Water storage through targeted infiltration in the soil

With the help of the Frankfurt Bridges, up to 2 million cubic meters of water can be collected. Since near-surface storage capacities are exhausted relatively quickly, the project aims to infiltrate this water into the groundwater body. In this way, large volumes of water from precipitation or from excavation pits can be collected, selectively infiltrated, and withdrawn as needed. The present groundwater modeling has been done for areas close to the city and has been limited to the absolutely necessary 600,000 cubic meters of water. By adding more seepage areas slightly further away that are reached by the bridge arms, initial estimates suggest that the full 2 million cubic meters of collected water could also be infiltrated or stored in the area of Frankfurt or close by.

Content: 600,000 cubic meters of water can be stored in groundwater close to the city

Under certain conditions, these enormous quantities can be infiltrated into the groundwater close to the city and enrich the groundwater body to such an extent that it is permissible to withdraw water from it again.

The following explains which soil quality standards must be met in order to be allowed to infiltrate into groundwater.

Potential infiltration areas close to the city were selected for the Frankfurt Bridges. Taking into account the existing groundwater level, a groundwater model was used to simulate by how many centimeters the groundwater level would rise with certain amounts of infiltration.

A prerequisite for any groundwater recharge is thorough special water treatment. The timing and location of extraction is also subject to strict regulation by the relevant authorities.

With the help of the bridges, more than two million cubic meters of water can be collected - storing such volumes is a major challenge

To have enough water for irrigation, several water sources can be used: Rainwater can be collected, groundwater can be pumped from excavations, and water can be taken from the Main River when it carries a lot of water.

This means that even theoretically more than the 600,000 to 800,000 cubic meters of water needed for irrigation can be collected over the year. But when exactly is it needed? How long do you have to store it?

The planned storage capacity was designed within the framework of the Frankfurt Bridges to be able to irrigate all green areas in question from April to September. This already takes into account the fact that the growing seasons in Germany have been extended by two weeks forward and two weeks backwards in recent years. This means that plants begin to sprout earlier in the spring and have longer foliage in the fall. This trend is likely to continue.

Consequently, sufficient water must be available at the end of March to ensure that green spaces and trees that are precious to the city's climate are protected against potential long dry spells: a Herculean task.

For such quantities of irrigation water, storage in the aquifer is more sustainable than the creation of huge artificial water reservoirs

In principle, several types of storage are possible: for example, large underground structures (similar to the 11 rainwater retention basins in Frankfurt) in which water is collected; or surface waters such as lakes near the city. The end section of a harbor basin on the Main River could also be considered.

But all of these reservoirs would require a great deal of construction. Moreover, due to the lack of space, it is not easy to provide large water storage facilities in Frankfurt's urban area. However, there is a large existing storage facility located directly underground: groundwater.

The principle is simple: the water is infiltrated into the groundwater. With regard to the quality of the discharged water, the legally stipulated prohibition of deterioration must be observed, i.e. the quality of the groundwater must not deteriorate as a result of the infiltration of water into it.

Subject to compliance with all conditions, the "groundwater body" can be enriched by infiltration, and water can be drawn again from nearby wells as needed without lowering the groundwater level - a storage principle that is otherwise only permitted in Germany if it can be ensured that the groundwater level is not endangered.

This is also how drinking water extraction works for Frankfurt in the Hessisches Ried region, and similar to that there, infiltration pits and infiltration shafts can also be installed close to the city in Frankfurt, for example in municipal parks.

Infiltration of the irrigation water enriches the groundwater body

Infiltration can best take place where an unsaturated layer still exists in the soil above the current groundwater level. Infiltration gradually fills the free spaces (pores) between the individual earth and rock particles in the unsaturated layer. The groundwater level rises. Once the layer is completely saturated, it cannot absorb any more water. If seepage were to continue, the soil there would soon be under water.

Here, the soil properties determine the infiltration potential

The decisive factor for the suitability of an area for groundwater storage is thus the "thickness" of the water-permeable soil layer above the groundwater; the experts speak of the "thickness" of the "unsaturated zone". This varies within the Frankfurt urban area: In Oberrad, the unsaturated zone is rather low in thickness; the groundwater lies more densely below the surface. In Eschersheim, on the other hand, the zone is thicker and the groundwater lies deeper.

Soil composition also varies greatly within Frankfurt: In the south of the city, the soil is sandy, while in the north it tends to be loamy. Accordingly, the percolating water takes different lengths of time to reach the groundwater.

The soil must not be contaminated with contaminating sites

This is because these contaminations would otherwise enter the groundwater through the infiltration processes. This means that some areas that used to be commercial areas (such as the Riederwald) are no longer potential infiltration areas, even though they appear to be completely clean.

The area must have space for water treatment devices and containers

Water may only be infiltrated if its quality is not worse than that of the underlying groundwater. But even if it is taken from the nearest wells, it may have to be treated again before it is allowed back into the ring main. And any kind of water treatment needs certain space.

The soil must be permeable to water

Traditionally, water seepage takes place in the city forest in the south of Frankfurt, where the soil is very permeable. North of the Main River, on the other hand, one quickly encounters the clay layer that lies beneath Frankfurt in large parts and makes seepage difficult.

The soil must have sufficient storage capacity

The unsaturated layer in the soil must be thick enough to absorb sufficient groundwater without allowing the water table to rise to such an extent that existing buildings or trees are endangered.

There are basically two different options for infiltration: Trench or shaft

For percolation close to the city, no parks or meadows within the city are flooded, but special percolation facilities are created: For one, infiltration trenches can be constructed. These can be easily hidden under walking paths.

On the other hand, there is the possibility of building deeper infiltration shafts at specific points.

The choice depends on the location: If you have enough space, you choose infiltration trenches, while in confined spaces the usually deeper infiltration shafts are more suitable.

Hessenwasser GmbH & Co. KG/Regierungspräsidium Darmstadt

If the infiltration takes place via coverable trenches into the groundwater reservoirs these trenches appear from the outside like ordinary walkways

Infiltration trenches make it possible to slowly channel large quantities of water into the ground while taking up as little space as possible. To do this, they are excavated a few meters deep and then filled with gravel. The water is introduced and fills the voids between the pebbles. While it stands in the pit, it gradually penetrates the soil.

To avoid people trampling over the gravel and also to prevent contamination, the infiltration trenches are covered. Thus, they become almost invisible and can serve as walking paths.

So while Frankfurt's population strolls along walkways in parks and enjoys the greenery, their city's aquifers are gradually filling up under their feet.

Risks and issues related to infiltration of water into groundwater must be clarified during the preliminary planning phase of the Frankfurt Bridges and examined by the authorities

Before infiltration into groundwater can take place, important prerequisites must be clarified:

Where can seepage take place in Frankfurt's urban area or close to the city? What is the soil quality and composition there?

What degree of treatment is required? Purification to drinking water quality or less?

At what local distance from the infiltration source may water be extracted?

At what temporal distance from the infiltration source may water be extracted?

Experts on this Topic

Untere Wasser- und Bodenschutzbehörde Frankfurt - Universität Heidelberg Institut für Geowissenschaften - Forschungsgruppe Hydrogeochemie und Hydrogeologie - Technische Universität Darmstadt Institut für Hydrogeologie - Hessenwasser nachhaltige Wasserversorgung - Hessisches Landesamt für Naturschutz, Umwelt und Geologie

Taking into account the local soil conditions and with the help of a simplified groundwater model, the areas in Frankfurt were identified in which water can probably be infiltrated without hesitation

In the present groundwater model, potential infiltration sites for large amounts of water were identified: The geology and groundwater situation of an area are decisive for the effects of infiltration there. Significant amounts of water can only be infiltrated in an area if the level of the aquifer affected there is not yet too high and therefore only rises to a certain extent. Otherwise, there is a risk that an excessively high groundwater level will cause damage to structural elements or plants.

In a first step, suitable areas were selected from all potential infiltration areas and unsuitable areas were excluded

Potential parks and green areas where infiltration could take place under walking paths certainly exist throughout the city. But not all of them are suitable for the infiltration of large quantities of water.

Selection parameters

Information about soil contamination
Relief of the project area (elevation profiles)
Geological conditions (clay, sand, rock, etc.), i.e. water permeability and storage capacity of the soil.
Current level of groundwater and corresponding increase in case of infiltration - only identifiable with the help of the groundwater model

(1) Contaminated sites in the model area

The identified contaminated sites preclude their use as infiltration areas, as this would impair the quality of the groundwater.

(2) The digital terrain model illustrates the elevation profile of the study area

The digital terrain model of the study area with the entire route of the bridge and the Main River gives an impression of the relief of the city and serves as a data basis for the creation of the 3D groundwater model. The considered infiltration areas are additionally shown in neon green.

(3) Quaternary thickness (in the model structure) indicates the water permeability and storage capacity of the soil

The so-called "Quaternary thickness" describes how thick the layer in the soil is that is capable of holding groundwater.

Put simply: The lower the Quaternary thickness of a site, the "thinner" the aquifer there

(4) Current groundwater levels are important input parameters for the groundwater model

In the 2D groundwater model, the current groundwater elevations can be read out in a very differentiated manner. This is an important basis for simulating the infiltration of additional water.

The initial situation of groundwater recharge in the model area

Groundwater recharge is defined as "access of infiltrated water to groundwater" (DIN 4049-3). Low or negative groundwater recharge rates can tend to be associated with a lot of evaporation, whereas high groundwater recharge rates indicate increased groundwater recharge. Suitable infiltration areas depend primarily on the particular subsoil structure and terrain sealing.

Infiltration areas for the Frankfurt Bridge World additionally contribute to groundwater recharge

In the groundwater model, existing groundwater recharge is combined with recharge from the infiltration areas. The areas that are particularly suitable for infiltration are: the Niddapark, parts of the Anlagenring, the Rebstockpark, the Heinrich-Kraft-Park and the Stadtwald.

Procedure: In the groundwater model, groundwater levels were calculated for the entire model area before and after infiltration through the main of the Frankfurt Bridges. Even though the modeling results are only an initial estimate, they provide an important basis before simulating the influence of infiltration of additional water.

A hydrogeological structure model is used for the - highly simplified - analysis of groundwater dynamics

With the help of the - publicly available - borehole points of the digital terrain model, a hydrogeological structure model was spatially (3D) interpolated. It represents the essential hydrostratigraphic formations of the Quaternary and Tertiary in a highly simplified way.

The figure also shows the discretized mesh of finite elements at whose transitions the differential equations of groundwater flow are solved. At known extraction wells, the model mesh is more finely discretized in order to represent the groundwater dynamics there in greater local detail.

The 3D groundwater model shows the current calculated groundwater levels (contour lines of groundwater levels) in the shallow Quaternary aquifer

The groundwater model shows the increases in groundwater levels after infiltration of an additional 600,000 m3 of water per year

The additional infiltration is expected to raise groundwater levels by only 25 to 50 centimeters over a large area. This means that up to 600,000 cubic meters of water can be infiltrated close to the city without affecting existing buildings or plants. If the model areas are expanded, up to two million cubic meters of water can be stored by infiltration in the immediate vicinity of Frankfurt.

Experts on this Topic

Universität Leipzig Institut für Infrastruktur und Ressourcenmanagement - Professur für Wassermanagement und Klimaanpassung - Universität Leipzig Institut für Infrastruktur und Ressourcenmanagement - Professur für Wassermanagement und Klimaanpassung - Universität für Bodenkultur Wien Institut für Ingenieurbiologie und Landschaftsbau AG Nature-based Solutions - Universität Potsdam - Institut für Umweltwissenschaften und Geographie - AG Wasser- und Stofftransport in Landschaften - Christian-Albrechts-Universität zu Kiel Sektion Geowissenschaften - AG Geohydromodellierung

Model annual balance infiltration and irrigation

The figure below shows an example of the annual cycle of infiltration and irrigation in an idealized year based on an irrigation demand of 600,000 m³/yr. Irrigation takes place over 6 months in summer and increases in quantity with daytime temperature. Infiltration occurs year-round and was estimated here proportionally from long-term monthly precipitation. The total balance is zero, accordingly.

Infiltration and withdrawal in the seasonal course

If the infiltration and irrigation values are plotted cumulatively over the year, the maximum required storage, in this case groundwater volume, can be determined from the largest difference between the two curves at a given period. In this example (irrigation volume 600,000 m³), the maximum in August and September is about 150,000 m³. For an irrigation volume of 1,000,000 m³/a, the required storage is about 250,000 m³. It can be concluded that even with a very high degree of certainty, for extreme years, the 600,000 m³ storage capacity of the groundwater is sufficient for the irrigation needs of the bridges, greenery and parks in Frankfurt.

One challenge: determining withdrawal locations and times in relation to infiltration locations and times.

In addition, the model can be used to calculate the flow and velocity of the groundwater. In this way, not only the best infiltration locations, but also the best extraction locations for the groundwater are determined.

Ideally, you should promptly withdraw as much water as you previously percolated - so in July, for example, you withdraw the water you allowed to percolate in February.

Before infiltration into the groundwater, the water from the ring main may have to be treated a second time. This is to be done in purification containers (probably by means of activated carbon filters)

Conclusion: In the area close to the city center, at least 600,000 m3 can be stored by infiltration

With the help of an initial rough groundwater model, it was possible to estimate the feasibility of storing larger volumes of water by infiltration in the Frankfurt urban area. The model showed that in selected infiltration areas located along Frankfurt's bridges, at least 600,000 m3 can be easily infiltrated without raising the groundwater level by more than 25 cm to 50 cm. This storage is sufficient for irrigation needs.

As irrigation needs increase due to climate changes or more extensive unsealing with planting, additional infiltration areas can be identified in the vicinity of the city.

Preapring the City for Challenges

The Bridge Ring Line

Harvesting" rainwater instead of discharging it into the canal

Groundwater from excavation pits should be used

The city of the future wastes no water

Unsealing of the city center

Vitalization of the urban green

The green metropolis of the future