Groundwater Processes Notes

Groundwater Overview

  • Groundwater is part of the catchment hydrological cycle but is often overlooked.
    • Definition: Subsurface water beneath the water table in rocks and soils (Ward & Robinson, 2000).

Importance of Groundwater

  1. Catchment Runoff:

    • Maintains river flows and wetlands when there is no precipitation.
    • Important source of storm runoff.

  2. Water Resources:

    • Accounts for 94% of global freshwater.
    • 35% of public water supply in England and Wales comes from groundwater.
    • Provides good quality drinking water.

Groundwater Storage

  • Groundwater is Earth's largest accessible store of freshwater, accounting for approximately 95% of all freshwater.
  • Abundant resources exist in many regions around the world.
  • Acts as a major regulator of the global water cycle, with an average residence time of around 300 years (MacDonald et al., 2012).
  • Dependency: 2 billion people rely on groundwater for drinking; 40% of the world's food is produced from irrigated agriculture.

Aquifers

  • Types: Confined and unconfined aquifers.
    • Groundwater is a dynamic system varying in time and space (Robinson & Ward, 2017).

Confined vs. Unconfined Aquifers

  • Confined Aquifers:

    • Water table is pressurized.
    • Water can be accessed via artesian wells, where pressure allows water to rise from the aquifer without pumps.

  • Unconfined Aquifers:

    • Water is more readily available for discharge; influenced by surface conditions.

Groundwater Movement and Properties

  • Aquifers as Reservoirs:
    • Act as both reservoirs and channels for groundwater movement.
    • Key properties affecting water-holding capacity include:
    • Porosity – the ratio of void spaces in the material.
    • Specific Yield – amount of groundwater that can be readily extracted.
    • Coefficient of Storage – measures the volume of water a saturated aquifer releases from storage per unit decline in hydraulic head.

Porosity

  • Various materials have different porosity percentages:
    • Fine sandy alluvium: 45-52%
    • Gravel: 25-40%
    • Shale rock: 5-15%
    • Crystalline limestone: 1-10%
  • Divided into primary and secondary porosity.

Specific Yield and Storage

  • The specific yield measure relates to the interconnectivity of pore spaces, essential for sustaining groundwater discharge.
  • Water retention is influenced by capillary action due to cohesion (water molecules attracting to each other) and adhesion (water molecules attracting to surfaces).
  • A trade-off exists between retention forces and porosity.

Coefficient of Storage

  • Influenced by compressibility and elasticity of confined aquifers.
  • Water withdrawal affects porewater pressure and intergranular pressures, impacting aquifer storage and pressure distribution in the system.

Groundwater Balance

  • Discharge and Recharge:
    • Discharge includes evapotranspiration, seepage to surface water, and abstraction.
    • Recharge comes from precipitation, seepage from surface water, and human-induced methods such as irrigation.

River Discharge Relations

  • Groundwater significantly contributes to base flow, sustaining river ecosystems.
  • Relationship dynamics between rivers and groundwater recharge are critical, with potential for widespread streamflow losses into underlying aquifers in the USA (Jasechko et al., 2021).

Issues with Groundwater

  • Overabstraction: Leads to significant problems including:
    • Cone of depression formation from excessive pumping.
    • Saline intrusion, particularly in coastal aquifers.

Management Strategies

  • The 1963 Water Act and subsequent regulations in England & Wales were established to control abstraction licenses due to previously over-generous licensing leading to low flows.
  • Case study of the Misbourne in Buckinghamshire highlights the consequences of excessive water abstraction, with recovery measures showing positive trends in aquatic life post-intervention.

Conclusion

  • Groundwater is a critical resource requiring careful management to sustain its availability and quality for future generations. Understanding its dynamics, movement, and properties is essential for effective water resource management and environmental conservation.