Groundwater Study Guide

Groundwater Study Guide

Water Properties

  • Specific Heat:

    • Water has a very high specific heat capacity, which means it requires a substantial amount of energy to change its temperature.

    • This property plays a crucial role in stabilizing Earth’s climate and allows aquatic environments to maintain stability against rapid temperature fluctuations.

  • Density When Frozen:

    • Unlike most substances, water expands when it freezes.

    • Ice is less dense than liquid water, which results in ice floating on water bodies. This phenomenon is critical as it protects aquatic life by insulating lakes and ponds during winter months.

Phases of Water

  • Water naturally exists in three main phases:

    • Solid (Ice)

    • Liquid (Water)

    • Gas (Water Vapor)

  • The transitions between these phases are fundamental drivers of the hydrologic cycle.

Water Distribution

  • Saltwater vs. Freshwater:

    • Approximately 97% of Earth's water is saltwater found in oceans.

    • Around 3% is freshwater:

    • Most (about 69%) is stored in ice sheets and glaciers.

    • Approximately 30% is groundwater.

    • Less than 1% is available in rivers and lakes.

Hydrologic Cycle

  • The hydrologic cycle is powered entirely by solar energy and entails several stages:

    • Water evaporates from oceans and lakes.

    • Transpiration occurs from plants.

    • Water vapor condenses into clouds.

    • It precipitates as rain or snow.

    • Water infiltrates into soil.

    • It runs off into rivers.

    • Water is stored in various reservoirs (air, ground, ice, oceans).

  • Atmospheric Entry:

    • Mainly through evaporation and transpiration processes.

  • Return to Earth:

    • Occurs through precipitation in forms like rain, snow, sleet, or hail.

Water Storage

  • Vadose Zone:

    • Also referred to as the unsaturated zone.

    • Water enters this zone through infiltration, where the pores contain both air and water, thus remaining unsaturated with respect to water.

  • Phreatic Zone:

    • Known as the saturated zone where all pore spaces are filled with water. The upper boundary of this zone is termed the water table.

  • Pressure Head:

    • The height of water above a given point in the aquifer, which influences groundwater pressure and the direction of flow.

  • Groundwater Movement:

    • Driven by gravitational forces and hydraulic pressure gradients.

  • Storage:

    • Groundwater is stored underground in aquifers. Not all aquifers have equal conditions; their quality depends on factors like porosity and permeability.

  • Porosity vs. Permeability:

    • Porosity: Percentage of pore space available in a material.

    • Permeability: The ability of a material to allow fluids to pass through it.

    • An aquifer with high porosity but low permeability (such as clay) is not effective for groundwater storage.

Aquifers

  • High Plains/Ogallala Aquifer:

    • The largest aquifer in North America, located beneath the central United States.

    • Provides essential irrigation water for agriculture but is being depleted more rapidly than its rate of natural recharge.

  • Crystalline Rocks:

    • Comprised primarily of igneous and metamorphic rocks that typically exhibit low porosity and permeability.

    • Water is stored in the fractures of these rocks.

  • Appalachian Aquifer:

    • Although composed of crystalline rocks, the presence of fractures and joints facilitates water storage and flow.

Types of Aquifers

  • Aquifer:

    • Defined as any rock or sediment that stores and transmits water.

  • Aquitard:

    • An impermeable layer that restricts the flow of water.

  • Unconfined Aquifer:

    • An aquifer that is open to the earth's surface, which can be recharged directly by rainfall and infiltration.

  • Confined Aquifer:

    • Separated from the surface by an aquitard; its recharge area is more limited, often located far from the aquifer.

  • Artesian Aquifer:

    • A type of confined aquifer that is under pressure; if a well is drilled below the potentiometric surface, water will rise naturally without the need for pumping.

Darcy’s Law

  • The equation for calculating discharge (flow rate) in an aquifer is represented as: (Q = K imes A imes ( rac{igtriangleup h}{L}))

    • Q = discharge (flow rate)

    • K = hydraulic conductivity (ease of flow)

    • A = cross-sectional area of flow

    • Δh = difference in hydraulic head

    • L = flow length

  • A steeper gradient (i.e., $ rac{igtriangleup h}{L}$) leads to a faster flow rate.

Fenomenons Related to Groundwater

  • Cone of Depression:

    • A feature that occurs when the water table is lowered around a well that is being actively pumped.

    • When pumping ceases, the water table typically recovers.

  • Relations Between Streams and Groundwater:

    • Gaining Stream: A stream that receives water from groundwater flow.

    • Losing Stream: A stream that loses water to the groundwater by infiltration.

    • The designation of gaining or losing is dependent on the water table elevation relative to the stream.

Springs and Water Tables

  • Springs:

    • Natural features where the water table intersects the land surface, allowing groundwater to flow out naturally.

  • Perched Water Table:

    • A localized saturated zone that exists above the main water table, separated by an impermeable layer.

Groundwater Use and Issues

  • Groundwater Uses:

    • Applications include drinking water, irrigation, and industrial uses. The largest utilization is for agricultural irrigation.

  • Highest Usage States:

    • States with significant agricultural activity such as California, Texas, and Nebraska exhibit the highest consumption of groundwater.

  • Depletion Issues:

    • Over-pumping groundwater can lead to lowered water tables, drying up wells, and reducing flow in streams and lakes.

  • Saltwater Intrusion:

    • As freshwater is less dense than saltwater, it tends to float atop saltwater. However, excessive pumping in coastal areas can cause saltwater to encroach inland, contaminating freshwater wells.

  • Land Subsidence:

    • The phenomenon of land sinking occurs when aquifers compact due to water extraction. In areas such as the San Joaquin Valley in California, land subsidence has been recorded to drop several meters, damaging infrastructure in the process.

    • Coastal areas are particularly vulnerable to the impacts of subsidence which exacerbates flooding issues.

  • Contamination Risks:

    • Groundwater contamination poses significant concerns because ground water flows at a slow rate, allowing pollutants to remain for decades, with cleanup being both difficult and expensive.

Karst Topography

  • Limestone Composition:

    • Composed of calcium carbonate, limestone easily dissolves in acidic water, leading to distinctive landforms.

  • Dissolution Features:

    • Cavities and caves formed through chemical weathering processes.

  • Karst Features:

    • Include sinkholes, caves, disappearing streams, and springs all resulting from the dissolution of soluble rocks like limestone.

  • Speleothems:

    • Deposits formed from dripping water in caves, exemplified by stalactites hanging from ceilings and stalagmites rising from the floor.

  • Sinkholes:

    • Result from the dissolution of limestone or structural collapse; these formations are prevalent in areas such as Florida and Georgia.

  • Karst in Georgia:

    • Predominantly found in the northwestern regions of the state, featuring characteristic karst landforms.

  • Geysers:

    • Natural phenomena that occur when groundwater is heated by nearby magma and erupts when pressure builds sufficiently.