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Report: Chemainus and Bonsall Watersheds: Preliminary Desktop Assessment of Hydraulic Connection

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The report summarizes the likelihood of hydraulic connection between groundwater and surface water in the Chemainus and Bonsall watersheds. Results identify stream reaches where hydraulic connection between surface water and groundwater is more likely and delineates groundwater management areas associated with those major connected stream reaches.

Author:  Tim Sivak and Mike Wei

Date Published:  Jan 2022

Report ID:  59390

Audience:  Government and Public

The Chemainus River and Bonsall Creek on the east coast of Vancouver Island have been identified as vulnerable to low flows during the dry season, which may affect the habitat and survival of aquatic life. Determining the likelihood of hydraulic connection of wells to streams in the Chemainus and Bonsall watersheds is vital to effective management of environmental flows and access to water for users. This desktop study identified stream reaches within the Chemainus and Bonsall watersheds where hydraulic connection with underlying aquifers and depletion of streamflow from pumping from those aquifers is most likely to occur. These connected stream reaches were identified by mapping the likely locations of a vadose zone and low--permeability sediments (i.e., till, silt, or clay) underneath streams using available well records, digital stream and topographic elevation data, as well as from published geological mapping. Preliminary groundwater management areas, based mostly on groundwater elevation mapping, were delineated in the study area for the Chemainus River and Bonsall Creek for both the unconsolidated and bedrock aquifers. Groundwater management areas identify the portion of the unconsolidated and bedrock aquifers that potentially contributes to streamflow depletion from well pumping upstream and upgradient of specific points along Chemainus River and Bonsall Creek and serves as a tool to inform allocation of water and management of groundwater use within the study area.

Report Type
  Region - Vancouver Island
  Watershed Groups - 920 - Vancouver Island (East) Rivers
  Water Information - Groundwater
  Water Information - Water Management

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