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TitleWhat pore water chemistry in Champlain Sea muds reveals about hydrogeology, marine salinity, and sensitivity to landslides
AuthorHinton, MORCID logo; Alpay, SORCID logo; Crow, HORCID logo
SourceGAC-MAC-IAH 2019: where geosciences converge/AGC-AMC-AIH 2019 : où les géosciences convergent; GAC-MAC-IAH Joint Meeting, Abstract volume vol. 42, 2019 p. 109-110 Open Access logo Open Access
LinksOnline - En ligne (complete volume, volume complet, PDF, 6.08 MB)
Alt SeriesNatural Resources Canada, Contribution Series 20180422
PublisherGeological Association of Canada
MeetingGAC-MAC-IAH 2019 / AGC-AMC-AIH 2019; Québec, QC; CA; May 12-15, 2019
Mediaon-line; digital
File formatpdf (Adobe® Reader®)
NTS31F/08; 31F/09; 31G/05; 31G/12
Lat/Long WENS -76.5000 -75.5000 45.7500 45.2500
Subjectssurficial geology/geomorphology; geochemistry; hydrogeology; geophysics; engineering geology; Nature and Environment; Science and Technology; Health and Safety; marine sediments; muds; sensitive clays; pore water samples; water geochemistry; salinity; landslides; groundwater regimes; groundwater flow; geophysical logging; modelling; geochemical dispersion; sea water geochemistry; chlorine geochemistry; bromine geochemistry; sodium geochemistry; isotopic studies; oxygen isotopes; Champlain Sea Sediments; Leda Clay; St. Lawrence Lowlands; Phanerozoic; Cenozoic; Quaternary
ProgramPublic Safety Geoscience Assessing Earthquake Geohazards
Released2019 05 01
AbstractChamplain Sea muds, also known locally as Leda Clays, form a regionally extensive aquitard in the St. Lawrence Lowlands and the Ottawa Valley. Investigations of pore water chemistry, borehole geophysics, and geotechnical sensitivity (ratio of undisturbed to remoulded compressive strength) within a thick sequence of Champlain Sea muds provide insights into groundwater flow across the aquitard, original salinity, and development of landslide susceptibility. Typically, the assumption is that diffusion alone controls solute transport through Champlain Sea muds. However, this study uses 1D groundwater modelling to demonstrate that both advection (Darcy flux of approx. 4 mm/a) and diffusion contribute to solute transport at a site located in thicker (~ 75 m) Champlain Sea mud deposits at Breckenridge, QC, northwest of Ottawa. Previous hydrogeological studies estimate Champlain Sea salinity to be approximately 33 % of contemporaneous ocean salinity. However, in this study, where the sequence of Champlain Sea muds is thicker, the peak measured ionic concentrations in pore water represent at least 60 % of relic seawater salinity. Geophysical logs of bulk apparent conductivity corroborate this finding. Additionally, transport modelling constrains the Champlain Sea concentration of seawater constituent ions (e.g. [Cl-]), and hence, the original salinity. Although the concentrations of seawater ions (Cl-, Br-, Na+) allow estimation of the seawater fraction, when combined with isotopic signatures (e.g. 18O), these multiple tracers form the basis to estimate relative proportions of seawater, meteoric fresh water, and fresh glacial melt water of the Champlain Sea episode. Champlain Sea salinity, along with the combined effects of advection, diffusion, and thickness of the muds, controls the current pore water chemistry, which in turn, influences sensitivity development of Leda Clays and susceptibility to landslides.
Summary(Plain Language Summary, not published)
Pore water was extracted from 75 m thick muds (Leda clays) deposited in the former Champlain Sea at a site near Breckenridge, Quebec. Results of chemical analyses show that the pore water contains at least 60% ocean water. Models of groundwater and chemical transport provide an estimate of groundwater flow (4 mm/year) and diffusion of saline water in the muds. The original and current salinity of pore water influences the sensitivity of Leda clay to landslides.

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