Quantifying Hyporheic Exchange Fluxes And Residence Times Using Environmental Tracer

Author: Roger Harvard Cranswick

Cranswick, Roger Harvard, 2014 Quantifying Hyporheic Exchange Fluxes And Residence Times Using Environmental Tracer, Flinders University, School of the Environment

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Abstract

Hyporheic exchange is a process in which water leaves a river through underlying or adjacent sediments and then returns to the river. This is now widely recognized as a critical process for nutrient cycling and river health but it remains a challenge to adequately characterize the spatial and temporal scales at which hyporheic exchange occurs. The method traditionally used to quantify hyporheic exchange is the applied tracer test. This approach characterizes the bulk exchange occurring within the river and riverbed sediments between locations separated longitudinally along a river. This means that the spatial variability of hyporheic exchange within each reach is not well described. Additionally, the exchange that occurs over longer temporal scales than the sampling period is not captured within the analysis. More broadly, it is not well understood how the scale and magnitude of hyporheic exchange compares with other surface water - groundwater exchange processes. The key objectives of this thesis are to investigate the use of environmental tracers (temperature and radon-222) for estimating hyporheic exchange fluxes and residence times along a pool - riffle sequence. The research also explores the relative scales and magnitudes of hyporheic and river - aquifer exchange fluxes. A review of exchange flux estimates found in the literature shows that hyporheic exchange fluxes are generally one order of magnitude larger than river - aquifer exchange fluxes. If methods are applied that cannot specifically distinguish between sources of water (e.g. seepage meters and other point measurements) there is the potential for large hyporheic exchange fluxes to be misinterpreted as river-aquifer exchange fluxes. This would have clear implications for water resource management where quantifying surface water - groundwater interaction is important. We outline the spatial and temporal scales at which common methods are applied. Then we discuss the importance of considering the scale of measurement and the use of multiple methods to successfully differentiate between exchange flux processes. A field investigation on the Haughton river in northeastern Australia, allows us to explore the use of temperature, radon-222 and electrical conductivity to characterize the hyporheic exchange processes occurring along a pool - riffle sequence. A 1D numerical approach is developed and validated by comparison with two synthetic 2D flowfields before being applied to interpret raw temperature data from the field. We show that the flux calculated between the surface and an observation depth is representative of the mean vertical component of flux along the flowpath that the water has traveled. Thus without informing us about the horizontal component of flow, the vertical 1D approach inherently contains a 'spatial footprint'. Simple analysis of the tracer data collected in a series of vertical profiles, allows us to identify the depth of circulation and calculate residence times within the hyporheic zone. Residence times derived from temperature and radon-222 data were compared directly and did not show a clear agreement. We suggest that small scale heterogeneity may have a different influence on each of these tracers to cause the disparity in flux and residence time estimates. We demonstrate the value of using temperature and radon-222 in combination, as they allow us to quantify hyporheic residence times from 0.01 - 15 days.

Keywords: Hyporheic Exchange,Surface Water - Groundwater Interaction,Environmental Tracers,Residence Time
Subject: Environmental Science thesis

Thesis type: Doctor of Philosophy
Completed: 2014
School: School of the Environment
Supervisor: Peter G. Cook