Hydraulic conductivity, symbolically represented as , is a property of vascular plants, soils and rocks, that describes the ease with which a fluid (usually water) can move through pore spaces or fractures. It depends on the intrinsic permeability of the material, the degree of saturation, and on the density and viscosity of the fluid. Saturated hydraulic conductivity, Ksat, describes water movement through saturated media. By definition, hydraulic conductivity is the ratio of velocity to hydraulic gradient indicating permeability of porous media.
There are two broad categories of determining hydraulic conductivity:
The experimental approach is broadly classified into:
The small scale field tests are further subdivided into:
The methods of determination of hydraulic conductivity and other related issues are investigated by several researchers.
A pedotransfer function (PTF) is a specialized empirical estimation method, used primarily in the soil sciences, however has increasing use in hydrogeology. There are many different PTF methods, however, they all attempt to determine soil properties, such as hydraulic conductivity, given several measured soil properties, such as soil particle size, and bulk density.
There are relatively simple and inexpensive laboratory tests that may be run to determine the hydraulic conductivity of a soil: constant-head method and falling-head method.
The constant-head method is typically used on granular soil. This procedure allows water to move through the soil under a steady state head condition while the volume of water flowing through the soil specimen is measured over a period of time. By knowing the volume of water measured in a time , over a specimen of length and cross-sectional area , as well as the head , the hydraulic conductivity, , can be derived by simply rearranging Darcy's law:
In a constant head experiment, the head (difference between two heights) defines an excess water mass, , where is the density of water. This mass weighs down on the side it is on, creating a pressure differential of , where is the gravitational acceleration. Plugging this directly into the above gives us
If we define the hydraulic conductivity to be related to the hydraulic permeability as
then we have our result. QED
In the falling-head method, the soil sample is first saturated under a specific head condition. The water is then allowed to flow through the soil without adding any water, so the pressure head declines as water passes through the specimen. The advantage to the falling-head method is that it can be used for both fine-grained and coarse-grained soils. . If the head drops from to in a time , then the hydraulic conductivity is equal to
Proof: As above, Darcy's law reads
The decrease in volume is related to the falling head by . Plugging this relationship into the above, and taking the limit as , we find the differential equation
which has the solution
Plugging in and rearranging, we have our result.QED
In compare to laboratory method, field methods gives the most reliable information about the permeability of soil with minimum disturbances. In laboratory methods, the degree of disturbances affect the reliability of value of permeability of the soil.
Pumping test is the most reliable method to calculate the coefficient of permeability of a soil. This test is further classified into Pumping in test and pumping out test.
There are also in-situ methods for measuring the hydraulic conductivity in the field.
When the water table is shallow, the augerhole method, a slug test, can be used for determining the hydraulic conductivity below the water table.
The method was developed by Hooghoudt (1934) in The Netherlands and introduced in the US by Van Bavel en Kirkham (1948).
The method uses the following steps:
where: horizontal saturated hydraulic conductivity (m/day), depth of the waterlevel in the hole relative to the water table in the soil (cm), at time , at time , time (in seconds) since the first measurement of as , and is a factor depending on the geometry of the hole:
where: radius of the cylindrical hole (cm), is the average depth of the water level in the hole relative to the water table in the soil (cm), found as , and is the depth of the bottom of the hole relative to the water table in the soil (cm).
The picture shows a large variation of -values measured with the augerhole method in an area of 100 ha. The ratio between the highest and lowest values is 25. The cumulative frequency distribution is lognormal and was made with the CumFreq program.
The transmissivity is a measure of how much water can be transmitted horizontally, such as to a pumping well.
An aquifer may consist of soil layers. The transmissivity for horizontal flow of the soil layer with a saturated thickness and horizontal hydraulic conductivity is:
Transmissivity is directly proportional to horizontal hydraulic conductivity and thickness . Expressing in m/day and in m, the transmissivity is found in units m2/day.
The total transmissivity of the aquifer is:
The apparent horizontal hydraulic conductivity of the aquifer is:
where , the total thickness of the aquifer, is , with .
Influence of the water table
When a soil layer is above the water table, it is not saturated and does not contribute to the transmissivity. When the soil layer is entirely below the water table, its saturated thickness corresponds to the thickness of the soil layer itself. When the water table is inside a soil layer, the saturated thickness corresponds to the distance of the water table to the bottom of the layer. As the water table may behave dynamically, this thickness may change from place to place or from time to time, so that the transmissivity may vary accordingly.
In a semi-confined aquifer, the water table is found within a soil layer with a negligibly small transmissivity, so that changes of the total transmissivity () resulting from changes in the level of the water table are negligibly small.
When pumping water from an unconfined aquifer, where the water table is inside a soil layer with a significant transmissivity, the water table may be drawn down whereby the transmissivity reduces and the flow of water to the well diminishes.
The resistance to vertical flow () of the soil layer with a saturated thickness and vertical hydraulic conductivity is:
Expressing in m/day and in m, the resistance () is expressed in days.
The total resistance () of the aquifer is:
where signifies the summation over all layers:
The apparent vertical hydraulic conductivity () of the aquifer is:
where is the total thickness of the aquifer: , with
The resistance plays a role in aquifers where a sequence of layers occurs with varying horizontal permeability so that horizontal flow is found mainly in the layers with high horizontal permeability while the layers with low horizontal permeability transmit the water mainly in a vertical sense.
When the horizontal and vertical hydraulic conductivity ( and ) of the soil layer differ considerably, the layer is said to be anisotropic with respect to hydraulic conductivity.
When the apparent horizontal and vertical hydraulic conductivity ( and ) differ considerably, the aquifer is said to be anisotropic with respect to hydraulic conductivity.
An aquifer is called semi-confined when a saturated layer with a relatively small horizontal hydraulic conductivity (the semi-confining layer or aquitard) overlies a layer with a relatively high horizontal hydraulic conductivity so that the flow of groundwater in the first layer is mainly vertical and in the second layer mainly horizontal.
The resistance of a semi-confining top layer of an aquifer can be determined from pumping tests.
When calculating flow to drains or to a well field in an aquifer with the aim to control the water table, the anisotropy is to be taken into account, otherwise the result may be erroneous.
Because of their high porosity and permeability, sand and gravel aquifers have higher hydraulic conductivity than clay or unfractured granite aquifers. Sand or gravel aquifers would thus be easier to extract water from (e.g., using a pumping well) because of their high transmissivity, compared to clay or unfractured bedrock aquifers.
Hydraulic conductivity has units with dimensions of length per time (e.g., m/s, ft/day and (gal/day)/ft² ); transmissivity then has units with dimensions of length squared per time. The following table gives some typical ranges (illustrating the many orders of magnitude which are likely) for K values.
Hydraulic conductivity (K) is one of the most complex and important of the properties of aquifers in hydrogeology as the values found in nature:
Table of saturated hydraulic conductivity (K) values found in nature
Values are for typical fresh groundwater conditions — using standard values of viscosity and specific gravity for water at 20 °C and 1 atm. See the similar table derived from the same source for intrinsic permeability values.
|Unconsolidated Sand & Gravel||Well Sorted Gravel||Well Sorted Sand or Sand & Gravel||Very Fine Sand, Silt, Loess, Loam|
|Unconsolidated Clay & Organic||Peat||Layered Clay||Fat / Unweathered Clay|
|Consolidated Rocks||Highly Fractured Rocks||Oil Reservoir Rocks||Fresh Sandstone||Fresh Limestone, Dolomite||Fresh Granite|
Source: modified from Bear, 1972
An anchialine pool or pond (pronounced "AN-key-ah-line", from Greek ankhialos, "near the sea") is a landlocked body of water with a subterranean connection to the ocean. Anchialine pools are a feature of coastal aquifers which are density stratified, with the water near the surface being fresh or brackish, and saline water intruding from the coast below at some depth. Depending on the site, it is sometimes possible to access the deeper saline water directly in the anchialine pool or sometimes it may be accessible by cave diving.Water levels in anchialine pools often fluctuate with tidal changes due to the coastal location and the connection with the ocean. The range in water levels fluctuations will be decreased (damped) and delayed compared to the range and time observed for the adjacent tide. The primary controls on the damping and lag are the distance from the coast, and the hydraulic conductivity of the geological materials.
Anchialine pools are extremely common worldwide especially along Neotropical coastlines where the geology and aquifer system are relatively young, and there is minimal soil development. Such conditions occur notably where the bedrock is limestone or recently formed volcanic lava. Many anchialine pools are found on the coastlines of the island of Hawaii, and on the Yucatán Peninsula, where they are locally called cenotes, as well as Christmas Island. The Sailor's Hat crater created by an explosives test in 1965 is an anchialine pool.Ecological studies of anchialine pools frequently identify regionally rare and sometimes endemic species. In Hawaii, the pools are home to the ʻōpaeʻula (Hawaiian shrimp, Halocaridina rubra). In karst anchialine pools and the caves these may be connected to, the fauna are diverse and include crustaceans, including remipedia and copepods, and among the vertebrates are several species of blind cave fish.Bioclogging
Bioclogging or biological clogging is clogging of pore space in soil by microbial biomass; their body and their byproducts such as extracellular polymeric substance (EPS). The microbial biomass blocks the pathway of water in the pore space, forming a certain thickness of impermeable layer in soil, and it reduces the rate of infiltration of water remarkably.
Bioclogging is observed under continuous ponded infiltration at various field conditions such as artificial recharge ponds, percolation trench, irrigation channel, sewage treatment system and landfill liner. It also affects groundwater flow in aquifer, such as permeable reactive barrier and microbial enhanced oil recovery. In the situation where infiltration of water at appropriate rate is needed, bioclogging can be problematic and countermeasures such as regular drying of the system are taken. In some cases bioclogging can be utilized to make impermeable layer to minimize the rate of infiltration.Conductivity
Conductivity may refer to:
Electrical conductivity, a measure of a material's ability to conduct an electric current
Conductivity (electrolytic), the electrical conductivity of an electrolyte in solution
Ionic conductivity (solid state), electrical conductivity due to ions moving position in a crystal lattice
Hydraulic conductivity, a property of a porous material's ability to transmit water
Thermal conductivity, an intensive property of a material that indicates its ability to conduct heatConsolidation (soil)
In. soil mechanics, consolidation refers to the process by which soil changes volume gradually in response to a change in pressure. This happens because soil is a two-phase material, comprising soil grains and pore fluid, usually groundwater. When soil saturated with water is subject to an increase in pressure, the high volumetric stiffness of water compared to the soil matrix means that the water initially absorbs all the change in pressure without changing volume, creating excess pore water pressure. As water diffuses away from regions of high pressure due to seepage, the soil matrix gradually takes up the pressure change and shrinks in volume. The theoretical framework of consolidation is therefore closely related to the diffusion equation, the concept of effective stress, and hydraulic conductivity.
In the narrow sense, "consolidation" refers strictly to this delayed volumetric response to pressure change due to gradual movement of water. Some publications also use "consolidation" in the broad sense, to refer to any process by which soil changes volume due to a change in applied pressure. This broader definition encompasses the overall concept of soil compaction, subsidence, and heave. Some types of soil, mainly those rich in organic matter, show significant creep, whereby the soil changes volume slowly at constant effective stress over a longer time-scale than consolidation due to the diffusion of water. To distinguish between the two mechanisms, "primary consolidation" refers to consolidation due to dissipation of excess water pressure, while "secondary consolidation" refers to the creep process.
The effects of consolidation are most conspicuous where a building sits over a layer of soil with low stiffness and low permeability, such as marine clay, leading to large settlement over many years. Types of construction project where consolidation often poses technical risk include land reclamation, the construction of embankments, and tunnel and basement excavation in clay.
Geotechnical engineers use oedometers to quantify the effects of consolidation. In an oedometer test, a series of known pressures are applied to a thin disc of soil sample, and the change of sample thickness with time is recorded. This allows the consolidation characteristics of the soil to be quantified in terms of the coefficient of consolidation () and hydraulic conductivity ().CumFreq
In statistics and data analysis the application software CumFreq is a tool for cumulative frequency analysis of a single variable and for probability distribution fitting.Originally the method was developed for the analysis of hydrological measurements of spatially varying magnitudes (e.g. hydraulic conductivity of the soil) and of magnitudes varying in time (e.g. rainfall, river discharge) to find their return periods. However, it can be used for many other types of phenomena, including those that contain negative values.Disc permeameter
The disc permeameter is a field instrument used for measuring water infiltration in the soil, which is characterized by in situ saturated and unsaturated soil hydraulic properties. It is mainly used to provide estimates of the hydraulic conductivity of the soil near saturation.Drainage equation
A drainage equation is an equation describing the relation between depth and spacing of parallel subsurface drains, depth of the watertable, depth and hydraulic conductivity of the soils. It is used in drainage design.
A well known steady-state drainage equation is the Hooghoudt drain spacing equation. Its original publication is in Dutch. The equation was introduced in the USA by van Schilfgaarde.Geosynthetic clay liner
Geosynthetic clay liners (GCLs) are factory manufactured hydraulic barriers consisting of a layer of bentonite or other very low-permeability material supported by geotextiles and/or geomembranes, mechanically held together by needling, stitching, or chemical adhesives. Due to environmental laws, any seepage from landfills must be collected and properly disposed of, otherwise contamination of the surrounding ground water could cause major environmental and/or ecological problems. The lower the hydraulic conductivity the more effective the GCL will be at retaining seepage inside of the landfill. Bentonite composed predominantly (>70%) of montmorillonite or other expansive clays, are preferred and most commonly used in GCLs. A general GCL construction would consist of two layers of geosynthetics stitched together enclosing a layer of natural or processed sodium bentonite. Typically, woven and/or non-woven textile geosynthetics are used, however polyethylene or geomembrane layers or geogrid geotextiles materials have also been incorporated into the design or in place of a textile layer to increase strength. GCLs are produced by several large companies in North America, Europe, and Asia. The United States Environmental Protection Agency currently regulates landfill construction and design in the US through several legislations.Groundwater discharge
Groundwater discharge is the volumetric flow rate of groundwater through an aquifer.
Total groundwater discharge, as reported through a specified area, is similarly expressed as:
For example, this can be used to determine the flow rate of water flowing along a plane with known geometry.Groundwater model
Groundwater models are computer models of groundwater flow systems, and are used by hydrogeologists. Groundwater models are used to simulate and predict aquifer conditions.Interflow
In hydrology, interflow is the lateral movement of water in the unsaturated zone, or vadose zone, that first returns to the surface or enters a stream prior to becoming groundwater. Interflow is sometimes used interchangeably with throughflow; however, throughflow is specifically the subcomponent of interflow that returns to the surface, as overland flow, prior to entering a stream or becoming groundwater. Interflow occurs when water infiltrates (see infiltration (hydrology)) into the subsurface, hydraulic conductivity decreases with depth, and lateral flow proceeds downslope. As water accumulates in the subsurface, saturation may occur, and interflow may exfiltrate as return flows, becoming overland flow.Lugeon
A Lugeon is a unit devised to quantify the water permeability of bedrock and the hydraulic conductivity resulting from fractures; it is named after Maurice Lugeon, a Swiss geologist who first formulated the method in 1933. More specifically, the Lugeon test is used to measure the amount of water injected into a segment of the bored hole under a steady pressure; the value (Lugeon value) is defined as the loss of water in litres per minute and per metre borehole at an over-pressure of 1 MPa.
Although the Lugeon test may serve other purposes, its main object is to determine the Lugeon coefficient which by definition is water absorption measured in litres per metre of test-stage per minute at a pressure of 10 kg/cm2 (1 MN/m2).Macropore
In soil, macropores are defined as cavities that are larger than 75 μm. Functionally, pores of this size host preferential soil solution flow and rapid transport of solutes and colloids. Macropores increase the hydraulic conductivity of soil, allowing water to infiltrate and drain quickly, and shallow groundwater to move relatively rapidly via lateral flow. In soil, macropores are created by plant roots, soil cracks, soil fauna, and by aggregation of soil particles into peds.
Macropores may be defined differently in other contexts. Within the context of porous solids (i.e., not porous aggregations such as soil), colloid and surface chemists define macropores as cavities that are larger than 50 nm.Permeability (earth sciences)
Permeability in fluid mechanics and the earth sciences (commonly symbolized as k) is a measure of the ability of a porous material (often, a rock or an unconsolidated material) to allow fluids to pass through it.
The permeability of a medium is related to the porosity, but also to the shapes of the pores in the medium and their level of connectedness.Pore space in soil
The pore space of soil contains the liquid and gas phases of soil, i.e., everything but the solid phase that contains mainly minerals of varying sizes as well as organic compounds.
In order to understand porosity better a series of equations have been used to express the quantitative interactions between the three phases of soil.
Macropores or fractures play a major role in infiltration rates in many soils as well as preferential flow patterns, hydraulic conductivity and evapotranspiration. Cracks are also very influential in gas exchange, influencing respiration within soils. Modeling cracks therefore helps understand how these processes work and what the effects of changes in soil cracking such as compaction, can have on these processes.Porosity
Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measure the "accessible void", the total amount of void space accessible from the surface (cf. closed-cell foam). There are many ways to test porosity in a substance or part, such as industrial CT scanning. The term porosity is used in multiple fields including pharmaceutics, ceramics, metallurgy, materials, manufacturing, hydrology, earth sciences, soil mechanics and engineering.Puddling (civil engineering)
Puddling is both the material and the process of lining a water body such as a channel or pond with puddle clay (puddle, puddling) - a watertight (low hydraulic conductivity) material based on clay and water mixed to be workable,Stagnosol
A Stagnosol in the World Reference Base for Soil Resources (WRB) is soil with strong mottling of the soil profile due to redox processes caused by stagnating surface water. Stagnosols are periodically wet and mottled in the topsoil and subsoil, with or without concretions and/or bleaching. The topsoil can also be completely bleached (albic horizon). A common name in many national classification systems for most Stagnosols is pseudogley. In the USDA soil taxonomy, many of them belong to the Aqualfs, Aquults, Aquents, Aquepts and Aquolls.
They are developed in a wide variety of unconsolidated materials like glacial till, and loamy aeolian, alluvial and colluvial deposits and physically weathered siltstone. Stagnosols occur on flat to gently sloping land in cool temperate to subtropical regions with humid to perhumid climate conditions.
The agricultural suitability of Stagnosols is limited because of their oxygen deficiency resulting from stagnating water above a dense subsoil. Therefore, they have to be drained. However, in contrast to Gleysols, drainage with channels or pipes is in many cases insufficient. It is necessary to have a higher porosity in the subsoil in order to improve the hydraulic conductivity. This may be achieved by deep loosening or deep ploughing. Drained Stagnosols can be fertile soils owing to their moderate degree of leaching.
Stagnosols cover 150–200 million ha worldwide. For the greater part in humid to perhumid temperate regions of West and Central Europe, North America, southeast Australia and Argentina. Here Stagnosols are associated with Luvisols as well as silty to clayey Cambisols and Umbrisols. They also occur in humid to perhumid subtropical regions, where they are associated with Acrisols and Planosols.
with a light-coloured, coarse-textured, surface horizon that shows signs of periodic water stagnation and abruptly overlies a dense, slowly permeable subsoil with significantly more clay than the surface horizon. In the US Soil Classification of 1938 used the name Planosols, whereas its successor, the USDA soil taxonomy, includes most Planosols in the Great Groups Albaqualfs, Albaquults and Argialbolls.Throughflow
In hydrology, throughflow, a subcomponent of interflow, is the lateral unsaturated flow of water in the soil zone, where a highly permeable geologic unit overlays a less permeable geologic unit, and which returns to the surface, as return flow, prior to entering a stream or groundwater. Once water infiltrates into the soil, it is still affected by gravity and either infiltrates to the water table or travels downslope. Throughflow usually occurs during peak hydrologic events, and flow rates are dependent on the hydraulic conductivity of the geologic medium.