The two most important forces controlling water movement in rock are gravity and molecular attraction. Gravity causes water to infiltrate until it reaches impermeable zones where it is diverted laterally. Gravity generates the flow of springs, rivers, and wells. If the pores in rocks and sediments are connected, gravity allows the water to move slowly through them. However, the smaller the opening, the harder it is for gravity to cause water movement. The second force, molecular attraction, slows the flow of water through small pores. Water is attracted to the surface of every particle with which it comes in contact. The force results from the attraction of the molecules of two substances for each other.
The attraction between water and soil or rock particles is termed adhesion. It is effective only over short distances. Thus, only a thin film of water is locked to the outside of each grain resisting the flow downward in response to gravity. It is this adhesion that helps hold water in the soil for plants. If gravity were the only force involved, all water would drain through the soil to some depth. In fine-grained sediments such as silt and clay, the aggregate surface area which can attract water molecules is very great. Fine-grained materials hold more water over a longer period of time than the same volume of coarse-grained materials such as sand or gravel.
Surface tension and capillarity
The attraction of water molecules for each other is termed cohesion. It can be demonstrated by immersing a pencil in water and noting the drop that remains at the base of the pencil, seemingly held there by the water above it. This attraction is due to the surface tension characteristic of water, caused by cohesion. Water will also rise in a small tube if it is immersed. This phenomenon is called capillary action or capillarity. The smaller the tube, the higher the water will rise. In The Occurrence of Ground Water in the U.S. with a Discussion of Principles, USGS Water-Supply Paper 489 (1923), Oscar E. Meinzer states the reason for this attraction of the water for the walls of the tube as follows: “The water in a capillary tube is held up not only by the attraction of the walls of the tube for the water but by this attraction acting through the cohesion of the water, whereby the influence of the attraction of the water was extended far beyond the range of molecular forces.” Capillary action is important in rocks and sediments because pores immediately above the saturated zone are filled with capillary water. The more fine-grained the sediment or rock, the higher the water is pulled. The diameter of the pore opening and the degree of connection with the saturated zone is very important. So much water is drawn into the pores above the water table that this zone is given a special term, the capillary fringe.
Permeability of rocks
Permeability is the capacity of a rock to transmit water under pressure. If no pressure exists, a static equilibrium is present and there is no tendency for water to move. This condition is very rare in nature. Most water can be thought to be in a dynamic state or moving in response to a pressure gradient.
Meinzer defines permeability as follows: “The permeability of a rock is measured by the rate at which it will transmit water through a given cross-section under a given difference of pressure per unit of distance.” In a sequence of sedimentary rock with varying permeability, it commonly can be shown that horizontal permeability or permeability that is parallel to the bedding of rocks such as sandstone and conglomerate is greater than permeability at right angles to bedding. This is because some beds in the sequence have such low permeability that vertical infiltration is slow whereas lateral permeability in units below confining beds is good.
No rocks near the surface of the earth are impermeable if enough pressure is applied in forcing the water through the natural openings in the rock. However, the forces generated by nature are insufficient in some cases to produce detectable permeability and rocks with such characteristics are said to be relatively impermeable. Examples of such rocks are found in shales that contain clays that swell on wetting and thus close off natural openings that may exist when the rock is dry. On the other hand, coarse, clean gravel contains such large openings that it readily transmits water. Ordinarily, such deposits function as the best aquifers where they can be easily recharged. Dirty or clay-rich gravels have much less permeability because the fine silt and clay between the larger particles effectively slow down or block completely the flow of water through some of the pores between the sand grains.
Coefficient of permeability
The coefficient of permeability (P) used by the USGS may be expressed as the number of gallons of water a day, at 60 °F, that is conducted laterally through each square foot of water-bearing material (measured at right angles to the direction of flow), under a hydraulic gradient of 1 foot per foot. It has the units of gallons per day per square foot (gpd per sq. ft.).
For analyzing field tests involving flow through the entire thickness of aquifers, it is generally more convenient to use the coefficient of transmissivity (T) of C.V. Theis (1935, The relation between lowering of the piezometric surface and rate and duration of discharge of a well using groundwater storage, Transactions of the American Geophysical Union 16, 519-524). Theis expressed as “T = coefficient of transmissiblity of aquifer, in gallons a day, through each 1-foot strip extending the height of the aquifer, under a unit gradient—this is the average coefficient of permeability (Meinzer) multiplied by the thickness of the aquifer.” It is expressed in gallons per day per foot (gpd per ft.). Both definitions are based upon Darcy’s law.*
Darcy’s law states that the rate of movement of water through porous media is proportional to the hydraulic gradient:
q = k × dh/dl
in which q = velocity of movement; k = constant of proportionality, which is the hydraulic conductivity; and dh/dl = hydraulic gradient, expressed as a change in head (dh) over a given change in flow length (dl).
For review, hydraulic gradient is the change in static head per unit of distance in a given direction, usually the direction of maximum decrease. Hydraulic gradient may be expressed in ft. per ft. or cm, per meter, etc., in the same way slope may be written.
*Groundwater hydrology began as a quantitative science when Henry Philibert Gaspard Darcy (1803-1858), a French hydraulic engineer, published a report on the water supply of Dijon, France. Darcy’s law is a foundation stone for several fields of study including groundwater hydrology, soil physics, and petroleum engineering.