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Oscar E. Meinzer's discussion of the occurrence of groundwater in the United States (1923, The Occurrence of Ground Water in the U.S. with a Discussion of Principles, USGS Water-Supply Paper 489) is a classic in the science of groundwater and geology.  It is an excellent reference for the broadest possible view of the principles of the science.  Meinzer’s work is abstracted and the key points presented below.

Rocks as receptacles of water

All rocks contain openings through which water will flow, especially near the surface of the earth.  In some cases, the rate of flow is almost immeasurable because these openings are so minute or disconnected.  The size, shape, and arrangement of the openings are highly variable.  Technically, the openings are called voids, pores, or interstices.

Secondary openings may develop after rocks are formed.  This type of opening is commonly larger and more linear, but not necessarily better connected than the primary opening.  Secondary openings include caves, tubes, fracture zones, and openings along joints and faults.  Some secondary openings develop as recrystallization of solution of original material takes place.

Porosity of rocks

rock_porosityThe porosity of rock is its property of containing open spaces and can be expressed as the ratio of the total volume of its pore spaces to its total bulk rock volume.  Thus, porosity is expressed as a percentage.  If all the pores are filled, the rock is saturated.  So porosity in the saturated zone is the percentage of the total volume of the rock which contains water.

The photo at right is a computer microtomograhy image showing the porosity of a sample rock core.  Image courtesy of Tom Kotzer, Canadian Light Source Inc.

Conditions affecting porosity in a sedimentary deposit

  • The shape and arrangement of its constituent particles

  • The degree of assortment of its particles

  • The cementation and compacting to which a rock is subjected

  • The removal of mineral matter through solution

  • The fracturing of the rock, resulting in joints.

Sorting

Agents that transport sediment as grains — or aggregates of grains or particles — tend to sort them according to their size, shape, and specific gravity (density).  Particle size can vary greatly in a sedimentary deposit.  Good sorting results in particles of approximately equal size, whether large or small, throughout the sample.  Well-sorted deposits lacking cement have high porosities.  Poorly sorted deposits (mixtures of two or more particle sizes) have less porosity than the same volume of well-sorted material.

Thus, a glacial deposit that is a mixture of clay, boulders, and silt has a much lower porosity than the same volume of well-sorted sand.  However, porosity can change after the sediment is deposited.  Pores can be filled with cement, thus reducing porosity.      Porosity can be increased as the solution removes some of the original material, a situation common in limestone or dolomite terrains.  Hard igneous or metamorphic rocks can develop porosity as they become fractured under stress or as they weather in response to the wear and tear of the elements.

Porosity of granular deposits

Granular deposits consist of individual grains, or aggregates of grains, which are deposited by the action of running water or wind.  The percentage of porosity (pore space) in a granular deposit is determined largely by the way the particles come to rest and what happens to them during compaction.  If we assume the particles are spheres that come to rest with their centers above each other along a vertical axis, the porosity of this arrangement would be 47.64 percent.  This arrangement is highly unlikely as this condition is unstable.  However, if the particles come to rest so that they are arranged in the most compact manner possible, the porosity is reduced to 25.95 percent.  This provides a starting point for the range of porosity possible in a granular deposit which is well-sorted and in which the grains are well-rounded.  Of course, it assumes that no cement exists in the pores.  Any addition of cement would reduce the porosity drastically.

Relation of porosity to the size of grains

If other conditions are the same, a material will have the same porosity regardless of whether it consists of all large or all small grains.  If the particles are well-sorted, the porosity of a deposit which consists of 100 percent silt will be the same as a deposit which consists of 100 percent of the same grain size of sand, providing their volumes are the same.

Relation of porosity to shape of grains

The shapes of individual grains can vary greatly.  The porosity of a deposit consisting of angular grains is greater than the same volume of well-rounded grains providing the sorting is the same in each case (rarely the case). 

Relation of porosity to degree of assortment

It has been noted that deposits consisting of mixtures of grains of different sizes, or of pebbles and sand, will have lower porosities than equal volumes of particles of the same size.  The addition of a large rock to a sand deposit will reduce the total pores considerably.  Solid rock in the form of pebbles or boulders simply occupies spaces formerly containing interstices.  A mechanical analysis is commonly conducted to determine which grain sizes are present in a sample and the percent of each.


The divisions between gravel, sand, and silt are arbitrary, and meant for classification only.  It is rather hard to accept a handful of grains 2 mm in diameter as gravel when accustomed to thinking in terms of somewhat larger pebbles.  In addition, it is rarely necessary to conduct a detailed sieve analysis on a job site, except to select a gravel pack material size.

For logging purposes, fine pebble gravel is 4 mm or approximately one-third to one-half of your smallest fingernail.  Sand is a matter of judgment unless run through standard sieves.  The division between very fine sand and silt is by feel.  If it feels gritty when rubbed between your fingers, it is probably still sand.  Silt would feel smooth to the touch, but gritty to the teeth.  Clay particles are talc-like and feel smooth both to the fingers and teeth.

This sort of judgment is very inexact and subject to personal opinion.  A small metric ruler graded in millimeters is a useful tool for coarse sand and gravel judgment.  Cards with particles of a sieve-determined size glued on may be used for reference.

Consolidated rocks made of the various sizes of particles are conglomerate (gravel), sandstone (sands), mudstone and siltstone (very fine sands, silt), shale (silt, clay), and soapstone (clay).  Many sedimentary deposits, especially glacial ones, are poorly sorted and include several sizes of particle.  Adjectives have to be added then to identify “silty sand,” “sandy gravel,” etc.

Methods of determining porosity

Many methods for determining porosity are available.  A common one is to measure the quantity of water required to saturate a known volume of the dry material.  Another is to compare the specific gravity of a dry sample with that of a saturated sample of the same material.

Meinzer states the procedure for using the specific gravity method.  "The specific gravity of a dry sample of coherent rock can be obtained by coating the sample with paraffin and then weighing it in air and in water.  The specific gravity of the sample is its weight in air divided by its loss of weight in water.  The specific gravity of a dry sample of incoherent material can be obtained by weighing a measured volume of the material and dividing this weight by the weight of an equal volume of water.  The specific gravity of a saturated sample is equal to the weight of the saturated sample divided by the weight of an equal volume of water.  The determination of this value involves saturation of the sample and determination of its volume."

Methods of making mechanical analyses of granular materials

A mechanical analysis consists of:

  • Separating into groups the grains of different sizes in a given volume of material

  • Calculating the weight of each group of grain sizes.

Normally, the following grain sizes are recognized by geologists: gravel, very coarse sand, coarse sand, medium sand, fine sand, very fine sand, silt, and clay.  These grain sizes are defined in terms of size ranges.  By agitating a sample of the deposit in a shaking device (Ro-Tap machine) and catching the particle sizes retained on sieves with different mesh size openings, the particle size groups can be separated.  Other techniques are listed in the USGS Water-Supply Paper 489 by Meinzer.

The above information is excerpted in large part from Chapter 12 of the 1999 NGWA Press publication, Ground Water Hydrology for Water Well Contractors