Municipal Groundwater Flow Modelling Study. Town of Torbay, NL

Transcription

1 Municipal Groundwater Flow Modelling Study Town of Torbay, NL October 19, 2015

2 Agenda 1 Purpose 2 What is a Groundwater Model 3 Approach 4 Conceptual Model & GW Flow Model Construction 5 Model Predictions Three Scenarios 6 Model Results & Future Applications

3 1 Purpose Groundwater significant source of potable water for the Town; 70% of the Town relies on private water wells. In recent years, significant growth and expansion out into unserviced areas of the municipality. Concerns that increased development may have adverse affect on groundwater resources. Fall 2014, the Town of Torbay commissioned a groundwater modelling study to evaluate these concerns: Characterize existing groundwater conditions Predicte the cumulative impacts of future residential subdivision and commercial development on groundwater resources.

4 2 What is a Groundwater Model Water well Computer generated replica of a real-world groundwater system. Site-specific Used to simulate and predict aquifer conditions.

5 3 Approach

6 4 Conceptual Model & GW Flow Model Construction Descriptive representation of groundwater system Interpret geological and hydrological conditions What processes are important? What are the boundaries? What parameter values are available? Water well

7 Model Boundaries Surface water watersheds used to define the flow divides of the groundwater system - Topographic analysis Atlantic Ocean Model Domain (combined sub-watersheds) Town Boundary Atlantic Ocean Constant Head (ocean) No Flow (watershed limits)

8 Groundwater System effective recharge* ponds and brooks wells constant head boundary discharge to ocean no-flow boundary input output * Estimated at 10% of annual precipitation. Accounts for runoff and evapotranspiration.

9 Wells - Unserviced Areas Determine the number of houses in defined unserviced areas. Estimate annual household groundwater use (329 L/day x days/year = 120,000 L/year = 120 m 3 /y Include industrial users

10 Hydrogeologic Parameter Estimates Bedrock Geology Model Layers Geological Survey Branch, Department of Mines and Energy, Government of Newfoundland and Labrador

11 Groundwater Model - Under Construction

12 Groundwater Model - Under Construction no flow pond (general head) ocean (constant head)

13 How does it work? The model runs once with the initial parameter values A comparison is made between the target values and the new modeled values. 132 head targets 105 from well logs where the location was confirmed 27 from Phase 2 reports 6 flow targets Baseflow estimated from existing daily flow data from hydrometric stations on the Avalon Peninsula Model Calibration

14 Calibrated Model Base case - existing conditions Considers existing development as of last Google Earth image (late 2014) Assumes a per house daily pumping of 1360 L (4 persons)

15 5 Model Predictions 3 Scenarios Scenario 1 Completion of existing subdivisions Forest Landing, Eagle Nest / Scenic View, Pine Ridge & Logy Bay Drawdown is typically confined to the footprint of the development Maximum impact to existing users might be along Flora Drive (Scenic View/Eagle Nest)

16 5 Model Predictions 3 Scenarios Scenario 2 New development A. A new Forest Landing at Bauline Line & Jones Pond east extension (same building density, water use and footprint) B. Pine River Valley residential C. Pine River Valley residential + commercial

17 Prediction Scenario 2A maximum corrected drawdown ~ 2.1 m uncorrected maximum corrected drawdown ~ 3.7 m 17

18 Prediction Scenario 2B uncorrected maximum drawdown ~ 0.4 m 18

19 Prediction Scenario 2C Proposed commercial area is 35,000 m 2 (8.6 acres) Typical free-standing McDonalds restaurant acres (including parking) Annual water consumption 4100 m 3 /y (8.3x a 4- person house) Assume 6 McDonaldstype restaurants are built beside the area (worst case as many other commercial uses wouldn t require this much water) 19

20 Prediction Scenario 2C uncorrected commercial wells maximum corrected drawdown in residential wells ~ 1.5 m 20

21 Prediction Scenario 2C How does this type of commercial water demand compare to reported well yields in the area? Average for Torbay wells L/min ( From the well record database (n=420) Min = 0.5 L/min Max = 450 L/min (seems very high) Average = 15.4 L/min 50% less than 8.1 L/min 90% less than 30 L/min 21

22 Prediction Scenario 2C Residential wells at Pine Ridge have yields ranging from 0.5 to 99 L/min with an average of 16.5 L/min A McDonalds-type commercial facility well would need a well yield of 7.8 L/min for operations This suggests that commercial development may be possible with minimal impact on existing users 22

23 Prediction Scenarios Scenario 3 Municipal well field Wells located around North Pond Sized to supply 400 homes (400 homes x 1360 L/d/home = 544 m 3 /d) 23

24 Prediction Scenario 3 Municipal wells The average existing drilled residential well has a yield of L/min which is equal to 20.8 m 3 /d 400 homes would require 544 m 3 /d You would need ~26 wells to meet this capacity assuming the same yield 24

25 Prediction Scenario 3 What would happen if 4 wells with a 10 - diameter were drilled near North Pond, 154 m deep Each well would pump 136 m 3 /d (94.4 L/min) Head correction for the pumping well would be 102 m 25

26 Prediction Scenario 3 uncorrected Corrected well drawdown ~110 m 26

27 More Frequency Prediction Scenario Histogram of Available Drawdown 100% 90% 80% 70% 90% of existing wells don t have this much available drawdown 25 60% 50% 20 40% Frequency Cumulative % 15 30% 10 20% 5 10% 0 0% Available Drawdown, meters 27

28 Prediction Scenario 3 Municipal wells are not likely feasible based on Typical well yield Typical available drawdown Further investigation would be needed to identify potential exploration targets based on known water-bearing geologic contacts and fracture/fold features 28

29 Model Results & Future Applications Predictive simulations show impact is low in terms of reduced available drawdown or dewatering of water-bearing features. Drawdown have been determined to be localized within the immediate vicinity of new development. Supplementing municipal surface water supply with a groundwater source is not likely feasible because of low well yields and available drawdown constraints. The developed groundwater model is designed to be a tool for adaptive groundwater resource management and land use planning.

30 Questions?