Water Stewardship Information Sources
| Citation | Waterline Resources Inc. 2013. Aquifer mapping study, Town of Gibsons, British Columbia. Prepared for Town of Gibsons. |
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| Organization | Town of Gibsons |
| URL | http://www.gibsons.ca/include/get.php?nodeid=216 |
| Abstract/Description or Keywords | The Town of Gibsons is located at the southern part of the Sunshine Coast, British Columbia and a short ferry ride from the City of Vancouver. It has a population of about 4400 residents. Approximately 73% of its residents are supplied groundwater from a confined sand and gravel beneath the Town. The groundwater supply is not chlorinated prior to distribution. The remaining one-third of Gibsons residents are supplied with treated surface water by the Sunshine Coast Regional District. As part of its long-term plan to protect the aquifer, the Town commissioned an aquifer mapping study in order gain a better understanding of aquifer boundaries, hydraulic properties, and recharge and discharge to and from the aquifer. The intent of the study was to expand on previous hydrogeological studies and develop a conceptual model of the aquifer to help identify flow characteristics and assess the spatial and temporal variations of key aquifer elements. Waterline Resources Inc was retained to complete the aquifer mapping project. Phase 1 included data compilation, field surveys, and drilling and well installation. Phase 2 involved the University of British Columbia who completed an environmental tracer study and computer modelling. A community outreach program was also completed by Gordon Groundwater Consultancy of Gibsons under contract to Waterline. In order to evaluate the aquifer, Waterline developed an ARC-GIS geodatabase and established a monitoring well network that included existing and new wells within the Town boundary and adjacent SCRD lands. The monitoring well network was instrumented with data loggers to assess water level changes over time. The Town wells exhibit flowing artesian conditions in Lower Gibsons. The primary goal of monitoring the aquifer was to assess aquifer response to climate, creek runoff, barometric and tidal fluctuations, and human activities such as pumping and diversion of groundwater. The Gibsons Aquifer is composed of a sand and gravel deposit that extends from beneath Gibsons Harbour to the base of Mt. Elphinstone. The aquifer is capped with a low permeability basal Capilano/Vashon till deposits that creates a confining layer over the aquifer. Although the aquitard is extensive, it is suspected to be discontinuous in creek valleys or beneath Upper Gibsons where younger Capilano deposits may have eroded through the aquitard unit. These areas are believed to form “recharge windows” to the underlying Gibsons Aquifer. Groundwater level monitoring suggests that water levels in the aquifer are stable, indicating that recharge to the aquifer is likely greater than discharge from the aquifer. Estimates of groundwater flow within the Gibsons Aquifer indicate an average linear groundwater velocity of about 400 m/year. Based on these flow estimates, the time for groundwater to travel from the recharge area near the base of Mt. Elphinstone to the discharge area in Lower Gibsons is approximately 9 years. This is consistent with water chemistry data that also suggests a short residence time for groundwater to travel from Upper to Lower Gibsons. Groundwater chemistry indicates that the Gibsons Aquifer is dominantly sodium-calcium-magnesium bicarbonate type water with a neutral pH and low TDS. Dissolved chloride and fluoride concentrations are elevated in samples collected from wells completed in the unconfined Capilano Aquifer near the Aquatic Centre. It is unclear if the elevated chloride/fluoride is related to operations at the Aquatic Center or the pool facility located at 913 Gibsons Way. Aluminum concentrations in groundwater samples collected from WL10-01, WL10-02, MW06-01A, MW06-1B and MW06-2B exceed the Canadian Drinking Water Quality Guideline. Aluminum, iron and manganese likely represent trace metal constituents present in suspended sediment particles. Groundwater samples collected from the Town wells 1, 3 and 4 contained low concentrations of total aluminum, iron and manganese. Arsenic concentrations have increased over the last three sample events in groundwater from WL10-01, completed near Upper Gibsons, deep in the aquifer near the overburden-bedrock interface. In 2011 and 2012, arsenic concentrations exceeded the Canadian Drinking Water Guideline. The elevated arsenic likely originates from arsenopyrite minerals present in the bedrock. As the Town wells are completed near the top of the aquifer and not near the bedrock interface, no significant risk to the water quality is anticipated to the Town’s water supply. The drilling of deep supply wells near the bedrock contact should be avoided. Arsenic levels in groundwater should continue to be monitored. A groundwater tracer study was completed by the University of British Columbia to assess recharge to the Gibson Aquifer. Tritium, noble gases, oxygen and deuterium stable isotopes, and tritium/helium tracers were assessed. In general, the data suggests that most samples consist of a mixture of sub-modern (older) and modern (more recent) water. The deepest wells completed in the Gibsons Aquifer indicated the oldest groundwater age, coolest recharge temperatures, and the highest estimated recharge elevation. Samples from the shallow unconfined Capilano Aquifer exhibit the youngest apparent ages, the warmest recharge temperatures and the lowest estimated recharge elevation. Although analysis of chlorofluorocarbons (CFCs) in groundwater had limited value in the groundwater tracer study, it was noted that Town Wells 2 and 3 contained trace concentrations of CFC (12) but elevated 25 to 350 times background levels. The CFC (12) concentrations in the Gibsons Aquifer may be related to sewage effluent or to a refrigerant source. It should be noted that the Town’s new sewage treatment plant is located only 180 m up gradient of Town Wells 2 and 3. Further investigation is required to determine the source of the trace levels of CFCs in groundwater. Three significant aquifer recharge mechanisms have been recognized for the Gibsons Aquifer. These include mountain block recharge, creek recharge, and recharge windows through the Capilano Alluvium. The environmental tracer study indicates that the Mt. Elphinstone mountain block potentially contributes 55% of the recharge to the Gibsons Aquifer. This recharge estimate will vary with climate and anticipated climate change thought to be caused by global warming. However, climate change predictions and potential effects on aquifer recharge are somewhat uncertain and can only be quantified by long-term monitoring trends and assessment of cause and effect response in the aquifer. For the purposes of the predictive analysis, only the worst case climate change scenarios were considered in the model simulations completed by UBC where less recharge to the aquifer is assumed, and the sea level is assumed to rise to an extreme case of about one meter above the current level. Computer model simulations indicate that the Gibsons Aquifer should be able to meet future demand where the Town is anticipated to grow to 10,000 residents. This assumes that 73% of the population obtain water from Town wells. This is also true under worst case climate change conditions although a new well(s) will eventually need to be drilled some distance from the coast to meet the additional demand for water The following table summarizes the study recommendations and approximate implementation timeline. |
| Information Type | report, mapping |
| Regional Watershed | Howe Sound & Sunshine Coast |
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| Comments | |
| Project status | complete |
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