Krčmář D., Hodasová K., Zatlakovič M., Bahnová N. & Ondrejková I., 2025: MODFLOW 6 GWE and API: New possibilities for modeling geothermal systems and heat pumps. Acta Geologica Slovaca, 17, 2, 77–85.

MODFLOW 6 GWE and API: New possibilities for modeling geothermal systems and heat pumps

Dávid Krčmář, Kamila Hodasová, Martin Zatlakovič, Natália Bahnová & Ivana Ondrejková

Comenius University Bratislava, Faculty of Natural Sciences, Department of Engineering Geology, Hydrogeology and Applied Geophysics, Ilkovičova 6, 842 15 Bratislava, Slovakia; david.krcmar@uniba.sk

Abstract

Accurate numerical simulation of subsurface heat transport is fundamental for the design and management of shallow geothermal systems, such as Aquifer Thermal Energy Storage (ATES) and Borehole Heat Exchangers (BHEs). Historically, groundwater heat transport was modeled using solute-transport codes (e.g., MT3DMS) by applying mathematical analogies between mass and energy conservation equations. While functional, this proxy approach relied on calculating equivalent retardation factors to mimic thermal storage in the solid matrix and struggled to accurately represent heat conduction in unsaturated or hydraulically inactive zones. This paper reviews the transition to the newly released Groundwater Energy (GWE) code for MODFLOW 6, which provides a rigorous, physically based formulation for simulating heat transport through both fluid and solid phases. Furthermore, we demonstrate the transformative capabilities of the MODFLOW 6 Application Programming Interface (API) for overcoming the limitations of traditional file-based simulations. Through a case study of a geothermal well doublet, we show how the API’s Extended Model Interface (XMI) enables dynamic feedback loops, allowing for the simulation of heat pumps operating with a constant temperature difference () rather than fixed injection temperatures. The results illustrate that coupling the explicit physics of the GWE code with the flexible control of the API significantly improves simulation accuracy and offers new possibilities for optimizing geothermal system operations under transient conditions.

Key words: subsurface heat transport, shallow geothermal energy, aquifer thermal energy storage (ATES), MODFLOW 6 GWE, Python API

Manuscript received: 2025-11-25

Revised version accepted: 2025-12-23