Identification of Lateral Macropore Flow in a Forested Riparian Wetland through Numerical Simulation of a Subsurface Tracer Experiment

Abstract

Understanding wetland hydrogeology is important as it is coupled to internal geochemical and biotic processes that ultimately determine the fate of potential contaminant inputs. Therefore, there is a need to quantitatively understand the complex hydrogeology of wetlands. The main objective of this study was to improve understanding of saturated groundwater flow in a forested riparian wetland located on a golf course in the Lower Pee Dee River Basin in South Carolina, USA. Field observations that characterize subsurface wetland flow critical to solute transport originating from storm-generated runoff are presented. Monitoring wells were installed, and slug tests were performed to measure permeabilities of the wetland soil. A field-scale bromide tracer experiment was conducted to mimic the periodic loading of nutrients caused by storm runoff. This experiment provided spatial and temporal data on solute transport that were analyzed to determine travel times in the wetland. Furthermore, a 3-D numerical, steady-state flow model (MODFLOW) was developed to simulate subsurface flow in the wetland. A particle tracking model was subsequently used to calculate solute travel times from the wetland inlet to the outlet based on flow modeling results. It was evident that observed tracer breakthrough times were not typical of these measured wetland soil matrix conductivity values. Based on surface water sampling results at the wetland outlet, tracer arrival time was about 9 h after the injection of the tracer. These results implied an apparent mean K value of 2,050 m/day, which is 152 times larger than the mean of the measured values using slug tests (13.4 m/day). Modeling efforts clearly demonstrated this implied preferential flow behavior; particle travel times resulting from the calibrated flow model were in the order of hundreds of days, while actual travel times in the wetland were in the order of hours to a few days. This significant difference in travel times was attributed to the presence of macropores in the form of dead root channels and cavities forming a pipe-flow network. The analyses presented in this study resulted in an estimate of the ratio of matrix permeability to matrix plus macropore permeability of approximately 1/150. Eventually, the tracer test and resulting travel times between various points in the wetland were critical to understanding the true wetland flow dynamics. The final conceptual model of the hydraulic properties of the wetland soils comprised a low permeability matrix containing a web of high K macropores. Simulation of tracer transport in this system was possible using a flow model with significantly elevated K values.

Macropores as preferential flow paths in meander bends

“…Our results show that macropores in a meander bend act as preferential flow paths by increasing advective transport rates through the meander bend (Table ). This trend agrees with previous findings from deeper groundwater that fractures can have flow rates that are orders of magnitude higher than the adjacent matrix (Elci and Molz, ; Geiger et al ., ; Qian et al ., ). However, the magnitude of the effect on velocities in our data is less (roughly 1 order of magnitude, Table ) than in many of these other studies.…”

Roots‐Enhanced Preferential Flows in Deciduous and Coniferous Forest Soils Revealed by Dual‐Tracer Experiments

“…In forest soils, these interrelationships are particularly important as root channels are considered to be primary preferential pathways (Elçi and Molz, 2009; Laine‐Kaulio et al, 2015; Zhang et al, 2016). Vegetation species in a forest can be diverse, including shrubs, coniferous and deciduous tree species, and so on (Jia et al, 2017).…”

An experimental investigation to characterise soil macroporosity under different land use and land covers of northeast India

“…Use of ion tracers can also provide spatial and temporal data on solute transport and their travel time. Elçi and Molz (2009) used bromide tracer to measure actual travel time of water flow in macroporous soils. They found that the soil matrix permeability was much less than the measured soil matrix plus macropore permeability.…”

“…These parameters can be used to compare, evaluate, and quantify the differences in soil macroporosity under different land use, land cover, and crop management practices. The quantitative information collected about the macropore flow patterns may be used in different fields of science and engineering applications such as ground water pollution and water quality modelling due to leaching of solutes, pesticides, and nitrates in macroporous soils (Jarvis et al 1991;Flury 1996;Jarvis 1998;Jarvis 1999, 2000;Mooney and Morris 2008), study of soil structural properties and infiltration behaviour (Olsen and Børresen 1997;Buttle and McDonald 2000), identification of lateral macropore flow responsible for flash floods (Anderson et al 2008;Elçi and Molz 2009), and hydrological investigations of rainfall-runoff processes. Weiler (2005) developed a hydrological model based on the data collected from similar experimental investigations.…”