WHEN GEOCHEMICAL INDICATORS OF DEEP GLACIAL RECHARGE AREN'T: INFORMATION FROM DEEP INVESTIGATIONS OF PERMAFROST AND METHANE HYDRATES

Isotopic (d²H, d¹⁸O) and chemical compositions are commonly utilized as indicators of glacial recharge to groundwater, however little attention has been given to the geochemical effects of permafrost formation. Permafrost often forms in front of advancing and retreating glaciers. Today, permafrost is known to extend to depths of 600m in the arctic of North America. In areas with significant gas concentrations, gas hydrates form at low subsurface temperatures and moderate pressures. Permafrost and gas hydrate formation were explored in the laboratory, field, and with numerical models. Laboratory investigations during ice column formation indicate similar chemical and isotopic compositions can result from in situ permafrost formation or glacial meltwater recharge. Isotopic and chemical behaviors are also similar during methane hydrate formation, and all three processes result in fluid dilution. The laboratory data collected clearly indicate permafrost and methane hydrate formation have a similar effect as glacial meltwater intrusion. Fluid and isotopic compositions were analyzed from data collected at previously studied Canadian Shield sites. Isotopic data at sites currently located in permafrost areas indicate the waters have been influenced by freezing. At one site, geochemical data clearly indicate in situ freezing during permafrost formation affected groundwater chemical composition. At other sites, it was not possible to distinguish between glacial recharge, and permafrost/methane hydrate formation due to fluid mixing. Mixing at several sites was aggravated by mining. Often permafrost is assumed impermeable, and an argument for deep glacial meltwater recharge is provided through permafrost melting beneath warm-footed glaciers. However, permafrost taliks are common, providing conduits for hydraulic flow to the subsurface. Modeling indicates methane hydrates form at shallower depths beneath glaciers largely due to increased hydrostatic pressures, reducing fracture and pore permeabilities even as permafrost melts beneath a warm-footed glacier. This investigation suggests permafrost and methane hydrates should be incorporated into models of glacial effects on groundwater flow. Caution is also warranted when interpreting geochemical data for deep glacial recharge in the Canadian Shield or areas with large gas deposits.