The Ambient Groundwater Geochemistry Program: pilot project in northern Ontario Precambrian aquifers

Abstract

The first implementation of the Ambient Groundwater Geochemistry Program (AGGP) in northern Ontario was completed in 2016 in the Sudbury area. The Sudbury program was designed to test if the AGGP could be successful in delineating the effect, on groundwater chemistry, of host rock lithology in northern Precambrian shield aquifers as has been already demonstrated for Paleozoic aquifers of southern Ontario. A second objective of this project was to test the potential for the routine sampling of radiological and radiochemical parameters, including radon and gross alpha-particle and gross beta-particle activity concentrations. Since 2016, the focus of the project has been analyzing the data from the Sudbury area survey and completing a 2017 follow-up project along the north shore of Lake Huron, including Manitoulin Island. In total, 93 overburden and 246 bedrock wells were sampled. This poster illustrates the regional trends and groundwater characteristics observed in the AGGP northern Ontario data thus far. In general, the data indicate fewer water quality related exceedences in the Sudbury area than were observed in southern Ontario. However, a few elements increase in concentration in the Precambrian aquifers of the Sudbury area such as U and Cd. Regional variation in groundwater chemistry was observed and may be controlled by the Precambrian geologic provinces. Bedrock well data from the Grenville Province indicate higher Ca, K and U concentrations than samples north of the Grenville Front. Samples from rocks of the Huronian Supergroup and the Sudbury Basin show relatively high Co, As, Pb and Cu concentrations compared to the Grenville Province. Radioactivity parameters appear to be controlled by the presence of clay rich overburden units rather than bedrock geology. The samples taken from wells that penetrate clay overburden are more alkaline because these older, deeper waters are disconnected from lower pH surface waters. Groundwater samples indicating a road salt NaCl source, as indicated by Cl-Br ratios, have the highest average dissolved oxygen percentage and total coliform relative frequency suggesting interaction with the surface. Some high chloride waters were noted on Manitoulin Island related to connate water in Paleozoic rocks. Future work on Precambrian aquifers will seek to determine if regional geochemical signatures are controlled by lithology, traces of deep formational waters, anthropogenic influences, or smaller scale regional geologic features such as the Sudbury Igneous Complex and associated mineralization.

Electron donor‐driven bacterial and archaeal community patterns along forest ring edges in Ontario, Canada

“…The expected fractionation due to bacterial reduction of aqueous sulfate is at least −25‰ (Habicht and Canfield, ), which would lead to a starting isotopic composition for SO 4 2− of approximately +6.5‰, which is much higher than observed. However, the anticipated starting range matches well with the range for δ 34 S‐SO 4 2− in shield terrain (+6.0‰ ± 5.9‰, n = 159, unpublished data), found in a large regional groundwater study near Sudbury (Dell et al ., ) and in local studies near mineral deposits in northeastern Ontario (+10.3‰ ± 4.3‰, n = 16; Fritz et al ., ). A likely scenario is that the observed deep H 2 S originally resulted from the bacterial reduction of aqueous SO 4 2− (δ 34 S‐SO 4 2− of approximately +6.5‰) in the ambient groundwater of the regional aquifers, yielding the observed (average) δ 34 S‐S 2− value of −18.5‰ for aqueous H 2 S. The methane commonly observed in groundwater in the fine‐grained glacial sediments of the Cochrane Till (Hamilton et al ., ,b,c) is evidence that reducing agents, likely in the form of buried organic material, are present in these glacial sediments.…”