Groundwater Supply for the Gulf Islands in BC

Community discussions regarding local groundwater supplies can often be complicated by conflicting claims about the nature of groundwater and how human activities may or may not affect groundwater supplies. This is not surprising. None of us can see the underground groundwater system. What we know about groundwater comes by observation of wells andsprings. So groundwater remains a rather mysterious subject. 

For example, some Gulf Island residents believe that their groundwater flows underground from far away – either from Mt. Baker or Vancouver Island or the Mainland. Scientific evidence, however, would suggest something quite different – island groundwater is rainwater that has percolated down into the ground. In other words, each island produces its own groundwater. Is the “Mt. Baker” myth just a harmless tale? Perhaps it is not. A community that imagines its groundwater flowing up to their island from far away might not see a connection between land use practices on their island and the quality and quantity of their groundwater resource. 

The purpose of the Gulf Islands Waterscape poster is to clarify what we know about Gulf Island groundwater and the potential impacts on groundwater by human activities, based on our best available science.

The poster was developed by federal and provincial agencies in partnership with the Islands Trust, groundwater professionals, and Gulf Island community members. The agency partners are Natural Resources Canada (NRCan), Environment Canada, and BC Ministry of Water Land Air Protection. Support also came from the BC Groundwater Association, the Canadian Water Resources Association, and BC Ferries. Community consultation included two public design workshops attended by over 70 people, and ongoing participation from an advisory committee. The poster content is available on the internet at www.islandstrust.bc.ca (see link under “Stewardship Programs”) and can be ordered through the Geological Survey of Canada in Vancouver at (604) 666-0529 or gscvan@nrcan.gc.ca. Further information on groundwater is available at wlapwww.gov.bc.ca/wat/gws/gwis.html and www.bcgwa.org/education.htm

The Gulf Islands water poster is the second in a new water poster series by NRCan and partners on community water issues. The first poster was for Bowen Island (www.bowenisland.info/ waterscapes). The water poster series follows similar public education posters by Natural Resources Canada and partners on community geoscience issues (www.geoscape.nrcan.gc.ca) and regional climate change issues (www.adaptation.nrcan.gc.ca). Similar water posters are under development for the Bow River Basin (Calgary region) and the Okanagan Basin. 

So what did we consider the essential generalizations about groundwater on the Gulf Islands? All of these are illustrated on the poster. We list them below. 

#1 All our water comes from the sky.

Rainwater feeds the fresh groundwater zone under every island. 

#2 Groundwater and surface water are one connected, interdependent system.

Surface water feeds the groundwater system through percolation. Likewise, groundwater springs feed most streams. That’s why streams continue to flow long after the last rain. 

#3 We have a storage problem rather than a supply problem.

We get lots of winter rain. But most rainwater flows quickly to the sea in streams, or is lost to the atmosphere by evaporation and transpiration of forests. Only a small amount is stored in lakes and wetlands, percolates into groundwater storage, or is captured in roof-fed cisterns. Harvesting rainfall from roofs, a practice common in many dry climate communities, is an effective way for individual homeowners to deal with summer water shortages.

#4 A limited zone of fresh groundwater underlies each island.

The fresh groundwater zone is surrounded and underlain by salty groundwater. This zone of fresh water expands as it is fed by winter rains, and shrinks in the summer as water is lost through springs, forest plant use, and wells. 

#5 During our dry summers, we depend on stored winter water.

And our islands go dry just as our visitors arrive. So demand for water peaks just as our supplies dwindle. 

#6 Conservation is crucial. 

The easiest way to increase supply is to decrease demand. We need to shift from wasteful homes to conserver homes. 

#7 Nature needs water too!

Nature has specific water needs – stream riparian and wetland communities depend on a certain seasonal water supply. Excessive pumping of wells can reduce groundwater flow to streams and wetlands. These ecosystems provide us valued services – water filtration, water storage, wildlife and fish habitat, aesthetic values. 

#8. There are no underground lakes and streams.

Groundwater occurs within small cracks in rock, and small pores in gravel and sand. Water-filled fractures are very irregularly distributed underground. When nearby wells produce very different amounts of water, this likely reflects the different number and connection of water-bearing cracks intersected by each well. 

#9 An island’s groundwater system is a shared resource.

Excessive pumping of a well or wells can cause the water table to fall excessively, causing nearby wells or streams to go dry. 

#10 Saltwater intrusion

It can occur where excessive pumping of fresh groundwater from wells causes the boundary between fresh and salty groundwater to move inland or upwards, allowing salty groundwater into wells. 

#11 By protecting our land, we protect our water. 

Each water-supply lake, reservoir, or stream has an area of land referred to as its watershed area. The water supply is fed by surface runoff from this watershed. By protecting the land of the watershed area, we protect the water supply. Likewise, each groundwater well has a capture zone. Within the capture zone area, surface waters that percolate down into the groundwater system may enter the well. So by protecting the capture area, the well waters are protected. A groundwater professional may be required to determine the capture zone of a particular well. 

#12 There are many ways to increase storage.

For example, protecting island forest lands maintains the recharge of surface waters to island aquifers. Roadditch water can be directed to infiltration pits that recharge groundwater, rather than directing ditch water to streams that flow to the sea. Increasingly popular are households that harvest their roof-top water into cisterns. This supply is available to any island household! 

#13 Keeping surface water clean is just common sense. 

Best practices exist for almost all human activities. These “best practices” reduce the risk of contamination to surface and groundwaters. For example, livestock should be fenced out of streams, runoff from road and site construction should have silt traps, pesticide and herbicide use should be reduced or eliminated, and heating oil tanks should be above ground and include containment. 

#14 Groundwater is often contaminated by poor well-head protection.

If a septic field becomes plugged, septic effluent can rise to the surface and flow overland. If it encounters an unsealed well, it can flow down the well and enter the groundwater supply. 

#15 There are safe wells and unsafe wells.

A safe well has a well cap, is sealed by grout, and protrudes from a mound so that surface waters flow away from the well head. 

***

Bob and Murray are scientists, and Shannon is a GIS specialist, with Natural Resources Canada in Vancouver. Richard is a graphic artist in North Saanich, B.C.

[From WS January/February 2005]

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