Manual packing and soil reuse effects on determination of saturated hydraulic conductivity of a loam soil
Introduction
Laboratory determination of saturated, Ks, or near-saturated, K, hydraulic conductivity of repacked soil samples represents the connecting link between the studies dealing with purely ideal data, such as those generated numerically or analytically, and the investigations performed in the laboratory or the field on real and nearly undisturbed soil. Therefore, working on a repacked porous medium allows the soil scientist to gradually move from theory to practice, that is from a rigid, homogeneous and isotropic porous medium to a soil which does not have the aforementioned characteristics (e.g., Verbist et al., 2009). Repacked soil samples have been used, and still continue to be used, for a large variety of scientific purposes that include, as an example, testing theories, devices and procedures for determining Ks (Reynolds and Elrick, 1987), studying factors influencing the Ks values of specific types of soil (Moutier et al., 1998), establishing wastewater reuse effects on Ks (Viviani and Iovino, 2004), relating Ks or K with the bulk soil physical properties (Assouline, 2006), testing soil seal development effects on K (Armenise et al., 2018), determining the relationship between Ks and the soil microstructure (Xu et al., 2021), evaluating the effects of rock fragments on Ks (Wu et al., 2021).
Working on repacked soil samples implies making a decision on the packing method to be applied, also depending on the available equipment and the size of the sample that has to be prepared. A variety of choices can be made at this purpose. For example, Laliberte et al. (1966) obtained different densities by vibrating the soil columns after packing them with air-dried soil. Reicovsky et al. (1980) obtained various degrees of compaction by means of a standard laboratory press with pistons that pressed moist soil samples slightly from both ends simultaneously. Bagarello et al. (2004) prepared samples of 5 cm in diameter and 4 or 5 cm in height with a step-by-step procedure, pouring at each step 1 cm of soil that was then manually compacted. The soil samples used by Bagarello et al., 2006, Concialdi et al., 2020 had a diameter of nearly 9 cm and a height of 32 cm or more. They were prepared by filling a column with 40 cm of soil that was then dropped repeatedly from a height of 5 cm until compaction ceased. Soil samples of 5.1 cm × 20 cm and 10 cm × 12 cm were prepared by Moody et al. (2009) by introducing soil in 5–10 increments. Additionally Zhang et al. (2017) prepared 30 cm × 60 cm samples with a step-by-step procedure. In this case, each step involved pouring 5 cm of soil followed by compaction. The soil samples by Ghosh and Pekkat (2019), having a diameter of 20 cm and a height of 30 cm, were obtained by packing the soil in three layers and imparting an equal number of blows on each layer, depending on the desired state of compaction. In some investigations, however, the applied packing method was not described (e.g., Moutier et al., 1998, Assouline and Narkis, 2011).
The link between the applied method to prepare repacked soil samples and the determination of Ks has rarely been considered although a recent investigation has demonstrated that the packing procedure can have a large impact on the Ks results. In particular, Teng et al. (2019) recognized that Ks can vary by even an order of magnitude due to the dependence of the pore size distribution on the applied packing method. Other investigations dealing with packing methods can be found in the literature but they are not expressly focused on measurement of Ks (Oliviera et al., 1996, Lewis and Sjöstrom, 2010, Banzhaf and Hebig, 2016). Therefore, the knowledge of the effect of the applied packing method on laboratory determination of Ks is still incomplete and additional investigations are necessary to fill this gap.
Another factor to be considered is the possibility to reuse the same sieved soil for different experiments. Using a mass of soil twice or more times has the practical advantage that less efforts have to be made to collect soil in the field and to perform the initial pre-treatment, such as removal of stones or vegetation residues. However, when a sieved soil mass is used for the first time, it is subjected to several solicitations. For example, a mechanical breakdown of the small aggregates in the sieved soil could occur during packing, depending on the applied forces. Changes in the soil are also possible during the Ks measurement stage due, for example, to slaking when the initially dry soil is suddenly wetted or to weakening of the interparticle binding forces promoted by a prolonged contact with water (Le Le Bissonnais, 1996, Dikinya et al., 2008), depending on the applied Ks measurement method. Generally, no detailed information is provided in the investigations making use of repacked soil samples about the history of the used soil mass for a given experiment although there are a few exceptions. For example, Moody et al., 2009, Moody et al., 2019 reused the same soil for subsequent experiments while Ryżak et al. (2015) reported that a given soil sample was only used once in their experiments. To the best of our knowledge, the impact of the repeated use of a sieved soil mass for determination of Ks has still to be investigated.
An interest of the scientific community for the simplified falling head (SFH) technique for determining Ks (Bagarello et al., 2004) is documented in the literature (Keller et al., 2012, Angulo-Jaramillo et al., 2016, Biddoccu et al., 2016, Preti et al., 2018). Probably, a point of attractiveness of this technique is that it is simple, rapid and usable both in the laboratory and the field. Part of the potential and limitations of the SFH technique have been investigated in the laboratory, working on repacked soil samples (Bagarello et al., 2006, Concialdi et al., 2020). However, the effects of the packing method and the soil reuse were not considered. Therefore, the sensitivity of the Ks values obtained with the SFH technique to both the applied packing method and the reuse of a given mass of soil is still unknown.
The objective of this investigation was to establish the impact of both the applied packing method and the repeated use of the same soil mass for the preparation of a soil sample on determination of saturated hydraulic conductivity of a loam soil with the simplified falling head technique. The specific objectives were to i) determine changes in Ks among four different packing methods that do not require specialized equipment; and ii) determine if and how Ks changes when the same soil mass is used more than once.
Section snippets
Soil
On November 2019, approximately 100 kg of soil were collected from the surface layer in an orchard established at the Department of Agricultural, Food and Forest Sciences of the Palermo (Italy) University (38°06′24′’ N, 13°21′06′’ E). The mean organic carbon content, OC (%), of the upper few centimeters of the soil, determined with the Walkley-Black method, was equal to 3.1%.
Eight soil samples were collected from the upper 0.1 m of the profile to determine the soil textural characteristics in
Dry soil bulk density and antecedent soil water content
The soil columns prepared for this investigation had a dry bulk density, ρb, varying from 1.24 to 1.27 g cm−3 (sample size, N = 135; mean = 1.25 g cm−3; coefficient of variation, CV = 0.6%) and an antecedent soil water content, θi, ranging between 0.028 and 0.045 m3m−3 (mean = 0.041 m3m−3; CV = 11.7%). The ρb and θi values were nearly constant for the first two experiments since, for the eight developed datasets (R0 and R1 experiments; P1, P2, P3 and P4 packing methods; N = 15 for each
Conclusions
The expectation of an experiment with a nearly homogeneous porous medium is to obtain nearly stable saturated hydraulic conductivity, Ks, data between replicates since the variability typical of real soils is purposely removed or minimized.
For a loam soil, this investigation demonstrated that both the packing method and the number of times a given soil mass is used to prepare a soil sample influence determination of Ks with the simplified falling head (SFH) technique.
Among the tested packing
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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