丁香园AV

Globally, coastal areas support the largest populations, thus the demand for freshwater is high. In many areas around the world, and indeed in Canada, groundwater is the primary source of freshwater (e.g. Prince Edward Island is 100% reliant on groundwater, and the Gulf Islands in BC rely ~80% on groundwater). In coastal aquifers, there is a sensitive balance between the amount of recharge to the aquifers and the position of the saltwater-freshwater interface. Various factors can lead to the interface moving inland and threatening groundwater quality through salinization. Consequently, coastal aquifers are potentially vulnerable to salinization. Bedrock aquifers, in particular, present challenges for groundwater management due to heterogeneities imparted by fractures, which can lead to uncertainties in how the groundwater system will respond to change. Given uncertainties in climate change, population growth, and the hydrogeology, risk assessment methods are ideally suited to informing decision making concerning groundwater resources in coastal aquifers.

Purpose of Study

The aim of this project was to develop a risk assessment methodology suitable for coastal aquifers that is both straightforward and standardized, and based on common risk assessment principles. A risk framework, spatial risk maps and the set of indicators that can be used to establish guidelines for drilling, monitoring and well operation (groundwater development and use) were developed. Ultimately, these deliverables can be used as tools for land use planning, direct monitoring efforts, and to build community preparedness (i.e. risk management).

The Study Area

The Gulf Islands are a group of 40+ islands situated within the Strait of Georgia east of Vancouver Island, in southwestern British Columbia. The islands range in area from ~1-75 sq.km, range in elevation from roughly 100-200m, and are characterized by a generally northwest-southeast trend and elongation defined by linear ridges and valleys. Coastlines are typically rocky, with either long expanses of low relief bedrock sloping shallowly into the ocean or, alternatively, steep cliffs and narrow rocky beaches. Groundwater for domestic supply is derived from the sandstone- or mudstone-dominant formations comprising the Upper Cretaceous Nanaimo Group, although Salt Spring Island also has older volcanic rocks. Structurally, the outer Gulf Islands are characterized by gentle folds with bedding that dips in the range of 5-15 degrees, with numerous small- and large-scale discrete fractures and faults. Structural elements, such as fracture zones and faults, and fractures associated with folds, offer secondary permeability in an otherwise very low permeability matrix.

Hydrogeological Characterization

The hydrogeological characterization of the Gulf Islands' aquifers builds on our previous research. Specific to this study we used Salt Spring Island as the pilot study area for developing a spatial risk assessment framework. Hydrogeological characterization included an assessment of:

1. The Geological Framework
2. The Aquifer Hydraulic Properties
3. The Groundwater Chemistry
4. Numerical Modeling

Salt Spring Island Report : Larocque, I., Allen, D.M., Kirste, D. (2015). The Hydrogeology of Salt Spring Island: A summary of research conducted by Simon Fraser University as part of a project “Risk Assessment Framework for Coastal Bedrock Aquifers”. Department of Earth Sciences, 丁香园AV, Burnaby, BC, 62 pp.

The Geological Framework

The fractured rock aquifers of the Gulf Islands are broadly comprised of sandstone and mudstone, with some volcanics (on Salt Spring Island).

The geological map of Salt Spring Island was prepared by Greenwood and Mihalynuk (2009).

Bedrock chip samples were logged during drilling of the research monitoring well.

Consulting the Bedrock Geology Map

Logging Chip Samples During Drilling

The Aquifer Hydraulic Properties

The aquifer properties for the Gulf Islands were assessed using 129 constant discharge pumping tests and 29 tidal response tests. Transmissivity values range from 10-4 to 10-5 m2/s. The results show remarkable consistency between test type and rock type.

Measuring Water Levels During a Pumping Test

Installing a Transducer in the Ocean for Tidal Response Testing

Tidal Response Data

The Groundwater Chemistry

The groundwater chemical evolution was previously described for Saturna Island by Allen and Suchy (2001). This study has combined all the groundwater chemistry data for the Gulf Islands and confirmed a similar overall chemical composition for the other Gulf Islands.

High salinities, associated with mixing with salt water at depth, were identified in over 100 wells across the islands. Most wells, however, are relatively fresh. Cation exchange is a dominant process on the Gulf Islands, which leads to high sodium concentrations in most wells.

Water Chemistry Sampling

Piper Plot for the Gulf Islands

Klassen, J. and Allen, D.M. (2015) Risk of Saltwater Intrusion in Coastal Bedrock Aquifers: Gulf Islands, BC. Final report submitted to BC Ministry of Forests, Lands and Natural Resource Operations, BC Ministry of Environment, and Islands Trust, 40 pp.

Aquifer Susceptibility

Aquifer vulnerability maps for the Gulf Islands were generated in previous research (Denny et al., 2007). However, these aquifer vulnerability maps focus on the susceptibility of the aquifer to chemical hazards associated with different activites at surface that can lead to the vertical migration of contraminants downward to the water table. (e.g. pesticides, chemical spills).

For coastal aquifers, the susceptibility of the aquifer to saltwater intrusion should be assessed considering the multiple directions (from the ocean side, from underneath due to upconing, and from overtop due to storm surge). This requires a different conceptualization of aquifer susceptibility. This project developed an approach for mapping aquifer susceptibility that can be used to assess risk of saltwater intrusion in coastal aquifer settings.

The Hazards:

1. Coastal Hazards (Sea Level Rise and Storm Surge) - coastal hazards were only evaluated on Salt Spring Island because high resolution Lidar data were available.
2. Pumping Hazards - pumping hazard was assessed based on well density on all Gulf Islands.

Coastal Hazards on Salt Spring Island

Well Density throughout the Gulf Islands

Vulnerabiltiy:

Vulnerability was assessed in two ways:

1. by combining pumping hazards and susceptibility (for all Gulf Islands). Areas with high EC in wells tend to also have a high vulnerabilty rating, while areas with low EC tend to have a low vulnerability rating.

2. by combining coastal hazards, pumping hazards and susceptibility (for Salt Spring Island only)

Electrical Conductivity (EC) of groundwater compared to Pumping Vulnerability assessed on the basis of Well Density.

For Salt Spring Island, coastal hazards were combined with pumping hazards to assess Total Vulnerability.

Risk

Risk was evaluated by combining Loss (the consequence of losing a groundwater supply due to salinization) with the Pumping Vulnerability.

The final Risk map is highly dependent on how Loss is evaluated. Alternative approaches for evaluating Loss will lead to different results.

Aquifer Response to Stressors

Numerical models can be used to understand hydrogeological processes. In this study, fresh water models and density-dependent flow and transport models were generated for the Swan Point area of Salt Spring Island. The fresh water models treated the ocean boundary as a seepage face, and the steady-state model was calibrated to observed heads using representative values for recharge and hydraulic conductivity. A transient fresh water model was also run to simulate the tidal response in a coastal well, so as to further constrain the aquifer properties.

SEAWAT models were then developed to simulate the freshwater-saltwater lens position, which was near vertical at the study site location due to the steep local topography. Under climate change condition, sea level rise and a small predicted increase in recharge of 1.5% had little impact on the salinity distribution.

Future research will continue to develop and calibrate SEAWAT models for the other Gulf Islands using refined estimates of recharge.

Collaboration and Funding

This project benefited from engagement with the Salt Spring Island Water Council.

The project partners include 丁香园AV and BC Ministry of Environment, and BC Ministry of Forests, Lands and Natural Resource Operations.

Project funding was by a grant from Natural Resources Canada, and from the Pacific Institute for Climate Solutions.

Presentation to Salt Spring Island Water Council July 2013

Group Discussion on Historical Storm Surge