Tier 1 Water Budget Model for the Conservation Halton Area

Earthfx completed multiple modelling assessments for Halton Region beginning with the development of models for the Milton area, which were used in the Halton Aquifer Management Plan (Holysh, 1995). This work continued and expanded with development of a geologic database for the Region (2004) and a regional groundwater flow model (2006).

Earthfx developed a distributed water balance model for a large area encompassing the watersheds in the Halton and Hamilton Source Protection Region. The model was based on the U.S. Geological Survey PRMS code. The water budget model computed components of the water balance (rainfall, interception, runoff, infiltration, evapotranspiration, groundwater recharge, and groundwater discharge to streams) on a daily basis for each subwatershed. Values were aggregated on a monthly and annual basis. Results of this model were used by Conservation Halton (CH) in preparing their Tier 1 Water Budget and Stress Assessment Report.

A refined groundwater model was developed for the South Halton area. The model area straddled the Niagara Escarpment, a prominent geologic feature that marks a sharp transition between two very different geologic environments: fractured bedrock aquifers dominate above the Escarpment while till plains and bedrock valley aquifer systems dominate below. The bedrock system above the Escarpment was characterized by Earthfx geologists in 2005 and many of the findings paralleled recent re-thinking of the geology and stratigraphy by the Ontario Geological Survey (OGS).

The groundwater model was calibrated to match observed water level data (MOE WWIS data along with other geotechnical and consultant wells, and average water levels from long-term monitoring sites). Another target for model calibration was estimates of annual average baseflow at the EC gauges. The calibrated model produced a detailed groundwater budget, simulated heads and flow rates in each model layer, and estimates of groundwater discharge to all streams in the study area.

Results of this model were used by CH in preparing their Tier 1 study and assessment report. Recharge estimates were subsequently used in the Georgetown/Acton, South Halton, and North Hamilton groundwater models. Detailed studies were conducted and a local scale model was developed for the Cedarvale wellfield in Georgetown (Earthfx, 2008). This model was refined and used for Source Water Protection Studies delineating wellhead protection areas (WHPA) and assessing wellfield vulnerability to contamination from groundwater and surface water sources in the Georgetown and Acton areas (Earthfx, 2010a). Similar studies were conducted for the wellfields in the north Hamilton area (Earthfx, 2010b).