Allochthonous sources of iodine and organic carbon in an eastern Ontario aquifer

Abstract

Regional geochemical characterization of groundwaters in a bedrock aquifer in the Ottawa – St. Lawrence Lowlands of eastern Ontario has identified an iodine (I) anomaly, with values regularly exceeding 150 μg/L and a maximum observed concentration of 10 812 μg/L. The spatial distribution, enrichment mechanisms, and sources of I and organic matter were investigated using geochemical and isotopic data. High-I groundwaters (>150 μg/L) are prevalent in Na–Cl-type groundwaters at low bedrock elevations in areas overlain by thick layers of glacial sediments. I is thought to be linked to massive muds in the glacial sediments overlying the aquifer, deposited during the postglacial incursion of the Champlain Sea 12–10 ka BP. Principal component analysis of I and 18 other chemical parameters revealed correlations among I, salinity, and indicators of microbial oxidation of organic matter, suggesting that the intrusion of saline pore waters affected by decomposition of organic matter such as marine phytoplankton in the massive muds is the dominant process controlling I enrichment in groundwater. 129I/127I ratios in the pre-modern waters vary between near-marine values of 460 × 10−14 and 5 × 10−14, demonstrating that older allochthonous I derived from the surrounding Paleozoic sedimentary terrain also contributed to the I pool in the Champlain Sea basin. 14C ages and δ13C signatures for dissolved organic carbon in groundwater and disseminated organic carbon within the glaciomarine muds highlight an allochthonous source of terrestrial organic carbon predating the Champlain Sea incursion, likely transported via glacial meltwaters in tandem with I to the Champlain Sea basin.