8 groundwater
Hydrosphere: Groundwater
Learning Outcomes
By the end of this section, students will be able to:
Define porosity and permeability and describe how they affect groundwater flow.
Define groundwater and groundwater zones.
Describe the groundwater table and its relationship to groundwater flow.
Describe differences between aquicludes, confined aquifers, and unconfined aquifers.
Describe how dissolution and precipitation by groundwater affect landscapes and rock features, as well as how groundwater chemistry changes.
Storage of Water on Earth
Total volumes of different water sources:
Ocean: 1,335,000,000 km³
Ice: 24,364,000 km³
Groundwater: ~23,400,000 km³
Surface water: 190,000 km³
Atmosphere: 12,900 km³
Water movement statistics:
Evapo-transpiration from land: 70,000 km³/year
Evaporation from oceans: 430,000 km³/year
Precipitation on land: 110,000 km³/year
Precipitation on oceans: 390,000 km³/year
Runoff and groundwater flow: 40,000 km³/year
Definition of Groundwater
Groundwater: Water located below the Earth’s surface (in the Geosphere) that occupies spaces in regolith, soil, and rock.
Comprises less than 2% of Earth’s total water; exact quantification is challenging, especially at greater depths.
Importance of Groundwater
Groundwater is essential for:
Municipal water supplies
Agriculture
Industry
Groundwater may have over 100 times more resources than surface water.
A significant concern is that groundwater can become contaminated due to human activities, often occurring unseen below the surface.
Water in Sediments and Rocks
Porosity:
Definition: The fractional amount of pore space in a geological medium (regolith, sediment, or rock), specified as a fraction (0 to 1) or percentage (0% to 100%).
Types:
Primary Porosity: Found in loosely packed sediments with significant spaces between grains.
Tends to decrease in poorly sorted sediments (varied grain sizes).
May decrease over time if cementation occurs.
Secondary Porosity: Results from later formations such as cracks and cavities forming in rocks.
Permeability:
Definition: A measure of how easily a fluid can flow through a material.
Factors affecting permeability include:
Porosity
Size of pores
Connectedness of pores
Measurement units: Darcy or millidarcy.
Example of permeability:
Loose sand: approximately 500 millidarcy.
Fine-grained mud (clay): high porosity but low permeability.
High porosity does not always correlate with high permeability.
Filling Pore Spaces
Materials that may occupy pore spaces in sediments and rocks include:
Ice (permafrost)
Hydrocarbons
Natural gas reservoirs
Oil reservoirs
Zones of Groundwater
Vadose Zone:
Definition: The zone where pore spaces are partially filled with water; also known as the aerated or unsaturated zone.
Saturated Zone:
Definition: The zone where pore spaces are completely saturated with water.
Water Table:
The boundary where the saturated zone meets the vadose zone.
Below this boundary, surface water may drain into wells.
Above the water table, water is retained by surface tension on sand grains.
Wells drilled into saturated zones will fill with groundwater, and the upper surface of this water defines the water table.
Form of the Water Table
Topography:
Traditionally mimics the landscape: higher beneath hills, lower in valleys.
Intersects the Earth's surface where rivers or lakes occur.
Groundwater Recharge
Recharge:
The process by which water enters the subsurface and becomes part of the groundwater system.
Groundwater may return to the surface at springs or as base flow into streams.
This often occurs where the water table intersects the surface, predominantly in elevated regions.
Changes in Water Table Position
Seasonal Variability:
The water table fluctuates with precipitation levels.
During periods of low precipitation or infiltration, the water table drops; during recharge events (e.g., rain, spring melt), it rises.
Groundwater Flow
Driving Forces:
Groundwater moves from higher to lower elevations and from higher to lower pressure zones.
These processes are described through hydraulic potential or hydraulic head, representing the height of a column of water that generates similar driving forces.
Flow typically occurs from areas of higher water table to lower water table regions.
It is important to note that groundwater flow is considerably slower than river flow, with travel times ranging from days to millennia depending on depth.
Springs
Definition: Locations where groundwater re-emerges at the Earth's surface, typically occurring where the water table or a permeability boundary intersects the land surface.
Groundwater and Streams
Groundwater interacts with surface water:
Gaining Stream: A stream that receives water from the groundwater system, contributing to its base flow.
Losing Stream: A stream that loses water to the groundwater system, which may vary throughout the stream's length.
Streams disconnected from groundwater may dry up if the water table falls below their beds.
Aquifers and Confining Layers
Aquifers:
Geological units (rock/sediment) with adequate capacity and permeability to provide water at usable rates for humans.
Confining Layers (Aquicludes):
Geological units that significantly slow or halt water flow due to low permeability.
Types of Aquifers
Unconfined Aquifer:
Bound by the water table; can freely receive and discharge water.
Confined Aquifer:
Surrounded by a confining layer restricting water flow above the aquifer.
Wells in Unconfined Aquifers
Withdrawal of groundwater through pumping creates a cone of depression around the well.
This cone deepens until the water inflow rate equals the withdrawal rate.
Excessive withdrawal may cause the water table to drop below the well base, leading to difficulties in extracting water.
Contamination Risks
Groundwater contamination may result from unintentional effects associated with the cone of depression, potentially introducing contaminants into drinking water supplies.
Confined Aquifers and Artesian Wells
Groundwater in confined aquifers is often under pressure. When a well is drilled into a confined aquifer, water may rise to levels close to that of the recharge area’s water table, creating an Artesian well.
Solution and Precipitation by Groundwater
Groundwater plays a significant role in the geological landscape through dissolution and the precipitation of minerals.
Solution by Groundwater:
Dissolves materials it comes in contact with, affected by groundwater composition, surrounding rock type, temperature, and pH levels.
Calcium carbonate rocks (limestone, marble) are particularly susceptible to acid solutions, resulting in Karst topography.
Karst Topography:
Formed through dissolution processes, leading to cave systems, sinkholes, and isolated rock pillars.
Example areas include Tsingy de Bemaraha (Madagascar) and Bear Rock Sinkhole (Northwest Territories).
Composition of Groundwater
Salinity: Concentration of dissolved minerals, measured in ppm (parts per million).
Fresh groundwater: < 1000 ppm
Brackish water: 1000 - 10,000 ppm
Saline water: 10,000 - 35,000 ppm (similar to seawater)
Brine: > 35,000 ppm
Salinity is influenced by groundwater flow velocity, rock types, pH, and temperature.
Commonly, deep, slow-moving groundwater is saline, including connate water trapped in sedimentary deposits.
Precipitation of Minerals
Occurs when dissolved ions in groundwater revert to solid form due to changes in conditions (e.g., pH, temperature, pressure, evaporation).
Resulting mineral deposits, known as cement, can transform loose materials into harder geological structures, such as:
Sand into sandstone (lithification)
Aquifers into aquicludes
Potential reservoirs into non-porous substrates.
Dissolved carbonate ions may form stalactites and stalagmites in caves via evaporation processes.
Hydrothermal Systems
Hot Springs & Geysers: Formed when hydrothermal water escapes the Earth’s surface, heated by geothermic energies.
Related Visuals
Various diagrams and images enhance understanding of the concepts about groundwater flow, aquifers, and related geological processes, ensuring contextual grasp on the interactions between groundwater and geological structures.
References
The content presented references various educators and instructors at the University of Alberta and other mentioned sources from the transcript, ensuring academic validity and credibility.