Figure 2: Schematic drawing of Induced bank filtration at the Szigetszentmiklós wellfield. (Source: Simonffy Z. 2002).
3. Brief description of the situation:
Hungary is located in the deepest part of the Carpathian basin. The majority of surface water arrives from the surrounding countries. Three large rivers leave the country after collecting the Hungarian runoff, which is the smallest in Europe and only adds 5% to the total outflow of these rivers. Potential evapotranspiration exceeds the precipitation from April to October, and consequently the runoff in summertime, when the peak of water demand occurs, is very low. Furthermore, the relief of the country is not very favourable for reservoirs.
These conditions have resulted in the present situation, where 70% of the total abstraction in Hungary is from groundwater. Abstraction of groundwater for public use even amounts to 94% of the total abstraction. This leads to a high pressure on the groundwater resources. Induced bank infiltration is an artificial recharge technique that offers a mitigation measure to these issues. More than one third of the total Hungarian groundwater abstraction for public use is being supplemented by induced bank infiltration. The drinking water supply of Budapest completely relies on this type of resource.
This case study describes the induced bank infiltration system installed at Csepel Island, Budapest (Figure 1). It is one of the two induced bank infiltration systems supplying drinking water to the 1.7 million inhabitants of Budapest. The Csepel island can be found downstream of Budapest. Because there is a potential pollution hazard related to wastewater from the city of Budapest, the situation is more sensitive than at other induced bank infiltration sites in Hungary. The island consists of a sandy-gravely deposit which is very suitable for bank infiltration. The coarse material offers a very high permeability, while the fine sediment of the riverbed provides an efficient natural filter for the infiltrating surface water.
4. Brief description of the technique:
Bank infiltration schemes commonly consist of a gallery or a line of boreholes at a short distance from, and parallel to the bank of a surface water body. Pumping of the boreholes lowers the water table adjacent to the river or lake, inducing this water to enter the aquifer system. During the passage of water through the riverbed (or lake bottom) and aquifer, dissolved and suspended contaminants as well as pathogens are removed due to a combination of physical, chemical, and biological processes. Induced bank infiltration systems are typically installed near perennial streams and lakes that are hydraulically connected to an aquifer through the permeable, unconsolidated deposits that form the stream bed or lake bottom.
5. Attributes overview:
||Domestic water supply
||Groundwater storage control
|Source of water
The wells are placed on an island, to allow for abstraction of a high portion of bank filtrate. Wells are grouped into galleries and oriented parallel to the river bank.
The thickness of the aquifer ranges between 3 and 15 m. Because the gravel terrace is not very thick at some locations, exploitation of these sections of the aquifer would require a lot of traditional tube or shaft wells. Therefore special large shaft wells with horizontal screens have been constructed here.
At other locations on Csepel island, such as the Szigetszentmiklós wellfield, bank filtration is combined with artificial recharge from infiltration ponds (Figure 2). This is done to improve the water quality.
The distance between wells in this system and the river bank ranges between 5 and 100 m. The proximity of wells to the river bank, in combination with the high permeability of the aquifer, results in very short travel times of the river water to the wells.
System capacity of the Csepel Island bank infiltration system is 400 Mm3/day, or 146 Mm3/year. The actual abstraction in 2002 was 250 Mm3/day, or 91 Mm3/year. This is contributing 40% of the drinking water supply to Budapest. The extra capacity of the system is designed to allow for increased abstraction in the future.
The capacity of the shaft wells with horizontal screens is ranging between 10 - 20 Mm3/day.
8. Experiences with Operation and Maintenance:
At Szigetszentmiklós wellfield, the bank-filtered water is moving through an artificially made bank, constructed of dredged sediment from the Danube. Because the dredged sediment contains a relatively high content of organic material the water is reduced, and consequently high iron and manganese content can be present in the wells. As active protection, recharge ponds have been constructed between the bank and the wellfield. These ponds infiltrate water pumped from the Danube, providing sufficient dilution with the bank filtered water in the aquifer. The reed cover of the infiltration ponds has been found very efficient to maintain a high infiltration capacity.
9. Experiences with Monitoring and Evaluation:
Monitoring is showing that although the travel time is very short, the bank filtered water does not need other treatment than disinfection. The natural filtration capacity of the exploited river sections are very efficient, no micro-pollutants have been found in he abstracted water. Because travel times are short (ranging between 5 and <1000 days), monitoring is especially important to ensure the good quality of the water abstracted from the wells.
10. Experiences with related subjects (e.a. erosion prevention, quality of drinking water, ..):
Water quality: An international commission for the protection of the river Danube was founded in 1994. The activities of this commission, together with the activities of waterworks, authorities, industries and transboundary programmes, resulted in a significant improvement of river water quality. In Budapest, drinking water is being continuously tested and is of good quality.
11. Experiences with management:
Induced bank filtration systems are complex, large scale and high cost projects. The Csepel bank infiltration is managed by the water authorities of Budapest, and funded by the state.
The Csepel island induced bank filtration system is an expensive large scale system, involving a high number of wells. However, the large quantities of water abstracted from the wells allow for a relatively low unit price of water. Because the bank filtered water is already of good quality, the water does not need other treatment than disinfection. This also aids in keeping the unit price of water abstracted here relatively low.
13. Generic factors of success and traps ('do's and don'ts'): If aquifer thickness is small, horizontal well screens allow for high abstraction rates without the need for larger and more expensive wellfields using traditional vertical screens. 14. What can be used elsewhere, under which conditions:
Induced riverbed infiltration systems are typically installed near perennial streams that are hydraulically connected to an aquifer through the permeable, unconsolidated deposits that form the stream channel. The quantity of surface water that can be induced to recharge the aquifer varies with:
Silt deposition, the primary cause of decreased stream bed permeability, can be avoided by placing pumping facilities near stream reaches having adequate velocity to prevent deposition, such as the outer edge of a bend in the stream.
- Amount and proximity of surface water
- Hydraulic conductivity of the aquifer
- The area and permeability of the stream bed (or lake bottom)
- The hydraulic gradient created by pumping
15. Advantages and disadvantages:
- The possibility to extract large volumes of water is the biggest advantage. The abstracted amount is limited by the infiltration capacity of the river bank only, because the discharge of the river is an order of magnitude greater than the abstracted amount.
- Compared to surface water abstraction, the treatment requirements of the water are reduced. The natural filtration capacity of the exploited river sections in Budapest is very efficient, and no micro-pollutants have been found in the abstracted water. .
- Long term contamination of river water by persistent organic compounds (such as pesticides and pharmaceuticals) may contaminate the groundwater.
16. Links to detailed information of this project / technique:
- Grischek T., Schoenheinz D., Worch E. 2002. Bank filtration in Europe - An overview of aquifer conditions and hydraulic controls. Management of Aquifer Recharge for Sustainability. Proceedings of ISAR-4, Adelaide, South Australia.
- Gale I. 2005. Strategies for Managed Aquifer Recharge (MAR) in semi-arid areas. IAH - MAR, UNESCO IHP. Paris, France.
- Simonffy Z. 2002. Enhancement of groundwater recharge in Hungary - 'Bank infiltration for drinking water supply'. In Management of Aquifer Recharge and Subsurface Storage NNC-IAH publication #4. Utrecht, The Netherlands