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Canada/U.S. transboundary geological maps of the Richelieu/Lake Champlain and Yamaska basins

Aquifers of the Richelieu/Lake Champlain and Yamaska watersheds are being characterized in the context of a regional groundwater assessment. The study area, which covers 16,500 km2 (55 % in Québec and 45 % in the United States), extends from the St. Lawrence River southward into
northern Vermont and New York. For the needs of this study, bedrock and surficial geology maps were compiled at a scale of 1:250 000. The bedrock map is based on compilation maps from the Quebec Department of Natural Resources (DNR) and on state survey maps in the U.S. (Vermont: 1:100 000 and NY:
1:250 000 scale); a small scale Canada/U.S. map (1:1000 000) was also used. The surficial geology map is based on 1:50 000 scale maps produced by the Quebec DNR specifically for this regional assessment and on 1:62 500 scale maps for VT and 1:250 000 for NY. The bedrock map describes the main
lithologic assemblages of each formation or group for the four geological contexts recognized in the study area: Adirondacks, St. Lawrence Lowlands, Appalachians and Cretaceous intrusive rocks. The map also shows the main fractured/faulted zones, as these may have a large impact on groundwater flow.
The surficial geology map subdivides Quaternary sediments on the basis of stratigraphy, origin and lithofacies. Due to the different scales and origins of the source maps used to produce these new transboundary maps, map units and legends had to be harmonized to recent standards and classification
schemes. Source maps were either available in a digital format or had to be digitized first. Representative cross-sections, based on borehole and geophysical data, were also prepared on the Canadian side to gain a better understanding of surficial sediment stratigraphy and architecture. The
surficial cover, consisting mainly of glacial sediments and minor glaciofluvial and glaciolacustrine sediments, is thin and discontinuous in uplands, but may reach a thickness of 50 m in the north-western part of the study area where the glacial sediment suite is covered by thick marine silty clays.
These transboundary geological maps and cross-sections are key tools for characterizing hydrogeological contexts, in particular to better understand groundwater flow dynamics, to assess hydraulic properties and recharge rates, as well as to develop transboundary groundwater flow numerical

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