Thermal Impact Assessment of Below-Water-Table Aggregate Extraction

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.


Site Location:


Pit

Aggregate Operations


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


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


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


Site Conceptualization


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


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


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


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


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


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


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 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 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


https://youtu.be/Cqj2kAfDzJ4?t=1m25s



Good calibration to deep plume attenuation pattern

Immediately downgradient from pit



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


Confirmatory Field Observations

Model suggests plume is deflected south by the till barrier.

Field investigations identified warm groundwater discharge to ditch in January


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



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 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



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.


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


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


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 Assessment of Below-Water-Table Aggregate Extraction

Environment

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