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Use of Radon to characterise surface water recharge to groundwater.


River recharge inputs constitute a significant proportion of the water balance in many New Zealand aquifers. There is a large uncertainty regarding estimates of river-groundwater fluxes based on hydraulic measurements alone, particularly for large braided river systems that present a number of technical challenges for accurate flow measurement. Hydrochemistry information, such as radon gas activity, provides an additional secondary means for measuring river - groundwater exchange at the riparian margins of a fluvial system. In this study we examined the usefulness of Radon-222 for determining the rates at which the Waimakariri River recharges the Canterbury Plains aquifer at Halkett and Crossbank. Radon concentrations measured in the Waimakariri River were very low and concentrations increased in shallow groundwater with increasing distance from the river, which was consistent with radon ingrowth processes and prior knowledge about the site hydrology. The data were modelled using the ingrowth equation for radon to determine values for the equilibrium radon value and groundwater velocity near the river, using simplified assumptions about transport flowpaths. The estimated groundwater seepage velocities of 350 m/day and 390 m/day at the Halkett and Crossbank sites, respectively, are relatively high but feasible near a large braided river. There was no significant variation in radon concentrations (differences were within analytical error) in the shallow groundwater with flow in the Waimakariri River, which ranged from about 50 to 250 m3/s. Translating groundwater velocities to effective recharge fluxes requires simplifying assumptions concerning the dimensions of the effective recharging area, whether recharge is constant along a particular reach, and on the estimated effective porosity of the groundwater system. None of these properties are reliably known at the study site and this precluded any recharge flux estimations from the radon data. Despite these limitations, radon can be used to infer useful knowledge about riparian aquifer systems. For example, continuous measurement of radon at a single observation well would provide information about how recharge at a particular location varies with time and river flow. Estimation of groundwater velocities using radon measurement from wells located at regular intervals down a river could give information on the likely variation in recharge amounts, and intensive monitoring of radon over a small spatial region could provide detail on preferential flow paths in a riparian zone.

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