In-Depth Notes on Groundwater Resources

Groundwater Resources

Learning Objectives

  • Explain the nature and variations of groundwater, including composition and flow rates.

  • Characterize key properties: porosity and permeability.

  • Differentiate aquifers from aquitards and understand the water table's nature.

  • Describe various wells and springs in groundwater access.

  • Sketch the origin of hot springs.

  • Explain damage and depletion of groundwater supplies with mitigation strategies.

  • Describe formation and evolution of caves and karst landscapes.

The Hydrologic Cycle

  • Groundwater plays an integral role in the hydrologic cycle, contributing to surface and subsurface water systems.

Porosity

  • Porosity refers to the percentage of empty spaces (pores) in rock or sediment, affecting water containment.

    • Total volume of open pore spaces = Porosity.

  • Pores can contain water, air, or minerals like oil and gas.

Secondary Porosity

  • New pore spaces formed after rock formation due to:

    • Fractures

    • Fault breccia

    • Solution cavities.

Permeability

  • Permeability is the measure of a material’s ability to transmit fluids.

    • High permeability: Water flows easily.

    • Low permeability: Water flows slowly.

Aquifers and Aquitards

  • Aquifer: A rock with high porosity and permeability that transmits water easily.

  • Aquitard: A rock with lower permeability that restricts water flow.

Water Table

  • The boundary between the saturated and unsaturated zones in groundwater.

    • Vadose zone: Unsaturated.

    • Phreatic zone: Saturated.

  • Depth varies by climate:

    • Humid areas: water table close to the surface.

    • Dry areas: water table deep below.

  • Water flow is influenced by land topography; water table mimics surface elevations.

Perched Water Table

  • Occurs when groundwater is trapped above a discontinuous aquitard, lying above the regional water table.

Hydraulic Head

  • The potential energy that drives groundwater flow:

    • Affected by elevation and water pressure.

    • Measured by a piezometer.

Recharge and Discharge

  • Groundwater moves from recharge areas (higher elevations) to discharge areas (lower elevations).

Groundwater Flow

  • Can vary significantly in scale and time; deeper paths take longer for water to flow through.

Darcy’s Law

  • Describes flow rates in porous mediums:

    • Flow rate ∝ Permeability x Hydraulic gradient

    • Hydraulic gradient defined as HG = \frac{(h1 - h2)}{j}, where:

    • h1 and h2 are hydraulic heads at two points,

    • j is the distance between them.

Cone of Depression

  • Created when groundwater is pumped faster than it can be replenished, lowering the water table and affecting nearby wells.

Artesian Wells and Potentiometric Surface

  • Artesian wells tap into confined aquifers under pressure, allowing water to flow without pumping due to the potentiometric surface.

Springs

  • Natural discharge points where the water table meets the surface, often occurring in valleys.

  • Indicators include wetland vegetation, perpetual water flow, nonfreezing ground, etc.

Structural Springs

  • Springs develop under conditions where groundwater reaches impermeable barriers, forcing it outward or appears at the intersection of perched water tables.

Hot Springs and Geysers

  • Hot Sprung Origins:

    • Deep groundwater heated by geothermal activity or volcanic heat.

  • Eruptions in geysers occur due to pressure build-up from boiling water.

  • Geothermal Activity Examples: Blue Lagoon (Iceland), Yellowstone (Wyoming).

Groundwater Problems

  1. Supply Issues: Groundwater depletion alters surface water environments.

  2. Reversing Flow: Over-extraction can reverse hydraulic gradients, leading to contamination.

  3. Saline Intrusion: Coastal over-pumping causes saltwater to mix with groundwater sources.

  4. Land Subsidence: Groundwater withdrawal leads to land surface collapse.

  5. Contamination:

    • Human activities can create contaminant plumes in groundwater.

    • Cleanup is challenging and costly; bioremediation techniques exist.

Caves and Karst Landscapes

  • Formed from limestone dissolution by acidic groundwater:

    • \text{H}2\text{O} + \text{CO}2 \rightarrow \text{H}2\text{CO}3 (carbonic acid).

  • Caves contain speleothems, which are mineral deposits from dripstone formation.

  • Karst terrain features:

    • Caves, sinkholes, vanished streams, springs, and unique landscape formation.