Preferential flow systems amended with biogeochemical components: imaging of a two-dimensional study
Authors: Pales Ashley R. , Li Biting , Clifford Heather M. , Kupis Shyla , Edayilam Nimisha , Montgomery Dawn , Liang Wei-zhen , Dogan Mine , Tharayil Nishanth , Martinez Nicole , Moysey Stephen , Powell Brian , Darnault Christophe J. G. 
Published: April 2018

Abstract. The vadose zone is a highly interactive heterogeneous system through which
water enters the subsurface system by infiltration. This paper details the
effects of simulated plant exudate and soil component solutions upon unstable
flow patterns in a porous medium (ASTM silica sand; US Silica, Ottawa, IL,
USA) through the use of two-dimensional tank light transmission method
(LTM). The contact angle (θ) and surface tension (γ) of two
simulated plant exudate solutions (i.e., oxalate and citrate) and two soil
component solutions (i.e., tannic acid and Suwannee River natural organic matter, SRNOM) were analyzed to determine the liquid–gas and liquid–solid
interface characteristics of each. To determine if the unstable flow
formations were dependent on the type and concentration of the simulated
plant exudates and soil components, the analysis of the effects of the
simulated plant exudate and soil component solutions were compared to a
control solution (Hoagland nutrient solution with 0.01 M NaCl). Fingering
flow patterns, vertical and horizontal water saturation profiles, water
saturation at the fingertips, finger dimensions and velocity, and number of
fingers were obtained using the light transmission method. Significant
differences in the interface properties indicated a decrease between the
control and the plant exudate and soil component solutions tested;
specifically, the control (θ= 64.5∘ and γ= 75.75 mN m−1) samples exhibited a higher contact
angle and surface tension than the low concentration of citrate (θ= 52.6∘ and γ= 70.8 mN m−1). Wetting front
instability and fingering flow phenomena were reported in all infiltration
experiments. The results showed that the plant exudates and soil components
influenced the soil infiltration as differences in finger geometries,
velocities, and water saturation profiles were detected when compared to the
control. Among the tested solutions and concentrations of soil components,
the largest finger width (10.19 cm) was generated by the lowest tannic acid
solution concentration (0.1 mg L−1), and the lowest finger width
(6.00 cm) was induced by the highest SRNOM concentration (10 mg L−1).
Similarly, for the plant exudate solutions, the largest finger width
(8.36 cm) was generated by the lowest oxalate solution concentration
(0.1 mg L−1), and the lowest finger width (6.63 cm) was induced by the
lowest citrate concentration (0.1 mg L−1). The control solution
produced fingers with average width of 8.30 cm. Additionally, the
wettability of the medium for the citrate, oxalate, and SRNOM solutions
increased with an increase in concentration. Our research demonstrates that
the plant exudates and soil components which are biochemical compounds
produced and released in soil are capable of influencing the process of
infiltration in soils. The results of this research also indicate that soil
wettability, expressed as (cos θ)1∕2, should be
included in the scaling of the finger dimension, i.e., finger width, when
using the Miller and Miller (1956) scaling theory for the scaling of flow in
porous media.