Ch 10

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Essentials of Geology, 9e:

Essentials of Geology, 9e Groundwater Chapter 10

Importance of groundwater:

Importance of groundwater Groundwater is water found in the pores of soil and sediment, plus narrow fractures in bedrock Groundwater is the largest reservoir of fresh water that is readily available to humans

Importance of groundwater:

Importance of groundwater Geological role of groundwater As an erosional agent, dissolving groundwater produces Sinkholes Caverns Groundwater serves as an equalizer of streamflow

Slide4:

Table 10.1

Distribution of groundwater:

Distribution of groundwater Belt of soil moisture – water held by molecular attraction on soil particles in the near-surface zone Zone of saturation Formation Water not held as soil moisture percolates downward

Distribution of groundwater:

Distribution of groundwater Zone of saturation Formation Water reaches a zone where all of the open spaces in sediment and rock are completely filled with water Water within the pores is called groundwater Water table – the upper limit of the zone of saturation

Distribution of groundwater:

Distribution of groundwater Capillary fringe Extends upward from the water table Groundwater is held by surface tension in tiny passages between grains of soil or sediment

Distribution of groundwater:

Distribution of groundwater Zone of aeration Area above the water table Includes the capillary fringe and the belt of soil moisture Water cannot be pumped by wells

Features associated with subsurface water:

Features associated with subsurface water Figure 10.2

The water table:

The water table The water table is the upper limit of the zone of saturation Variations in the water table Depth is highly variable Varies seasonably and from year to year Shape is usually a subdued replica of the surface topography

The water table:

The water table Variations in the water table Factors that contribute to the irregular surface of the water table Water tends to “pile up” beneath high areas Variations in rainfall Variations in permeability from place to place

The water table:

The water table Interaction between groundwater and streams Constitutes a basic link in the hydrologic cycle Three types of interactions Gaining streams – gain water from the inflow of groundwater through the streambed Losing streams – lose water to the ground-water system by outflow through the stream-bed

Gaining and losing streams:

Gaining and losing streams Figure 10.4

The water table:

The water table Interaction between groundwater and streams Interactions A combination of the first two – a stream gains in some sections and loses in other areas

Movement of groundwater:

Movement of groundwater Darcy’s Law Hydraulic gradient – the water table slope, determined by dividing the vertical difference between the recharge and discharge points by the length of flow between these points Hydraulic head – the vertical difference between the recharge and discharge points

Movement of groundwater:

Movement of groundwater Exceedingly slow – typical rate of movement is a few centimeters per day Energy for the movement is provided by the force of gravity Darcy’s Law – if permeability remains uniform, the velocity of groundwater will increase as the slope of the water table increases

Movement of groundwater:

Movement of groundwater The movement of groundwater is measured directly using Various dyes Carbon-14

Factors influencing the storage and movement of groundwater:

Factors influencing the storage and movement of groundwater Porosity – percentage of total volume of rock or sediment that consists of pore spaces Determines how much groundwater can be stored Variations can be considerable over short distances

Factors influencing the storage and movement of groundwater:

Factors influencing the storage and movement of groundwater Permeability, aquitards, and aquifers Permeability – the ability of a material to transmit a fluid Aquitard – an impermeable layer that hinders or prevents water movement (such as clay) Aquifer – permeable rock strata or sediment that transmits groundwater freely (such as sands and gravels)

Features associated with groundwater:

Features associated with groundwater Springs Occur where the water table intersects Earth’s surface Natural outflow of groundwater Can be caused by an aquitard creating a localized zone of saturation which is called a perched water table

Springs may result from a perched water table:

Features associated with groundwater:

Features associated with groundwater Wells To ensure a continuous supply of water, a well must penetrate below the water table Pumping of wells can cause Drawdown (lowering) of the water table Cone of depression in the water table

Formation of a cone of depression in the water table:

Formation of a cone of depression in the water table Figure 10.7

Features associated with groundwater:

Features associated with groundwater Artesian well – a situation in which groundwater under pressure rises above the level of the aquifer Types of artesian wells Nonflowing – pressure surface is below ground level Flowing – pressure surface is above the ground Not all artesian systems are wells, artesian springs also exist

An artesian well resulting from an inclined aquifer:

An artesian well resulting from an inclined aquifer Figure 10.8

Problems associated with groundwater withdrawal:

Problems associated with groundwater withdrawal Treating groundwater as a nonrenewable resource In many places the water available to recharge the aquifer falls significantly short of the amount being withdrawn Subsidence Ground sinks when water is pumped from wells faster than natural recharge processes can replace it (San Joaquin Valley of California)

Groundwater contamination:

Groundwater contamination One common source is sewage Extremely permeable aquifers, such as coarse gravel, have such large openings that groundwater may travel long distances without being cleaned Sewage often becomes purified as it passes through a few dozen meters of an aquifer composed of sand or permeable sandstone

Groundwater contamination:

Groundwater contamination Sinking a well can lead to groundwater pollution problems Other sources and types of contamination include substances such as Highway salt Fertilizers Pesticides Chemical and industrial materials

Hot springs and geysers:

Hot springs and geysers Hot springs Water is 6-9oC warmer than the mean annual air temperature of the locality The water for most hot springs is heated by cooling of igneous rock

Hot springs and geysers:

Hot springs and geysers Geysers Intermittent hot springs Water erupts with great force Occur where extensive underground chambers exist within hot igneous rock Groundwater heats, expands, changes to steam, and erupts

Old Faithful geyser in Yellowstone National Park:

Hot springs and geysers:

Hot springs and geysers Geysers Chemical sedimentary rock accumulates at the surface Siliceous sinter (from dissolved silica) Travertine (from dissolved calcium car-bonate)

Geothermal energy :

Geothermal energy Tapping natural underground reservoirs of steam and hot water Favorable geologic factors include A potent source of heat Large and porous reservoirs with channels connected to the heat source A cap of low permeability rocks Geothermal energy is not inexhaustible

Slide34:

Table 10.2

Geologic work of groundwater:

Geologic work of groundwater Groundwater dissolves rock Groundwater is often mildly acidic Contains weak carbonic acid Forms when rainwater dissolves carbon dioxide from the air and from decaying plants Carbonic acid reacts with calcite in limestone to form calcium bicarbonate, a soluble material

Geologic work of groundwater:

Geologic work of groundwater Caverns Most caverns are created by acidic groundwater dissolving soluble rock at or just below the surface in the zone of saturation Features found within caverns They form in the zone of aeration

Geologic work of groundwater:

Geologic work of groundwater Caverns Features found within caverns Composed of dripstone (travertine) Calcite deposited as dripping water evaporates Collectively, they are called speleothems Includes stalactites (hanging from the ceiling) and stalagmites (form on the floor of a cavern)

Geologic work of groundwater:

Geologic work of groundwater Karst topography Landscapes that to a large extent have been shaped by the dissolving power of groundwater Some common features include Irregular terrain Sinkhole or sinks (formed by groundwater slowly dissolving the bedrock often accompanied by collapse) Striking lack of surface drainage (streams)

Features of karst topography:

Features of karst topography Figure 10.17

End of Chapter 10:

End of Chapter 10