Eric Rosa

A Review and Evaluation of the Impacts of Climate Change on Geogenic Arsenic in Groundwater from Fractured Bedrock Aquifers

Climate change is expected to affect the groundwater quality by altering recharge, water table elevation, groundwater flow, and land use. In fractured bedrock aquifers, the quality of groundwater is a sensitive issue, particularly in areas affected by geogenic arsenic contamination. Understanding how climate change will affect the geochemistry of naturally occurring arsenic in groundwater is crucial to ensure sustainable use of this resource, particularly as a source of drinking water. This paper presents a review of the potential impacts of climate change on arsenic concentration in bedrock aquifers and identifies issues that remain unresolved. During intense and prolonged low flow, the decline in the water table is expected to increase the oxidation of arsenic-bearing sulfides in the unsaturated zone. In addition, reduced groundwater flow may increase the occurrence of geochemically evolved arsenic-rich groundwater and enhance arsenic mobilization by reductive dissolution and alkali desorption. In contrast, the occurrence of extreme recharge events is expected to further decrease arsenic concentrations because of the greater dilution by oxygenated, low-pH water. In some cases, arsenic mobilization could be indirectly induced by climate change through changes in land use, particularly those causing increased groundwater withdrawals and pollution. The overall impact of climate change on dissolved arsenic will vary greatly according to the bedrock aquifer properties that influence the sensitivity of the groundwater system to climate change. To date, the scarcity of data related to the temporal variability of arsenic in fractured bedrock groundwater is a major obstacle in evaluating the future evolution of the resource quality.

Mobility and speciation of geogenic arsenic in bedrock groundwater from the Canadian Shield in western Quebec, Canada

High arsenic concentrations occur in groundwater collected from a fractured crystalline bedrock aquifer in western Quebec (Canada). Sampling and analysis of water from 59 private wells reveal that more than half of the bedrock wells exceed the Canadian guideline value of 10μg/l for arsenic, whereas shallow wells in unconsolidated surficial deposits are not affected by the contamination. The weathering of arsenic-bearing sulfides present along the mineralized fault zone is considered to be the primary source of arsenic in groundwater. High-arsenic wells are generally characterized by mildly reducing conditions (Eh<250mV), weak alkaline conditions (pH>7.4), low Ca/Na ratios, elevated dissolved Fe and Mn concentrations and high proportions of As(III). Private bedrock wells are open boreholes that likely receive groundwater from multiple contributing fractures. Hence, it is proposed that dissolved arsenic is mainly derived from the contribution to the well discharge of reducing and alkaline geochemically evolved groundwater that contains arsenic as As(III). Geochemically evolved groundwater provides favorable conditions to release arsenic by reductive dissolution of iron and manganese oxyhydroxides and alkaline desorption from mineral surfaces. Thus, high-arsenic wells would contain a high proportion of geochemically evolved groundwater, while oxidizing low-pH recharge water causes dilution and sequestration of arsenic. In relation with the chemical evolution of groundwater along the flow path, most contaminated wells are located in confined areas whereas most of the wells located in unconfined recharge areas are not contaminated. The occurrence of boreholes with high dissolved arsenic as As(V) and oxidizing conditions is attributed to extensive sulfide oxidation and alkaline desorption. This work shows that the determination of arsenic speciation provides a valuable tool to investigate the behavior of arsenic in bedrock groundwater.

Geochemical and isotopic mass balances of kettle lakes in southern Quebec (Canada) as tools to document variations in groundwater quantity and quality

Given increasing anthropogenic and climatic pressures on water resources, groundwater and surface water need to be better managed and preserved. As these two water stocks can be connected to each other, their evolutions are linked and need to be considered as such. However, interactions between lakes and groundwater are not well understood and, most of the time, are not taken into account. Therefore, establishing a comprehensive approach to quantify groundwater and lakes’ hydrogeochemical interactions in various settings is of foremost importance for assessing the sensitivity of lakes to groundwater evolution. In this study, small kettle lakes set in fluvioglacial deposits and that are most likely well connected to shallow unconfined aquifers are specifically targeted. Geochemistry and isotopic results highlight that groundwater flux to the lakes is generally the dominant parameter of the lake water budget. The 222Rn results in particular suggest that 38% of the studied lakes have a high proportion of groundwater in their balances. It appears that the different tracers are complementary: geochemistry is influenced by groundwater inflows, reflecting its quality and the local geology, whereas water stable isotopes correspond directly to the volumetric lake water budget, and both of these tracers are impacted by in-lake processes. Moreover, the third tracer considered, 222Rn, highlights the location of groundwater inputs in space and time. Finally, the studied kettle lakes are characterized by a short to medium flushing time by groundwater. As a result, these lakes can be highly sensitive to environmental and climate changes affecting groundwater.

Using water stable isotopes for tracing surface and groundwater flow systems in the Barlow-Ojibway Clay Belt, Quebec, Canada

This study aims to improve the understanding of surface and groundwater flow systems based on water stable isotope data in a 19,549 km2 region of the Barlow-Ojibway Clay Belt, in western Quebec, Canada. The available geochemical database contains 645 samples including precipitation, snow cores, surface waters, groundwater and springs. All samples were analyzed for water stable isotopes (δ2H-δ18O) and complementary tritium analyses were conducted on 98 groundwater and spring samples. Precipitations depict a clear temperature-dependent seasonal pattern and define a local meteoric water line (LMWL) without a latitudinal trend in δ2H-δ18O. Samples collected from the snowpack plot on the LMWL, suggesting that the bulk snowpack preserves the isotopic composition of precipitation throughout the frozen period, prior to the spring snowmelt. Surface water samples define a local evaporation line (LEL), and evaporation over inflow (E/I) ratios range between 0 and 36%. Groundwater and spring samples are evenly distributed around the LMWL, suggesting that evaporation processes are limited prior to infiltration and that surface waters do not significantly contribute to groundwater recharge. Shallow unconfined aquifers present a greater variability in δ2H-δ18O compared to confined aquifers located farther down gradient, suggesting the mixing of varied recharge waters along the regional groundwater flow system. A three-component mixing model based on isotopic and specific electrical conductivity data allows the quantification of such mixing processes. The interpretation of isotopic data constrains a regional-scale conceptual model of groundwater flow systems and describes processes related to the timing of recharge, evaporation, mixing and discharge.

A graphical approach for documenting peatland hydrodiversity and orienting land management strategies

This study focuses on the development of an approach to document the hydrological characteristics of peatlands and understand their potential influence on runoff processes and groundwater flow dynamics. Spatial calculations were performed using geographic information systems data in order to evaluate the distribution of peatlands according to (a) neighbouring hydrogeological units and (b) their position within the hydrographic network. The data obtained from these calculations were plotted in a multiple trilinear diagram (two ternary plots projected into a diamond‐shaped diagram) that illustrates the position of a given peatland within the hydrogeological environment. The data allow for the segregation of peatlands according to groups sharing similarities as well as the identification of peatlands that are most likely to have similar hydrological functions. The approach was tested in a 19,549 km2 region of the southern portion of the Barlow‐Ojibway Clay Belt (in Abitibi‐Temiscamingue, Canada) and lead to a conceptual model representing the hydrological interactions between peatlands, aquifers, and surface waters. This approach allows for a geographic information systems‐based differentiation of headwater peatland complexes that are likely to interact with aquifers and to supply continuous baseflow to small streams from lowland peatland complexes of the clay plain that are isolated from surrounding aquifers but that can act as storage reservoirs within the hydrographic network. The typology is further used to discuss land management strategies aimed at preserving peatland hydrodiversity within the study region. The proposed approach relies on widely applicable hydrogeological and hydrographic criteria and provides a tool that could be used for assessing peatland hydrodiversity in other regions of the planet.

Stratigraphic sequence map for groundwater assessment and protection of unconsolidated aquifers: A case example in the Abitibi-Témiscamingue region, Québec, Canada

Quantifying water resources at various scales is crucial for ensuring safe access to potable water for present and future generations. Fitting into this framework, this study presents a GIS-based approach aimed at allowing the evaluation of available groundwater resources within unconsolidated aquifers set in vast and heterogeneous shield regions. The approach was developed in a 19,397 km2 region located in Abitibi-Témiscamingue (Québec, Canada) where unconsolidated aquifers are set in an irregular geologic framework owing to the rugged Canadian Shield topography and to the diversity of glacial and post-glacial events that shaped the landscape. Sparse and unevenly distributed stratigraphic borehole data were used for constructing a GIS-based model representing the total overburden thickness (unconsolidated sediments covering the bedrock), as well as the thickness and extent of the fine-grained deep-water sediments (silt-clay) of the Barlow-Ojibway proglacial Lake on a 100 m × 100 m mesh at the regional scale. The study suggests a new set of functions to map drift thickness in areas of highly discontinuous drift cover, using GIS tools. These data were used jointly with surficial deposits maps in order to develop a regional-scale two-dimensional model where 15 distinct stratigraphic sequences allow representation of the architecture of unconsolidated geological units. The approach allows simplifying data representation and ensuring the consistency between various regional-scale hydrogeological maps. The data is used to produce a stratigraphic sequence map for representing the architecture of aquifer– aquitard systems at the regional scale in a manner that is intelligible to non-specialists. The map and related data are first discussed for documenting the extent and volume of regional aquifers. The stratigraphic sequence map is subsequently discussed as a tool for supporting political decision makers dealing with issues related to groundwater resource assessment, protection and sustainable development.

Processes governing the stable isotope composition of water in the St. Lawrence river system, Canada†

ABSTRACT Linkages between δ18O–δ2H and hydrological processes have been investigated from isotopic time series recorded in the St. Lawrence River basin. Three stations were monitored from 1997 to 2008. They include the Ottawa River, the St. Lawrence River main channel at Montreal and the fluvial estuary. All sites depict seasonal isotopic cycles characterized by heavy isotope depletions during the snowmelt period and heavy isotope enrichments throughout the ice-free period. The data define δ2H–δ18O regression lines falling below the meteoric water line. In the Ottawa River, calculations suggest that approximately 8 % of the total inflow to the basin is lost through evaporation. In the St. Lawrence River main channel, seasonal isotopic fluctuations most likely reflect hydrological processes occurring within the Great Lakes and mixing with tributaries located downstream. In the St. Lawrence River fluvial estuary, isotopic data allow partitioning streamflow components and suggest that the recorded seasonal variations mainly respond to mixing processes.

Results from the Quebec Groundwater Knowledge Acquisition Program

Results from the Quebec Groundwater Knowledge Acquisition Program Marie Larocque* , Vincent Cloutier , Jana Levison and Eric Rosa Département des sciences de la Terre et de l’atmosphère, Université du Québec à Montréal, Montréal, Canada; Centre de recherche GEOTOP, Université du Québec à Montréal, Montréal, Canada; Groupe de recherche sur l’eau souterraine, Institut de Recherche en Mines et en Environnement, Université du Québec en Abitibi-Témiscamingue, Amos, Canada; School of Engineering, University of Guelph, Guelph, Canada