SSI Water Council Society, in partnership with Islands Trust, supported the thesis work of Simon Fraser University graduate student Isabelle Larocque focussing on “The Hydrogeology of Salt Spring Island” under the supervision of Dr. Diana Allen.
We are pleased to invite you to attend a public meeting of the Water Council Society for an account of finished and future work presented by Professor Allen and her research team.
When: at 2 pm on January 27, 2015
Where: at the Fritz Theatre.
We hope to see you there. This meeting is open to the public, so please feel free to circulate this information.
Link to: Thesis summary by Dr. Diana Allen (PDF)
Isabelle Larocque’s 255 page thesis, with 34 tables and 85 figures, focusses on assessing the sensitivity of our island coastal aquifer to stressors that include sea level rise, changes in precipitation, and development related to pumping from wells. This work is a continuation of research by Dr. Allen into the groundwater hydrology of the Gulf Islands. The research gives us new quantitative data on which to base planning for future demands on our water resource. Findings from the thesis work include:
- The aquifer on Salt Spring Island consists of a network of fractures in sedimentary rocks of the Nanaimo Group in the north part of the island and fractures in older granitic, volcanic, and sedimentary rocks in the south part of the island.
- Recharge of the groundwater is entirely through precipitation and the chemistry of the water is the result of chemical interaction between the rocks and the water. Ion exchange reactions involving sodium and calcium dominate in the Nanaimo rocks, but not in the older rocks of the south end.
- Discharge of the precipitation is achieved by surface runoff, evaporation, subsurface flow of groundwater to the sea, and by pumping. The overall distribution of groundwater flow is outlined from an analysis of water table data from wells and shows island-scale groundwater and flow directions. These are controlled mainly by topography. Future studies of the useable quantity of water on the island will be aided by this island-scale analysis.
- Pump tests on Salt Spring Island are analysed, including results from other Gulf Islands and combined with analysis of well responses to tidal fluctuations to determine representative hydraulic properties of the aquifer. Average hydraulic properties are summarised and it is interesting that the properties on Salt Spring Island are similar to those on the other islands and that they are not widely different for the different rock types. This suggests that properties derived for Salt Spring Island may be of wide application and that future hydrologic modelling will be a promising avenue to pursue. The tables of data constitute a valuable resource for future work.
- The effect of tidal variation on groundwater flow was studied using the Swan Point area near Booth Canal and a combination of numerical modelling and calibration from existing well data. An interesting and unexpected result has been that the salt-fresh interface is steep in this area of high topographic relief but that strong pumping may result in salt intrusion. Areas of low topographic relief may still have a shallow saltwater lens at depth.
In summary, the research provides solid new data on the hydrologic properties of Salt Spring Island aquifers, it provides an island-wide model of the general behaviour of the island’s groundwater, and assesses the potential future effects of sea level rise. The fresh-salt interface is shown to be steep in areas of high topographic relief and is expected to be shallow in areas of low relief.
The Water Council Society is grateful to the LTC for their support of this research and particularly grateful to Professor Allen for her continuing work in the Gulf Islands and on Salt Spring Island particularly.