Influence of Streambed Heterogeneity on Hyporheic Flow and Sorptive Solute Transport

Yuanhong Liu,Mohamad Reza Soltanian,Dipankar Dwivedi,Faranak Behzadi,Yaoquan Zhou,Lianqing Xue,Reza Ershadnia,Corey D Wallace

doi:10.3390/w12061547

Abstract

The subsurface region where river water and groundwater actively mix (the hyporheic zone) plays an important role in conservative and reactive solute transport along rivers. Deposits of high-conductivity (K) sediments along rivers can strongly control hyporheic processes by channeling flow along preferential flow paths wherever they intersect the channel boundary. Our goal is to understand how sediment heterogeneity influences conservative and sorptive solute transport within hyporheic zones containing high- and low-K sediment facies types. The sedimentary architecture of high-K facies is modeled using commonly observed characteristics (e.g., volume proportion and mean length), and their spatial connectivity is quantified to evaluate its effect on hyporheic mixing dynamics. Numerical simulations incorporate physical and chemical heterogeneity by representing spatial variability in both K and in the sediment sorption distribution coefficient ( K d ). Sediment heterogeneity significantly enhances hyporheic exchange and skews solute breakthrough behavior, while in homogeneous sediments, interfacial flux and solute transport are instead controlled by geomorphology and local-scale riverbed topographies. The hyporheic zone is compressed in sediments with high sorptive capacity, which limits solute interactions to only a small portion of the sedimentary architecture and thus increases retention. Our results have practical implications for groundwater quality, including remediation strategies for contaminants of emerging concern.

Highlights

River bedforms enhance the exchange of water and solutes between the surface water and shallow riverbed, known as hyporheic exchange
To better understand the impacts of heterogeneity in the equilibrium sorption distribution could be used to model sorption (e.g., [67,68]), but our goal was to address the lack of understanding coefficient, three sorption scenarios were simulated in the present study: 1) a conservative case with no sorption, 2) a low sorption case where the of high-K facies was ten-fold lower than of how simple sorption processes affect solute transport within the hyporheic zone
The average interfacial flow increased with connectivity, consistent with previous studies (e.g., [4,26]). The results for both percolating and non-percolating cases are consistent with prior work on the impacts of heterogeneity on the dynamics of hyporheic exchange (e.g., [19,22,23,69,70,71,72,73])

Summary

Introduction

River bedforms enhance the exchange of water and solutes between the surface water and shallow riverbed, known as hyporheic exchange. The architectural and textural characteristics of sediment facies (i.e., three-dimensional (3D) sediment bodies [4]) across different scales control the distribution of both the physical (e.g., hydraulic conductivity (K), porosity, grain size, and bulk density) and chemical (e.g., mineralogical composition, equilibrium sorption distribution coefficient (Kd ), and reactive surface area) properties of the subsurface [5,6,7,8,9,10,11,12,13,14,15] These properties in turn influence fluid flow and solute mass transport. 88% of whole-stream ecosystem respiration has been attributed to processing within the hyporheic zone (HZ) [18]

Objectives

Methods

Results

Conclusion