A spectrum of preferential flow alters solute mobility in soils

Abstract

Preferential flow reduces water residence times and allows rapid transport of pollutants such as organic contaminants. Thus, preferential flow is considered to reduce the influence of soil matrix-solute interactions during solute transport. While this claim may be true when rainfall directly follows solute application, forcing rapid chemical and physical disequilibrium, it has been perpetuated as a general feature of solute transport—regardless of the magnitude preferential flow. A small number of studies have alternatively shown that preferential transport of strongly sorbing solutes is reduced when solutes have time to diffuse and equilibrate within the soil matrix. Here we expand this inference by allowing solute sorption equilibrium to occur and exploring how physiochemical properties affect solute transport across a vast range of preferential flow. We applied deuterium-labeled rainfall to field plots containing manure spiked with eight common antibiotics with a range of affinity for the soil after 7 days of equilibration with the soil matrix and quantified preferential flow and solute transport using 48 soil pore water samplers spread along a hillslope. Based on > 700 measurements, our data showed that solute transport to lysimeters was similar—regardless of antibiotic affinity for soil—when preferential flow represented less than 15% of the total water flow. When preferential flow exceeded 15%, however, concentrations were higher for compounds with relatively low affinity for soil. We provide evidence that (1) bypassing water flow can select for compounds that are more easily released from the soil matrix, and (2) this phenomenon becomes more evident as the magnitude of preferential flow increases. We argue that considering the natural spectrum preferential flow as an explanatory variable to gauge the influence of soil matrix-solute interactions may improve parsimonious transport models.

Soil organic matter can delay—but not eliminate—leaching of neonicotinoid insecticides

“…Even though our study focused on a single growing season, the results provide evidence that interactions with SOC may be most effective for reducing the rate of TMX and CLO transport, rather than total loads. This behavior fits the conceptual framework that solute‐solid phase interactions delay, but do not fully prevent, chemical transport (Cameron & Klute, 1977; Radolinski et al., 2022). Here we note that our lysimeters were installed at relatively shallow depths (with the collection surfaces ∼0.27 m below the soil surface), so the effective transport distance was much less than the length of a typical rooting zone.…”

“…The post‐planting concentrations of TMX detected in our soil were similar to those measured in production corn fields in Canada (Schaafsma et al., 2015), confirming realistic TMX loading in our experiment. We also focused our analysis on vertical leaching, which is an important mechanism by which contaminants such as insecticides and antibiotics become transported from crop fields (Flury, 1996; Radolinski et al., 2022; Willkommen et al., 2021). Leaching is particularly important in low‐relief topographies and landscapes with drain tiles (Blann et al., 2009; Dolliver & Gupta, 2008) and can also drive lateral subsurface flow in sloping fields (e.g., Radolinski et al., 2019).…”

Preferential flow in the shallow vadose zone: Effect of rainfall intensity, soil moisture, connectivity, and agricultural management

“…It is crucial to nearly all forms of subsurface contaminant transport, as it drastically reduces the residence time of water in the subsurface causing faster and less attenuated transport of pollutants (Sprenger et al, 2019). Indeed, preferential flows have been linked to the rapid transport of nutrients (King et al, 2015: Pluer et al, 2020; Williams et al, 2016), pesticides (Klaus et al, 2014; Radolinski et al, 2022), and particulate matter (Mohanty et al, 2016; Nazari et al, 2022) through the subsurface. Despite its ubiquity and relevance for subsurface flow generation, the transit time distribution of water in the soil, and contaminant transport, preferential flow effects are often ignored or generalized with broad assumptions in hydrologic models (Ford et al, 2017; Radcliffe et al, 2015; Weiler, 2017).…”

Subsurface Manure Injection Reduces Surface Transport of Antibiotic Resistance Genes but May Create Antibiotic Resistance Hotspots in Soils

“…Further, antimicrobials themselves are not always fully metabolized, meaning high levels (typically 1–10 mg kg –1 or L –1 or even greater than 200 mg kg –1 or L –1 ) , of antimicrobials can still be present in their waste. , Large-scale application of animal manure as fertilizer and soil amendments , thus represents a critical pathway for ARE introduction into agroecosystems. Once in agroecosystems, AREs can be taken up by plants, transported into groundwater or surface water bodies via preferential flow, or (if kinetically favorable , ) retained in the soil. − …”

“…14,15 Largescale application of animal manure as fertilizer and soil amendments 16,17 thus represents a critical pathway for ARE introduction into agroecosystems. Once in agroecosystems, AREs can be taken up by plants, transported into groundwater or surface water bodies via preferential flow, 18 or (if kinetically favorable 19,20 ) retained in the soil. 21−24 In 2013, the US Food and Drug Administration (US FDA) introduced a final guidance for industry to phase out the use of human medically important antibiotics in animal production, and in 2015, these products were not to be used for production (e.g., growth promotion) purposes and may be used only under the authorization of a licensed veterinarian.…”