Impacts of Streambed Heterogeneity and Anisotropy on Residence Time of Hyporheic Zone

Abstract

The hyporheic zone (HZ), which is the region beneath or alongside a streambed, plays an important role in the stream's ecology. The duration that a water molecule or a solute remains within the HZ, or residence time (RT), is one of the most common metrics used to evaluate the function of the HZ. The RT is greatly influenced by the streambed's hydraulic conductivity (K), which is intrinsically difficult to characterize due to its heterogeneity and anisotropy. Many laboratory and numerical studies of the HZ have simplified the streambed K to a constant, thus producing RT values that may differ from those gathered from the field. Some studies have considered the heterogeneity of the HZ, but very few have accounted for anisotropy or the natural K distributions typically found in real streambeds. This study developed numerical models in MODFLOW to examine the influence of heterogeneity and anisotropy, and that of the natural K distribution in a streambed, on the RT of the HZ. Heterogeneity and anisotropy were both found to shorten the mean and median RTs while increasing the range of the RTs. Moreover, heterogeneous K fields arranged in a more orderly pattern had longer RTs than those with random K distributions. These results could facilitate the design of streambed K values and distributions to achieve the desired RT during river restoration. They could also assist the translation of results from the more commonly considered homogeneous and/or isotropic conditions into heterogeneous and anisotropic field situations.

Numerical modelling to evaluate the nitrogen removal rate in hyporheic zone and its implications for stream management

“…Due to the particles' different starting positions, they rarely have the same RT, and thus, the mean and median RTs are two common indicators used to describe the performance of particles remaining in the HZ. Liu and Chui () found that the median RT was more stable than the mean RT among different stochastically generated cases, with consideration of heterogeneity and anisotropy of hydraulic conductivity. Therefore, in this study, median RT is used as the indicator to describe total reaction time in the HZ.…”

“…The hyporheic zone (HZ hereafter), which is the region beneath or alongside a streambed where active groundwater and surface water mix, plays an important role in the aquatic environment and stream ecology (Brunke & Gonser, ; Hester & Gooseff, ; Liu & Chui, ; Peralta‐Maraver, Reiss, & Robertson, ; Peterson, Sickbert, & Moore, ). The HZ forms a unique environment between surface water and groundwater for macroinvertebrates, and it is the locus of a number of important biogeochemical reactions; for these reasons, it is vital to the biological and ecological functioning of streams (Montgomery, Buffington, Peterson, Schuett‐Hames, & Quinn, ; Soulsby, Malcolm, & Youngson, ).…”

Controls of River Dynamics on Residence Time and Biogeochemical Reactions of Hydrological Exchange Flows in A Regulated River Reach

“…Even short-term perturbations (e.g., flooding) can have long-lasting influences (Gomez-Velez et al, 2017). The shapes of RTDs in real dynamic river corridors can be very complex since RTDs are influenced by both subsurface physical features and hydrologic forcings (Boulton et al, 1998;Gomez & Wilson, 2013), including sediment permeability and porosity (Cardenas et al, 2004;Liu & Chui, 2018;Salehin et al, 2004), river morphology (e.g., riffles, bars, and dunes) (Buffington & Tonina, 2009;Cardenas, 2008;Cardenas & Wilson, 2007;Stonedahl et al, 2013), naturally occurring hydrologic processes and events (e.g., flooding, evapotranspiration, recharge, snowmelt and tidal cycles) (Gomez-Velez et al, 2017;Gomez-Velez & Wilson, 2013;Larsen et al, 2014), and anthropogenic activities (e.g., dam-induced stage fluctuations) (Shuai et al, 2019;Song et al, 2018). The dynamic hydrologic fluctuations can produce equivalently complex pathways and RTDs as complex geomorphic features (Schmadel et al, 2016).…”

Empirical Equations to Predict the Characteristics of Hyporheic Exchange in a Pool‐Riffle Sequence

“…The dominating driver of HE is the variation of pressure along the streambed (Tonina and Buffington ), which is in turn induced by heterogeneities in hydraulic conductivity (Vaux ; Ward et al ; Herzog et al ; Liu and Chui ) and the interactions between surface water and streambed topography, such as dunes, steps, ripples, pool‐riffles, or flow obstacles (Tonina and Buffington ). Among the various morphological features, pool‐riffle sequences have received vast attention because they are ubiquitous (Buffington et al ) and may account for a large portion of HE in many streams (Harvey and Wagner ).…”