Geochemical evolution of groundwater on Saturna Island, British Columbia

Abstract

A detailed geochemical study of surface waters, spring waters, and groundwaters was undertaken to examine the geochemical evolution of groundwater on Saturna Island, British Columbia. The purpose of the study was to characterize the nature and occurrence of saline waters and to provide insight on chemical processes that lead to salinization in the fractured sedimentary bedrock aquifers of this small island. Major ion chemistry shows that groundwater is recharged locally but mixes with saline waters that occur at depth or near the coast. Simple mixing is complicated by cation exchange (between calcium-rich waters and sodium-rich exchange sites offered by mudstone beds) and results in a spatially variable hydrochemical composition that is dependent on the island topography and geological framework (structural, sedimentological, and glacial), in combination with groundwater use patterns. Sodium, present at exchange sites, is speculated to be a remnant of ocean water intrusion during the Pleistocene, when the island was submerged. As a result of its high mobility and conservative nature, chloride (and sulphate) has been flushed from the shallow bedrock during a process of natural desalinization but may remain trapped in the pores and fractures at depth. Modern salt-water intrusion, brought about by increased development on the island, is now competing with natural desalinization along the coast and has left many drinking-water supplies contaminated.Résumé : Une étude géochimique détaillée des eaux de surface, des eaux de sources et des eaux souterraines a été entreprise pour examiner l'évolution géochimique de l'eau souterraine sur l'île de Saturna, en Colombie Britannique. Le but de l'étude était de caractériser la nature et l'occurrence d'eau saline et de comprendre les processus chimiques qui conduisent à la salinisation des aquifères situés dans la roche mère sédimentaire fracturée de cette petite île. La chimie des ions majeurs montre que l'eau souterraine est rechargée localement, mais qu'elle se mélange aux eaux salines qui se trouvent en profondeur ou près de la côte. Le simple mélange se complique par l'échange de cations (entre les eaux riches en calcium et les sites d'échanges riches en sodium des lits de mudstone); il en résulte une composition hydrochimique à variation spatiale qui dépend de la topographie de l'île et de son cadre géologique (structural, sédimentologique et glacial) combinés aux patrons d'utilisation de l'eau souterraine. Nous proposons que le sodium, présent aux sites d'échanges, est un restant d'intrusion d'eau océanique au cours du Pléistocène, lorsque l'île était submergée. En raison de sa grande mobilité et de sa nature conservatrice, le chlorure a été retiré de la roche-mère à faible profondeur durant un processus de dessalement mais, en profondeur, il peut demeurer emprisonné dans les pores et les fractures (idem pour le sulfate). L'intrusion d'eau saline moderne, amenée par la croissance du développement sur l'île, est maintenant en compétition avec le dessalement naturel le long...

Response of a fractured bedrock aquifer to recharge from heavy rainfall events

“…Because the coarser-grained formations are more resistant to weathering than the finer-grained formations (Allen and Suchy, 2001), topographic high points at the centre of the island (~160 masl) are comprised of the Gabriola Formation. Steep relief has formed along contacts where the less-resistant Northumberland and Spray Formations have eroded, leaving behind the Geoffrey and Gabriola Formations (Scibek et al, 2013).…”

“…This area is well suited for this research because the hydrogeology and hydrogeochemistry of the fractured bedrock aquifers has been extensively studied (Allen and Matsuo, 2002;Allen et al, 2003;Allen and Suchy, 2001;Appaih-Adjei, 2006;Dakin et al, 1983;Mustard, 1994;Staples, 2005;Larocque, 2014;; rainfall is the dominant form of precipitation (so as to avoid snowmelt processes);…”

“…In general, recharge occurs across the majority of the land surface, and groundwater discharges along the coast or in large valleys, near streams (Allen and Suchy, 2001;Allen and Matsuo, 2002). Furthermore, the water table roughly follows land surface topography (Hodge, 1995).…”

“…Significant deposition occurred during the last glaciation approximately 20,000 BP (Clague, 1986). Due to erosion and mass wasting, sediment has accumulated and is thickest in low lying areas and valleys, while it thinly covers hills and ridges (Allen and Suchy, 2001). The relative thinness of surficial sediment is thought to not contribute significantly to groundwater storage A rain shadow effect from the Insular Mountains to the west and Olympic Mountains to the south partially controls climate on the islands (Kenney et al, 1990).…”

Historical temporal trends in groundwater levels from British Columbia, Canada

“…Aquifer types in the province have been classified [3] and various investigations have employed case-study type approaches to determine the status of groundwater resources, developed rigorous quantitative models of representative aquifers with predictive capacity, and attempted to assess future changes due to natural and/or anthropogenic forcings (see, e.g., ref. [4][5][6][7][8][9][10][11][12][13][14][15][16] and references therein). In our previous work, we examined temporal trends in groundwater monitoring wells for the western Canada prairie province of Saskatchewan [17].…”