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. Nebraska is particularly rich in and dependent on groundwater resources. We will step through a number of questions to get a handle on groundwater.

Content:


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?

Paddlers playing in one of the "boiling" sand springs along the Dismal River in the Sand Hills of central Nebraska. The river flows to the left in the background. The Dismal River is largely goundwater fed, and this spring is one of the more visible additions to its flow. One can drop a weight several tens of feet down through the sand suspended by the upward moving water. Rain water that seeped into the sand dunes on either side is returning to the surface environment here.

What are important variables that influence seepage?

The Nebraska Sand Hills landscape is unique in several ways in large part due to the distinctive seepage characteristics of the ground (more on this later).

Artificial recharge basins and other strategies to enhance seepage and feed the underlying aquifers:


How much water can the ground hold?

Porosity: percentage of void space.

Diagram showing the concept of the water table. The unsaturated zone above the water table 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. The water table is not fixed with time, but moves up and down depending on recharge versus discharge rates. Source: http://pubs.usgs.gov/gip/gw_ruralhomeowner/

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

What determines intergrain porosity?

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:

How to measure porosity (in class demonstration)?


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 some pumice floats. Pumice is a volcanic rock type where abundant gas bubbles are trapped in the solidifying frothy lava (more info from the USGS on pumice). 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

Transmissivity: takes into account both permeability and aquifer thickness/size.


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.” Source: USGS http://capp.water.usgs.gov/aquiferBasics/index.html .
A formation here means a defined rock unit that shares characteristics. For example in the Omaha area there is the distinctive brown colored Dakota Sandstone, which is a formation. .

Aquiclude: The opposite of an aquifer, a low permeability unit that acts as a barrier to groundwater flow. An aquitard is somewhere between.

There are many different types of aquifers. Their complexity is due to the great variety of different rock types and different arrangements of those rock types. 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. With an iceberg only 10% is above and the rest is below. 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 consequences?

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 (such as the one depicted above) 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. Which types of sediments are more prone to compaction? Muds can easily compact 2 to 1 as they are buried. So if you have 100 feet of mud the overall compaction you get can be 50'. Sands compact much less.

Water tends to 'support' sediment and its removal can increase or hasten compaction with the following consequences:

Diagram source: Ground Water Atlas of the United States, USGS, http://ca.water.usgs.gov/groundwater/gwatlas/valley/landsub.html. Note that this is 1983 data - subsidence has continued since then.

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. In addition to groundwater withdrawal, pumping out gas and oil may contribute to ground surface subsidence.Note the year of publication. How has this influenced drainage and flooding patterns? Source: http://pubs.usgs.gov/ha/ha730/ch_e/E-text6.html

The caption provides an explanation. Source: USGS http://pubs.usgs.gov/ha/ha730/ch_e/E-text6.html . Muddy coastlines in particular are naturally prone to subsidence and related flooding such as this, as as the muds get buried by more sediment the water gets slowly squeezed out and the muds compact causing flooding. Pumping out the water helps to speed this natural process up.

 


Page created 09/2009, modified 9/17by H. D. Maher.