Chapter 11 - Groundwater and Water Resources

Water as a Resource

  • Water is crucial for domestic use, agriculture, and industry.
  • Water availability can limit the development of other resources, like fossil fuels.
  • The discussion encompasses surface water, groundwater, and ice.

Distribution of Water in the Hydrosphere

  • Table 11.1 illustrates water distribution in the hydrosphere.
  • There is a limited amount of fresh liquid water on Earth.
  • Over half the fresh water is trapped as ice, primarily in polar ice caps.
  • Not all groundwater is fresh.
  • Freshwater use requires restraint.
  • Water is a renewable resource geologically, but local supplies can be inadequate or nonrenewable in the short term.
  • Groundwater is a significant potential water source due to its volume.
  • Accessibility and availability are influenced by the geologic setting.
  • Most groundwater is in small cracks and pores; soluble rocks can have large caverns.

Fluid Storage and Mobility: Porosity and Permeability

  • Porosity and permeability determine a material's ability to contain and transmit fluids.
  • Porosity is the void space proportion in a material, indicating fluid storage capacity.
    • Expressed as a percentage (e.g., 1.5%) or decimal fraction (e.g., 0.015).
    • Pore spaces may contain gas, liquid, or both.
  • Permeability measures how readily fluids flow through a material.
    • Dependent on pore interconnection and size; larger pores reduce friction.
  • Porosity and permeability are affected by grain shape, size range, and arrangement (Figure 11.1).

Rock Types and Fluid Storage

  • Igneous and metamorphic rocks typically have low porosity and permeability due to interlocking crystals, unless fractured or weathered.
  • Chemical sedimentary rocks also have low porosity unless dissolution creates cavities.
  • Clastic sediments can have more open pore space, even after compaction.
  • Well-rounded, similar-sized grains result in high porosity and permeability, like in many sandstones.
  • Mixed grain sizes reduce porosity as finer materials fill gaps between coarser grains.
  • Flat, platelike grains (e.g., clay minerals) can create porous but low-permeability sediments like shale, especially perpendicular to the plates.

Porosity and Permeability Values

  • Figure 11.2 shows typical porosity and permeability ranges for geologic materials.
  • Porosity ranges (blue bars) and permeabilities (brown bars) vary among materials.
  • These properties are relevant to groundwater availability, stream flooding, petroleum resources, water pollution, and waste disposal.

Subsurface Waters

  • Infiltration occurs if soil is permeable enough.
  • Gravity draws water down until an impermeable layer causes accumulation.
  • The saturated zone (phreatic zone) is where water fills all accessible pore space above the impermeable material.
  • Groundwater is located in the saturated zone, generally a few kilometers into the crust due to pressure closing pores at greater depths.
  • The unsaturated zone (vadose zone) lies above the saturated zone, with pore spaces partly filled with water and air.
  • Soil moisture in the unsaturated zone is vital for agriculture.
  • Subsurface water includes groundwater, soil moisture, and water in unsaturated rocks.
  • The water table is the top surface of the saturated zone, except where confined by impermeable rocks.
  • Figure 11.3 illustrates these relationships.

Water Table Dynamics

  • The water table isn't always below ground; it forms the surface of lakes, streams, springs, and wetlands where it intersects the surface.
  • Below the surface, the water table undulates with topography and permeable/impermeable rock distribution.
  • Water table height varies, peaking in spring with heavy rain or snowmelt.
  • It drops during dry seasons or with intensive groundwater use.
  • Groundwater flows laterally from high to low elevations or pressure, from infiltration areas to drier ones, or away from areas of little use toward areas of heavy use.
  • Groundwater can contribute to or be replenished by streamflow (Figure 11.4).
  • Recharge is the process of groundwater replacement via infiltration and percolation.
  • Groundwater discharge is when groundwater flows into a stream, emerges as a spring, or exits the aquifer.
  • Darcy's Law further describes water moving through an aquifer.

Aquifers

  • Porosity and permeability are critical for well water supply.
  • Water is dispersed in pore spaces, typically a few percent of rock volume, though some sands and gravels can reach 50%.
  • Permeability controls withdrawal and recharge rates.
  • An aquifer is a rock or soil that stores and transmits water rapidly enough to be useful.
  • Sandstones and coarse clastic sedimentary rocks are often good aquifers, but any rock type can serve if sufficiently porous and permeable.
  • Aquitards, like shales, store water but have very low permeability.

Aquifer Geometry and Groundwater Flow

Confined and Unconfined Aquifers

  • Groundwater behavior is controlled by aquifer geology and geometry.
  • Unconfined aquifers are directly overlain by permeable rocks and soil (Figure 11.5).
  • They can be recharged by infiltration across their entire area.
  • Water rises in a well in an unconfined aquifer to the water table level and requires pumping to the surface.
  • Confined aquifers are bounded by low-permeability rocks (aquitards).
  • Water in confined aquifers may be under pressure due to elevation differences within the aquifer.

Artesian Systems

  • Wells drilled into confined aquifers can experience water rising above the aquifer level due to hydrostatic pressure, forming an artesian system (Figure 11.6).
  • Water may or may not reach the surface naturally, sometimes requiring pumping.
  • The potentiometric surface represents the height to which water would rise if unconfined, often above the aquifer top and potentially above ground level.
  • Artesian water is chemically the same as other groundwater, just under pressure.
  • Water towers create a similar effect artificially, increasing pressure in water delivery systems.
  • A simple demonstration involves holding a water-filled hose at an angle; a hole in the lower end will shoot water up to the water level in the hose.

Recharge and Implications for Confined Aquifers

  • Overlying aquitards prevent recharge from above in confined aquifers.
  • Replenishment relies on lateral flow from elsewhere in the aquifer; modern recharge may be limited or nonexistent.
  • This poses significant implications for water use from confined aquifer systems.

Darcy's Law and Groundwater Flow

  • Groundwater movement depends on permeability and hydraulic head differences.
  • Water flows from higher to lower hydraulic head areas.
  • The water table (unconfined aquifer) or potentiometric surface (confined aquifer) reflects hydraulic head.
    Q = K \cdot A \cdot \frac{\Delta h}{\Delta l}
  • Where:
    • Q = discharge
    • A = cross-sectional area
    • K = hydraulic conductivity (permeability, fluid viscosity, density)
    • (\frac{\Delta h}{\Delta l}) = hydraulic gradient (hydraulic head difference (\Delta h) divided by distance (\Delta l)).
  • Hydraulic conductivities vary widely among geologic materials.
  • The hydraulic gradient is analogous to a stream's gradient (Figure 11.7).
  • Darcy's Law applies to other fluids like oil.
  • The equation resembles stream discharge, with flow velocity represented by (K \cdot A).

Other Factors in Water Availability

  • Complex local geology can complicate groundwater availability assessments.
  • Perched water tables can form above impermeable rock lenses within permeable rocks (Figure 11.8).
    • These create localized saturated zones above the true water table.
    • Wells drilled here might find little water, sensitive to precipitation levels.
    • Long-term supply requires drilling to the regional water table at a greater cost.
  • Groundwater availability depends on flow paths and recharge zone locations.
    • Consumption close to the recharge area can quickly exhaust stored water if exceeding the recharge rate.
    • Extraction far from the recharge zone may tap into a larger water reserve.
  • Like stream systems, aquifers also have divides that determine the tappable area.
    • Groundwater divides can separate polluted and unpolluted waters within the same aquifer system.

Consequences of Groundwater Withdrawal

Lowering the Water Table

  • Pumping water from an aquifer often results in slower inflow than extraction.
  • In unconfined aquifers, this causes a circular lowering of the water table around the well, known as a cone of depression (Figure 11.9A).
  • Overlapping cones of depression from closely spaced wells further lower the water table between wells (Figure 11.9B).
  • Regional water tables drop when withdrawal rates consistently exceed recharge rates.
  • A sign of this is the need to periodically deepen wells.
  • Cones of depression can also develop in potentiometric surfaces of artesian wells.

Groundwater Mining

  • Deepening wells isn't limitless due to impermeable rock depths.
  • Groundwater flow rates are often slow (meters or tens of meters per year).
  • Recharge, particularly to confined aquifers, can take decades or centuries.
  • Excessive withdrawal leads to