nd
Session S5.1 - Aqua 2015 - 42
IAH Congress
953 - 222RN AS A HYDROGEOLOGIC TRACER OF GROUNDWATER INPUTS TO SURFACE WATERS: A
STUDY CASE EXPLORING THE IMPORTANCE OF AN ACCURATE DETERMINATION OF THE GAS
EXCHANGE RATE CONSTANT
Luisa Stellato
Dipartimento di Matematica e Fisica - Seconda Università degli Studi di Napoli,Caserta
Fabio Marzaioli
Dipartimento di Matematica e Fisica, Seconda Università degli Studi di Napoli, Caserta, Italy
Harald Hofmann
School of Earth Sciences, The University of Queensland, St Lucia, Australia
Laurent Pfister
Geo-hydrosystems and land use management, Centre de Recherche Public Gabriel Lippmann, Belvaux, Luxembourg
Zornitza Tosheva
Radiation Physics Laboratory, University of Luxembourg, Luxembourg
Antoine Kies
Radiation Physics Laboratory, University of Luxembourg, Luxembourg
Filippo Terrasi
Dipartimento di Matematica e Fisica, Seconda Università degli Studi di Napoli, Caserta,
Fulvio Celico
Dipartimento di Fisica e Scienze della Terra Macedonio Melloni, Università degli Studi di Parma, Parma, Italy
The main advantages of the use of 222Rn (radon, Rn) as a tracer of groundwater (gw)-surface water (sw)
exchanges are that: i) radon occurs naturally in all gw systems, although its concentrations may vary considerably
between aquifers, depending on lithology and geologic structure; ii) in-stream radon activity concentrations are
usually several orders of magnitude lower than the associated gw concentrations because of degassing; iii) 222Rn
sampling and measurement procedures are fast, low-cost and well established. For a quantitative estimation of the
gw contribution to the stream discharge an accurate determination of the Rn degassing constant is needed. To this
purpose a headwater stream has been intensively sampled over three one-day sampling campaigns for radon,
major ions, T, EC, and pH. A constant-rate NaCl injection has been performed to obtain the amount and the location
of lateral inflows to the creek. These data were supported by discharge and rain data. Radon behaviour along the
creek has been numerically modelled taking into account the lateral groundwater inflows to estimate the Rn
degassing constant. The proposed model well reproduced the experimental data within the observed uncertainties
and allowed the indirect measurement the gas exchange rate constant for the considered stream reaches. A
comparison between produced experimental results and the empirical models of gas exchange for degassing
constant estimations (i.e., Thin film and Surface renewal models) has been performed evidencing that the
agreement was verified for the reaches characterized by a regular streambed, while systematic underestimation of
the actual degassing for reaches with an irregular streambed was verified.
Natural streams are generally different from each other and unique in their hydraulic characteristics. It is
therefore recommended if possible to measure stream reaeration.
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