Stone, A. E. C., & Edmunds, W. M. (2014). Naturally-high nitrate in unsaturated zone sand dunes above the Stampriet Basin, Namibia. Journal of Arid Environments, 105, 41–51.
Abstract: Elevated groundwater nitrate levels are common in drylands, often in excess of WHO guidelines, with concern for human and animal health. In light of recent attempts to identify nitrate sources in the Kalahari this paper presents the first unsaturated zone (USZ) nitrate profiles and recharge rate estimates for the important transboundary Stampriet Basin, alongside the first rainfall chemistry records. Elevated subsurface nitrate reaches 100–250 and 250–525 mg/L NO3–N, with NO3–N/Cl of 4–12, indicating input above evapotranspiration. Chloride mass balance recharge rates range from 4 to 27 mm/y, indicating a vertical movement of these nitrate pulses toward the water table over multi-decadal timescales. These profiles are sampled from dune crests, away from high concentrations of animals and without termite mounds. Given low-density animal grazing is unlikely to contribute consistent spot-scale nitrate over decades, these profiles give an initial estimate of naturally-produced concentrations. This insight is important for the management of the Stampriet Basin and wider Kalahari groundwater. This study expands our knowledge about elevated nitrate in dryland USZs, demonstrating that it can occur as pulses, probably in response to transient vegetation cover and that it is not limited to long-residence time USZs with very limited downward moisture flux (recharge).
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Vogel, J. C., Talma, A. S., & Heaton, T. H. E. (1981). Gaseous nitrogen as evidence for denitrification in groundwater. Journal of Hydrology, 50, 191–200.
Abstract: By investigating the nitrate, oxygen, nitrogen and argon concentrations and 15N14N ratios in artesian groundwater with radiocarbon ages ranging up to 27,000 yr. a process of very slow denitrification in a confined aquifer is demonstrated. The calculated nitrogenisotope fractionation factor associated with this reaction is comparable to that reported for bacterial cultures in vitro and in vivo.
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Doulgeris, C., Tziritis, E., Pisinaras, V., Panagopoulos, A., & Külls, C. (2020). Prediction of seawater intrusion to coastal aquifers based on non-dimensional diagrams. In EGU Geophysical Abstracts (4073).
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Külls, C. (2001). Groundwater of the North-Western Kalahari, Namibia: estimation of recharge and quantification of the flow system. Doctoral thesis, Hydrogeologie und Umwelt, .
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Döll, P., Krol, M., Fuhr, D., Gaiser, T., Herfort, J., Höynck, S., et al. (2003). Integrated scenarios of regional development in Ceará and Piauí. In Global Change and Regional Impacts (pp. 19–41). Springer, Berlin, Heidelberg.
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Voerkelius, S., Külls, C., Santiago, M. M. F., Frischkorn, H., dos Santos Semrau, L. A., Heinrichs, G., et al. (2003). Investigations on water management and water quality in Picos/PI and Tauá/CE. In Global change and regional impacts (pp. 173–184). Springer, Berlin, Heidelberg.
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Külls, C., Leibundgut, C., Schwarz, U., & Schick, A. P. (1995). Channel infiltration study using dye tracers. IAHS Publications-Series of Proceedings and Reports-Intern Assoc Hydrological Sciences, 232, 429–436.
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Zagana, E., Külls, C., & Udluft, P. (2000). Der Wasserhaushalt des Aliakmonas. Vom Wasser, 94, 29–39.
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Külls, C., Adar, E. M., & Udluft, P. (2000). Tracer Studies and Modelling of Regional Groundwater Systems-Resolving patterns of groundwater flow by inverse hydrochemical modelling in a semiarid Kalahari basin. IAHS Publications-Series of Proceedings and Reports-Intern Assoc Hydrological Sciences, 262, 447–452.
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Klock, H., Külls, C., & Udluft, P. (2001). Quantification of Groundwater Recharge-Estimating recharge values using hydrochemical and geological data: A case study from the semiarid Kalahari catchment of northern Namibia. IAHS Publications-Series of Proceedings and Reports-Intern Assoc Hydrological Sciences, 269, 25–32.
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