Tier 3 Water Budget and Local Area Risk Assessment for the Greensville Groundwater Municipal System

Tier 2 Water Budget analysis completed under the Clean Water Act identified Middle Spencer Creek for further analysis at the Tier 3 Risk Assessment level. Earthfx was hired by the Halton–Hamilton Source Protection Region to complete assess the water quantity risks and drought sustainability the Greensville Municipal well. Major features for in the study area include the Niagara Escarpment, large quarry operations and growing urban development in the North Hamilton area.

To complete the Risk Assessment an integrated groundwater/surface water model was developed using the USGS GSFLOW model. The model covers a 165 km2 area and the subsurface is represented by 11 hydrostratigraphic layers. The model grid was refined around the municipal well, quarries, and in the shallow subsurface to represent all mapped streams, wetlands, reservoirs and quarry drainage infrastructure.

The municipal well is completed in the shallow weathered bedrock aquifer that is partially confined by both tills and less permeable bedrock formations to the west. Quarry operations have locally compromised the confining units and future developments will significantly deepen the operations. Significant effort was required to fully represent quarry operations and the complex drainage infrastructure that is used to manage the various inflows. The integrated model allowed the integrated analysis of both surface water and groundwater inputs, without many of the compromises that are needed when separate SW and GW analysis techniques are used.

The results of the study demonstrated the overall resilience of the Greensville municipal well to water quantity threats related to future land use change, increased competition from additional private residential wells, and the eventual build out of the quarry operations to the north. Under the transient 10-year drought conditions, the Greensville municipal well was still able to meet the water demands on the system, both under existing conditions and with the consideration of future quarry, land use change, and additional private well takings.

The resilience of the municipal well can be attributed to two key factors identified through the Tier 3 modelling study: the presence of a groundwater divide between the Greensville well and the quarry operations; and the hydraulic separation of the shallow production aquifer from the deeper Goat Island/Gasport aquifers by shale confining beds of the Vinemount member in the vicinity of the well. The simulations demonstrate that future quarry developments will create a large drawdown in the aquifers beneath the Municipal well aquifer, but the intervening confining layers should isolate the well from those impacts. The effect of flow across the Niagara Escarpment was simulated but did not appear to affect the well during drought conditions.

Overall, this study presented a unique challenge because of the need to consider both quarry engineering and wellfield scale issues within a larger watershed drought response framework. The integrated modelling approach proved essential to the representation of competing SW and GW water use. This study has significantly improved our understanding of the risks to the Greensville Municipal well.