent age and on-site helium analysis to identify groundwater flow processes and transport of perchloroethylene (PCE) in an urban area

Christian Moeck,Andrea L Popp,Matthias S Brennwald,Rolf Kipfer,Mario Schirmer

doi:10.1016/j.jconhyd.2021.103773

Abstract

Urban groundwater management requires a thorough and robust scientific understanding of flow and transport processes. 3H/3He apparent ages have been shown to efficiently help provide important groundwater-related information. However, this type of analysis is expensive as well as labor- and time-intensive, and hence limits the number of potential sampling locations.To overcome this limitation, we established an inter-relationship between 3H/3He apparent groundwater ages and 4He concentrations analyzed in the field with a newly developed portable gas equilibrium membrane inlet mass spectrometer (GE-MIMS) system, and demonstrated that the results of the simpler GE-MIMS system are an accurate and reliable alternative to sophisticated laboratory based analyses. The combined use of 3H/3He lab-based ages and predicted ages from the 3H/3He–4He age relationship opens new opportunities for site characterization, and reveals insights into the conceptual understanding of groundwater systems.For our study site, we combined groundwater ages with hydrochemical data, water isotopes (18O and 2H), and perchloroethylene (PCE) concentrations (1) to identify spatial inter-aquifer mixing between artificially infiltrated groundwater and water originating from regional flow paths and (2) to explain the spatial differences in PCE contamination within the observed groundwater system. Overall, low PCE concentrations and young ages occur when the fraction of artificially infiltrated water is high. The results obtained from the age distribution analysis are strongly supported by the information gained from the isotopic and hydrochemical data. Moreover, for some wells, fault-induced aquifer connectivity is identified as a preferential flow path for the transport of older groundwater, leading to elevated PCE concentrations.

Highlights

Increasing population and water demands for industrial, agricul tural, and household uses, combined with climate change, is leading to an imbalance of water supply and demand in many regions, and chal lenging water resource management (e.g. Burri et al, 2019; de Graaf et al, 2014; Gleeson et al, 2012; Minnig et al, 2018)
Contaminated sites are often the foremost concern for groundwater quality. It is problematic if these sites are located in the vicinity of drinking water production areas, poten tially leading to decreased water quality and posing a potential risk to both human health and the environment (Baillieux et al, 2015; Karges et al, 2018)
A linear relationship between the field-based 4He concentrations collected with the gas equilibrium membrane inlet mass spectrometer (GE-MIMS) system and the estimated laboratory

Summary

Introduction

Increasing population and water demands for industrial, agricul tural, and household uses, combined with climate change, is leading to an imbalance of water supply and demand in many regions, and chal lenging water resource management (e.g. Burri et al, 2019; de Graaf et al, 2014; Gleeson et al, 2012; Minnig et al, 2018). In urban areas with industrial zones, the supply of drinking water from groundwater sources is further complicated due to potential ground water contamination (Bertrand et al, 2016; Navarro and Carbonell, 2007) Water management in such areas is recognized as a very complex task, both in terms of different spatial and temporal scales (Schirmer et al, 2013; Vazquez-Sun ́e et al, 2005). Cirpka and Kitanidis, 2000; Koltermann and Gorelick, 1996; Moeck et al, 2020; Poeter and Gaylord, 1990; Regli et al, 2003), which cannot be fully explored (Fienen et al, 2009; Kitanidis, 2015; Renard, 2007) As a result, both the spatial dis tribution of contaminants within an aquifer and the time needed for water quality changes to propagate through the system are not completely understood (Szabo et al, 1996). Strategies to protect drinking water production sites are often based on limited data and limited system knowledge (Mackay et al, 1985; Moeck et al, 2016)

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