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

Ian Cartwright,Harald Hofmann

doi:10.5194/hess-20-3581-2016

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 222Rn 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 222Rn budget. The distribution of 222Rn 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 222Rn 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 222Rn activities due to 222Rn emanation from the alluvial sediments. If the high 222Rn 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 222Rn 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 222Rn from parafluvial flow will result in overestimating groundwater inflows to rivers.

Highlights

Quantifying groundwater inflows to streams and rivers is critical to understanding hydrogeological systems, protecting riverine ecosystems, and managing water resources (e.g. Winter, 1999; Sophocleous, 2002; Brodie et al, 2007)
This paper examines groundwater–river interaction in the Avon River, southeast Australia, primarily using 222Rn as a tracer
The variation in 222Rn activities and Cl concentrations clearly define the reaches of the Avon River that are gaining

Summary

Introduction

Quantifying groundwater inflows to streams and rivers is critical to understanding hydrogeological systems, protecting riverine ecosystems, and managing water resources (e.g. Winter, 1999; Sophocleous, 2002; Brodie et al, 2007). Quantifying groundwater inflows to streams and rivers is critical to understanding hydrogeological systems, protecting riverine ecosystems, and managing water resources Groundwater inflows may form the majority of water in gaining rivers during periods of low streamflow, and riverine ecosystems are commonly sustained by groundwater inflows at those times (Kløve et al, 2011; Barron et al, 2012; Cartwright and Gilfedder, 2015). Understanding the distribution and magnitude of groundwater inflows is important for managing and protecting these commonly vulnerable ecosystems. Documenting the distribution and quantity of groundwater inflows to rivers is required for flood forecasting, understanding the impacts of contaminants on rivers, and assessing the potential impacts of climate or land use changes on river systems

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