Submarine and Lacustrine Groundwater Dis- Charge: Localization and Quantification Using Radionuclides and Stable Isotopes As Environ- Mental Tracers
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Fakultät Umweltwissenschaften Submarine and Lacustrine Groundwater Dis- charge: Localization and Quantification using Radionuclides and Stable Isotopes as Environ- mental Tracers DISSERTATION Zur Erlangung des akademischen Grades Doctor rerum naturalium (Dr. rer. nat.) vorgelegt von Dipl.-Geograph ERIC PETERMANN Geboren am 16. Januar 1985 in Gera Tag der Verteidigung: 14. März 2018 Gutachter: Prof. Dr. Rudolf Liedl Technische Universität Dresden, Institut für Grundwasserwirtschaft Prof. Dr. Holger Weiß Helmholtz-Zentrum für Umweltforschung, Department Umweltinformatik Prof. Dr. Johannes Barth Friedrich-Alexander Universität Erlangen-Nürnberg, Lehrstuhl für Angewandte Geologie For Milena, Sarah, Jonas & Emilia. Erklärung des Promovenden Die Übereinstimmung dieses Exemplars mit dem Original der Dissertation zum Thema: „Submarine and Lacustrine Groundwater Discharge: Localization and Quantification using Radionuclides and Stable Isotopes as Environmental Tracers“ wird hiermit bestätigt. Leipzig, 18.5.2018 Ort, Datum Unterschrift Abstract ABSTRACT The discharge of groundwater into surface water bodies is a hidden, but significant path- way for the input of water and matter into lakes, rivers, estuaries and the coastal sea. Since groundwater is most often characterized by higher levels of nutrients or heavy metals, its discharge has often a crucial effect on the surface water body´s chemistry and the ecosys- tem health as well as on the related ecosystem service supply. For instance, groundwater- derived nutrient inputs are essential to fuel primary productivity, but if critical thresholds are exceeded groundwater-derived nutrient inputs can cause eutrophication, which may trigger harmful algal blooms or the creation of oxygen minimum zones – a serious threat to aquatic life. This thesis focuses on quantifying submarine and lacustrine groundwater discharge by applying environmental tracer based methods with emphasis on radionuclide (radon and radium isotopes) and stable water isotope (δ18O, δ2H) techniques. These tracers are suit- able for determining groundwater discharge as they show distinct concentration and iso- tope ratio gradients between groundwater and the receiving surface water. Four studies are presented in this thesis: (1) The quantification of the response delay of the mobile radon detector RAD7 applied for radon-in-water mapping. The response delay of the mobile radon-in-air detector RAD7 is determined for two de- tection set-ups (radon extraction via RADaqua and via a membrane module) as well as for a range of water flow rates. For the membrane module the response delay is less pro- nounced compared to the RADaqua. For instance, at a water flow rate of 1 l min-1 the peaks of the instruments recordings lag behind the radon-in-water concentrations by ~10 min for the membrane module and by ~18 min for the RADaqua. Further, it was demonstrated that faster water flow rates decrease the response delay. An algorithm is presented that allows the inverse calculation of radon-in-water concentrations from RAD7 records for the described detection set-ups and water flow rates. Thus, it allows a more precise local- ization of radon-in-water anomalies and, consequently a more precise localization of groundwater discharge areas. (2) Determination of submarine groundwater discharge into a large coastal bay (False Bay, South Africa) SGD consists generally of two components: (a) fresh terrestrial SGD (FSGD) driven by the inland hydraulic gradient and (b) seawater re-circulation (RSGD) through the coastal aq- uifer driven by seaward effects such as tidal pumping. A bay-wide radon mapping resulted in identification of a SGD site, where subsequently detailed investigations were con- ducted. At this SGD site a salt and a radon mass balance were applied consecutively for determining FSGD and total SGD, respectively. RSGD was inferred from the difference be- tween FSGD and total SGD. For the radon mass balance, new approaches for calculating the radon degassing and mixing loss were proposed. The tracer mass balance revealed median FSGD of 2,300 m³ d-1 or 0.9 m³ d-1 per m coastline and median RSGD of 6,600 m³ d-1 or 2.7 m³ d-1 per m coastline. The FSGD rate was validated using (a) a hydrological model for calculating the groundwater recharge rate and (b) a groundwater flow model 1 Abstract for delineating the subsurficial FSGD capture zone. This validation supported the tracer based findings. The relevance of this study is foremost the presentation of new methodo- logical approaches regarding the radon mass balance as well as the validation of FSGD under consideration of hydrological and hydrogeological information. (3) Differentiation of fresh and re-circulated submarine groundwater discharge in an estu- ary (Knysna Estuary, South Africa)Knysna Estuary is a more complex system than False Bay since besides seawater, FSGD and RSGD also river water mixes within the estuary. Both FSGD and RSGD were differentiated by applying a mixing analysis of the estuary wa- ter. For this purpose, an end-member mixing analysis (EMMA) was conducted that simul- taneously utilizes radon and salinity time series of estuary water to determine fractions of the end-members seawater, river water, FSGD and RSGD. End-member mixing ratio uncertainty was quantified by stochastic modelling (Monte Carlo simulation) under con- sideration of end-member characterization uncertainty. Results revealed highest FSGD and RSGD fractions in the estuary during peak low tide. Median fractions of FSGD and RSGD were 0.2 % and 0.8 % of the estuary water near the mouth over a 24 h time-series. In combination with a radon mass balance median FSGD of 46,000 m³ d-1 and median RSGD of 150,000 m³ d-1 were determined. By comparison to other sources, this implies that the SGD is a significant source of dissolved inorganic nitrogen (DIN) fluxes into the estuary. This study demonstrates the ability of EMMA to determine end-member fractions in a four end-member system under consideration of end-member uncertainty. Further, the importance of SGD for the water and DIN budget of Knysna Estuary was shown. (4) Quantification of groundwater discharge and water residence time into a groundwater- fed lake (Lake Ammelshainer See, Germany). The presented approach utilizes the stable isotopes of water (δ18O, δ2H) and radon for determining long-term average and short-term trends in groundwater discharge rates. The calculations were based on measurements of isotope inventories of lake and ground- water in combination with climatic and isotopic monitoring data (in precipitation). The results from steady-state annual isotope mass balances for both δ18O and δ2H are con- sistent and reveal an overall long-term average groundwater discharge that ranges from 2,800 to 3,350 m³ d-1. These findings were supported by the good agreement of the simu- lated annual cycles of δ18O and δ2H lake inventories utilizing the determined groundwater discharge rates with the observed lake isotope inventories. However, groundwater dis- charge rates derived from radon mass balances were significantly lower, which might in- dicate a distinct seasonal variability of the groundwater discharge rate. This application shows the benefits and limitations of combining δ18O/δ2H and radon isotope mass bal- ances for the quantification of groundwater connectivity of lakes based on a relatively small amount of field data accompanied by good quality and comprehensive long-term meteorological and isotopic data (precipitation). This thesis presents important methodological achievements with respect to radon and stable water isotope mass balances, uncertainty quantification, geochemical differentia- tion between FSGD and RSGD and validation of FSGD. Further, first SGD estimates are re- ported for False Bay and Knysna Estuary in South Africa. 2 Kurzfassung KURZFASSUNG Der Austritt von Grundwasser in Oberflächengewässer stellt einen unsichtbaren Eintrags- pfad von Wasser und Stoffen in Seen, Flüsse, Ästuare und das küstennahe Meer dar. Die Konzentrationen vieler Stoffe wie beispielsweise von Nährstoffen und Schwermetallen ist im Grundwasser im Allgemeinen signifikant höher als in Oberflächengewässern. Daher können selbst volumetrisch verhältnismäßig kleine Grundwasseraustritte entscheiden- den Einfluss auf Wasserchemie und den Gesundheitszustand des aquatischen Ökosystems haben, womit Auswirkungen auf die Bereitstellung von Ökosystemleistungen verbunden sein können. Beispielsweise sind grundwasserbürtige Nährstoffeinträge eine entschei- dende Steuergröße für die Primärproduktivität. Überschreiten diese grundwasserbürti- gen Nährstoffeinträge jedoch einen Schwellenwert, kann es zur Eutrophierung des Ober- flächengewässers kommen. Dies wiederum kann toxische Algenblüten oder die Entste- hung von Sauerstoffminimumzonen zur Folge haben und das aquatische Leben bedrohen. Diese Dissertation beschäftigt sich mit Methoden zur Quantifizierung von Grundwas- sereinträgen in den küstennahen Ozean, Ästuare und in Seen. Dabei stützt sich diese Ar- beit primär auf Umwelttracer, vor allem auf Radionuklide (Radon- und Radium-Isotope) sowie die stabilen Isotope des Wassers (δ18O, δ2H). Diese Umwelttracer sind für die un- tersuchten Systeme in besonderer Weise geeignet, da zwischen Grundwasser und Ober- flächenwasser ein ausgeprägter Gradient hinsichtlich Konzentration bzw. Isotopensigna- tur besteht. Vier Einzelstudien stellen den Kern dieser Arbeit dar: (1) Die Quantifizierung der Antwortverzögerung des mobilen Radon-Detektors RAD7, an- gewendet für die Radon-in-Wasser-Kartierung.