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Thermal Impact Assessment of Below-Water-Table Aggregate Extraction
Dirk Kassenaar E.J. Wexler, Mike Takeda, Pete Thompson
CWRA Conference May 25, 2016
Thermal Impacts of Development
Most water management issues include both SW and GW issues
▪ Earthfx specializes in fully-integrated SW/GW modelling
▪ Other talks: Tomorrow I will be presenting watershed scale integrated analysis of:
• Sustainability of water use in the oilsands
• Integrated simulation of moisture-demand based agricultural water use
▪ This talk: Demonstrate what can be done at a local, high resolution scale
Simulating thermal effects of land development on streams and wetlands is natural extension of integrated SW/GW flow modelling
▪ Many ecosystem functions (fisheries) are temperature sensitive
▪ We can simulate the routing of water through both domains, but can we reliably route the heat?
Thermal Impact Modelling of Hillsburgh Pit 2
Project Setting and “Challenges”:
Our client: Gravel Pit Operator
▪ Gravel pit already licenced to extract aggregate from below water table.
▪ Pit extraction paused a few years ago after “some concerns” identified at the site
Three site concerns and challenges: ▪ Hillsburgh Meltwater Channel ANSI (Area of Natural Scientific Interest)
• A large outwash glacial spillway which forms a lowland valley occupied by the south flowing Credit River (Erin Branch)
▪ Alton-Hillsburgh Wetland Complex ESA (Environmentally Significant Area)
• Significant wetland protected under Credit Valley Conservation Authority (CVC) policy.
Thermal Impact Modelling of Hillsburgh Pit 3
Site Location:
Thermal Impact Modelling of Hillsburgh Pit 4
Pit
Aggregate Operations
Thermal Impact Modelling of Hillsburgh Pit 5
Pit
Final “Challenge”: Caledon Mountain Trout Club
Caledon Mountain Trout Club (CMTC) operates a hatchery immediately south of the gravel pit
▪ Private, exclusive fish farm founded in 1901
Thermal Impact Modelling of Hillsburgh Pit 6
Trout Hatchery
Project Objectives:
Evaluate whether additional aggregate extraction below the water table will impact the trout hatchery and ecologically significant wetland complex.
Approach: Integrated simulation of SW+GW+Heat transport from the pit, through the groundwater system, and into the wetland
Thermal Impact Modelling of Hillsburgh Pit 7
Assessment Steps:
Develop model of existing site and pond configuration
▪ 3-D Geologic model development: Represent aquifer layering and till barrier
▪ Surface water model development: Estimate GW recharge
▪ Groundwater flow model development: Estimate steady-state average flow
▪ Transient thermal modelling:
• Variable pond temperature input - based on measured pond temperature
• Variable GW recharge temperature input – based on monthly seasonal estimate
Calibrate to flow and temperature observations
Assessment: Simulate effects of enlarged pond
Thermal Impact Modelling of Hillsburgh Pit 8
Site Conceptualization
Thermal Impact Modelling of Hillsburgh Pit 9
Glaciofluvial/lacustrine sediments of Orangeville Moraine overlie drumlinized Catfish Creek Till.
Hillsburgh Meltwater Channel cut into moraine east of the site.
Conceptual Cross-Section
A
A’
A A’
Site Conceptualization: Till Barrier
Thermal Impact Modelling of Hillsburgh Pit 10
Drilling and mining operations indicate a possible buried drumlin or “till barrier” located along the south-eastern portion of the site ▪ Likely truncated by meltwater channel formation
Water levels appear to decline by 6 m across the barrier
The barrier likely has a significant impact on both groundwater flow patterns and off-site impacts
Site Monitoring Data
Thermal Impact Modelling of Hillsburgh Pit 11
Static and transient groundwater levels from 33 site monitors (existing and historical).
Temperature data from 11 site monitors from 2005 to 2009.
Site Data: Shallow and Intermediate Depth
Thermal Impact Modelling of Hillsburgh Pit 12
Shallow system suggests a consistent seasonal response to recharge at multiple on-site locations
Intermediate depth monitor shows a lagged response
Observed Temperature Data
Intermediate depth monitor
Site Data: Pond and Deep System Temperature
Thermal Impact Modelling of Hillsburgh Pit 13
Deep system monitors show a pond thermal plume that attenuates with distance from the pond
Observed Temperature Data
Pond
Surface Water, Groundwater and Thermal Transport Models
Thermal Impact Modelling of Hillsburgh Pit 14
USGS Precipitation-Runoff Model (PRMS) Computes water balance for every cell in a 50-m
grid overlain on study area. Includes interception, snowmelt, runoff, infiltration,
ET, and groundwater recharge. Model inputs are rainfall, temperature, and solar
radiation. Other information: topography, soils, and land use.
USGS MODFLOW-NWT Computes groundwater levels in each aquifer across
the study area. Model inputs are groundwater recharge, aquifer and
aquitard properties.
USGS Version of MT3D Uses groundwater flow model
results to simulate solute or thermal transport.
Latest version includes interaction with lakes and streams.
MODFLOW model results read directly (recharge rates and flows to/from lakes).
GW Recharge
GW
Flu
x
Site Conceptualization: Model Representation
Thermal Impact Modelling of Hillsburgh Pit 15
Our interpretation is that the till barrier is related to a buried Catfish Creek drumlin structure.
The southern and eastern extent of the till barrier is extrapolated
Explains sharp drop in water levels around BH3 and BH4.
Till barrier represented as both layering and area of low hydraulic conductivity.
GW Flow Model Calibration
Steady-state groundwater model calibrated to match groundwater levels at MOE wells, on-site monitors, lake stage, and stream gauges.
Thermal Impact Modelling of Hillsburgh Pit 16
Thermal Transport Model Inputs
Thermal model inputs ▪ Average (background) groundwater temperature: 8.2 C
▪ Monthly recharge water temperature: Estimated from air temperature
▪ Monthly pond temperature: Estimated from measured pond temperature
Thermal Impact Modelling of Hillsburgh Pit 17
Thermal Transport Model Calibration
Animation shows simulated temperature in Layer 3 for existing pond conditions.
Pulses of high and low temperature water moving southeast from the pond.
Shallow system (Layer 1) is much less affected by pit plume than deeper system (Layer 3). ▪ Shallow system response due to seasonal
changes in temperature of recharge.
Click for Animation
Thermal Impact Modelling of Hillsburgh Pit 18
Thermal Transport Model Calibration
Thermal Impact Modelling of Hillsburgh Pit 19
Good calibration to deep plume attenuation pattern
Immediately downgradient from pit
Thermal Transport Model Calibration
Thermal Impact Modelling of Hillsburgh Pit 20
Shallow system response to GW recharge
Deep plume attenuation matched by model
Model Insights: Existing Conditions
Model calibration to existing site conditions: ▪ Good calibration to water levels
▪ Impressive calibration to thermal plume migration – very sensitive to flow velocity
Interpretation ▪ Model results confirm understanding of site geology and thermal plume dynamics
▪ Direction and rate of groundwater flow strongly influenced by till barrier
Insight: Simulations clearly identify shallow and deeper response ▪ Shallow wells affected by seasonal recharge with little lag and attenuation.
▪ Deeper wells affected by on-site pond that creates a lagged and attenuated plume response
Thermal Impact Modelling of Hillsburgh Pit 21
Confirmatory Field Observations
Model suggests plume is deflected south by the till barrier.
Field investigations identified warm groundwater discharge to ditch in January
Thermal Impact Modelling of Hillsburgh Pit 22
Confirmatory Field Observations
Model suggests plume is deflected south by the till barrier.
Field investigations identified warm groundwater discharge to ditch in January Click for Animation
Thermal Impact Modelling of Hillsburgh Pit 23
Predictive Simulation of Expanded Pond
Future build-out of Stage 3 pond
Simulation objective: Will expanded pond create a thermal plume that will impact the trout farm and wetlands?
Thermal Impact Modelling of Hillsburgh Pit 24
Thermal Impact Results
Animation of simulated temperature in Layer 3 under full build-out of pond.
Results show seasonal pulsing of plume, but deep thermal plume is attenuated
Click for Animation
Thermal Impact Modelling of Hillsburgh Pit 25
Predicted Impact of Enlarged Pond
Maximum temperature range predicted to increase by 1.9ºC in model Layer 3.
Peak arrives 2 months earlier.
Shorter travel distance for plume from pond.
▪ Pond closer to boundary
Less opportunity for attenuation and dispersion of plume.
Thermal Impact Modelling of Hillsburgh Pit 26
2 Months Earlier
1.9ºC Warmer
Peak
Annual Temperature Fluctuations in deep system
at CMTC property
Interpretation and Conclusion
Expansion of pond will speed up arrival time at site boundary and decrease attenuation
Despite changes to plume, effect on shallow spring at CMTC likely minimal
▪ Thermal plume from the enlarged pit is in the deeper system, and deep plume is deflected south by till barrier
▪ Trout water supply intake is shallow – already sees natural seasonal fluctuations
Thermal Impact Modelling of Hillsburgh Pit 27
Integrated Thermal Impact Modelling
In addition to magnitude of thermal changes, this example shows how we must also consider plume arrival “lag” from an ecological perspective ▪ Natural patterns in the shallow system:
• Seasonal High: 10 deg. C in September
• Seasonal Low: 5 deg. C in March
▪ This site with full build pond: Pattern in the deep system • Seasonal High: 12 deg. C in mid-April
• Seasonal Low: 6 deg. C in mid-October
▪ At discharge point, warmest water now arrives in mid-April, not September • Lag could be eco-significant depending on site geometry and ecology of
discharge location
Thermal Impact Modelling of Hillsburgh Pit 28
Integrated Thermal Impact Modelling
Thermal modelling is a natural extension of integrated SW/GW modelling
Unlike contaminate transport modelling, the “source” (climate and water temp.) is relatively well known and measureable
New models can simulate, or “route”, flow and heat movement in both the SW and GW system
Thermal Impact Modelling of Hillsburgh Pit 29