Dynamic river–groundwater exchange in the presence of a saline, semi‐confined aquifer

Abstract

Understanding groundwater–surface water exchange in river banks is crucial for effective water management and a range of scientific disciplines. While there has been much research on bank storage, many studies assume idealized aquifer systems. This paper presents a field‐based study of the Tambo Catchment (southeast Australia) where the Tambo River interacts with both an unconfined aquifer containing relatively young and fresh groundwater (<500 μS/cm and <100 years old) and a semi‐confined artesian aquifer containing old and saline groundwater (electrical conductivity > 2500 μS/cm and >10 000 years old). Continuous groundwater elevation and electrical conductivity monitoring within the different aquifers and the river suggest that the degree of mixing between the two aquifers and the river varies significantly in response to changing hydrological conditions. Numerical modelling using MODFLOW and the solute transport package MT3DMS indicates that saline water in the river bank moves away from the river during flooding as hydraulic gradients reverse. This water then returns during flood recession as baseflow hydraulic gradients are re‐established. Modelling also indicates that the concentration of a simulated conservative groundwater solute can increase for up to ~34 days at distances of 20 and 40 m from the river in response to flood events approximately 10 m in height. For the same flood event, simulated solute concentrations within 10 m of the river increase for only ~15 days as the infiltrating low‐salinity river water drives groundwater dilution. Average groundwater fluxes to the river stretch estimated using Darcy's law were 7 m3/m/day compared with 26 and 3 m3/m/day for the same periods via mass balance using Radon (222Rn) and chloride (Cl), respectively. The study shows that by coupling numerical modelling with continuous groundwater–surface water monitoring, the transient nature of bank storage can be evaluated, leading to a better understanding of the hydrological system and better interpretation of hydrochemical data. Copyright © 2015 John Wiley & Sons, Ltd.

Sources and mean transit times of stream water in an intermittent river system: the upper Wimmera River, southeast Australia

“…This has been documented in other Australian catchments (e.g. the Tambo River: Unland et al, 2015) and is a common feature in many river systems (sometimes referred to as the old water paradox: Kirchner, 2003;. Unlike in some catchments (Tsujimura et al, 2007;Birks et al, 2019;Jung et al, 2019), the major ion and stable isotope geochemistry of regional groundwater and near-river water are similar (Figs.…”

Tracing Bank Storage and Hyporheic Exchange Dynamics Using 222Rn: Virtual and Field Tests and Comparison With Other Tracers

“…This is a promising result. While other tracers, such as temperature (Briggs et al., 2013; Hatch et al., 2006), conservative solutes (Larsen et al., 2014; McCallum et al., 2010; Reddy et al., 2008; Unland et al., 2015), and isotopes of the water molecule (Gooseff et al., 2003; Krest & Harvey, 2003) have different advantages, they also have limitations in characterizing hydrologic exchange flows. Robust temperature signals in banks are typically limited to several decimeters to a few meters away from the river‐groundwater interface and these may also persist long after a flood, making their interpretation of tagging actively exchanged water or the exchange zone somewhat challenging; this limitation had also been demonstrated through observations (Watson et al., 2018).…”

Application of multi-method approach to assess groundwater–surface water interactions, for catchment management

“…For the sampling of groundwater, where the rope attached to the bailer was insufficient, a submersible pump was inserted into the well. Thereafter the required three borehole volumes to be removed were calculated with the formula according to (Triplett et al 2006): where r is the radius of the well under investigation and h is the length of the borehole. By setting the pump to pump at 6ℓ/s, it became apparent that a time frame of approximately 15 min was required for the sufficient well volumes to be removed.…”