Water Quantity Risk Assessments: Protecting Municipal Drinking Water Supplies

Paul J. Martin, M.Sc., P.Eng.

WATERtech 2012 April 13, 2012

Source Protection - Impetus

• Walkerton Tragedy

• O’Connor Recommendations – Drinking water focused

– Quality Management

– Quantity Management

– Watershed Scale Analysis • Understand Big Picture, manage drinking water

system within natural setting

National Round Table on the Environment and Economy

Source Protection – Generalized Approach

• Quality – Contaminant Sources Inventory

– Surface Water / Groundwater Vulnerability • SW – Intake Protection Zones

• GW - Intrinsic Susceptibility & Capture Zones

– Contaminant Threats Ranking

• Quantity – Water Demand Assessment

– Water Budget / Stress Assessment • SW – Streamflow, Recharge, etc.

• GW – Groundwater Flow

– Quantity Threats Assessment

Threats

Management

Threats

Management

Water Quantity Risk Assessment

• Framework Focus:

– Evaluate municipal water supply sustainability

• Current and Planned water demands

• Surface Water Intakes or Wellheads

– Help managers understand risk

• Approach

– Tiered Approach

– Screening tools detailed water budget tools

Water Budget – Tiered Assessment Process

5,000-10,000 km2

500-1,000 km2

Tier 1 &Tier 2 Assessment Approach

Tier 1

Assessments use simplified, yet structured means to estimate water supply volumes and compare that to consumptive demand. The ratio of demand to supply defines the “potential for hydrologic stress” within a subwatershed.

• Existing Scenario

• Future Development Scenario

Tier 2

Assessments use more detailed and complex tools to estimate water supply volumes and compare that to consumptive demand. The ratio of demand to supply calculated in Tier 1 is repeated in Tier 2 with refined information.

• Existing Scenario

• Future Development Scenario

• Planned Scenario

including Drought & Climate Change }

Tier 1 & Tier 2 Stress Assessment

% Water Demand =

QDEMAND

x 100 QSUPPLY - QRESERVE

0 - 20 % Low

> 20% Moderate

50% Significant

Maximum

Monthly

% Water

Demand

Stress Level

Assignment

0 - 25% 0 - 10% Low

> 25% >10% Moderate

50% 25% Significant

Maximum

Monthly

% Water

Demand

Average

Annual

% Water

Demand

Stress Level

Assignment

Groundwater Stress Levels Surface Water Stress Levels

QGW-Out

QGW-In

QGW-R

QGW-D

QGW-C QSW-RO

QSW-C

QSW-In

QSW-Out

QP

QET

Groundwater

Surface

Water

Unsaturated

Zone

Saturated

Zone

Water Budget Components

Groundwater

Modelling

Surface Water Modelling

Integrated Modelling

Tier 2 - Example Application

~4,000 km2

Surface Water - Water Budget Tools

Quaternary Geology/Soils

DEM Land Cover

Channel Characteristics

Climate (Precip, Temp)

STP Discharges

Water Takings (SW and Shallow GW)

Surface Water Model

40 Year Continuous Simulation

Streamflow ET Runoff Groundwater

Recharge Soil Water

Content

Streamflow

Surface Water Calibration

Simulated Recharge

Groundwater - Water Budget Tools

Geology

DEM

Watercourses

Hydrogeologic Characteristics

Recharge (SW Model)

Boreholes (Water & Oil/Gas)

Water Takings Groundwater Model (FEFLOW / MODFLOW)

Steady State and Time Marching

Simulations

Groundwater Discharge

Flow Paths Groundwater

Elevations

Water Levels

Baseflow Estimates

Interbasin Flow

FEFLOW

NRMS: 5.3%

RMS: 7.3 m

Mean Error: +2.1m

Mean Absolute Error = 5.6m

Simulated Water Levels

Groundwater Discharge to Streams

QGW-Out

QGW-In

QGW-R

QGW-D

QGW-C QSW-RO

QSW-C

QSW-In

QSW-Out

QP

QET

Groundwater

Surface

Water

Unsaturated

Zone

Saturated

Zone

Water Budget Components

Tier 2 Scenarios

• Current and planned demand scenarios

• Drought: potential impacts under historic drought conditions

• Climate Change

Tier 2 Surface Water Stress Assessment

21

Municipal Intakes

Moderate Stress

Significant Stress

Tier 2 Groundwater Stress Assessment

Municipal Wells

Moderate Stress

Significant Stress

Tier Three Assessment

Focus shifts from subwatershed analysis to wellhead analysis.

Where?

Tier Two Subwatershed

Stress Assessment

classified the

subwatershed as having a

Moderate or Significant stress level or,

Historical issues with water

sources meeting demand.

Tier Three Risk Assessment

• Refine and Characterize the Hydrogeology/geology of the area

• Develop a Detailed Surface Water Model (recharge rates)

• Develop/ Refine a Groundwater Model

• Calibration the Groundwater Model (Well Field Scale)

• Characterize the Water Takings (municipal and non-municipal)

• Delineate the Vulnerable Areas

• Risk Assessment Scenarios

• Assign semi-quantitative ‘RISK’

Refine Characterization

Refine Modeling Tools

Define “Vulnerable

Area”

Assess Risk Level using Multiple

Scenarios

HSP-F Surface Water Model Domain

MODFLOW Groundwater Model Domain

Enhanced Model Calibration Baseflow Calibration

Risk Assessment Scenarios

Existing Land Cover & Existing Pumping Average & Drought Climate

Planned Land Cover & Planned Pumping Average Climate

Planned Land Cover & Planned Pumping Drought Climate

… Climate Change

Risk Assessment Scenario Results

WHPA-Q1 Area

Well Name

Safe Additional

Aquifer Drawdown

(2008)

MODFLOW Groundwater Model Scenario Drawdown (m)

Average Climate Drought

G(1) G(2) G(3) D H(1) H(2) H(3)

Recharge Reduction, Increased Demand

Increased Demand

Recharge Reduction

Existing Recharge, Demand

Recharge Reduction, Increased Demand

Increased Demand

Recharge Reduction

A1

Well 2A 4.7 3.1 2.2 0.8 1.2 4.3 3.3 2.0

Well 5/ 5A 3.2 3.7 0.6 2.5 1.3 5.31 2.0 4.0

Well 6 2.9 2.6 2.2 0.3 1.5 4.12 3.7 1.8

Well 7 8.7 4.8 3.9 0.7 1.2 6.0 5.0 1.9

Well 8B 7.4 1.9 1.3 0.5 0.9 2.9 2.2 1.4

Well 8C 7.5 2.0 1.4 0.5 0.9 3.0 2.3 1.4

Well 9A/ 9B 4.3 3.0 0.4 2.3 1.4 4.6 1.9 3.8

Well 11 6.2 3.1 2.7 0.3 1.6 4.7 4.3 1.9

Well 12 11.5 5.3 4.0 1.1 1.1 6.1 4.7 2.2

Pullen Well 30.4 3.5 2.6 0.8 1.2 4.8 3.9 2.0

A2

Cardinal Woods 1 4.8 0.7 0.4 0.3 0.8 1.5 1.1

1.1

Cardinal Woods 3 3.0 2.4 2.0 0.3 0.8 3.1

3 2.8

1.0

B Island Lake Wells 22.0 0.3 0.3 0.0 0.4 0.8 0.7

0.4

C Coles 1, 2 34.7 0.6 0.1 0.3 1.7 2.2 1.8 2.0

D Well 10 35.9 1.4 1.4 0.0 0.2 1.6 1.6 0.2

Pumped drawdown close to, or greater than, the safe additional available drawdown 1 Safe additional drawdown level predicted to be exceeded 76% of the time (35 years in the 46-year simulation).

2 Safe additional drawdown level predicted to be exceeded 32% of the time (19.1 years in the 46-year simulation).

3 Safe additional drawdown level predicted to be exceeded 7.2% of the time (3.3 years in the 46-year simulation).

Risk Assessment Scenario Results

Risk Assessment Scenario Results

Ecological Impacts: 1) Change in simulated discharge 2) Wetland Function

Summary

• Source Water Protection

– Water Quality

– Water Quantity

• Water Quantity undertaken as a Tiered Approach – Successively evaluate Risk using more refined modelling tools

– Modelling tools used to guide decision-making for Water Managers

• Risk Level – Tier 2 – percent water demand

– Tier 3 – sustainability of pumping rates, impact on environment

• Determine and Rank “Threats” – Define appropriate risk management measures for each threat.

Evaluate Risk Level and Identify Threats

Acknowledgements

Co-Authors: – Dave Van Vliet, Sam Bellamy, Paul Chin

• AquaResource, A Division of Matrix Solutions

– Mike Garraway, Ontario Ministry of Natural Resources

• Resource: waterbudget.ca