Dynamic Coupling of Sewer Exfiltration and Groundwater Response using a Simplified Approach

Christian Engelmann1, Mike Müller2, Martin Binder3
1 Technische Universität Dresden, Institute of Groundwater Management, Bergstraße 66, 01069 Dresden
2 hydrocomputing GmbH & Co. KG Leipzig, Zur Schule 20, 04158 Leipzig
3 Department Environmental Informatics, Helmholtz-Centre for Environmental Research – UFZ, Permoserstraße 15, 04318 Leipzig, Germany

2.2 in Wasserquantitäts- und qualitätsmodellierung auf regionaler Skale - Herausforderungen und neue Ansätze

Groundwater is an essential source for drinking and irrigation water supply worldwide. In the light of rapid urbanization, groundwater resources are more and more affected by anthropogenic influences, especially in strongly urbanized areas with municipal and private sewer systems. Large amounts of nutrients and highly persistent emerging pollutants may be distributed in aquifers through sewage exfiltration, posing tremendous threats for secure water supply and ecosystems. Quasi-continuous monitoring in combination with model-based simulations is needed to predict the fate of contaminants and to identify adequate strategies for risk assessment.

Urban environments are often complex systems. Especially the unsaturated transition zone between sewer and groundwater is subject to frequent impacts by both hydraulic stress and solute concentrations. Sub-daily and seasonal changes of sewage exfiltration can lead to highly dynamic distributions of water content and contaminant concentrations. These processes lead to non-uniform shapes of contaminant plumes in the aquifer. Holistic modeling approaches may adequately represent hydraulic and transport processes. However, such approaches frequently result in high levels of complexity and largely suffer from uncertainties. Further, simulation run times typically exceed applicable time frames. Therefore, approaches with reduced levels of complexity may lead to comparably plausible modeling predictions. For example, the transport through the vadose zone is represented by an analytical equation.

This study shows a dynamic coupling of sewer exfiltration and groundwater response. The model uses the newly developed software ueflow [1] that makes MODFLOW 6 [2] available via pymf6 [3]. The modeler can implement a plug-in written in Python [4]. This plug-in is then called by MODFLOW at each time step during the model run. This way the plug-in can introduce dynamic boundary conditions, i.e., specified values depending on internal model conditions.

Here, we present a synthetic setup of dynamic boundary conditions for sewage exfiltration that are defined along a single pipe with multiple defects (after [5]). These boundary conditions couple sewer pipe outflow rates with the vadose zone´s response. The exfiltration rates are defined by varying hydraulic states controlled by the pipe water level and colmation layer thickness. The vadose zone response is represented by a simplified hydraulic transition from sewer pipe to groundwater surface. The coupling process was checked for plausibility by volume balancing and visual inspection. Besides that, the modeling results were compared to the results of a reference model setup [5].

In further works the model setup may be extended to larger urban scales to evaluate interferences by several sewer network features such as multiple sewer pipes with different exfiltration properties. Also, it is planned to implement conservative transport to delineate contaminant plumes emerging from leaky sewer systems.

[1] Müller, M.; Engelmann, C.; Binder, M.; Händel, Falk (2019). From Modeler to Programmer - Enabling Groundwater Modelers with Python. EuroSciPy Conference 2019, September 2-6, Bilbao, Spain

[2] Langevin, C.D.; Hughes, J.D.; Banta, E.R.; Provost, A.M.; Niswonger, R.G.; Panday, S. (2018). MODFLOW 6 Modular Hydrologic Model version 6.0.3: U.S. Geological Survey Software Release, 9 August 2018, https://doi.org/10.5066/F76Q1VQV

[3] Müller, M.; Engelmann, C.; Binder, M.; Händel, Falk (2019). Pymf6 - Python wrapper for MODFLOW 6. Available under https://github.com/hydrocomputing/pymf6 (last accessed on 30/10/2019)

[4] PSF (2019) Python Software Foundation. Python Language Reference, Version 3.7. Available under http://www.python.org (last accessed on 30/10/2019)

[5] Binder, M.; Engelmann, C.; Rojas, K. L.; Liedl, R.; Schirmer, M.; Walther, M. (2019). Simulation of Leaky Sewer Pipes on Different Scales. Groundwater Quality Conference 2019, September 9-12, Liége, Belgium