Groundwater dating with3H and SF6 in relation to mixing patterns, transport modelling and hydrochemistry

Abstract

Abstract:A comprehensive study of a sandy aquifer of deltaic origin in southern Poland included water chemistry, isotopes, dissolved trace gases and transport modelling. Tritium, sulphur hexafluoride (SF 6 ) and freons (F-11, F-12 and F-113) showed the presence of modern waters in the recharge areas and shallow confined parts of the aquifer. The presence of older Holocene waters farther from the recharge areas was indicated by lack of 3 H, SF 6 contents Ä0Ð02 fmol l 1 and relatively low 14 C values. The discharge from the system is by upward seepage in the valley of a major river. Pre-Holocene waters of a cooler climate, identified on the basis of υ 18 O, υ 2 H, 14 C, Ne and Ar data, were found in some distant wells. Concentrations of N 2 , Ne and Ar determined by gas chromatography were used for calculating the noble gas temperatures, air excess needed for correction of SF 6 , and nitrogen content released by denitrification process. The time series of 3 H content available for some wells supplied quantitative information on age distributions and the total mean ages of flow through the unsaturated and saturated zones. The derived 3 H age distributions turned out to be very wide, with mean values in the range of about 30 to 160 years. For each well with determined 3 H age, the SF 6 data showed either a lower age range or the possibility of a lower age as expected due to shorter travel times of SF 6 through the unsaturated zone, which most probably also resulted in different types of age distributions of these tracers. Freons appeared to be of little use for individual age determinations. A quantitative estimation of two-component mixing from SF 6 -3 H relations is not possible unless the travel time of 3 H through the unsaturated zone is comparable to that of SF 6 . The ratio of integrals of the response function over the age range with tracer and the whole response function yields the ratio of water with tracer to total flow of water. That ratio is a tracer-dependent function of time. Transport modelling of SF 6 tracer done with MT3D code yielded initially large discrepancies between calculated and measured tracer concentrations. Some discrepancies remained even after calibration of the transport model with SF 6 . Simulation of tritium contents with a calibrated transport model yielded reasonable agreement with measured contents in some wells and indicated a need for further investigations, particularly in the eastern part of the aquifer. The existence of distinct hydrochemical zones is consistent with the tracer data; young waters with measurable 3 H and SF 6 contents are aerobic and of HCO 3 -Ca or HCO 3 -SO 4 -Ca types. Slightly elevated Na and Cl contents, as well as the highest concentrations of SO 4 and NO 3 within this zone are due to anthropogenic influences. Anaerobic conditions prevail in the far field, under the confining cover, where pre-bomb era Holocene waters dominate. In that zone, dissolved oxygen, NO 3 and U contents are reduced, and Fe, Mn and NH 4 contents increase. In the third zone, earl...

Residence times and mixing of water in river banks: implications for recharge and groundwater–surface water exchange

“…as discussed by Cartwright et al, 2013). A similar covariance of 14 C and 3 H activities would be obtained using a dispersion model (Zuber et al, 2005) or the renewal rate model of Le Gal La Salle et al (2001;Fig. 9).…”

“…Lumped-parameter models are commonly used to describe groundwater flow in shallow unconfined and semi-confined aquifers Zuber, 1991, 1982;Morgenstern et al, 2010;Zuber et al, 2005). Piston flow models assume that no mixing takes place between recharge and water in the aquifer and are suitable for settings where dispersion is low.…”

“…where T is the mean residence time and f is the ratio of exponential flow to piston flow for the total flow volume (Zuber et al, 2005). f was estimated at 0.8 for shallow bores neighbouring the Tambo River on the basis of bore depth, screen length and aquifer lithology (cf.…”

Residence time, mineralization processes and groundwater origin within a carbonate coastal aquifer with a thick unsaturated zone

“…It is also a powerful integrative approach of groundwater origin and flow path organisation within aquifers (Anders et al, 2014;Bertrand et al, 2010;Gooddy et al, 2006;Jaunat et al, 2012;Kamtchueng et al, 2015). Furthermore, groundwater residence time provides information to constrain recharge area, discharge rate, flow directions and velocities, all necessary to either quantitatively validate a numerical or conceptual hydrogeological model Post et al, 2013;Turnadge and Smerdon, 2014;Zuber et al, 2005). When groundwater residence times are over timescale of 0-70 years, dating tools such as tritium, chlorofluorocarbons (CFCs) or sulphur hexafluoride (SF 6 ) may be used (Aeschbach-Hertig et al, 1998;Alvarado et al, 2005;Cook and Solomon, 1997;Corcho Alvarado et al, 2007;Delbart et al, 2014).…”

“…Thus this residence time should be mostly interpreted as a weighted mean of idealized residence times (Bethke and Johnson, 2008;Goode, 1996;Jodar et al, 2014;Suckow, 2014;Torgersen et al, 2013;Turnadge and Smerdon, 2014). The major advantage of CFCs and SF 6 resides in the possibility to discriminate different water bodies considering various mixing models (piston flow, binary or exponential), providing information on aquifer functioning (Cook et al, 1995;Jodar et al, 2014;Kashiwaya et al, 2014;Zuber et al, 2005). CFCs and SF 6 are commonly considered as pertinent dating tracers but there are nevertheless some limits to their use (Kashiwaya et al, 2014).…”