Interpretive Glossary of Water-related Terms and Expressions - H

hardness: A term that refers to the relative concentrations of calcium ions and magnesium ions in water. See hard water and water analysis.

hard water: A water solution that contains significant concentrations of ions from calcium and magnesium compounds, carbonates and/or sulfates, particularly calcium bicarbonate (Ca(HCO3)2).

hard rock: A well cemented, consolidated sedimentary rock.

head: See hydrostatic head.

headgate: A physical structure on a reservoir, river or stream through which water is diverted into a ditch.

heaving shale: Shale that becomes hydrated and/or breaks free of the formation by the action of drilling mud, the drill bit, and drill pipe during the drilling process.

historic use: The documented diversion and use, by a water right holder, of water in a ditch over a period of years. Douglas Co.

hydraulic: Of or pertaining to the mechanics of water to provide pressure and force. In water wells it is the form of pressure that drives water to seek relief in an environment at lower pressure. This is the force that drives water to a well bore. See hydraulic pressure, hydrostatic pressure and hydrostatic load.

hydraulically connected: A condition where two or more aquifers are in communication with each other. This condition can be recognized when the static water levels are nearly the same. See water level (1).

hydraulic conductivity: In 100% water saturated aquifers, hydraulic conductivity is the ease with which water can move through the interstices of the aquifer under a specific pressure gradient. Applies to either vertical or lateral flow. A part of Darcy's equation. If vertical hydraulic conductivity differs from lateral or horizontal hydraulic conductivity, the aquifer is said to be anisotropic.

Darcy’ linear equation is:

                                    q = ( kA / μ ) × ( Δpd )                                 (1)

where q = fluid flow rate, k = absolute permeability to water, A = cross-sectional area of cumulative flow paths within the aquifer, μ = viscosity of fresh water, Δp = initial pressure difference that provides the energy for ground water to flow across distance Δd. This is an equation that is commonly applied to liquid flow rates in linear lengths of rock. This equation can be rewritten as:

                                    q = K (A × Δp / Δd )                                            (2)

where K = k / μ is the hydraulic conductivity for the water in a saturated aquifer. This relationship also is the equation for mobility, the ease with which any specific geofluid can be moved through a porous medium. In aquifers, it is the ease with which ground water can be moved. Here, hydraulic conductivity is equal to, and the same thing, as mobility. The term Δp / Δd is the hydraulic pressure gradient. If Eq.(2) above is solved for K, an alternate equation for hydraulic conductivity is

K = ( q / A ) × ( Δd / Δp )

which is equal to the flow rate per unit cross-sectional area divided by the hydraulic pressure gradient,  Δp / Δd.

Transmissivity is defined as K times the net saturated thickness of the aquifer determined from petrophysical well logs.

For any specific potentiometric surface measurement, the measurement is site specific. The reference datum level is the elevation of either the perforations or the level where water emerges from the aquifer in a specific water well.

The expression Δp / Δd is the pressure gradient or hydraulic pressure gradient where the initial energy is related to Δp measured between the elevation of the water table at the recharge area and the elevation of the perforations or the level where water emerges from the aquifer. At distance Δd, the distance from the recharge area to the well site, the potentiometric surface has declined and the energy remaining for production,  Δp, has declined accordingly. At the producing well site,  reduced Δp is the hydrostatic head that determines both the amount of overpressure of the water in the aquifer and the flow rate. Here, the potentiometric surface has fallen to a lower elevation and is the water level within a static well bore that balances the formation-water pressure. The energy remaining for production,  reduced Δp, is measured between the elevation of the new potentiometric surface at the well site and the elevation of the level where water emerges from the aquifer. See potentiometric surface, and basic explanation of Darcy’s equation at Darcy’s equation.

hydraulic fracturing: The process of stimulating a formation by creating additional permeability through manmade fractures. A water compatible with the formation is injected into a formation exhibiting low permeability. Because the formation has low permeability the injection fluid will have difficulty penetrating the rock and the injection pressure will rise. When the injection pressure exceeds the strength of the rock and overcomes internal stresses, the rock will fracture or split. The fracture must remain open for the stimulation to be successful, therefore, clean sand, aluminum pellets, glass beads, ceramic grains (radioactive or nonradioactive) or other similar materials are pumped into the fracture as propping material, or proppants, to hold the faces of the fracture apart. Hydraulic fracturing is similar in practice to injection of waste liquids. Except in the case of hydraulic fracturing, much greater injection pressures are used. In such cases, high pressure injection has caused lubrication of fault interfaces as well as fractures with the result of minor earthquakes. Also see formation damage.

The hydraulic liquid usually is water based. Often the water solution will contain appropriate chemicals necessary to increase the flow rate of the produced fluid. The chemicals also might be designed to prevent swelling of clays and to control wettability of the rock. See discussions under recovery.

hydraulic gradient: Hydraulic pressure gradient. See equation form as it appears in Darcy’s equation under hydraulic conductivity. See pressure gradient, also see drift.

hydraulic head: Total pressure head. Non-directional. Sum of both vertical and horizontal components of pressure. Compare hydrostatic head and hydraulic pressure.

hydraulically connected: A condition where two or more aquifers are in communication with each other. This condition can be recognized when the static water levels are nearly the same. See water level (1).

hydraulic pressure: The kind of pressure that exists in the water stream of an aquifer at any specific time. Hydraulic pressure is non-directional, but sometimes is considered to be acting primarily in lateral directions as opposed to vertical directions. Derives its energy from hydrostatic load. Expressed in psi.

(1) In aquifers, backpressure and resistance to flow must be overcome by hydraulic pressure for water production to take place. Regardless of the source of the energy, it is hydraulic pressure that provides sufficient lateral pressure gradient to force the formation water into the lower pressure environment of the well bore of a water well.

(2) Pascal's Law states that pressure applied to any part of a free but confined liquid (i.e. water) exerts the same pressure to every other part. In a confined aquifer, either an increase in pressure or a decrease in pressure can be transmitted to all other parts of the permeable aquifer system, if they are hydraulically connected, and subject to permeability, until any difference between water pressures reaches equilibrium. As a result, any decrease in water pressure caused by production at remote wells causes a decrease in water pressure at local wells with the consequent decrease in the life of local water production. See depleted (2).

hydrogen sulfide: The poisonous and corrosive gas of hydrogen and sulfur (H2S). Occurs naturally in some earth formations.

hydrogeology: The study of ground water and its relationship to geology. Also sometimes known as geohydrology. GWAC.

O. E. Meinzer is considered to be the father of hydrogeology. He derived and defined many of the terms used in the study of ground water and hydrogeology (1923). His work preceded any petrophysical well logs and, as a result, he and his followers had no means to describe the characteristics and properties of aquifers by remote, downhole methods. The first petrophysical well log was not performed until Sept. 5, 1927. See well log and petrophysical well log.

hydrograph: A graph showing stage, flow, velocity, or other characteristics of water with respect to time. A hydrograph commonly shows rate of flow; a groundwater hydrograph shows water level depth and hydrostatic head in a well. GWAC.

hydrologic: Pertains to the study of the movement of and distribution of water on the earth. Douglas Co.

hydrologic budget, hydrologic balance: Accounting of the inflow to, outflow from, and storage in a hydrologic unit such as a drainage basin, aquifer, soil zone, lake, or reservoir; the relationship between evaporation, precipitation, runoff, and the change in water storage, expressed by the hydrologic equation. GWAC.

hydrologic cycle: The cycle of water movement from the atmosphere to the earth and back to the atmosphere through evaporation, transpiration, condensation, precipitation, percolation, runoff, and storage. CSU. Renewable water can be renewed only through the hydrologic cycle of water movement of which precipitation is the key to the renewal and sustainability of our water supply.

hydrology: The science and study of underground water resources and the study and treatment of above-ground water, its systems and cycles. Hydrology pertains to the science of surface water and ground water, whereas hydrogeology focuses on ground water.

hydrostatic head: Generic. (1) In a water well, the pressure of a static column of water in a well bore from the water surface to any depth of interest. The water surface can be any depth depending on formation pressure, and is not necessarily the water table. The hydrostatic head of a confined aquifer, for example, is a measurement between the elevations of the potentiometric surface and the elevation of the level that the water emerges from the aquifer. That pressure for fresh water is 0.433 (psi) per foot of column height. See potentiometric surface (1).

(2) In a water well, the pressure of a static column of water in a well bore from the water surface to the depth of water emergence from the aquifer face. The hydrostatic head in the well bore of a water well is the artesian head. See artesian water and artesian head.

(3) In a drilling well, the hydrostatic pressure of the drilling mud from the ground surface to the depth of interest.

hydrostatic load: Sometimes called hydraulic head. Specific. The total pressure of a column of formation fluid, usually water, standing between the water table and the depth of interest. Expressed in psi (pounds per square inch of area). The load is 0.433 (psi per vertical foot of depth) times the specific gravity of the actual water of interest (a ratio) multiplied by the depth of interest (feet) measured from the water table. The product 0.433 (psi per foot of depth) times the specific gravity of the actual water of interest will be a relatively unvarying hydrostatic pressure gradient in water wells. See also normal pore pressure. Also see compaction (2).

In an aquifer, hydrostatic load, by virtue of its height and weight, provides the energy to transmit pressure from the water table to a higher pressure environment at depth where the water is then forced by hydraulic pressure to seek relief in the lower pressure environment of the well bore of a water well. Also see pressure gradient. See also artesian head and potentiometric surface.

hydrostatic pressure: One of a number of different forms of drive mechanisms. A static pressure and the vertical component of hydraulic pressure. Hydrostatic load generates hydrostatic pressure and is the natural source of the energy in hydraulic pressure. See aquifer (2), hydrostatic head, hydrostatic load, and hydraulic pressure (2).


Compiled and Edited by Robert C. Ransom


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