Groundwater Extraction at the Zagreb Water Supply Facilities
The water supply for the city of Zagreb and parts of Zagreb County is secured through the extraction of groundwater at active water supply facilities. Existing piezometers are constructed such that the piezometric pipe is open throughout the entire interval of the aquifer. Sampling groundwater from these piezometers provides data on the average values of the tested parameters.
The project for constructing piezometric fields aims to open the installed piezometric pipes at characteristic hydraulic intervals, thereby enabling groundwater sampling from specific hydrogeological environments. This approach allows for the collection of data on the hydrochemical stratigraphy of the aquifer. Information regarding the water quality of the aquifer facilitates the design of wells (pump intakes) in less stressed intervals of the aquifer and planning for rehabilitation and/or remediation of the aquifer or the water catchment area. The piezometric field is also used for measuring the groundwater level.
To carry out the construction of an exploratory piezometric borehole, in accordance with the applicable Water Act and the Regulation on the Issuance of Water Management Permits, it was necessary to prepare a Work Program for obtaining water management conditions from Hrvatske Vode before commencing the work.
Drilling Technology for Exploratory Piezometric Boreholes
The drilling of an exploratory borehole is conducted using the direct drilling method with continuous coring, utilizing a core drilling apparatus with a diameter of ∅113 mm, along with the necessary use of flushing and the installation of protective casing. The excavated material (core) is placed in designated core boxes measuring 1.0 meter in length, ensuring that the depth equivalent of the core at 1.0 m corresponds to 1.0 m in the length of the box, labeled with the borehole identifier and depth interval. Other methods will be employed to sample representative samples of the excavated material (core) according to the USCS system and hydrogeological categorization of each lithological member of the exploratory piezometric borehole. This includes modal analysis of the heavy and light mineral fractions of clay and silt, as well as qualitative analysis of the mineral composition of clay and silty-sandy material using diffraction methods. Laboratory tests on these samples are conducted in accordance with the standard HRN EN ISO/IEC 17025.
Technical Construction and Backfill
A technical construction had to be installed in the drilled exploratory piezometric borehole. The technical construction of the exploratory piezometric borehole was made of stainless steel. The screen (filter) of the piezometer is also made of stainless steel. Below the screen, a sediment trap and a solid stainless steel pipe were installed. The exact arrangement and depth of the installation of the screen (filter) is determined by the client’s representative based on the characterization of the excavated material (core).
After the installation of the piezometric construction, the annular space between the piezometric structure and the borehole was backfilled with glass beads along the entire profile of the piezometer. The granulometric composition of the gravel is determined based on the granulometric characteristics of the aquifer and the openings in the filter section. Above the gravel, a clay-bentonite seal had to be installed up to the ground surface to prevent the infiltration of precipitation and potential contaminants into the aquifer.
Cleaning and Development of the Piezometer through Pumping Tests
After the installation of the technical construction, cleaning and removal of fine particles and sand that could affect the pump's operation was initiated. The cleaning and development of the piezometer were performed using a compressor with the air-lift method, which involves alternating the application of static and dynamic operation (pulsing) of the compressor. The air-lift construction consists of pressure and air pipes connected to the compressor. The piezometer was cleaned until the sand was no longer being discharged and the water was clear.
Stabilization of the Borehole Entrance
The protective casing on each piezometer is a 1-meter-high piezometric construction pipe. The securing of the exploratory piezometric borehole was executed as per the client's request by constructing a concrete block measuring 0.75 x 0.75 x 0.50 m with a threaded cap (cover). An identification mark for the piezometer was placed on the cover. Finally, disinfection of the piezometer was conducted by pouring a disinfectant solution into the piezometric structure.