Climate Change: Implications for Groundwater Recharge and Saltwater Intrusion on the Gulf Islands Diana M. Allen Department of Earth Sciences Groundwater Resources Research Group Simon Fraser University SFU students: Dan C. Mackie Megan J. Surrette Emmanuel K. Appaih-Adjei Department of Earth Sciences
Outline Groundwater and climate a review of the basics with a focus on the Gulf Islands Estimating groundwater recharge Climate change models and regional forecasts Potential 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 Year to year Seasonal variations Snowmelt recharge is dominant in most parts of Canada
Climate of the Gulf Islands 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).
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 Hydrograph Hydrograph of Observation Well No. 290 Saturna Island, B.C. Water level (metres below ground level) -0.5 0.0 0.5 1.0 1.5 2.0 Water level based on month-end readings 2.5 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 Date 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 Hydrograph Hydrograph of Observation Well No. 290 Saturna Island, B.C. Water level (metres below ground level) -0.5 0.0 0.5 1.0 1.5 2.0 Water level based on month-end readings 2.5 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 Date 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 the Gulf Islands. Collect fracture data Generate fracture distributions using FRACMAN Determine K Z for fractured bedrock Map Recharge Zones Model Recharge for each Recharge Zone
Study Area Vancouver Vancouver Island
Interbedded Sandstones / Mudstones Sandstones 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 = 9.5 x 10-8 m/s Kz = 1.1 x 10-7 m/s Kz = 5.7 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 Spatially-distributed recharge zones (48) were mapped in GIS Soil Class Recharge Zones
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
January 0 mm/month 0.1-6 6-12 12-22 22-65 65-116 116-126 >126 July
Climate Change Models and Regional Forecasts
Increasing spatial resolution of global climate models (GCMs) since 1990 IPCC (2007) 4 th Assessment Report gives the most up-to-date climate change model results Source: IPCC 4 th Assessment Report, 2007
Observed temperature and difference between model and observed Observed (top) and modeled (bottom) precipitation Source: IPCC 4 th Assessment Report, 2007
Global Climate Models Source: IPCC 4 th Assessment Report, 2007
Scenarios and Model Uncertainty
Multi-Model Global Predictions Source: IPCC 4 th Assessment Report, 2007 Changes 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 4 th Assessment Report, 2007
What has been observed so far this past century?
Climate Change Projections for the Pacific Northwest Changes in Annual Mean Temperature Precipitation 2020s 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% 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 Climate Results 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 Map Low susceptibility Figure courtesy of Geological Survey of Canada
Conclusions Recharge Recharge to the Gulf Islands is local, and undergoes variability due to PDO and ENSO cycles Permeability Based 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.