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Article

Quality of Constructed Technogenic Soils in Urban Gardens Located on a Reclaimed Clay Pit

by
Dariusz Gruszka
,
Katarzyna Szopka
and
Cezary Kabala
*
Institute of Soil Science Plant Nutrition and Environmental Protection, Wroclaw University of Environmental and Life Sciences, Grunwaldzka 53, 50375 Wroclaw, Poland
*
Author to whom correspondence should be addressed.
Land 2025, 14(8), 1613; https://doi.org/10.3390/land14081613
Submission received: 30 June 2025 / Revised: 4 August 2025 / Accepted: 6 August 2025 / Published: 8 August 2025
(This article belongs to the Special Issue Soil Ecological Risk Assessment Based on LULC)
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Abstract

Urban gardening plays diverse social, cultural and economic roles; its further development appears to be worthwhile, provided that soil contamination does not compromise ecosystem services. This study was conducted at a complex of urban gardens in Wroclaw (Poland) where topsoil screening indicated significant spatial differentiation of trace elements content, presumably related to the history of the site. Urbic Technosols cover the reclaimed section of the gardens, where industrial and urban waste materials, such as ash, slag, construction and demolition, and household waste, were used to fill former clay and sand mines. Although the topsoil layers, comprised of transported external soil, exhibited beneficial physicochemical properties and high fertility, they were seriously contaminated with trace elements (up to 1700, 920, 740, 5.1, 7.4, and 5.1 mg kg−1 zinc, lead, copper, cadmium, mercury, and arsenic, respectively). The trace elements were likely transferred from technogenic materials used for mine infilling, which now underlie the thin humus layers of the garden soils. The results suggest that the quality of soils in urban gardens located at reclaimed post-mining sites, while seemingly beneficial for horticulture based on physicochemical soil properties and fertility indices, can be seriously and permanently compromised by soil contamination from inappropriate materials used for site reclamation, thereby affecting soil quality and posing potential health and ecological risks.

1. Introduction

Although gardening has long existed in various forms in urban areas, it has recently received special attention in many countries [1,2]. Urban gardens, along with urban parks and forests, contribute to the urban systems of green areas, which influence the microclimate and mitigate various effects [3]; affect the water cycle by disrupting surface sealing [4]; sequester carbon dioxide and improve air quality [5]; and improve the biodiversity of urban ecosystems [6]. Community or allotment gardens provide an alternative to public parks for family recreation, cultivation-oriented physical activity, and local community-building interactions [7,8,9]. The most commonly noted function of gardens is the cultivation of fruits and vegetables, considered as local, safe, and healthy products [10,11].
However, numerous studies have reported on the contamination of soils and vegetables in urban gardens, particularly those located in large cities and urban–industrial agglomerations [12,13,14,15,16,17]. Xenobiotics in garden soils pose a general environmental risk [18], including direct and indirect risks to human health [19,20]. Urban traffic and transport, along with local or regional industrial emissions, are among the most frequently identified sources of trace metal contamination of urban soils [21,22]. Kabala et al. [23] reported on the relationship between soil pH and heavy metal content in garden soils and the long-term application of contaminated lime (of industrial/smelting origin), identified as the source of soil pollution. Some types of household or industrial waste, including composts and sludges, although considered beneficial and cheap fertilizers, even if aligned with the principles of the circular economy, can deliver excessive amounts of trace elements and other xenobiotics [24,25]. Furthermore, urban garden soils can contain an admixture of construction and demolition debris, particularly if the city experienced war damage [26,27,28]. Although crushed bricks and concrete can increase the availability of water for plants and improve the physicochemical properties of sandy soils [29], other construction materials or additions, such as paints, painted wood (including joinery), glazed pottery, crystals, stained glass, and metal products, can release Cd, Cu, Zn, Hg, Ni, and other toxic substances into the soil [30,31].
Most urban gardens are located on ‘natural’ soils, which often contain admixtures of anthropogenic materials [30,32,33]. However, little is known about gardens located on artificially constructed soils at reclaimed post-mining sites such as former sand and clay pits [32,34]. Many cities in Europe and around the world consumed large volumes of construction materials excavated locally. Such rapid growth in the nineteenth century contributed to the creation of numerous suburban sand and clay pits, the latter often accompanied by brickyards. As cities continued to grow, they absorbed previous suburbs, increasing the need to reclaim depleted and abandoned mines [35]. Depending on the type of mine, topography, hydrological conditions, and local needs, the pits were converted into water bodies or, after being filled with construction and other urban debris, transformed into parks, sports fields, or residential areas [36,37]. Some reclaimed sites were converted into urban gardens, as the demand for gardens in urban spaces could not be met with ‘natural soils’, already consumed by residential or industrial projects [13,35].
Urban gardens in Wroclaw, Poland were first established in the mid-19th century and were rented to poor people and factory workers [38,39]. Since the beginning of the 20th century, many school gardens and the so-called Schreber gardens (education plus health promotion) have been established, with these gardens becoming an important source of food for urban populations during the First World War (1914–1918) [38]. During the economic crisis of the 1920s and 1930s, urban gardens served not only for food production, but even as provisional homes for unemployed and homeless people [39]. Many of the gardens established in Wroclaw (then called Breslau) in the 1920s–1930s now exist as ‘allotment gardens’, with many of them situated in reclaimed sand and clay pits (Figure 1).
Although many other cities in Poland and Germany share a similar scenario, little is known about the properties of constructed garden soils at reclaimed sites or about the impact of the materials used for reclamation on soil quality. The objective of this study was to analyze the physicochemical properties of constructed technogenic soils, including the content of selected trace elements, as a possible source of risk to human health, in a complex of allotment urban gardens situated in a reclaimed post-mining area in Wroclaw. Understanding the scale of contamination and its sources is essential for sustainable urban gardening on reclaimed lands.

2. Materials and Methods

2.1. Study Area

The study was conducted at the ‘Wytchnienie’ (‘Relaxation’) complex of allotment gardens, located in the north-east of Wroclaw, south-west Poland (Figure 1), where urban gardens (allotments) occur throughout the city and are particularly popular among retired people and families with small children [39].
Typical natural soils in the floodplains of the Odra River and its tributaries are Brunic Fluvisols and Fluvic Cambisols developed from stratified loamy and sandy alluvial sediments [41]. Beyond the Odra Valley, Luvisols and Brunic Arenosols developed from glacial tills dominate in the northern part of the city, while Phaeozems and Chernozems developed from thin loess covers on glacial sediments dominate in the southern part of the city [23,41]. The northern outskirts of the city have been extensively exploited for the mining of sand and clay/loam, and for brick processing, as illustrated on the 1886 map [40] (Figure 1C). Many other small sand and loam excavation sites are believed to have existed in the 19th century; however, these were not mapped due to their early reclamation [39]. Some of these sites have been converted into urban gardens, particularly in the north-east of Wroclaw (Figure 1C). The area of the current ‘Wytchnienie’ complex was reclaimed in the 1920s [42]; in the 1930s, it was converted into urban gardens [39].

2.2. Field and Laboratory Methods

The original project involved only topsoil (0–25 cm) sampling at 40 plots of the garden complex (Figure 1D) before the planned sampling of vegetable and fruit. However, an initial analysis of the concentration of five trace elements expressed a large difference in soil contamination between the northern and southern sections of the gardens, which could not be explained by the impact of flooding (the entire area experienced a ‘millennial flood’ in 1997 [43]) or by different management practices [23]. It was assumed that the higher soil contamination in the southern section was related to the reclamation of clay mines that existed in this area at the end of the nineteenth century and the beginning of the twentieth century. Therefore, three soil pits (P1–P3) were excavated and sampled in the southern part of the complex (Figure 1D). The extent of the clay mines (a pink polygon in Figure 1D) was approximated based on the historical topographical map [40].
Soil profiles were characterized (Figure 2) and classified according to the international soil classification system, the World Reference Base for Soil Resources (WRB) [44]. All collected soil samples were air-dried, crushed, and sieved (mesh: 2 mm). The particle size distribution was determined using a hydrometer and sieving methods, after organic matter removal, where necessary, and sample dispersion with Na-hexametaphosphate. Soil texture classes were distinguished according to the WRB system [44]. The soil pH in distilled water (1:2.5 suspension, v/v) and the electrical conductivity (EC) in a saturated paste were analyzed potentiometrically [45]. Soil organic carbon (SOC) and total nitrogen (Nt) were determined by high-temperature catalytic combustion (Vario MACROcube, Elementar Analysensysteme GmbH, Langenselbold, Germany) after carbonate removal. Calcium carbonate, expressed as CaCO3 equivalent, was analyzed by volumetric methods using 10% HCl. The exchangeable base cations (Caex, Mgex, Kex, and Naex) and cation exchange capacity (CEC) were extracted using 1 M ammonium acetate buffered at pH = 7 [45]. Element concentrations in the extracts, including Na as an indicator of CEC, were determined by microwave plasma atomic emission spectroscopy (MP-AES 4200, Agilent Technologies, Santa Clara, CA, USA).
The plant-available forms of Pav, Kav, and Mgav were extracted following the Mehlich-3 procedure [46] and determined by inductively coupled plasma spectrometry (ICP-OES, Thermo Scientific iCAP 7400, Waltham, MA, USA). Total concentrations of Zn, Cu, Pb, Cd, and Hg in topsoil samples from the reclaimed and unreclaimed sections of the gardens, as well as samples collected from soil profiles P1–P3, were determined by ICP-OES after sample digestion with aqua regia (ISO standard 54321:2020) [47]. Furthermore, total concentrations of Fe, Mn, Cr, Ni, and As were measured in dry, finely ground samples from soil profiles using portable X-ray fluorescence (pXRF; EDX Explorer 7000). The certified reference materials ISE 836, ISE 838, ISE 851, ISE 853, ISE 856, ISE 869, RTH 912, and RTH 953 (WEPAL, Wageningen, the Netherlands) were used for the validation of the ICP-OES analysis after aqua regia digestion and for the calibration and validation of the pXRF technique.
The geo-accumulation index (Igeo) was calculated for a comprehensive evaluation of soil pollution [48], following the formula: Igeo = log2(Ci/1.5*GB), where: Ci is a concentration of an individual element, and GB is the geochemical background value.
Median concentrations of trace elements in the plough layers, based on a long-term monitoring of arable soils in Poland (Table 1 [49]), were adopted as an ambient geochemical background (GB) for the Igeo calculations.
The legal concentration thresholds, the so-called ‘permissible levels’ of trace elements in the medium-textured arable soils of Poland [50], are given in Table 1 for further interpretation.

2.3. Statistical Analysis

Basic statistical calculations, including mean, median, and standard deviations were performed using the Statistica 13 software package (TIBCO Statistica, Santa Clara, CA, USA). The statistical significance of the differences between the mean values was verified using post hoc Tukey’s test.

3. Results

3.1. Trace Element Concentrations in the Topsoil Layer

The concentrations of trace elements in the topsoil layer (0–25 cm) of the allotment gardens varied widely (Table S1—Supplementary Materials). However, even the lowest concentrations of Zn, Pb, Cu, Cd, and Hg—measured as 186, 47.6, 26.7, 0.78 and 1.11 mg kg−1, respectively (Table 2)—were 4 to 5 times higher than their median values in arable soils of Poland (Table 1). Minimum, maximum, and mean concentrations of these elements were noticeably higher in the sites located in the reclaimed southern section (formerly clay pit) than in the unreclaimed northern section of the complex (Table 2). The 2.5–4-fold differences, statistically significant (at p < 0.05) for each element, were reported between the reclaimed and unreclaimed garden sections.
Significantly (at p < 0.05) higher mean geoaccumulation indexes (Igeo) for all trace elements (Table 2) confirmed greater contamination in reclaimed sections than in the unreclaimed sections and allowed for a relative assessment of the pollution level. If Igeo values in an unreclaimed part indicated ‘moderate to high soil pollution’, the values for the reclaimed part indicated ‘high to extremely high soil pollution’ [48]. However, an assessment based solely on Hg suggested ‘high pollution’ and ‘extremely high pollution’, in the unreclaimed and reclaimed sections, respectively.

3.2. Morphological Characterization of Soil Profiles on Reclaimed Clay Pit

All three soil profiles, situated in the marginal part of the former clay pit, were artificially constructed above the stratified alluvial sediments (horizons 4Cl) starting at a depth of 80–150 cm (Figure 2). As evidenced by the recently introduced symbol ‘τ’ (tau) for transported materials [44], the topsoil layers constructed during the pit reclamation consist of local translocated materials, free of technogenic artefacts. The underlying layers consist of, or are affected by, the waste materials used to fill the pit and shape the land surface. These technogenic materials consist of ash, slag, charred wood, and coarse fragments of bricks, concrete, stones, pottery, ceramic tiles, glass, metals (rods, cables, tools), roofing felt, animal bones, fabrics (Figure 2D) and other unidentified substances, mixed with quarzitic sand. The addition of artefacts is reflected by the symbol ‘u’ [44] in the horizon designations (Figure 2A). Infilling materials of variable origins were deposited in subsequent layers, as reflected in their current stratification. The layers containing larger admixtures of ash are partly cemented, while the strata consisting mainly of construction debris mixed with household waste are loose.
The topsoil horizons are 21–40 cm thick, black or very dark brown (Munsell colours 10YR 2/1-3, moist), rich in humified organic matter (SOC content of 5–11%, Table 3) and biologically active, as evidenced by numerous plant roots, earthworm channels/casts, and a well-developed, fine-to-medium granular structure. The transitional ABu horizons are still rich in humus but also rich in artefacts. The colors of the ABu and BCu horizons are grey-brown or strong brown due to the weathering of the artefacts and the release of iron oxides [27,28]. The structure of these subsoil horizons is blocky subangular; however, the abundance, size, and durability of structural aggregates is affected by the content and kind of artefacts, and is, thus, it can be poorly developed in case of predominance of coarse fragments.

3.3. Physicochemical Soil Properties in the Profiles P1–P3

The texture of the technogenic soils varied between loamy sand and sandy loam, with a very low clay content (Table 3). A coarser texture (sand class) was found only in the natural alluvial bedrock (horizons 4Cl), while fine-textured loam was only identified as the horizon 3BC in the P2 profile and most probably represents the loam residues of loam exploited in the former clay mine.
All horizons of the constructed soils, excluding the natural bedrock layers, were rich in organic matter (4.6–11.5% SOC) and nitrogen (0.25–0.48% N). In soils relatively poor in artefacts, the SOC and N contents were highest in the topsoil Ap horizons and decreased with depth. In soils rich in artefacts, particularly charred wood particles (such as the 5ACu layer in the P1 profile), the SOC content was even higher than in the topsoil horizons (Table 3). The high Nt content in these layers suggests the impact of household debris.
Although noticeably higher than in the arable soils of Poland [49], EC did not reach thresholds for natural or human-affected saline soils [44]. EC increased from 0.5–1.4 dS m−1 in the topsoil layers to 2.7–2.9 dS m−1 in the subsoil layers rich in artefacts (Table 3). The bedrock layers in the P1 and P2 profiles were characterized by elevated CE values of 1.4 to 2.1 dS m−1, probably affected by the overlying anthropogenic materials.
Calcium carbonate was present in the fine-earth fractions of all topsoil layers and in all layers containing artefacts. The highest CaCO3 contents, up to 9.5%, occurred in layers containing ash and slag. The presence of carbonates and other alkaline waste affected the soil pH, which was slightly alkaline (7.4–8.1) throughout the profiles, including the carbonate-free bottom layers (Table 3). The sum of base cations (BC) reached very high values, of up to 63 cmolc kg−1 (Table 4), clearly affected by calcium carbonate and other substances dissolved in buffered ammonium acetate. Therefore, BC values should be analyzed with caution, as they reflect ‘extractable’ rather than purely ‘exchangeable’ forms of Ca, Mg, K, and Na. However, the high CEC values, of up to 45.4 cmolc kg−1 (Table 4), suggest the high sorption capacity of the constructed soils, likely due to their high organic matter content and porous mineral particles in the silt fraction [30,33]. The lowest BC and CEC values, while still indicating high base saturation, were found in the alluvial sandy bedrock layers (Table 4).
Taking into account the thresholds for arable soils in Poland [46], the topsoil layers in the constructed soils were very rich in plant-available phosphorus, magnesium and potassium, with concentrations reaching 396, 612 and 266 mg kg−1, respectively (Table 4).

3.4. Trace Element Concentrations in the Profiles of Constructed Soils

Total concentrations (aqua regia-extractable) of Zn, Pb, Cu, and Cd in the natural alluvial 4Cl horizons did not exceed 50, 19, 16 and 0.4 mg kg−1, respectively (Table 5). In contrast, the topsoil layers contained 1000–1700, 300–500, 200–360, 2.5–3.9 and 1.2–3.1 mg kg−1 of Zn, Pb, Cu, Cd, and Hg, respectively. The concentrations of elements in the subsoil layers rich in ash and slag were at a similar level or higher than in the topsoil layers, reaching, as follows: 1500–2000 mg kg−1 Zn; over 500 mg kg−1 of Pb and Cu; and over 4 mg kg−1 of Cd. The highest concentrations of elements were found in the layers containing mixtures of ash, construction debris and household waste with up to 3500 mg kg−1 Zn; 1500 mg kg−1 Pb; 690 mg kg−1 Cu; 8 mg kg−1 Cd; and 4.7 mg kg−1 Hg (Table 5).
Furthermore, the total concentrations of Fe, Mn, Cr, Ni, and As, measured by pXRF (Table 5) were several times higher in layers containing artefacts than in the natural alluvial bedrock (horizons 4Cl). The Cr, Ni, and As concentrations in topsoil layers varied in the ranges of 60–90, 50–90, and 50–120 mg kg−1, respectively. Although these values were lower than those in the technogenic subsoil layers, they were markedly higher than the median values for arable soils in Poland (Table 1).
The classification of soil pollution levels based on the geo-accumulation index (Igeo) (Table 6) indicated ‘high to extremely high pollution’ of the topsoil horizons, and ‘extremely high pollution’ for the subsoil horizons containing anthropogenic artefacts, due to Hg, Zn, Pb, Cu, and As (Igeo of 4–5 and ≥5, respectively). The assessment indicated ‘moderate to high pollution’ based on Igeo for Cr and Ni (Igeo of 2–4) and ‘no pollution’ based on Mn (Igeo of 0–2).

4. Discussion

4.1. Classification of the Constructed Soils in Urban Gardens

The complete soil name in the WRB classification reflects the soil origin, its present functions and its health, as it is constructed based on soil morphology, physicochemical properties and contamination [44]. The WRB classification distinguishes two taxa at the highest classification level: Anthrosols for human-affected soils and Technosols for human-created soils. The name of the reference soil group is accompanied by principal and supplementary qualifiers that allow for a precise identification of their unique properties.
The soils studied have a well-developed structural, dark, rich in humus and nutrient Ap horizons, which meet the requirements for diagnostic hortic horizons. However, these hortic horizons are less than 50 cm thick, which is the minimum for Anthrosols [33,44]. The soils in profiles P1–P3 contained mineral waste materials, such as coal-ignition debris, construction and demolition debris, and household waste, which were used to fill the former clay pit. The content of these materials (artefacts) exceeded the required 20% (by volume) averaged over the upper 100 cm of soil profiles P1 and P3, or formed a layer ≥10 cm thick starting within 50 cm of the soil surface, with ≥80% artefacts (profile P2). This allowed for the soils to be classified as Technosols [44]. The complete soil classification included the following qualifiers:
  • Profile P1 (left side, Figure 2A): Urbic Technosol (Arenic, Calcaric, Hortic, Humic, Pyric, Endoskeletic, Transportic);
  • Profile P1 (right side, Figure 2A): Urbic Technosol (Arenic, Calcaric, Hortic, Humic, Mahic, Endoraptic, Transportic);
  • Profile P3: Urbic Technosol (Arenic, Calcaric, Hortic, Humic, Pyric, Skeletic, Transportic).
The principal qualifier Urbic reflects the prevalence of mineral artefacts of urban origin. Therefore, the garden soils represent the Urbic Technosols, the taxon commonly reported from urbanized areas in Europe [26,27,30,32,34]. Among the alphabetically listed supplementary qualifiers, Arenic indicates the prevailing sandy texture; Calcaric, the presence of calcium carbonate; Humic, the high SOC content (≥1%) to a depth of 50 cm; Pyric, the high (≥5%) black carbon (charcoal) content in a layer ≥ 10 cm thick; Skeletic, the high content (≥40%) of coarse fragments. The important qualifier Hortic reflects the presence of a dark, base-saturated (≥50%), biologically active topsoil layer rich in humus (≥1%) and plant-available phosphorus (≥120 mg kg−1) [30,33]. The Transportic qualifier refers to the external origin of the subsoil and topsoil layers that are not enriched with artefacts. Additionally, Endoraptic refers to the lithological discontinuity observed in profiles P1 and P2, where technogenic and transported materials overlie natural alluvial sediments.
Despite the high content of trace elements, the Toxic qualifier [32,44] was omitted from the soil names due to the high biological activity observed in the topsoil layers and the lack of evidence of toxic impacts on vegetation.

4.2. Quality of Reclaimed Garden Soils in Terms of Their Physicochemical Properties and Fertility

Although the soils in the reclaimed part of the gardens are characterized by predominantly loamy sandy and sandy loamy textures (Table 3), the high organic matter content and the well-developed granular structure in the thick A horizons may compensate for the low clay content, support water retention, and improve water availability to plants [51]. The presence of calcium carbonate, accompanied by a slightly alkaline pH, ensures a high calcium ion concentration and effective cation exchange and also stabilizes organic matter and soil structure [52]. Both the relatively high pH and the humus content can reduce the solubility and availability of toxic elements for plants and soil fauna [53]. Furthermore, high or very high concentrations of plant-available P, K, and Mg indicate high fertility and the overall high quality of reclaimed garden soils in terms of their suitability for fruit and vegetable production [54]. This high horticultural quality is similar to that reported for other garden soils in Wroclaw and elsewhere [23,27], regardless of the initial status of soils and terrains converted into gardens [32,33].

4.3. Quality of Reclaimed Garden Soils in Terms of Contamination with Trace Elements

The total concentrations of trace elements in the unreclaimed part of the garden complex under study were similar to the mean concentrations recorded in the gardens of Wroclaw [23]. On the contrary, the concentrations in the reclaimed part were significantly higher—up to four times higher for copper and lead—than the mean values for Wroclaw [23]. Furthermore, the element concentrations in the reclaimed section were noticeably higher than those reported for most gardens in both smaller towns [12,16,24,55,56,57,58] and large urban agglomerations [13,59,60,61], and were comparable to the concentrations in highly contaminated garden and arable soils in industrial regions of Australia, northern Germany, and southern Poland [15,62,63]. The potential risk was also confirmed by the geo-accumulation index (Igeo), which indicated the ‘high to extreme’ pollution level, rarely reported in urban gardens in Poland [16,48].
The mean concentrations of Zn, Pb, Cu, Hg, and As in the reclaimed garden soils studied exceeded the legal threshold limits (‘permissible levels’) for medium-textured cultivated soils in Poland by 2–6 times [50]. According to formal interpretations, concentrations exceeding these limits are considered to pose a potential risk to human health and require respective action [50,64]. In this case, a detailed risk assessment, followed by a respective soil remediation or a change in land use, is required to minimize the impact on human health [64].

4.4. Maintaining Urban Gardening in Reclaimed Mining Sites

Waste materials rich in trace elements—such as ash, slag, construction and demolition debris, and household waste—used for pit infilling and reclamation, then covered with only a thin layer of non-contaminated soil, pose a potential but persistent risk of topsoil contamination. This risk arises from the continuous transfer of elements from the subsoil, driven by: (a) mechanical mixing during earthworks or construction in the gardens; (b) zooturbation—deep soil mixing by earthworms, moles and other burrowing animals; and (c) bioaccumulation—the extraction of elements by roots, followed by their transfer to above-ground plant parts and their eventual return to the topsoil via leaf litter, post-harvest remains or garden-made compost [65].
The continuous transfer of contaminants from the shallow subsoil can render ineffective the remediation techniques intended to remove metals from the topsoil layer [17,24,26]. Decreasing the mobility and plant-availability of metals by liming and organic fertilization is considered one of the simplest methods to reduce the risk of metal transfer from contaminated soil to cultivated plants [12,21]. However, these practices may also reduce the availability of macronutrients, particularly phosphorus [46,53].
Covering the present surface with an additional layer of uncontaminated soil to a thickness of at least 30–50 cm to create a new rooting layer can be considered an alternative method of reducing metal transfer from the subsoil to the rooting zone [32]. However, raising the surface level involves reconstruction of the entire garden infrastructure (roads, fencing, buildings, etc.) and can threaten existing trees. Applying this remediation method would effectively mean re-establishing the gardens. This approach appears to be generally unrealistic owing to its financial and social costs [32].
Thus, the present study highlights the urgent need for assessing the quality of fruits and vegetables and the risk to human health in reclaimed gardens created on urban post-mining sites. Confirmation of this risk may justify revising the functions (services) of urban gardens and limiting fruit and vegetable production in favor of recreation in many urban gardens on reclaimed sites in Poland and Central Europe.

5. Conclusions

Intentionally constructed anthropogenic soils, Urbic Technosols, predominate in a reclaimed section of urban gardens in north-eastern Wroclaw, where industrial and urban waste materials, such as ash, slag, construction and demolition debris, and household waste, were used to fill former clay and sand pits (open-cast mines). Although the topsoil layers, formed from transported soil, were characterized by beneficial physicochemical properties and a high abundance of humus and plant-available macronutrients, soils were found to be severely contaminated with zinc, lead, copper, arsenic, and mercury, along with elevated concentrations of other metals (including cadmium, nickel and chromium), transferred from the technogenic materials used for mine infilling. These materials form the subsoil layers directly beneath the humus-rich topsoil. The concentrations of trace elements were found to exceed legally permissible levels and pose a potential risk to human health, questioning the main function of the gardens. The results highlight the urgent need for a risk assessment of similarly reclaimed urban gardens in Poland and elsewhere, followed by soil remediation or changes in garden services in the case of a confirmed health or ecological risk.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/land14081613/s1, Table S1: Pseudo-total concentrations of Zn, Pb, Cu, Cd and Hg and the geoaccumulation index (Igeo) for a topsoil layer (0–25 cm) of the allotment gardens complex Wytchnienie in Wroclaw (in the unreclaimed and reclaimed sections).

Author Contributions

Conceptualization, D.G., K.S. and C.K.; methodology, K.S. and C.K.; investigation, D.G. and C.K.; writing—original draft preparation, D.G.; writing—review and editing, K.S. and C.K.; funding acquisition, D.G. All authors have read and agreed to the published version of the manuscript.

Funding

This research was partly supported by the Wroclaw University of Environmental and Life Sciences (Poland) as the Ph.D. research program “Bon doktoranta SD UPWr” project no. N020/0004/22 and partly by city of Wroclaw, within a Student Activity Fund, grant number BWU-1/2024/F6 (edition 2024).

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Acknowledgments

The voluntary student research group (Studenckie Koło Naukowe Gleboznawstwa i Ochrony Środowiska) is acknowledged for extensive field and laboratory assistance.

Conflicts of Interest

The authors declare no conflicts of interest.

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Land 14 01613 g001
Figure 1. Location of the study site in Poland (A); study site in Wroclaw city (B); (C) historical map (1886 [40]) of the north-east forelands with marked sand and clay pits (pink areas). Present-day urban (allotment) gardens are bordered with blue dashed lines; and (D) the urban garden complex under study with marked reclaimed sand/clay pits (pink areas), topsoil (0–25 cm) sampling sites (small white dots, numbers 1–40), and soil profiles (large white dots, numbers P1–P3).
Figure 1. Location of the study site in Poland (A); study site in Wroclaw city (B); (C) historical map (1886 [40]) of the north-east forelands with marked sand and clay pits (pink areas). Present-day urban (allotment) gardens are bordered with blue dashed lines; and (D) the urban garden complex under study with marked reclaimed sand/clay pits (pink areas), topsoil (0–25 cm) sampling sites (small white dots, numbers 1–40), and soil profiles (large white dots, numbers P1–P3).
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Figure 2. Technogenic soil profiles P1, P2, P3 (A–C) situated in the reclaimed part of the allotment gardens; and (D) artefacts from the horizons 2BCu and 5ACu of the profile P1.
Figure 2. Technogenic soil profiles P1, P2, P3 (A–C) situated in the reclaimed part of the allotment gardens; and (D) artefacts from the horizons 2BCu and 5ACu of the profile P1.
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Table 1. Total concentrations of Zn, Pb, Cu, Cd, and Hg in the topsoil layers of arable soils in Poland as an ambient geochemical background and the legal thresholds (permissible levels) of trace elements in arable soils in Poland.
Table 1. Total concentrations of Zn, Pb, Cu, Cd, and Hg in the topsoil layers of arable soils in Poland as an ambient geochemical background and the legal thresholds (permissible levels) of trace elements in arable soils in Poland.
AreaTotal Concentration, mg kg−1
ZnPbCuAsCdHgCrNiMn
Geochemical background
for arable soils in Poland [49]		32.011.86.22.70.150.0210.26.4325
Legal permissible levels
in medium-textured soils in Poland [50]		5002501502034300150-
Table 2. Total concentrations (aqua-regia extractable) of Zn, Pb, Cu, Cd, and Hg and geo-accumulation index for the topsoil layers (0–25 cm) in the unreclaimed and reclaimed sections of the allotment gardens complex. Detailed results are presented in Table S1 (Supplementary Materials).
Table 2. Total concentrations (aqua-regia extractable) of Zn, Pb, Cu, Cd, and Hg and geo-accumulation index for the topsoil layers (0–25 cm) in the unreclaimed and reclaimed sections of the allotment gardens complex. Detailed results are presented in Table S1 (Supplementary Materials).
Site HistoryParameterTotal ConcentrationIgeo
ZnPbCuCdHgZnPbCuCdHg
mg kg−1
unreclaimed
area
n = 20		minimum18647.426.70.781.112.01.41.51.85.2
maximum7642451452.381.664.03.84.03.45.6
mean378 a109 a64.6 a1.34 a1.35 a2.8 a2.3 a2.4 a2.5 a5.4 a
SD18073.242.40.530.250.60.80.80.50.2
reclaimed
mine
n = 20		minimum38673.842.31.481.613.22.63.02.35.7
maximum23409227365.135.065.65.76.34.57.4
mean1120 b474 b276 b3.28 b3.30 b4.5 b4.6 b4.7 b3.8 b6.5 b
SD4202091450.891.530.50.70.70.50.7
Explanation: Igeo—geo-accumulation index; SD—standard deviation; a, b—indication of statistical difference between mean values for unreclaimed and reclaimed sites, checked by Tukey’s post hoc test at p < 0.05.
Table 3. Particle size distribution and selected physicochemical properties of soils in the reclaimed part the allotment gardens.
Table 3. Particle size distribution and selected physicochemical properties of soils in the reclaimed part the allotment gardens.
Soil
Profile		Soil
Horizon		DepthSand
2–0.05 mm		Silt
0.05–0.002		Clay
<0.002		Texture ClassCaCO3pHECSOCNt
cm%%dS m−1%
P1Apτ10–2573252LS3.67.41.15.90.368
Apτ225–4081172LS3.27.61.45.00.281
ABu40–5088111LS2.47.91.84.60.279
2BCu50–6580191LS5.87.82.97.80.428
3BCτ65–9084106LS07.91.91.50.118
4Cl90–1509811S08.02.10.20.014
5ACu60–11077221LS7.57.82.611.60.393
P2Apτ0–2173261LS4.17.70.911.50.336
BCu21–4781181LS9.57.91.311.00.329
2BCu47–55553015SL0.78.01.15.50.256
3BCτg55–70364123L07.91.22.10.171
4Cl70–1209811S07.91.40.30.024
P3Apτ10–2070291SL5.07.40.59.90.484
Apτ220–2767321SL4.97.51.09.70.436
ABu27–3576231LS7.17.81.57.70.318
2BCu35–6075241LS9.27.82.39.30.302
3ABu60–10074242LS9.58.12.79.80.312
Explanation: EC—electrical conductivity, SOC—soil organic carbon, Nt—total nitrogen; soil texture classes according to WRB classification [44]: S—sand, LS—loamy sand, SL—sandy loam.
Table 4. Cation exchange capacity and plant-available macronutrients in soils of the reclaimed part of the allotment gardens. n.d.—not determined.
Table 4. Cation exchange capacity and plant-available macronutrients in soils of the reclaimed part of the allotment gardens. n.d.—not determined.
Soil
Profile		Soil
Horizon		DepthExchangeable CationsBCCECBSPlant-Available Nutrients
CaexMgexKexNaexPavKavMgav
cmcmolc kg−1%mg kg−1
P1Apτ10–2520.71.350.360.0722.421.0100318242416
Apτ225–4021.21.410.290.0923.013.2100145170222
ABu40–5017.81.290.210.1219.411.710050111159
2BCu50–6527.62.290.350.2330.511.5100n.d.n.d.n.d.
3BCτ65–9014.10.840.130.1015.107.80100n.d.n.d.n.d.
4Cl90–1501.600.170.100.102.002.1395n.d.n.d.n.d.
5ACu60–11042.34.611.030.3848.329.1100n.d.n.d.n.d.
P2Apτ0–2139.23.420.270.1943.126.2100175162523
BCu21–4744.74.580.200.3149.825.0100191781030
2BCu47–5522.26.290.490.2129.228.410018217880
3BCτg55–7010.05.890.470.1816.516.699n.d.n.d.n.d.
4Cl70–1201.600.280.100.102.102.5585n.d.n.d.n.d.
P3Apτ10–2033.93.020.660.1437.736.8100396266612
Apτ220–2734.12.890.530.1537.636.2100373249542
ABu27–3550.73.630.660.3255.333.110048260569
2BCu35–6055.95.950.710.5363.145.4100n.d.n.d.n.d.
3ABu60–10052.96.310.980.5360.830.8100n.d.n.d.n.d.
Explanation: Caex, Mgex, Kex, and Naex—exchangeable Ca, Mg, K and Na, BC—sum of exchangeable base cations, CEC—cation exchange capacity, BS—base saturation.
Table 5. Total concentrations of trace elements and iron in soil profiles of the reclaimed part of allotment gardens. n.d.—not determined.
Table 5. Total concentrations of trace elements and iron in soil profiles of the reclaimed part of allotment gardens. n.d.—not determined.
Soil
Profile		Soil
Horizon		DepthTotal Concentration (Aqua Regia-Extractable)Total Concentration (XRF Technique)
ZnCuPbCdHgMnCrxrfNixrfAsxrfFexrf
cmmg kg−1%
P1Apτ10–2510802073112.501.204866050611.94
Apτ225–4012402033002.46n.d.4597352542.11
ABu40–5015303415353.55n.d.5107657502.23
2BCu50–65246054110705.46n.d.10902461201964.80
3BCτ65–90295621151.461.613944615231.66
4Cl90–15018650.25n.d.94371130.52
5ACu60–110358068915208.254.671050125831894.87
P2Apτ0–2114103364853.603.1394685661073.66
BCu21–4716606276714.075.0512701461431535.2
2BCu47–553801161292.10n.d.9169160413.07
3BCτg55–7010333921.32n.d.4757424282.82
4Cl70–1205016190.37n.d.13739930.57
P3Apτ10–2016403355063.811.6691393961083.30
Apτ220–2717003634993.892.0691796901203.38
ABu27–35200054512204.39n.d.11701211282924.81
2BCu35–6017003866734.87n.d.18102164471996.92
3ABu60–10015604086494.98n.d.14701371781455.45
Table 6. Mean geo-accumulation index (Igeo) for trace elements in the topsoil and subsoils layers of soil profiles in the reclaimed part of the allotment gardens.
Table 6. Mean geo-accumulation index (Igeo) for trace elements in the topsoil and subsoils layers of soil profiles in the reclaimed part of the allotment gardens.
Soil HorizonsZnCuPbCdHgMnCrNiAs
topsoil Ap horizons4.94.94.53.86.00.52.42.84.4
subsoil with artefacts5.35.75.54.57.31.23.13.75.0
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Gruszka, D.; Szopka, K.; Kabala, C. Quality of Constructed Technogenic Soils in Urban Gardens Located on a Reclaimed Clay Pit. Land 2025, 14, 1613. https://doi.org/10.3390/land14081613

AMA Style

Gruszka D, Szopka K, Kabala C. Quality of Constructed Technogenic Soils in Urban Gardens Located on a Reclaimed Clay Pit. Land. 2025; 14(8):1613. https://doi.org/10.3390/land14081613

Chicago/Turabian Style

Gruszka, Dariusz, Katarzyna Szopka, and Cezary Kabala. 2025. "Quality of Constructed Technogenic Soils in Urban Gardens Located on a Reclaimed Clay Pit" Land 14, no. 8: 1613. https://doi.org/10.3390/land14081613

APA Style

Gruszka, D., Szopka, K., & Kabala, C. (2025). Quality of Constructed Technogenic Soils in Urban Gardens Located on a Reclaimed Clay Pit. Land, 14(8), 1613. https://doi.org/10.3390/land14081613

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