Monitoring of Water and Solute Transport in the Vadose Zone: A Review

Abstract

Core Ideas Soil solution sampling is essential to better understand water and solute movement in soils. A review of different types of soil solution samplers is provided in this paper, including: drainage lysimeter or soil column, pan lysimeter, resin bags or membranes, wick lysimeters, suction cup, and suction plate. Recent developments, modifications, and recommendation criteria are provided for selecting appropriate soil solution extraction samplers. A number of contaminants including agrochemicals (fertilizers, pesticides), heavy metals, trace elements, and pathogenic microbes along with pharmaceuticals and hormones used in animal production move through the soil and are responsible for degradation of groundwater quality. Therefore, it is essential to sample soil solution for better understanding of movement and environmental fate of various contaminants in soils. We review different soil solution extraction samplers. The soil solution samplers discussed here are: drainage lysimeter or soil column, pan lysimeter, resin bags or membranes, wick lysimeters, suction cup, and suction plate. We have reviewed 304 journal articles representing a wide array of scientific disciplines. A brief history of soil solution monitoring and terminology used for describing various soil solution samplers is also provided. This review classifies literature on the basis of type of soil solution extraction samplers, soil type, land use–land cover (LULC), and analytes measured. Recommendation criteria are provided for selecting appropriate soil solution extraction samplers based on spatial and temporal variation, cost, soil type, amount of disturbance caused during installation of soil solution samplers, and monitoring of leachates involving different cations, anions, carbon, pH, EC, colloids, pesticides, and microbes. Use of advanced techniques with lysimeters for monitoring soil moisture content, soil water potential and flux is also discussed in this review.

Advanced In Situ Soil Water Sampling System for Monitoring Solute Fluxes in the Vadose Zone

“…Currently, the application of our approach is not suitable for soils prone to preferential flow (i.e., macroporous soils and soils with strong structural heterogeneities) because the preferential flow is likely to bypass the suction cup (Grossmann and Udluft, 1991) or the soil moisture sensor and hence the flow field will not be captured. An alternative would be to use sampling devices with a larger cross‐sectional sampling area, such as porous suction plates (Ciglasch et al, 2005; Singh et al, 2017; Weihermüller et al, 2007), instead of suction cups and to increase the number of soil moisture sensors.…”

“…In situ soil water sampling from the vadose zone can be grouped into active or passive sampling systems (Singh et al, 2017; Weihermüller et al, 2007). Passive sample systems, such as pan or zero‐tension lysimeters, have the drawback that zero‐tension lower boundary conditions cause a small saturated zone above the lower boundary and hence influence soil water fluxes and solute concentrations (Flury et al, 1999).…”

“…Passive sample systems, such as pan or zero‐tension lysimeters, have the drawback that zero‐tension lower boundary conditions cause a small saturated zone above the lower boundary and hence influence soil water fluxes and solute concentrations (Flury et al, 1999). Active soil sampling devices, such as suction cups, suction plates, a rhizon sampler, or a wick lysimeter need a hanging water column or a vacuum to extract soil water (Singh et al, 2017; Weihermüller et al, 2007). All vacuum‐controlled methods could operate in continuous/discontinuous or variable pressure modes possibly regulated by soil water tension.…”

“…A possibility to ensure soil water sampling of defined pore origin would be a continuous pressure application of a fixed vacuum to restrict sampling to the desired matrix potential. However, drawbacks of this operation mode are the risk to receive composite samples of more than one infiltration event if the samples cannot be collected in short time and the fact that continuous pressure application might initiate preferential flow paths in the surrounding substrate (Singh et al, 2017; Weihermüller et al, 2007). Moreover, Weihermüller et al (2007) concluded that it seems difficult or maybe even impossible to derive soil water samples unbiased by the sampling procedure and stated that each sample only temporally represents the sample location.…”

Simulating the Fate and Transport of Coal Seam Gas Chemicals in Variably-Saturated Soils Using HYDRUS

“…For example, agricultural operations now depend on a broad range of chemicals in plant and animal production such as fertilizers, pesticides, hormones, pharmaceuticals, pathogenic microorganisms, or fumigants, which have made agriculture one of the most important sources for non-point source pollution [1]. Examples of environmental problems resulting from non-agricultural activities are the disposal of brine/saline waters produced during extraction of coalbed or shale gas [2], releases from municipal or industrial landfills [3], acid mine drainage [4], contamination from nuclear activities [5,6], the formation of redox zones in organic-contaminated aquifers [7], and pollutant release from reactive permeable barriers used for aquifer remediation [8,9]).…”