Impact of the Nelson Aggregate quarry expansion on Lake Ontario

Nov 17

Written By Posted by Lake Ontario Waterkeeper

The proposed quarry would be located at the heart of Mount Nemo. Nelson’s property includes provincially significant wetlands and woodlands. Two species at risk (the Jefferson Salamander and the Butternut tree) have been documented on and adjacent to the Nelson property. The site is part of the Lake Ontario Watershed. Water resources that originate on Nelson’s property, or that are fed by groundwater and streams on Nelson’s property, feed and replenish Lake Ontario.

The Mount Nemo Plateau is the highest point of land in the area and is home to the headwaters of a number of creek systems, including the Mount Nemo Tributary West and East Branches, which flow into Grindstone Creek, and tributaries to Shoreacres Creek, Tuck Creek, Willoughby Creek (Bronte Creek), and Lowville Creek (Bronte Creek).

The proposed extraction would take place below the water table, which means that Nelson will be de-watering the quarry to keep it dry for excavation. The floor of the existing quarry will continue to be de-watered to allow it to be used for processing. A new quarry could capture groundwater, lower groundwater levels in the surrounding area and capture surface water run-off that would have contributed to watercourse downstream.

The existing Nelson quarry has been estimated to take 40% or more of the available annual water surplus (after evaporation) for the entire Milton Outlier. Adding a second quarry on the plateau could raise this amount to over 50%. The existing quarry already has significantly impacted flows to springs and headwater streams that feed Lake Ontario. The proposed new quarry would exacerbate these impacts.

The top layer of bedrock on the Milton Outlier is the Amabel Formation, a layer of dolostone that constitutes a valuable aquifer. The groundwater in the aquifer is replenished by rainwater. It then feeds at least 20 headwater streams along the slopes around the plateau. Some of the streams replenish ponds, others wetlands, and others feed into locally or regionally significant creeks. Wilf Ruland, Lake Ontario Waterkeeper’s hydrogeologist, suspects that many of these springs and streams have likely suffered reductions in flows due to the dewatering associated with the existing quarry.

All of the significant springs on the Milton Outlier are karst features. Karst features are formed when groundwater moves through limestone or dolostone bedrock, slowly dissolving the rock and increasing its permeability. As LOW’s hydrogeologist explains, “the Milton Outlier is significant and quite unique. Various tributaries of Bronte Creek, Grindstone Creek, and of several smaller creeks rise from karst springs along the slopes of the Outlier. They all have as their source the groundwater which has come from rainfall infiltrating into the plateau on top of the Outlier”.

Mount Nemo Plateau painting by Stewart Jones, courtesy of PERL