Controls on the spatial and temporal variability of 222Rn in riparian groundwater in a lowland Chalk catchment

Abstract

Radon is a powerful tracer of stream-aquifer interactions. However, it is important to consider the source and behaviour of radon in groundwater when interpreting observations of river radon in relation to groundwater discharge. Here we characterise the variability in groundwater radon concentrations in the riparian zone of a Chalk catchment. Groundwater 222 Rn (radon) concentrations were determined in riparian zone boreholes at two sites in the Lambourn catchment, Berkshire, UK, over a two year period. In addition, borehole core material was analysed for 226 Ra (radium) and to determine radon emanation. Radon and radium concentrations and radon emanation were found to change with depth and temporal variations in groundwater radon concentrations were found at different scales. The abundance of radium and emanation of radon increased nearer the surface leading to greater groundwater radon concentrations. It is shown that seasonal changes in water table elevation can to lead to variable radon concentrations in groundwater as zones of radon production become hydraulically active. Groundwater radon concentrations in shallow piezometers were found to respond to both seasonal changes in the water table and individual rainfall events. Riparian sources of radon can be variable and are therefore potentially influential in the radon signals observed in rivers and should be properly characterised when interpreting river radon inputs.

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

“…Finally, in rivers that run through coarse alluvial sediments, water from the hyporheic or parafluvial zones may provide a source of additional 222 Rn to groundwater inflow (Cook et al, 2006;Cartwright et al, 2014;Bourke et al, 2014a). As has been outlined in several studies, comparison of the calculated groundwater inflows from 222 Rn with those made from other geochemical tracers or with streamflow measurements is a crucial test of the calculations (Cook et al, 2003(Cook et al, , 2006Mullinger et al, 2007Mullinger et al, , 2009Cartwright et al, 2011Cartwright et al, , 2014McCallum et al, 2012;Unland et al, 2013). Carrying out studies at baseflow conditions when most of the water contributing to the streams is from groundwater inflows allows for a comparison between the calculated groundwater inflows and the observed increase in streamflows, which in turn provides for a test of the parameters used in the 222 Rn mass balance (Cartwright et al, 2014).…”

Radon in Groundwater of the Northeastern Gran Canaria Aquifer

“…The average concentrations range from 0.3 to 76.9 Bq/L; the arithmetic mean is 12.8 Bq/L; the median of the distribution is 5.3 Bq/L; and the standard deviation is 17.7 Bq/L. Several studies related to the use of radon as a hydrogeochemical tracer can be found in the literature, [7,34,35]. The main factors that influence the presence of 222 Rn in groundwater are the content of 226 Ra in the reservoir rock and its emanation coefficient, as well as the feasibility of mixing of various groundwater components [36,37].…”

Investigating the spatio-temporal variability in groundwater and surface water interactions: a multi-technique approach

“…Geochemical tracers including major ions, stable isotopes and radiogenic isotopes have been used to estimate groundwater fluxes in gaining rivers (Cartwright et al, 2008(Cartwright et al, , 2010Cook, 2012;Cook et al, 2003Cook et al, , 2006Durand et al, 1993;Genereux et al, 1993;Genereux and Hemond, 1990;Lamontagne et al, 2005Lamontagne et al, , 2008Lamontagne and Cook, 2007;Mullinger et al, 2007Mullinger et al, , 2009Négrel et al, 2003;Rhode, 1981;Ribolzi et al, 2000;Stellato et al, 2008). The utility of each of these tracers depends on a variety of factors including the difference between the concentration of the tracer in groundwater compared to surface water, its spatial and temporal variability, the accurate characterisation of its sources and sinks, and the potential for it to change by processes such as evaporation, precipitation, radioactive decay, degassing, or biogeochemical reactions.…”

“…Cook, 2012;Cook et al, 2006). The use of 222 Rn as a groundwater tracer has increased over the last two decades as methods for its measurement in the field have improved Cartwright et al, 2011;Cook et al, 2003;Ellins et al, 1990;Genereux and Hemond, 1990;Gilfedder et al, 2012;Hofmann et al, 2011;Mullinger et al, 2007Mullinger et al, , 2009Santos and Eyre, 2011). The short half-life (3.82 days) and degassing of 222 Rn from surface water makes it a particularly valuable groundwater tracer, as elevated 222 Rn activities will only occur close to zones of groundwater discharge.…”