Relating sequence stratigraphic and karstic controls of regional groundwater flow zones and hydrochemistry within the Early Silurian Lockport Group of the Niagara Escarpment, southern Ontario
Authors: Brunton F R , Priebe E H , Yeung K 
Published: March 2016

The Ontario Geological Survey has been mapping regional bedrock potable groundwater flow zones across the Niagara Escarpment region of Southern Ontario and Manitoulin Island. The sedimentary rocks that comprise the Niagara Escarpment are Early Silurian in age and
display a complex but predictable stratigraphic architecture that has been identified through detailed logging/sampling of cores and outcrops both within and away from the "Arch" or forebulge region. This study has developed a new paleogeographic / paleoenvironmental perspective, which provides
important insights into the controls on various carbonate bedrock fluid pathways and supports a predictive framework. Intermittent responses to far-field tectonics along the Appalachian Foreland basin influenced local carbonate ramp geometries and relative sea level fluctuations and differential
erosion regionally. Findings show that the more significant the time breaks within the stratigraphic architecture, the more regional and significant the extent of the flow zones. It also highlights the economic importance of characterizing forebulge-tectonic zones, and the value of geologic mapping
and acquisition of geologic data to successfully explore, characterize, and define bedrock flow zones in a cost-effective manner.
Due to regional stress fields and differential erosion of the Paleozoic strata, rock strata presently dip gently in a SW direction away from the topographic high of the erosional Niagara scarp face. The Cabot Head Fm shales of the Clinton Group form the regional aquitard to the potable water
supplies that reside in the overlying Lockport Group carbonates north of Hamilton; the slightly younger Rochester Fm shales of the Clinton Group form the regional aquitard between Hamilton and Niagara Falls.
Delineation of preferred bedrock groundwater flow zones required regional outcrop mapping, combined with examination of &amp;gt; 100 bedrock/overburden cores and geophysical-logs. The cores were logged and sampled for whole rock, trace element, and select REEs and isotopes (C, O, Sr), and
conodont biostratigraphy over a five year period (2009 through 2014). Key cored holes across the study area also had video logs, variable duration packer pumping tests, FLUTe K-profiling, select Heat Pulse and optical-acoustic televiewer profiling, and select dye tracer tests. Many of the key cores
integrated in this study were collected in collaboration with municipalities and other partners that both rely on bedrock ground waters and/or are exploring for new resources to meet future population and industry pressures.
The position and continuity of groundwater flow zones identified with the geological sequence stratigraphic model are currently being corroborated with hydrochemical, geochemical and isotopic tracers (natural, non-injected) and hydrogeochemical modelling. Isotopic and hydrochemical results provide
new insight into recharge timing and chemical distinctions between groundwater flow zones. A comparison of Oxygen-18 and deuterium values for pre-freshet composite snow columns, collected along a north-south transect of the study area, show distinct differences between the isotopic-signature of
groundwater in the carbonates versus the isotopic composition of the snow - the groundwater isotopic signature resembles that of the local fall season precipitation. Tritium isotopes, redox sensitive parameters and vertical gradient information have assisted in the identification of some areas of
deep (~100 m) and rapid recharge. Hydrochemistry results suggest formation- level variability in major and trace elements, which are being used to trace flow zones.