Hydraulic Conductivity, How it Is Measured and Why it Is Important for Transient Storage
The hydraulic conductivity of soil is related to its texture. The rate is generally higher in coarser soils, but it is also influenced by structure and can be profoundly influenced by soil management operations and the exchangeable cation status (Richards 1956). The quality of irrigation water is an important consideration when determining irrigation feasibility and permanence alternatives. Rather than hazards to irrigation agriculture from the soluble constituents of irrigation water, the main problems appear to be the accumulation of soluble salts and exchangeable sodium in soil. In this regard, the salinity of irrigation water has a direct impact on such factors as crop selection, the appropriate method of application of the water, as well as the leaching required to effectively manage salt accumulations in the soil, all of which are factors subject to constraints imposed by drainage conditions (Richards 1956). This paper provides an overview of hydraulic conductivity, how it is measured, and why it is important for transient storage. A review of current issues in hydraulic conductivity is followed by a summary of the research in the conclusion.
Review and Discussion
Background and Overview. The rate at which water flows through soil is dependent on the gradient of hydraulic potential (the sum of capillary potential and elevation) and the physical properties of the soil expressed in terms of a parameter called hydraulic conductivity, which varies with soil moisture in a nonlinear fashion (Beven 2004). The steady-state infiltration rate of water is equivalent to the saturated hydraulic conductivity of the soil surface (Ceballos, Cerda, & Schnabel 2000). Measured sample values of hydraulic conductivity have been shown to rapidly vary in space, making the application of measured point values for predictive purposes at larger scales subject to some degree of uncertainty. According to Beven, among other factors, water moves in soil because of differences in temperature and chemical concentrations of solutes in soil water; the latter can be expressed as an osmotic potential; this rate is especially important for the movement of water into plant roots due to high solute concentrations within the root water (2004). Richards reports that leaching is accomplished by the downward movement of water through soil; however, in order to be adequately drained, any water table that tends to form in irrigated land must be maintained well below the root zone. Effective management requires that outlets be available for the groundwater and that water transmission in the subsoil be appreciable. Based on its relation to water application methods and to leaching, the rate at which a soil will transmit water is one of its most important physical properties for irrigation applications (Edwards 1956). The French engineer Darcy formulated an empirical law in the mid-19th century that provided a macroscopic model for groundwater movement. According to Ford et al. (1987), Darcy's Law relates the rate at which groundwater flows across a surface to the rate of change of energy of the groundwater along the flow path. Under ideal 20 homogeneous isotropic geologic groundwater conditions, the average linear groundwater velocity ((v)) can be expressed using the Darcy relation as follows:
v = - [K dh/dl]/n where: v = the average linear groundwater velocity
K = hydraulic conductivity dh/dl = hydraulic gradient n = transport porosity (Ford et al. 1987)
Ford et al. note that the same equation is used to compute values for average linear groundwater velocity for both granular and fractured media, with the primary difference is in the value of n. "For granular media n nearly represents the total porosity; in fractured rock or clay n represents the total void space in connected fractures within a unit volume of media" (Ford et al. 20). When the values of K. And dh/dl are similar in media that is granular and fractured, the average linear groundwater velocity may be orders of magnitude larger in fractured media than in granular media; the velocity of groundwater in a horizontal sand and gravel aquifer is typically in the range of 0.05 to 1 meter per day (Ford et al. 1987).
In 2003, Thomas, Valetta, Webster and Mulholland (2003), reported they had developed the Regression Partitioning Method (RPM) for estimating the proportion of reactive solute uptake occurring within transient storage zones of streams. "The RPM is a technique for analyzing solute addition data in which whole stream uptake (mg m?2 d?1) is determined from the longitudinal pattern in plateau tracer concentrations. At one location, a time series of samples are collected that define the 'rising limb' of the solute breakthrough curve" (Thomas et al. 965). These researcher estimated the y-intercept by regressing a measure of reactive tracer availability (e.g., NO3 -- 15N:Cl ratio) as well as the percentage of tracer that has resided within, and returned from, the transient storage zone (i.e. hyporheic zone), which was then used to predict channel-specific NO3 uptake rates. The uptake within the transient storage zone of stream-derived material was calculated by difference.
A number of numerical steps were developed by these researchers that associated the link uptake rate estimates with first-order reaction rate constants (C and S, min?1) which have previously been more commonly used to describe solute behavior in one-dimensional transport models. According to Thomas et al., "The RPM was used to analyze the results of 2 stable isotope additions performed in Snake Den Branch, a small headwater stream in western North Carolina, USA. Channel-specific uptake rates (UC) ranged from 10.6 to 23.0 mg NO3 -- N m?2 d?1 and slightly exceeded uptake in the transient storage zone (U.S.), which varied from 10.1 to 18.2 mg NO3 -- N m?2 d?1" (Thomas et al. 965). These authors determined that the uptake rate within the transient storage zone represented 44 -- 49% of the total uptake; C. And S. estimates ranged from 0.023 to 0.034 min?1 and 0.011 to 0.024 min?1, respectively; they concluded that these processing rates corresponded to solute residence times of 30 -- 44 min and 41 -- 90 min in the channel and storage zones, respectively.
Current Hydraulic Conductivity Issues. In his essay, "The Search for a Landfill Site in the Risk Society," Ali (1999) reports that, "One of the most pressing environmental problems that modern society faces is waste disposal, and addressing this issue has become an urgent matter for many industrial societies because the space required for disposing of waste is quickly running out" (1). Resolving waste disposal issues has assumed new importance for many communities, particularly Ontario, Canada, where in 1993, 45% of the province's landfill capacity was lost due to the closing of three of its largest landfills. In response to the growing need, the tasked authorities developed a set of technical "Minimum Acceptability Standards" that related to the minimum hydraulic conductivity of soils around the newly selected landfill site, the minimum distance of the landfill from built-up areas and wells, the requirement that the maximum height of the landfill not exceed 11 meters, and the requirement that they exclude sites that were too small to accommodate a leachate treatment system (Ali 1999).
According to Robert N. Caldwell (1998), seawater intrusion in coastal regions is a common problem in the United States today. "Intrusion occurs when the hydraulic head of fresh groundwater that is in hydraulic continuity with the sea is reduced relative to that of seawater. This reduction in hydraulic head is usually caused by human activity, specifically, well pumping" (1099). Under natural conditions the altitude of the water table, or potentiometric surface, in a coastal aquifer is higher than sea level and it decreases toward the coast; the movement of fresh groundwater along this gradient is seaward; however, when the freshwater gradient is decreased or reversed, such as by the pumping of nearshore wells, the seaward flow of freshwater is decreased, and the…