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Report: Aquifer Mapping and Monthly Groundwater Budget Analysis for Aquifers on Salt Spring Island

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The groundwater resources of Salt Spring Island were evaluated through a two-phase study to review and update of the provincial aquifer mapping and classification for the Island (Phase 1) and to develop monthly groundwater budget estimates for the updated aquifers (Phase 2).

Author:  Golder Associates; Gorski, N. G. and J. P. Sacre

Date Published:  Feb 2019

Report ID:  56660

Audience:  Government and Public

Salt Spring Island is the most populated of British Columbia's southern Gulf Islands. Water supply for its 10,000 residents and an estimated 10,000 annual visitors is derived entirely from the Island's footprint, using both surface water and groundwater sources. The Island's groundwater supply faces challenges related to variable yields and increased demand due to development pressure, climate change, and saltwater intrusion. To support the sustainable management of the Island's groundwater resources, a two-phase study was undertaken to characterize the groundwater aquifers on the island, and to develop preliminary aquifer water budgets of the aquifers. The purpose was to improve understanding of groundwater processes, availability and demand on a regional aquifer basis, and to identify data limitations and constraints for groundwater management. Aquifer mapping was carried out by integrating available geological mapping, information contained in WELLS database, and other publicly available information into a spatial database and then visualized using Leapfrog Hydro, a commercially available hydrogeological interpretation software. Over 2500 logs from wells were standardized and used to develop a simplified understanding of subsurface conditions. This information, in combination with the results of hydrogeological testing and Island-wide interpretation of water table elevations was used to delineate key aquifer units. Four bedrock aquifers were delineated based on the hydraulic properties of the differing rock types and formations, geological orientation and structure (folding, deformation and weathering), and an assessment of the influence of geological structures (i.e., faults, contacts) on groundwater movement. Four overburden aquifers of limited extent were delineated in areas where there was evidence of a reasonable thickness of unconsolidated permeable material, evidence (or likelihood) that the permeable material is saturated, and that the permeable material is of sufficient lateral extent to be considered an aquifer. Seasonal groundwater shortages in the dry season are also observed across the island due to local exhaustion of groundwater storage. As a result, groundwater availability and the likelihood of supply problems on Salt Spring Island are likely more sensitive to the overall duration of the dry period as opposed to the amount of precipitation received during that dry period. In areas where the unconsolidated aquifers have been mapped, most drillers chose to complete wells in the underlying bedrock aquifers rather than the overlying surficial material. This, combined with the highly variable nature of the surficial material and its glacial origin, suggests that the unconsolidated aquifers may not be particularly productive. The unconsolidated aquifers may provide a source of minor, local groundwater supply, but should not be targeted for future groundwater development. Salt water intrusion represents a real concern to groundwater availability in certain areas of the Island, which is already observed and managed in some coastal water supply systems. Groundwater resources in areas susceptible to sea water intrusion must be carefully managed, particularly in coastal wells or high yielding wells with good connectivity to the sea. The leapfrog file is view able with a free viewer through leapfrog.

Report Type
  Region - Vancouver Island
  Water Information - Groundwater

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