Pilot area activities – #12 Thermal groundwater use in the urbanized area of Zagreb, Croatia

Groundwater temperature monitoring of two open loop systems

Zagreb is the capital of Croatia, and by far the largest urban agglomeration in the country (≈800 000 inhabitants). The city area can be divided into alluvial plain of the Sava River (ZG-AL, blue polygon in figure 1) and Podsljeme urbanized zone (ZG-PUZ) at the foothills of Medvednica Mt.

The aquifer system (ZG-AL) comprises two Quaternary aquifers. Hydrogeologically, the Quaternary deposits are divided into three basic units: An overburden of clay and silt; a shallow Holocene aquifer of medium-grained gravel mixed with sands; and deeper aquifers from the Middle and Upper Pleistocene, with frequent lateral and vertical alterations of gravel, sand and clay. The thickness of the unsaturated zone of the Zagreb aquifer system is 2-8 m going from SE to NW, predominantly gravels and sands, with sporadic clays.

Zagreb, as other larger urban areas, has significant heating and cooling loads, and it is known that many investors and developers opt for the installation of different types of heat pumps, instead of gas fired heating and typical air-source heat pump cooling systems. Unfortunately, the number of installed heat pump systems is unknown. There is no solution in sight for chronical lack of data, since the regulations on construction are becoming more lenient, rather than stricter.

Previous studies focused on the possibilities for closed loop heat pump utilization. The scope of MUSE is now to investigate do the open loop systems cause thermal pollution in the Zagreb aquifer, which is also used for water supply and, if they do, to what magnitude. Therefore, two test sites were chosen: One is a residential building, while the other is an IKEA store and accompanying facilities. We are considering a smaller and a larger system to test if there are there differences in their impact. Due to the large extent and thickness of the aquifer, our hypothesis is that we will not observe significant temperature deviations. However, that remains open.

We installed HOBO WATER LEVEL data loggers at both sites – at one piezometer in the smaller system (figure 2) and at one production and reinjection well in the larger system. The results of the field measurements will be presented in the next round of blogs about the MUSE pilot areas.

For more information, please contact:
Staša Borović, sborovic@hgi-cgs.hr

Figure 1: City of Zagreb in hydrogeological context. ZG-AL: Alluvial plain of the Sava River. ZG-PUZ: Podsljeme urbanized zone. Dark blue line: Outline of aquifer.
Figure 2: Observation borehole of smaller open loop system with installed water level logger.

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