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  • 1
    Language: English
    In: Journal of Hydrology, 27 November 2014, Vol.519, pp.3386-3399
    Description: The travel-time distribution between rivers and groundwater observation points and the mixing of freshly infiltrated river water with groundwater of other origin is of high relevance in riverbank filtration. These characteristics usually are inferred from the analysis of natural-tracer time series, typically relying on a stationary input–output relationship. However, non-stationarity is a significant feature of the riparian zone causing time-varying river-to-groundwater transfer functions. We present a non-stationary extension of nonparametric deconvolution by performing stationary deconvolution with windowed time series, enforcing smoothness of the determined transfer function in time and travel time. The nonparametric approach facilitates the identification of unconventional features in travel-time distributions, such as broad peaks, and the sliding-window approach is an easy way to accommodate the method to dynamic changes of the system under consideration. By this, we obtain time-varying signal-recovery rates and travel-time distributions, from which we derive the mean travel time and the spread of the distribution as function of time. We apply our method to electric-conductivity data collected at River Thur, Switzerland, and adjacent piezometers. The non-stationary approach reproduces the groundwater observations significantly better than the stationary one, both in terms of overall metrics and in matching individual peaks. We compare characteristics of the transient transfer function to base flow which indicates shorter travel times at higher river stages.
    Keywords: Travel-Time Distribution ; Bank Filtration ; Non-Stationarity ; Nonparametric Inference ; Geography
    ISSN: 0022-1694
    E-ISSN: 1879-2707
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  • 2
    Language: English
    In: Journal of Hydrology, 2011, Vol.402(3), pp.274-289
    Description: ► Using multiple isotopes to study groundwater flow in active rifts. ► Hydrochemical and isotopic evolution from escarpments to Rift floor. ► Mantle CO influences groundwater hydrochemistry. ► Apparent C ages are similar in Rift floor groundwater. ► Groundwater flow paths occur both longitudinal and transversal to rift axis. This study aims to investigate groundwater recharge and flow patterns in tectonically active rift systems, exemplified by a case study in the Main Ethiopian Rift. The chosen approach includes the investigation of hydrochemical parameters and environmental isotopes ( H, δ H, δ O, δ C-DIC, C-DIC, Sr/ Sr). Apparent groundwater ages were determined by radiocarbon dating after correction of C-DIC using a modified δ C-mixing model and further validation using geochemical modelling with NETPATH. Hydrochemical and isotopic data indicate an evolutionary trend existing from the escarpments towards the Rift floor. Groundwater evolves from tritium-containing and hence recently recharged Ca–HCO -type water on the escarpments to tritium-free Na–HCO groundwater dominating deep Rift floor aquifers. Correspondingly, rising pH and values coupled with increasingly enriched δ C signatures point to hydrochemical evolution of DIC and beginning dilution of the carbon isotope signature by other carbon sources, related to a diffuse influx of mantle CO into the groundwater system. Especially thermal groundwater sampled near the most recent fault zones in the Fantale/Beseka region displays clear influence of mantle CO and increased water–rock interaction, indicated by a shift in δ C and Sr/ Sr signatures. The calculation of apparent groundwater ages revealed an age increase of deep groundwater from the escarpments to the Rift floor, complying with hydrochemical evolution. Within the Rift, samples show a relatively uniform distribution of apparent C ages of ∼1800 to ∼2800 years, with the expected down-gradient aging trend lacking, contradicting the predominant intra-rift groundwater flow described in existing transect-based models of groundwater flow. By combining hydrochemical and new isotopic data with knowledge of the structural geology of the Rift, we improve the existing groundwater flow model and propose a new conceptual model by identifying flow paths both transversal and longitudinal to the main Rift axis, the latter being strongly controlled by faulted and tilted blocks on the escarpment steps. The connection between groundwater flow and fault direction make this model applicable to other active rift systems with similar structural settings.
    Keywords: Rift Tectonics ; Hydrochemistry ; Isotope Hydrology ; Groundwater Cycle and Dating ; 87sr/ 86sr ; 14c ; Geography
    ISSN: 0022-1694
    E-ISSN: 1879-2707
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