Variability of unsaturated Bromide fluxes as measured through a layered volcanic vadose zone in New Zealand

Abstract

Abstract:The measured drainage fluxes through a layered volcanic vadose zone exhibited high spatial variability as a consequence of heterogeneous flow conditions. The drainage flux variability was quantified using automated equilibrium tension lysimeters, installed in close-proximity and resulted in high variability in the Br masses recovered from a conservative tracer experiment. The primary cause of the heterogeneous flow was attributed to textural changes occurring at the interface between volcanic layers, resulting in development of funnel-flow patterns, and further enhanced by the existence of hydrophobic conditions. The Br recoveries in individual automated equilibrium tension lysimeters were used to determine the corresponding variable sizes of the surface areas contributing drainage to the lysimeters. The tracer experiment confirmed the existence of unsaturated lateral transport occurring at the sloping interface of the coarse Taupo Ignimbrite material with the silty Palaeosol layer at approximately 4.2 m depth. This study demonstrates that measurements of both flux and solute concentrations at multiple locations are essential when heterogeneous flow is suspected to be present, to be able to determine reliable estimates of contaminant leaching through the vadose zone at the plot scale.

Understanding Preferential Flow in the Vadose Zone: Recent Advances and Future Prospects

“…The importance of the spatial correlation structure was demonstrated by Schlüter et al (2012a), who concluded from numerical experiments that “increased isotropic, short‐range connectivity reduces non‐equilibrium, whereas anisotropic structures that are elongated in the direction of flow enforce it.” This blueprint for preferential flow is certainly matched by large structural pores (termed macropores ) that are almost ubiquitous in soil (Jarvis, 2007). Apart from macropore flow, funnel flow or focused recharge at the Darcy scale can also be induced by much larger scale structures of elongated geometry, such as tongue‐like soil horizon interfaces (e.g., Kulasekera and Parkin, 2011; Hardie et al, 2011) or inclined layers or lenses of contrasting permeability in the soil or vadose zone (e.g., Kung, 1990; Heilig et al, 2003; Barkle et al, 2014; Sohrt et al, 2014).…”

Evaluating the transferability of measurements from simple constructed non weighable gravitation lysimeters to predict the water regime on field scale—a case study

“…Their drainage flux and therefore their lower boundary has to proceed as in the natural soil, whereas the effect of the lower boundary got discussed controversially in literature. On the one hand, suction controlled devices got emphasized to work better than free draining lysimeters ( Vereecken and Dust , 1998; Zhu et al, 2002; Gee et al, 2009; Peters and Durner , 2009; Barkle et al, 2014) and on the other hand studies by Meissner et al (2010) yielded that there are no significant differences between both types for longer observation periods. Each of these effects or a combination of these problems could lead to deviations between the water regime of the lysimeter and the investigated field ( Hagenau et al, 2015).…”