Climate Change: Implications for Groundwater Recharge and Saltwater Intrusion on the Gulf Islands

Diana M. AllenDepartment of Earth SciencesGroundwater Resources Research Group Simon Fraser University

SFU students:Dan C. MackieMegan J. SurretteEmmanuel K. Appaih-Adjei

Department of Earth Sciences

Outline

Groundwater and climate – a review of the basics with a focus on the Gulf IslandsEstimating groundwater rechargeClimate change models and regional forecastsPotential impacts of climate change on groundwater resources of the Gulf Islands

Groundwater is an important component of the hydrologic cycle

Processes at the Land Surface

Climate Influences on Recharge• Changes to recharge rates are determined by spatial and

temporal changes in climatic factors and their interactions with surface and shallow subsurface conditions.

• Key climatic factors in determining recharge:

Variation in amount Timing Form

**** * **

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Year to year

Snowmelt recharge isdominant in most parts of Canada

Seasonal variations

Temperature• Cool, dry summers and humid, mild winters. • Mean monthly temperature ranges from:

– 3.66°C to 4.23°C from November to January– 16.98°C to 18.39°C from June to August.

Precipitation• Mean annual precipitation ranges from 658mm to

983mm. Most falls as rain.– On average, the lowest monthly precipitation occurs in

July (~23mm), and the maximum in November (~143mm).

Climate of the Gulf Islands

Average climate for all Gulf Islands Climate Stations is well represented by Victoria International Airport

Aquifer Responses

• All recharge to the groundwater system in the Gulf Islands comes from precipitation.

• Most recharge occurs in the late fall and winter months. • Aquifers respond to recharge (and discharge) cycles by

changing water levels. • We commonly show these variations on a well

hydrograph – BC Observation Well Network.• The magnitude of the fluctuations and the time lag

between precipitation event(s) and the aquifer response is determined by a number of factors.

Observation Well HydrographHydrograph of Observation Well No. 290 Saturna Island, B.C.

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0.0

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1.0

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2.51984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002

Date

Wat

er le

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Water level based on month-end readings

• At this observation well, we observe cyclicity in the data, indicating that the aquifer is recharged on an annual basis with a responding change in groundwater levels.

Average Annual Hydrograph

Long term climate variations

• The Pacific region is dominated by variations in precipitation over longer time scales (decades)

• These are the result of the PDO (Pacific Decadal Oscillation) and the ENSO (El Nino Southern Oscillation).

Observation Well HydrographHydrograph of Observation Well No. 290 Saturna Island, B.C.

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1.0

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2.51984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002

Date

Wat

er le

vel (

met

res

belo

w

grou

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Water level based on month-end readings

• Longer term cycles are evident in the historic record

Trends in groundwater level must be examined keeping in mind these variations.

Coastal Aquifers

• In coastal aquifers, there is a sensitive balance between the amount of recharge and the position of the saltwater interface.– A decrease in recharge will result in

encroachment of the interface.– A rise in sea level will also result in the

interface moving inland. – Increased pumping can also cause this

interface to shift.

So, can we predict recharge and possibly model how

recharge might vary under future climate conditions?

Research Objectives

To quantify the vertical hydraulic conductivity of the bedrock on the Gulf Islands using a fracture flow model

To determine spatially-distributed recharge for theGulf Islands.

Determine KZ for fractured bedrock

Map Recharge Zones

Model Recharge for each Recharge Zone

Collect fracture data

Generatefracture distributions

using FRACMAN

Study Area

Vancouver

VancouverIsland

Sandstones

Interbedded Sandstones / Mudstones

Fault / Fracture Zones

Figure courtesy of Geological Survey of Canada

Conceptual Model: Hydrostructural Domains

Fracture Data Collection

Fracture Data

4 Field Seasons 8 Islands>200 Outcrops>10,000 Fractures

Statistical Analysis

We undertook a statistical analysis to determine if the fracture characteristics within each hydrostructural domain were statistically similar to each other.

What we found…

Fractures within the same hydrostructural domain tend to have similar fracture characteristics, that are statistically different from those in other hydrostructural domains.

The next step was then to determine the permeability of each hydrostructural domain.

Potential Permeability

Kz = 1.1 x 10-7 m/s

Kz = 5.7 x 10-8 m/s

Kz = 9.5 x 10-8 m/s

Implications for Recharge

• The bedrock is variably fractured, and the IBMS-SS and FZ domains have higher permeability than the LFSS domain.

• This suggests that zones of high fracture intensity with sub-vertical joints and fault zones may be primary sites for recharge.

• To model recharge, the climate must be considered, as well as spatial variations in aquifer permeability, soil permeability, slope, and water table depth.

Spatial Variations in Aquifer Properties

• In order to determine how recharge might vary spatially, we considered a number of different spatial datasets:

• Soils• Aquifer media• Water table depth• Slope• Vegetation

Water Table Class Aquifer Class

Soil Class Recharge Zones

Spatially-distributed recharge zones (48) were mapped in GIS

Recharge Modeling

• USEPA HELP• Each column

consisted of two layers, with the soil layer overlying the aquifer media. The water table formed the base of the column.

Recharge Results

Modelled Range (mm)for different geologic media

% of Precip

PRECIPITATION 880 mm

RUNOFF 0-200 (HELP) 5%-35%

EVAPOTRANSP 385-500 49%

RECHARGE 170-500 45%-15.6%

Values remain highly uncertain

0 mm/month0.1-66-1212-2222-6565-116116-126>126

January

July

Climate Change Modelsand

Regional Forecasts

Increasing spatial resolutionof global climate models (GCMs) since 1990

IPCC (2007) 4th Assessment Report gives the most up-to-date climate change model results

Source: IPCC 4th Assessment Report, 2007

Source: IPCC 4th Assessment Report, 2007

Observed (top) and modeled (bottom) precipitation

Observed temperature and difference between model and observed

Global Climate Models

Source: IPCC 4th Assessment Report, 2007

Scenarios and Model Uncertainty

Multi-Model Global Predictions

Source: IPCC 4th Assessment Report, 2007Changes are annual means for the SRES A1B scenario (mid-line) for the period 2080 to 2099 relative to 1980 to 1999.

Predicted Changes for North America

Source: IPCC 4th Assessment Report, 2007

What has been observed so far this past century?

Changes in Annual Mean

Temperature Precipitation2020s

Low + 1.1ºF (0.6ºC) -9%

Average + 2.2ºF (1.2ºC) +1%

High + 3.4ºF (1.9ºC) +12%

2040s

Low + 1.6ºF (0.9ºC) -11%

Average + 3.5ºF (2.0ºC) +2%

High + 5.2ºF (2.9ºC) +12%

2080s

Low + 2.8ºF (1.6ºC) -10%

Average + 5.9ºF (3.3ºC) +4%

High + 9.7ºF (5.4ºC) +20%

Climate Change Projections for the Pacific Northwest

Source: Climate Change Group, University of Washington

Annual precipitation changes predicted by 5 different downscaled GCMs for the Gulf Islands Region

Recharge Predictions under Future Climate Change

• We started with the current recharge for each month and then ran computer simulations again using climate data predicted from a GCM

• We used only one GCM, however (CGCM1)

• Continued research will explore ranges of predictions.

Recharge Under Future ClimateResults from GCCM1

Sea Level Change

• Global sea level is predicted by rise anywhere from a few cm to close to 1 m.

• However, uplift in the Georgia Basin region due to subduction of the Juan de Fuca plate is causing uplift.

• So, the relative change in sea level in future in this region is uncertain.

Implications of Sea Level Rise on Saltwater Intrusion

• Difficult to say at this time, but likely only a small shift in the position of the interface.

• Low lying areas are particularly at risk, not only of seawater intrusion, but of land inundation.

• The greater risk is groundwater extraction due to pumping, particularly along the coast.

Aquifer Vulnerability MapLow

susceptibility

Figure courtesy of Geological Survey of Canada

RechargeRecharge to the Gulf Islands is local, and undergoes variability due to PDO and ENSO cycles

PermeabilityBased DFN modeling, the IBMS-SS and FZ domains are more permeable than the LFSS, suggesting they are likely the dominate water bearing aquifers on the southern Gulf Islands. Estimates of vertical permeability were used as input to a hydrologic model to estimate groundwater recharge

Conclusions

Recharge Modeling• Highest recharge on the islands is in December, whereas the

lowest rates occur between July and October, which is consistent with observed data.

• Spatially distributed mean annual recharge to the Gulf Islands was estimated to be in the range of 184 to 537 mm/year, but these estimates are highly uncertain and more research is needed.

Conclusions

Climate Change Impacts• Global climate models provide regional forecasts for shifts in

temperature and precipitation. Temperature is expected to increase, and precipitation may increase slightly, particularly during the winter months.

• Recharge to the Gulf Islands will likely not change by very much.

• Sea level may rise on the order of 0.5m, and possibly as high as 1m over the next century.

• This shift in sea level may result in some coastal wells that are already sensitive to salinity becoming more saline as the interface shifts slightly inland; however, more research is needed to better constrain this prediction.

Conclusions