Hydrochemical characterisation of groundwater to understand the flow dynamics of the Saint-Mathieu/Berry Esker, Abitibi, Québec Daniel Blanchette 1 , Vincent Cloutier 1 , René Lefebvre 2 and Ian Clark 3 1 Groupe de recherche sur l’eau souterraine, Département des sciences appliquées – Université du Québec en Abitibi-Témiscamingue, Campus d’Amos, Amos, Québec, Canada 2 Centre Eau Terre Environnement – Institut national de la recherche scientifique (INRS-ETE), Québec, Québec, Canada 3 Department of Earth Sciences – University of Ottawa, Ottawa, Ontario, Canada ABSTRACT In Abitibi-Témiscamingue, eskers having an exposed crest and edges confined by the Barlow-Ojibway Lake clay provide high quality groundwater resources. However, these aquifers are vulnerable to degradation by surface activities. It is thus necessary to develop a characterization approach providing the information needed by decision makers for sustainable groundwater management. This paper describes the initial hydrochemical characterization of the Saint-Mathieu/Berry esker near Amos, Québec. It is shown that the chemical composition of precipitations and groundwater can be used as tracers to efficiently understand the groundwater flow dynamics associated with esker aquifer systems. RÉSUMÉ En Abitibi-Témiscamingue, les eskers avec une crête libre et des flancs confinés par les argiles Barlow-Ojibway fournissent d’excellentes ressources en eau souterraine. Toutefois, ces aquifères sont vulnérables à la dégradation reliée aux activités de surface. Il est donc nécessaire de développer une approche de caractérisation qui fournisse aux gestionnaires les informations requises pour la gestion durable de ces ressources. Cet article décrit la caractérisation hydrochimique initiale de l’esker Saint-Mathieu/Berry près d’Amos au Québec. Il est montré que la composition chimique des précipitations et des eaux souterraines peut être utilisée en tant que traceur pour efficacement comprendre la dynamique de l’écoulement dans les systèmes aquifères des eskers. 1 INTRODUCTION The last glaciation and ice retreat, which ended around 8 ka in Abitibi, Québec, left several glaciofluvial granular deposits as eskers, which consist of sand and gravel. Many eskers in Abitibi are partly or totally covered by clay deposited in glacial Lake Barlow-Ojibway. Some of these eskers are aquifers that are supplying excellent quality groundwater to several municipalities and to a water bottling company. Despite this good aquifer potential, development of economic activities, such as gravel and sand pits and logging, could negatively impact these aquifers. Conflicts are thus emerging about human activities surrounding aquifers in the perspective of the sustainability and protection of these groundwater resources. Decision makers thus need reliable information to make sound decisions regarding economical activities, while also considering the sustainable development of groundwater resources. One way to provide better information about groundwater resources related to eskers is to develop multidisciplinary characterization approach to better understand these aquifer systems. This paper is related to a project aiming to develop such a characterization approach using the aquifer of the Saint-Mathieu/Berry esker located near the city of Amos as a natural hydrogeological laboratory (Figure 1). Groundwater is actively exploited from this esker, which has been initially characterized (Bolduc et al., 2004; Riverin, 2006). By integrating geological, geophysical, geochemical and hydrogeological methods, this project intends to better assess this aquifer system. More specifically, the project intends to make a more accurate estimation of groundwater recharge, better understand the flow dynamics in the esker and define the geochemical evolution of groundwater in the esker. This better assessment of the esker aquifer system should form the basis for the development of more realistic groundwater flow numerical models. Such models are important tools to quantitatively understand aquifers and thus better support groundwater resource management. As part of the general study related to the segment of the Saint-Mathieu/Berry esker being studied, an initial hydrochemical characterization was carried out. As part of that characterization, rain, snow and groundwater samples were collected in 2009 and 2010 and chemically characterized, including physico-chemical parameters, complete chemical analyses and selected isotopic analyses. This paper presents the results of this geochemical characterization with three main objectives: 1) depict the spatial variability of groundwater chemistry, 2) relate this variability to the sampling depth (and thus residence time) in the esker and, 3) use geochemistry as a natural tracer to help constrain the conditions of groundwater flow dynamics. This initial characterization will guide further work needing to be carried out for the development of the general hydrogeological assessment approach developed on the Saint-Mathieu/Berry esker.