Porosity, permeability and other groundwater basics

GEOL 1100 lecture outline

Intro statement: For this lecture we will focus on understanding groundwater as a resource. Remember from last lecture that compared to surface water a substantial amount of water resides in the ground, but it is mostly out of sight. In many semi-arid area the agricultural productivity depends on groundwater. Many cities, including Omaha, depend on ground water significantly or totally. In some ways, Nebraska is a groundwater state, rich, but dependent on groundwater resources. We will step through a number of questions to get a handle on groundwater.

How does water get into the ground?

On the one hand the answer is simple - it seeps into the ground. However, on the other hand, what determines how much seeps into the ground is more complex. The diagram below gives some feel. It is an older diagram I constructed, and one of these days it needs to be updated.

This is a simplified system diagram that looks in detail at how water gets into the ground and leaves it. Considering reservoirs, transfer processes, variables and rules, which components are shown here and how, and which components are missing?

Important variables that influence seepage:

• rainfall flux: slow gentle rain produces maximum seepage versus downpour and minimum seepage.
• seepage characteristics of soil
• slope
• temperature (what if frozen?)

Sand Hills landscape (more on this later) and seepage.

Long Island recharge basins.

How much water can the ground hold?

Porosity: percentage of void space.

Diagram showing the concept of the water table. The unsaturated zone is also known as the vadose zone, while the saturated zone is known as the phreatic zone. Also note the depiction of two different types of spaces in the ground that water can reside in. Source: http://pubs.usgs.gov/gip/gw_ruralhomeowner/

Types of porosity: Each type of porosity has its own characteristics.

• intergrain porosity
• fracture porosity
• solution porosity

What determines intergrain porosity?

• grain sorting
• grain shape
• amount of cement
• not grain size

and

Diagrams showing different arrangements of spheres in 2-D, and the different porosities. Remember that sand grounds are spheres and not circles and that in the 3-D world of intergrain porosity the actual amount of porosity will be different, but the general effect of sorting is the same.

Typical porosities:

• conglomerates and sandstones - 20-40%,
• muds - up to 60%,
• shales (lithified muds) up to 10%, often much less.

How to measure porosity?

How fast can water move through the ground?

In this simple diagram depicting isolated pore spaces, such as you might find in pumice, the porosity can be quite high, but the permeability quite low. One outcome of this is that pumice floats. In addition to porosity the geometry, especially the interconnectedness and size of pore spaces, is crucial to understanding how water can flow through the ground. Photo of floating pumice - http://www.hoax-slayer.com/new-pacific-island.shtml

Permeability: describes the ability of water to flow through a medium given a driving force (pressure differential)

Diagram of simple Darcy tube. This is nothing but a pipe filter, but also captures the essence of water moving through the ground.

Darcy's Law

• hydraulic conductivity
• historic development of idea.
  
• set-up and equation for.
• similarity to another basic law of flow?
  

Transmissivity, general flow rates.

What are different types of aquifers?

Aquifer: A formation, group of formations, or part of a formation that contains sufficient saturated, permeable material to yield significant quantities of water to wells and springs.”
A formation here means a defined rock unit that shares characteristics.
Source: USGS http://capp.water.usgs.gov/aquiferBasics/index.html

Aquifers, aquitards, and aquicludes.

There are many different types of aquifers. Their complexity is due to that of the geology. We will review some examples.

Simplified cross section diagram show a variety of different types of aquifers.

One of the most common aquifers utilized are shallow alluvial aquifers which are depicted above. Alluvial fill is just the gravels, sands, silts and muds a river can deposit in its own valley. the river sorts the sediment fairly well, and with the coarser fraction the porosity and permeability both can be relatively high. Diagram source: http://ga.water.usgs.gov/edu/watercycleinfiltration.html

Artesian wells and confined aquifers. If there is an upper bound, a seal, then it is a confined aquifer, and the water pressure can build up in the aquifer.

Simplified depiction of a confined aquifer, where the clay is playing the role of a seal. Source: http://geology.er.usgs.gov/eespteam/brass/aquifers/aquifersintro.htm

Water flowing out of a well that has tapped a confined aquifer at depth, where the water is under such pressure it flows out at the surface without any pumping. This is known as an artesian well. Source: http://ga.water.usgs.gov/edu/watercyclegwdischarge.html

Salt-fresh water aquifer geometries: These are of enormous importance to coastal communities. The key consideration is that a fresh water bubble floats on top of the denser salt water, with a zone of mixing between them. Pumping the groundwater can easily lead to saltwater intrusion, vertically or laterally.

Diagram showing the fresh water bubble and how the geometry changes with pumping. Source: http://pubs.usgs.gov/gip/gw/quality.html

Diagram showing the complexity of fresh-salt water aquifers in the Florida, Brunswick area. Diagram source: http://ga.water.usgs.gov/projects/brunswick/PosterAbstract.html

Groundwater withdrawal

When you pump the groundwater table is pulled down around the well head as depicted in the diagram below to form a cone of depression. What determines the size of your cone of depression?

Diagram source: http://ga.water.usgs.gov/edu/watercycleinfiltration.html

A contoured map of the composite cone of depression in the Chicago area that developed around well fields and due to groundwater withdrawals. How long would it take for this groundwater resource to replenish itself. Source: USGS http://ga.water.usgs.gov/edu/gwdepletion.html. Are you familiar with contour maps and how to read them? If not, please ask about them.

Subsidence related to groundwater withdrawal. Sediments are prone to compaction, and the amounts can be quite significant. Water tends to 'support' sediment and its removal can increase or hasten compaction with the following consequences.

• along coastlines can increase the extent of flooding
• differential movement causes earth fissures and surface faulting in some cases
• change drainage patterns (?)
• irreversible
• many examples including New Orelans. More below.

Diagram source: Ground Water Atlas of the United States, USGS, http://ca.water.usgs.gov/groundwater/gwatlas/valley/landsub.html

Fissure on Edwards Air Force Base attributed to groundwater withdrawal. Photo source: http://ca.water.usgs.gov/groundwater/gwatlas/valley/landsub.html

Map of subsidence pattern for Houston area. Source: http://pubs.usgs.gov/ha/ha730/ch_e/E-text6.html

Source: USGS http://pubs.usgs.gov/ha/ha730/ch_e/E-text6.html

Page created 09/2009 by H. D. Maher.