Radon in Chalk streams: Spatial and temporal variation of groundwater sources in the Pang and Lambourn catchments, UK

Abstract

and hydrological events. In the Lambourn, the radon budget is controlled by diffuse 10 groundwater inputs, supporting the hypothesis that the alluvial aquifer plays a greater 11 role during periods of high accretion. The Pang is more complex than the Lambourn 12 having a combination of diffuse and point source inputs, with spring inputs dominating 13 both flow and radon signatures in the lower part of the catchment. Significant temporal 14 and spatial variations were determined for C I in both catchments reflecting their 15 differing geologies and flow regimes. One use of radon in hydrology is the 16 determination of groundwater discharges to rivers, but the observed variations in C I 17 mean this approach may not be appropriate to all situations and that changes in source 18 need further evaluation. Nonetheless, radon is shown to be a useful tracer of flow paths 19 and processes within these catchments. 20 21

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.…”

“…The uncertainties associated with groundwater end members can be reduced by combining groundwater measurements with laboratory experiments in which the 222 Rn activity of water in equilibrium with the river sediments is determined (Burnett et al, 2008;Cook et al, 2006;Corbett et al, 1998;Martens et al, 1980;Peterson et al, 2010). Recent studies have also focussed on better quantifying processes such as hyporheic exchange and gas transfer, making the use of 222 Rn more reliable (Cook et al, 2006;Lamontagne and Cook, 2007;Mullinger et al, 2007).…”

“…Assuming that both the concentration of 222 Rn in the atmosphere and the ingrowth of 222 Rn in river water through the decay of 226 Ra in suspended sediment are negligible (Cook et al, 2006;Mullinger et al, 2007), the inflow of groundwater along a reach (I in m 3 m −1 day −1 ) may be calculated from changes in 222 Rn activity in the river c r (Bq m −3 ) with distance x (m) by Cook et al, 2006)…”

“…As such, gas transfer rates (k) were estimated using the O' Connor and Dobbins (1958) and Negulescu and Rojanski (1969) gas transfer models as modified by Genereux and Hemond (1992) and Mullinger et al (2007):…”