Using radon to understand parafluvial flows and the changing locations of groundwater inflows in the Avon River, southeast Australia

Abstract

Abstract. Understanding the location and magnitude of groundwater inflows to rivers is important for the protection of riverine ecosystems and the management of connected groundwater and surface water systems. This study utilizes 222 Rn activities and Cl concentrations in the Avon River, southeast Australia, to determine the distribution of groundwater inflows and to understand the importance of parafluvial flow on the 222 Rn budget. The distribution of 222 Rn activities and Cl concentrations implies that the Avon River contains alternating gaining and losing reaches. The location of groundwater inflows changed as a result of major floods in 2011-2013 that caused significant movement of the floodplain sediments. The floodplain of the Avon River comprises unconsolidated coarse-grained sediments with numerous point bars and sediment banks through which significant parafluvial flow is likely. The 222 Rn activities in the Avon River, which are locally up to 3690 Bq m −3 , result from a combination of groundwater inflows and the input of water from the parafluvial zone that has high 222 Rn activities due to 222 Rn emanation from the alluvial sediments. If the high 222 Rn activities were ascribed solely to groundwater inflows, the calculated net groundwater inflows would exceed the measured increase in streamflow along the river by up to 490 % at low streamflows. Uncertainties in the 222 Rn activities of groundwater, the gas transfer coefficient, and the degree of hyporheic exchange cannot explain a discrepancy of this magnitude. The proposed model of parafluvial flow envisages that water enters the alluvial sediments in reaches where the river is losing and subsequently re-enters the river in the gaining reaches with flow paths of tens to hundreds of metres. Parafluvial flow is likely to be important in rivers with coarse-grained alluvial sediments on their floodplains and failure to quantify the input of 222 Rn from parafluvial flow will result in overestimating groundwater inflows to rivers.

Explicit Modeling of Radon‐222 in HydroGeoSphere During Steady State and Dynamic Transient Storage

“…To represent low 222 Rn activities of infiltrating stream water, the river nodes were fixed to a constant value of 100 Bq/m 3 during the entire simulation. This is similar to the values measured in the Avon river by Cartwright and Hofmann (). In the model, 222 Rn emanation γ in the subsurface is constant (see Table ) and initial conditions for head potentials and 222 Rn concentrations result from a spin‐up simulation that was run until the model reached steady‐state base flow conditions.…”

“…About 30% of the catchment consists of the mountainous headwaters, while the remaining 70% comprises lowland plains with undulating hills. A pronounced flood regime has led to the development of a wide gravel, and in some places braided, river channel in the lowland section (Cartwright and Hofmann ). During most of the year the river only occupies a small portion of the channel and meanders through depressions in the gravel beds scoured during high flow events.…”

Global Isotope Hydrogeology―Review

“…More frequent research that combines geochemical‐based approaches with other techniques used to evaluate groundwater discharges could improve conceptual models describing groundwater connections with surface waters (geophysics—Harrington et al, ; temperature profiles—Xu et al, ; piezometric and solute concentration data—Rhodes et al, ; and measurements of well water levels along river corridors to quantify hydraulic gradients—Cook, ). Synoptic sampling of river waters has helped quantify groundwater discharges along rivers spanning approximately ones to tens of kilometers (Beisner et al, ; Campodonico et al, ; Harrington et al, ; Smerdon et al, ). Multiple geochemical tracers are measured and applied to assess groundwater inputs along rivers, including radiocarbon (Bourke et al, ), radon‐222 (e.g., Cook et al, , ; Cook, ; Cartwright et al, ; Campodonica et al, 2015; Cartwright & Gilfedder, ; Cartwright & Hofmann, ; Xie et al, ; Xu et al, ; Avery et al, ), helium‐4 (Gardner et al, ), and chloride or other major ion concentrations, strontium isotopes (e.g., Négrel et al, ; Shand et al, ). Where longitudinal river isotope surveys have been completed, groundwater discharges are frequently found to be densely distributed (e.g., Atkinson et al, ; Cook, ; Ellis & Jasechko, ).…”

Estimating retention potential of headwater catchment using Tritium time series

“…where τ m is the mean residence time and f the ratio of the volume of the aquifer that exhibits exponential flow to the total aquifer volume. The EPM is widely used to estimate transit times in catchments where the water in the stream follows flow paths of varying lengths but where parts of the aquifer are confined or where there is vertical recharge through the unsaturated zone above an aquifer that exhibits exponential flow Cartwright and Hofmann, 2016).…”