Groundwater as a Resource

Chapter 11: Groundwater as a Resource

Presented by: Gwenn Flowers, Simon Fraser University
Courtesy of course creator, Diana Allen

1. Groundwater Management in BC

Water Use

• Groundwater use is regulated in British Columbia (BC) since January 2016.

  • Requires a license for pumping groundwater, except for domestic users.

• Surface water use is also regulated and requires licenses.

  • Approximately 44,000 water licenses exist in BC for the diversion, use, and storage of water.

  • Licenses are granted based on the “first in time, first in right” (FITFIR) doctrine.

Groundwater Regulation

• New Groundwater Protection Regulation introduced in BC as of November 2004:

  • Establishes driller qualifications

  • Mandates well construction standards

Groundwater Protection

• Well head protection (WHP) programs are widely implemented in the U.S. and to a lesser extent in Canada.

  • Aim: Protection of groundwater quality through delineating well capture zones by estimating the cone of depression at different times:

  • 1 Year Capture Zone

  • 5 Year Capture Zone

  • 10 Year Capture Zone

2. Yield Concepts

Definitions

• Well Yield:

  • Maximum pumping rate that keeps the water level above the pump intake.

• Aquifer Yield:

  • Maximum sustained withdrawal rate with an “acceptable” decline in head, which depends on the number and spacing of wells.

• Basin Yield:

  • Maximum sustained withdrawal rate from the entire hydrogeologic system of a groundwater basin without causing “unacceptable” changes to any component of the hydrologic cycle, dependent on the number and spacing of exploited aquifers.

• Sustainable Yield:

  • An ambiguous term that needs clear definition concerning acceptable pumping rates compatible with hydrogeological conditions.

  • Aquifer levels decline upon pumping; defining “acceptable” levels of decline is necessary.

  • Utilizes a 20-year extrapolation of pumping test curves (as practiced in Alberta)

  • Also utilizes a 100-day extrapolation of pumping test curves (as practiced in British Columbia)

Determining Sustainable Yield

• It is critical to consider:

  • Variability in precipitation on an interannual basis.

  • Variability in recharge on an interannual basis.

  • Long-term climate effects that necessitate record-keeping over extended periods.

3. Impacts of Groundwater Extraction

• Well Interference:

  • Potential for interference between neighboring wells during pumping activities.

• Impact on Streams and Wetlands:

  • Extraction can lead to changes in the natural water table and associated ecosystems.

• Regional Drawdown:

  • A decrease in the overall water table can occur as a consequence of substantial groundwater extraction.

• Global Monitoring:

  • GRACE satellite project identifies groundwater losses worldwide.

  • Reference: http://youtu.be/zu4cBM4m5gU

4. Saltwater Intrusion

Overview

• Natural Flow:

  • Groundwater flows from inland areas towards the coast to prevent saline intrusion.

  • The shape and positioning of the salt/freshwater interface rely on the volume of freshwater discharge.

• Flow Rate Alteration:

  • Any change in groundwater flow rates can lead to shifts in salinity boundaries.

Types of Intrusion Mechanisms

• Passive Intrusion:

  • Caused by reduced freshwater recharge or rising sea levels, leading to a slow landward movement of the boundary while still maintaining a hydraulic gradient towards the sea.

  • Example Location: Miami, FL – altered drainage canals facilitating this effect.

• Active Intrusion:

  • Reversal of the natural hydraulic gradient due to excessive pumping causes rapid inland movement of the saltwater front.

  • Example Location: Brooklyn, N.Y. – observed a drop of 9-15 m in the water table.

Hydrostatic Analysis Conditions

1. Interface angle \alpha < 90^o

2. System operates under hydrostatic conditions.

Ghyben-Herzberg Relation

• The relationship is defined as: $\rho<em>s g z</em>s = \rho<em>f g (z</em>s + z_f)$ where:

  • $\rho_s = 1.025 ext{g/cm}^3$ (density of seawater)

  • $\rho_f = 1.000 ext{g/cm}^3$ (density of freshwater)

  • This can be simplified to yield:
    $z<em>s = \frac{\rho</em>f}{\rho<em>s - \rho</em>f} z_f$

  • For seawater and freshwater, this approximates to:
    $z<em>s = 40 z</em>f$

Upconing of the Sea Water Interface

• A common phenomenon when the water table drops in response to pumping, leading to the upconing of saline seawater beneath the pumping well.

5. Effects of Sea Level Rise

Groundwater Recharge and Intrusion Dynamics

• Increased sea levels contribute to a shift in the saline interface, creating a wedge of saline groundwater that encroaches on the freshwater lens.

• This scenario results in reduced volumes of freshwater available in coastal aquifers.

6. Managed Aquifer Recharge (MAR)

Importance

• As groundwater use nears available yield, effective management of water supplies becomes essential.

  • In certain circumstances, surface water may need to be imported.

  • Optimal water use involves a combination of groundwater, surface water, and reclaimed water.

• MAR involves:

  • Infiltration from a storage basin into high-permeability, unconfined alluvial aquifers through constructed basins.

  • Results in a "groundwater mound" formation.

Benefits

• External benefits include chemical and biological purification of water as it percolates through alluvial sediments.

  • Particularly applicable to Groundwater Under Direct Influence (GUDI) systems.

7. Induced Infiltration

Definition and Methodology

• Induced infiltration is a technique that combines the use of surface water and groundwater.

  • Involves pumping from alluvial sands and gravels that are hydraulically connected with nearby streams under constant head conditions.

  • However, caution is warranted as contamination from the stream may lead to groundwater pollution, presenting environmental risks.