**2. Materials and methods**

Despite its significant potential, blue and green infrastructure is still insufficiently researched in Polish conditions and, therefore, remains little used as a means of counteracting the effects of climate change and adapting our cities.

#### **2.1 Study area**

As the study area was selected the water system within the municipal park in Zduńska Wola (Poland) - 51° 35 '52.9 "N/18 ° 55' 58.2" E. The park is located in the very center of the city, in the valley of the Pichna River, flowing through the city in the east–west direction. It is the largest green area in the city, covering an area of about 10 ha. It was established at the turn of the twentieth century as a private garden of Zenon Anstadt, the owner of the adjacent brewery. The factory owner made the park available to residents over time.

The design concept assumed the rebuilding of municipal ponds together with the surrounding area, with particular emphasis on the optimal solution for sealing, filling, and maintaining the purity of water in the ponds. The park is characterized by varied topography, the highest point of which is in the north and slopes to the south, toward the Pichna River valley, supplying two park ponds that are the closure of the compositional axis. In terms of nature, the area has significant values due to old trees and the water system.

As part of the preparatory work for the revalorization of the park, a number of studies and analyses were carried out, including assessment of the sanitary state of the waters of the Pichna River that supplies reservoirs and flows through the park. On this basis, it turned out that degree of the river pollution (among others, as a result of the discharge of untreated water from nearby traffic routes) makes it impossible to restore the water system while further supplying the ponds with river water. Therefore, it was decided to use complex and modern technological solutions that enable the renovation of the water system to ensure a satisfactory degree of purity and transparency of the water in the ponds. The idea of rebuilding the existing ponds into scenic and recreational reservoirs was based on Western European solutions used in municipal swimming ponds with natural water purification. It consisted of, inter alia, on the appropriate shape of the reservoirs, the use of natural mineral and plant filters and proper water circulation in order to repeatedly intensify the natural processes of water self-purification. In addition, the appropriate shaping of the shoreline and the introduction of swamp vegetation ensured a natural appearance of the ponds, which blend harmoniously with the surrounding landscape, which significantly contributed to the attractiveness of the entire park.

The ponds cover the area of 10,067 m<sup>2</sup> and consist of a recreation and viewing area and a mineral and plant filter. The solution is based on the use of the sorption capacity of a substrate consisting of natural ion-exchange minerals and absorbents of nitrogen, phosphorus, and heavy metals, as well as aquatic and rush plants. Water filtration technology based on the German FLL [14] standards for swimming ponds was used to ensure the best water parameters. The water surface area in the viewing and recreation area is approx. 3952 m<sup>2</sup> , and the area covered with water and rush vegetation is 2664 m2 . The surface of the water in the pond with the mineral-plant filter is 757 m<sup>2</sup> , and the area of the mineral-plant filter planted with water and rush vegetation is 2694 m<sup>2</sup> , which is a total of 53% of the area planted with plants.

#### **2.2 Methodology**

The design concept assumed the rebuilding of municipal ponds together with the surrounding area, with particular emphasis on the optimal solution for sealing, filling, and maintaining the purity of water in the ponds. Therefore, in 2015, it was developed detailed design documentation, taking into account a series of processes that promote the natural purification of water in ponds. In spring 2017, the project was started. Prior to profiling the bottom of the ponds, the water and marsh vegetation as well as permanent weeds were removed, and the concrete slabs that had previously protected the pond slopes were removed. The subsequent step was to remove the layer of silts and peat from the bottom of the ponds and to prism them for the use in coastal areas. Appropriate control and drainage was installed at the bottom of the ponds to monitor subcutaneous waters during sealing of the ponds. Then, the bottom of ponds was properly shaped in accordance with the designed profiling. Each 30-cm layer of soil was compacted so that the formed slopes would not erode at a later stage of use.

Surface of the bottom of the pond had to be leveled and thoroughly compacted (larger depressions were buried in layers and compacted), free of sharp objects stones, glass, etc. Sealing of the pond consisted in spreading 400 g/m<sup>2</sup> of geotextile. For a proper sealing of the pond, EPDM foil 1.02 mm thick with a certificate of neutrality with respect to flora and fauna in the ponds was used (Photo 1). In order to maintain the water table at a constant, designed level, it was necessary to permanently stabilize the edges of the pond at the appropriate level (deviations from the level could not be more than 1 cm). The entire bottom of the tanks, including the pond shore (about 1 m wide), was lined with twice washed gravel (16–32 mm fraction) with a 20-cm-thick layer.

The key element of the entire water system conditioning the purification of water in the ponds was the implementation of the mineral-plant filter (regeneration tank). Purification of water in ponds is based primarily on the proper selection of filtration material (mineral in properly developed proportions adapted to the content of elements in the water supplying the tanks) and forcing water circulation in a closed system, equipped with pumps, overflows, mechanical-mineral filter and relevant from phyto- and rhizo-filtration point of view—swamp filter (mineral bed planted with plants).

Water quality, when using biologically active sorption filters, depends on the material used and its adsorption properties and intensity of biochemical processes. It is extremely important in the process of effective removal of organic compounds (above all, phosphorus and nitrogen compounds) as well as elements present in trace amounts (e.g. heavy metals) [15]. Therefore, the mineral substrate Biozamonit® (4–16 mm fraction) was used, which is the nitrogen, phosphorus, and heavy metals as well as parasites sorbent with the addition of FerroSorp® iron hydroxide (phosphorus absorbent). This trade name includes lime-silica rocks (bedrock), zeolite, dolomite, or limestone grits in appropriate proportions [16].

Due to the high calcium content, it is the bedrock that is one of the most effective reactive materials used to remove phosphorus from aqueous solutions [17]. Furthermore, during thermal treatment, its sorption capacity increases significantly (from 60.5 g P∙kg −1 at 250°C to 119.6 g P∙kg −1 at 1000°C) [18] due to the breakdown of calcium carbonate into calcium oxide and carbon dioxide. As the firing temperature increases, the reaction also increases: from 6.80 to 12.4 after roasting at 900–1000°C [18, 19].

*Natural Water Reservoirs as an Example of Effective Nature-Based Solutions (NBS) DOI: http://dx.doi.org/10.5772/intechopen.106070*

**Figure 1.** *Mineral-plant filter with visible plantings of purple loosestrife, bulrush, and Calamus (photo: W. Walczak).*

Water pollution can be removed using mechanical, physicochemical, and biological methods. Therefore, in parallel with the use of physicochemical methods, a biologically uncomplicated method, which is phytoremediation, was used. It is a technology for the purification of ground and surface waters, and even soil and air, which uses the natural predisposition of specific taxa of plants capable of growing and developing in ecosystems contaminated with inorganic and organic substances, as well as for their uptake, accumulation, or biodegradation [20].

Therefore, several habitat zones have been arranged in the pond. In the open water, plants with delicate, flabby stems, and fine leaves (adapting their requirements to the depth of the zone) were planted (**Figure 1**). Water lily with decorative leaves and flowers were planted in the bottom layer. There are also numerous zooplankton species in this zone, which very effectively support water filtration [21]. The banks were planted with iris, calamus, and other littoral species, as well as plants from wetland habitats—sedge, horsetail, and mint [22]. All species were selected according to the habitat requirements criterion.

### **3. Results and discussion**

The use of mineral-plant filtration bed in ponds is aimed at eliminating the most adverse biogenic compounds (phosphorus and nitrogen) and maintaining optimal physical and chemical parameters of water (similar to those found in natural oligotrophic reservoirs). Artificial stimulated conditions served to limit the development of unicellular and filamentous algae, determining the natural and scenic values of ponds (each additional gram of phosphorus above the allowed norm can generate the development of filamentous algae weighing 250 kg).

In the water reservoirs given in the analyses, a natural mineral substrate Biozamonit® was used as an absorbent of phosphorus, nitrogen, and heavy metals. Composition of the above mineral deposit contains, among others, bedrock, roasted rock, zeolite, calcareous grits, ferrosorp, and dolomite grits in appropriate proportions. Biozamonit® contains, among others, additions of roasted rock, which according to tests, has a contents of CaO and SiO2 of 238.6 and 550.1 g∙kg–1, respectively. Therefore, the efficiency of PO4-P removal from an aqueous solution with an initial concentration of 1.84 and 2.88 mg∙dm−3 by the roasted rock is 88% and 70%, respectively [23]. It is also important to place the substrate in the appropriate parts of the filter bed, because the filtration processes in the mineral-plant bed take place through the vertical forced flow of water through the mineral bed.

It was the filtration, ion exchange, and buffer properties that determined the use of the aforementioned mixture as the best for this type of pond, successfully used in other projects of reservoirs of a scenic and recreation nature. The filter bed is also a habitat supporting the growth of nitrifying bacteria that are very important in the process of ammonia decomposition.

In order to increase sorption properties, the bed was planted with water and marsh plants with the best properties of absorbing the biogenic compounds from water. Plants that are used for the biological purification of water belong to the group of the so-called repository plants, highly effective also in removing toxins, protecting banks, reclamation, and cleaning soil or water. They capture pollution, pathogens, and phosphates dissolved in water and at the same time strive to stabilize water chemistry. Among the most commonly used plants for this purpose, there are helophytes (marsh plants), emery hydrophytes, and submersible hydrophytes that


#### **Table 1.**

*Summary of water test results.*

#### *Natural Water Reservoirs as an Example of Effective Nature-Based Solutions (NBS) DOI: http://dx.doi.org/10.5772/intechopen.106070*

live completely under the water [24]. Both reservoirs were planted with aquatic and underwater plants, including those with a covering function.

As a result of the sorption properties of the mineral-plant filtration bed (regeneration tank) in the first and second year of using the ponds, the amount of dissolved phosphorus (P), one of the biogenic factors, decreased 10-fold compared to the amount contained in tap water used to fill the ponds. Thus, as shown in **Table 1**, it has come close to the European standards regarding the purity of water in bathing tanks. The amount of nitrates decreased fourfold and ammonium threefold. The carbonate hardness remains at the right level around 6 dH, and the pH of the water is optimal. The complete elimination of harmful nutrients is impossible, because they are periodically influenced by the evapotranspiration process, feces of water birds, plant pollen, or small particles of airborne pollutants. As experience from other water reservoirs with scenic and recreational functions shows, the use of effective mineral and plant beds with appropriately selected physicochemical properties contribute to the effective filtration of water in ponds.

### **4. Conclusions**

Significant benefits of the construction of retention ponds are the possibility of collecting water for use in periods of drought, providing habitats for wild plants and animals in urbanized areas, and functional and compositional enhancement of public green areas. An additional benefit is the ability to purify water from contaminants from surface runoff through sedimentation and phytoremediation.

In order to increase the usability of the pond, especially in urbanized areas (**Figure 2**), it is worth designing it in such a way as to enable recreational use by various age groups of users as well as to provide the opportunity to conduct environmental education. The location of the reservoir should contribute to the improvement of the continuity of naturally functioning areas and thus favor the ecological role of this element, significantly increasing biodiversity in the urban environment.

**Figure 2.** *Pond before renovation (photo: Milecka M.).*

**Figure 3.** *Recreational pond after 2 years of operation (photo: Walczak W.).*

The observations to date indicate a very high efficiency of natural self-purification of water in bathing reservoirs [7]. This is possible thanks to the enormous vital activity of the biocenosis of natural swamp systems—from microorganisms to plants and algae, which are in dynamic balance with each other. They trigger a number of physical, chemical, and biochemical processes such as sedimentation, adsorption, oxidation, and the exchange of volatile substances between the atmosphere and water (release of gaseous metabolic products into the atmosphere), which, by interacting with each other, maintain the biological balance in the pond (**Figure 3**). The reduction of coliforms or enterococci on properly designed filters is particularly effective, so much so that the standards required by sanitary regulations are usually achieved with reserve [25]. On the other hand, one of the most difficult technical challenges posed by such solutions is to maintain the phosphate concentration in the top-up and bathing water at the level characteristic of oligotrophic waters [7]. In terms of efficiency, the analyzed joints are cheaper to maintain and implement than conventional solutions.

*Natural Water Reservoirs as an Example of Effective Nature-Based Solutions (NBS) DOI: http://dx.doi.org/10.5772/intechopen.106070*
