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The most common method for in situ measurement of saturated hydraulic conductivity (Ksat) of the vadose zone is the constant head well permeameter method. Our general objective is to provide an empirical method for determining volume of water required for measuring Ksat using this procedure. For one-dimensional infiltration, steady state reaches as time (t) → ∞. For three-dimensional water flow from a cylindrical hole under a constant depth of water, however, steady state reaches rather quickly when a saturated bulb forms around the hole. To reach a quasi-steady state for measuring Ksat, we assume an adequate volume of water is needed to form the saturated bulb around the hole and increase the water content outside of the saturated bulb within a bulb-shaped volume of soil, hereafter, referred to as wetted soil volume. We determined the dimensions of the saturated bulb using the Glover model that is used for calculating Ksat. We then used the values to determine the volume of the saturated and wetted bulbs around the hole. The volume of water needed to reach a quasi-steady state depends on the difference between the soil saturated and antecedent water content (Δθ). Based on our analysis, between 2 and 5 L of water is needed to measure Ksat when Δθ varies between 0.1 and 0.4 m3 m−3, respectively. [ABSTRACT FROM AUTHOR]

There has been widespread interest in using compost to improve the hydrologic functions of degraded soils at construction sites for reducing runoff and increasing infiltration. The objective of this study was to determine the effects of compost amendment rate on saturated hydraulic conductivity (Ks) and water retention in order to identify target compost rates for enhancing soil hydrologic functions. Samples were prepared with three soil textures (sandy loam, silt loam, and sandy clay loam), amended with compost at 0%, 10%, 20%, 30%, 40%, and 50%. All soils were tested at a porosity of 0.5 m3/m3, and the sandy loam was further tested at high (0.55 m3/m3) and low (0.4 m3/m3) porosities. The Ks and water retention data were then used to model infiltration with HYDRUS‐1D. With increasing compost amendment rate, Ks and water retention of the mixtures generally increased at the medium porosity level, with more compost needed in heavier soils. As porosity decreased in the sandy loam soil, the amount of compost needed to improve Ks rose from 20% to 50%. Water distribution in pore fractions (gravitational, plant‐available, and unavailable water) depended on texture, with only the highest compost rates increasing plant‐available water in one soil. Results suggest soil texture should be taken into consideration when choosing a compost rate in order to achieve soil improvement goals. Hydrologic benefits may be limited even at a high rate of compost amendment if soil is compacted. [ABSTRACT FROM AUTHOR]

Featured Application: Results of this study can be used by health authorities to set limits on the thickness of saprolite material needed below on-site septic system drainfields in order to remove E. coli from home sewage. Results will also be of interest to field personnel evaluating sites for on-site wastewater disposal. Saprolite, weathered bedrock, is being used to dispose of domestic sewage through septic system drainfields, but the thickness of saprolite needed to remove biological contaminants is unknown for most saprolites. This study developed and tested a simple method for estimating the thickness of saprolite needed below septic drainlines to filter E. coli from wastewater using estimates of the volume of pores that are smaller than the length of the coliform (≤10 μm). Particle size distribution (texture) and water retention data were obtained for 12 different saprolites from the Piedmont and Mountain regions of North Carolina (N.C.). Saprolite textures ranged from clay loam to coarse sand. The volume of pores with diameters ≤10 μm were determined by water retention measurements for each saprolite. The data were used in an equation to estimate the saprolite thickness needed to filter E. coli. The estimated saprolite thicknesses ranged from 36 cm in the clay loam to 113 cm for the coarse sand. The average thickness across all samples was 58 cm. Saprolite thickness estimates increased as silt percentage decreased and as sand percentage and in situ saturated hydraulic conductivity increased. Silt percentage may be most useful for estimating appropriate saprolite thicknesses in the field. [ABSTRACT FROM AUTHOR]

• In 3-dimensional flow from a cylindrical hole steady-state reaches rather quickly. • At steady-state a saturated bulb of limited size forms around the cylindrical hole. • At surface of saturated bulb pressure head is zero and hydraulic gradient is one. • Surface area of saturated bulb can be determined empirically. • Models that include the sorptivity parameter produce unrealistic results. In the constant head well permeameter method the saturated hydraulic conductivity (K s) of the vadose zone is determined by the equation Q = A × K s , where Q is the steady state rate of water flow into the soil from a cylindrical hole of known diameter under a constant depth of water, and A is a factor that must be calculated by a model. The Glover model for determining factor A was developed approximately 70 years ago by considering the hydrostatic pressure and gravity in a saturated zone around the hole. This model does not depend on soil texture or structure. Newer models, on the other hand, consider both saturated and unsaturated flow of water, and include both K s and the capillary factor α (sorptive number) that must be determined independently or be estimated based on soil texture and structure. The main objective here is to compare these models and demonstrate that they yield similar results for coarse-textured soils (relatively large α), but those that consider both saturated and unsaturated flow yield unrealistic results for medium- to fine-textured soils with relatively small α values. The soil water profile around a cylindrical hole under a constant depth of water is initially composed of a bulb-shaped saturation zone (pressure head h > 0), saturation front (h = 0), transmission and wetting zones (h < 0), and wetting front. At steady state, a saturated bulb with a finite volume forms around the hole, and the impact of the unsaturated zone away from the hole on water flow within the saturated bulb diminishes. In addition, the surface area of the saturated bulb is numerically equal to the factor A. Using an empirical approach, the factor A determined by the Glover model and each of the other models for different soils (i.e., different α values) was matched against the surface area of a saturated bulb. Then, the volume of the saturated bulb for each case was determined. Since the factor A determined by the models that consider unsaturated flow is inversely related to α , the volume of the saturated bulb for each model increases unrealistically as the soil texture becomes finer. For soils with α < 0.12 cm−1, the volume of the saturated bulb, and the amount of water needed to reach steady state are unrealistically high making the models that depend on α unsuitable for determining K s by the constant head well permeameter method. [ABSTRACT FROM AUTHOR]

Polyacrylamide (PAM) is becoming a widely used soil conditioning and erosion control agent, and a better understanding of its transport is required to improve its use. In this study vertical PAM transport through a clay loam soil was investigated using thin soil columns (7.62-cm diameter × 2-cm thick) under saturated condition. The columns received a water-soluble, anionic PAM solution (16 Mg mol − 1 with 50 mol% charge density) under pulse and step (continuous) inputs using a constant-head method. The pulse input was 500 mg L − 1 PAM solution applied for 0.6 pore volume ( PV ), after which the input was switched to deionized (DI) water for 25 PVs . The step input was 25 mg L − 1 PAM solution applied continuously for 129 PVs . Saturated hydraulic conductivity ( K sat ) was measured prior to PAM application and was monitored during PAM and DI water leaching. Leachate samples were collected frequently with time from each column and analyzed for the dissolved PAM concentration. The PAM applications reduced K sat to 1% of the initial K sat (4 cm h − 1 ) under the pulse input and to 0.3% of the initial K sat under the step input. Transport of PAM was best-fitted with a two-region (dual-porosity) model. The fitted retardation factor ( R ) was more than two-fold greater for the step input ( R = 2695) than for the pulse input ( R = 1242). The results from transport modeling and pore size distribution analysis suggested that viscous PAM solution contributes to a mechanical entrapment of the PAM molecules, clogging most water-conducting pores smaller than 225–274 μm in diameter. Under saturated condition, either the pulse or step input of dissolved PAM could reduce seepage with limited mobility in the soil profile. [ABSTRACT FROM AUTHOR]