Origin of dissolved solids in groundwaters of Mayne Island, British Columbia, Canada

Abstract

Hydrogeological, hydrochemical and isotope techniques were applied to determine the origin of saline groundwaters on Mayne Island in the Strait of Georgia, British Columbia. The saline groundwater is a sodium chloride type, with concentrations of Na + and Cl − as high as 4367 and 8008 mgl −1, respectively. Leach tests on twenty rock samples from boreholes indicate that the shale strata in the Cretaceous bedrock have Cl − concentrations ranging up to 900 ppm by weight and Na + concentrations up to 2070 ppm. The salt probably originated in the shale during periods of post-depositional compaction, or during periods of immersion in ocean water during episodes of glacial loading and erosion during Pleistocene time. The present flow regime of active flushing of meteoric water through the surficial sediments and sedimentary bedrock of Mayne Island began as the island emerged from below sea level during the last deglaciation. The salt content of the present-day groundwater can be accounted for by the slow release, through molecular diffusion, of Na + and Cl − from the low-permeability matrix of the shale to the active paths of groundwater flow in fractures in the shale and in sandstone strata. Intrusion of ocean water under the present hydrologic regime on the island, and upward flow of brine from deep zones within the regional sedimentary basin are believed to be relatively unimportant contributors of salt to the active groundwater flow systems. 18O, deuterium and tritium concentrations are rather uniform for both surface waters and groundwaters on Mayne Island and a meteoric origin is suggested. Saline spring waters on an adjacent island were found to have high 18O and D concentrations ( δ 18 O = + 6.5‰ , δ O = − 24‰ ), suggesting a sedimentary basin brine, but there is no evidence that these brines are mixing with Mayne Island groundwater.