Küstennahe Grundwassersysteme: Kartierung der Chloridverteilung aus Bohrdaten und Geophysik

Information on chloride (Cl) distribution in aquifers is essential for planning and management of coastal zone groundwater resources as well as for simulation and validation of density-driven groundwater models. We developed a method to derive chloride concentrations from borehole information and helicopter-borne electromagnetic (HEM) data for the coastal aquifer in the Elbe-Weser region where observed chloride and electrical conductivity data reveal that the horizontal distribution of salinity is not uniform and does not correlate with the coastline. The integrated approach uses HEM resistivity data, borehole petrography information, grain size analysis of borehole samples as well as observed chloride and electrical conductivity to estimate Cl distribution. The approch is not straightforward due to the complex nature of the geology where clay and silt are present. Possible errors and uncertainties involved at different steps of the method are discussed.Informationen zur Chloridverteilung im Grundwasserleiter sind für Planung und Management von Grundwasserressourcen in Küstengebieten sowie für die Simulation und Validierung von dichtegesteuerten Grundwassermodellen von wesentlicher Bedeutung. Es wurde eine Methode zur Ableitung der Chloridkonzentration aus Bohrungen und hubschrauber-elektromagnetischen Daten (HEM) entwickelt und für den Küstengrundwasserleiter des Elbe-Weser-Dreiecks eingesetzt. Die dort beobachteten Chloriddaten und elektrischen Leitfähigkeitsdaten zeigen eine ungleichmäßige horizontale Verteilung des Salzgehalts, die nicht mit der Küstenlinie korreliert. Der integrierte Ansatz verwendet HEM-Daten des spezifischen elektrischen Widerstands, Bohrloch-Petrographie-Informationen, Korngrößenanalysen von Bohrlochproben sowie beobachtete Chlorid- und elektrische Leitfähigkeitswerte, um die Chlorid-Verteilung abzuschätzen. Mögliche Fehler und Unsicherheiten bei den verschiedenen Schritten des Verfahrens werden diskutiert

Monitoring freshwater–saltwater interfaces with SAMOS – installation effects on data and inversion

A major problem for the freshwater supply of coastal regions is the intrusion of saltwater into aquifers. Due to extensive extraction of freshwater to suffice increasing drinking water demands and/or in periods of reduced groundwater recharge, the equilibrium state may be disturbed. The result is an upconing or movement of the fresh–saline groundwater interface, which reduces the local drinking water resources at coastal regions or islands. The saltwater monitoring system (SAMOS) is a vertical electrode chain installed in a backfilled borehole. It provides a solution to observe the transition zone in detail, both temporally and spatially. We present monitoring data of the first year from three locations - with different geological conditions that show disturbances in the resistivity distribution that result from the drilling processes. A clayey backfilling, for example, can lead to beam-like artefacts, and a mixed fluid within the backfilling changes its bulk resistivity, both leading to misinterpretations. We performed data inversion under cylindrically symmetrical conditions in full-space in order to separate these resistivity artefacts from the undisturbed background. Data inversion reveals that it is possible to separate drilling effects on the resistivity distribution from the undisturbed background. Thus, an interpretation of the natural transition zones can be made immediately after the installation

Monitoring freshwater–saltwater interfaces with SAMOS – installation effects on data and inversion

A major problem for the freshwater supply of coastal regions is the intrusion of saltwater into aquifers. Due to extensive extraction of freshwater to suffice increasing drinking water demands and/or in periods of reduced groundwater recharge, the equilibrium state may be disturbed. The result is an upconing or movement of the fresh–saline groundwater interface, which reduces the local drinking water resources at coastal regions or islands. The saltwater monitoring system (SAMOS) is a vertical electrode chain installed in a backfilled borehole. It provides a solution to observe the transition zone in detail, both temporally and spatially. We present monitoring data of the first year from three locations - with different geological conditions that show disturbances in the resistivity distribution that result from the drilling processes. A clayey backfilling, for example, can lead to beam-like artefacts, and a mixed fluid within the backfilling changes its bulk resistivity, both leading to misinterpretations. We performed data inversion under cylindrically symmetrical conditions in full-space in order to separate these resistivity artefacts from the undisturbed background. Data inversion reveals that it is possible to separate drilling effects on the resistivity distribution from the undisturbed background. Thus, an interpretation of the natural transition zones can be made immediately after the installation. © 2020 The Authors. Near Surface Geophysics published by John Wiley & Sons Ltd on behalf of European Association of Geoscientists and Engineers

Aktuelle und zukünftige Grundwasserversalzung in der nördlichen Elbe-Weser Region

Salinization of the upper aquifer of the northern Elbe-Weser region almost extends to the surface. Chloride content exceeds 250 mg/l and the groundwater is therefore, according to the German Drinking Water Ordinance, not suitable as drinking water. The chloride content in the aquifer originates from early flooding with seawater which occurred during the Holocene sea level rise. Depth and extent of the salinization were mapped by airborne electromagnetic surveys and validated by groundwater analyses. In the transition zone between the marshlands and geest areas, the fresh-saline groundwater interface falls to a depth of > −175 m NHN. Due to the extensive drainage of the marshlands, seepage of fresh groundwater is impeded. Instead, an upconing of the fresh-saline groundwater interface appears due to an upwardly directed hydraulic gradient. Due to climate change, chloride concentrations will increase along the coastlines. Further inland, a decrease of chloride content in near-surface groundwater will occur.Interreg http://dx.doi.org/10.13039/100013276Landesamt für Bergbau, Energie und Geologie (LBEG) (4273

Modelling of the groundwater salinization in the coastal aquifers of Lower Saxony derived by airborne electromagnetics

Die Bundesanstalt für Geowissenschaften und Rohstoffe (BGR) führte in den Jahren 2000–2014 umfangreiche (etwa 5900 km2) aerogeophysikalische Erkundungen an der niedersächsischen Nordseeküste durch. Die Aeroelektromagnetik liefert Informationen über die elektrisch leitfähigen Strukturen im Erduntergrund und ermöglicht die Unterscheidung von Süß- und Salzwasser wie auch von Tonen und Sanden. Im Landesamt für Bergbau, Energie und Geologie (LBEG) sowie am Leibniz Institut für Angewandte Geophysik (LIAG) wurden diese Daten zur Kartierung der Tiefenlage der Süß‑/Salzwassergrenze genutzt. Dadurch ist unter anderem eine detaillierte Karte der Grundwasserversalzung innerhalb der küstennahen Aquifere entlang der niedersächsischen Nordseeküste im Maßstab 1:50.000 entstanden. Diese zeigt den aktuellen Stand der Grundwasserversalzung, abgeleitet aus den gewonnenen Elektromagnetik-Modellen. Die Modelle waren auch Grundlage für eine hydraulische Modellierung der Süßwasserlinse der Nordseeinsel Borkum. Darauf aufbauende Simulationen zeigen die Entwicklung der Grundwassersituation für den Zeitraum bis 2100.Landesamt für Bergbau, Energie und Geologie (LBEG) (4273

Geophysikalische Methoden zur Bewertung potenzieller Standorte für die Grundwassergewinnung

Complex geology in glacial terrain makes groundwater exploration challenging for water supply companies. Abrupt lateral changes of geological conditions, for example, raise the risk of unsuccessful exploration drilling. At some locations, local water supply companies experience a large number of dry wells, raising the need for better exploration approaches. The use of geophysical techniques enables the detection and characterization of subsurface structures in terms of aquifers and aquitards. We present two examples in which seismic and resistivity methods are combined for subsurface characterization. A thrust structure is detected in the Varde study area in Denmark and a channel structure is mapped in the Hamburg-Sülldorf study area in Germany. The different imaging characteristics of seismic P‑ and S‑waves are demonstrated. The combination of seismic and resistivity methods enables delineating groundwater-bearing and groundwater-barrier layers down to a depth of 150 m.Interreg North Sea Region ProgrammeLeibniz-Institut für Angewandte Geophysik (3464

Saltwater intrusion under climate change in North-Western Germany - mapping, modelling and management approaches in the projects TOPSOIL and go-CAM

Climate change will result in rising sea level and, at least for the North Sea region, in rising groundwater table. This leads to a new balance at the fresh–saline groundwater boundary and a new distribution of saltwater intrusions with strong regional differentiations. These effects are investigated in several research projects funded by the European Union and the German Federal Ministry of Education and Research (BMBF). Objectives and some results from the projects TOPSOIL and go-CAM are presented in this poster