Risk assessment
The Igeo indicates contamination of the flood sediments by Zn, Cu, Pb, Cd, and Sn. This enrichment in several trace metals is not unexpected, as the simultaneous enrichment of Pb, Cd, and Zn is common for zinc smelting areas [71] like the Inde catchment. The resulting industrial development leads to multi-element contaminations typical for regions with long industrial history [121]. Other relevant industries besides the coal and steel industry, some of which are still located in Eschweiler and Stolberg today, were glass production, the pharmaceutical industry, the chemical industry such as detergent production, and the textile industry [137]. These industries settled preferably to use the water on the Inde and Vicht rivers [103]. Sewage treatment plants, another typical source of trace metals, mainly emit Hg, Cu, Cd, Zn, Pb, Cr, and Ni [138]. The Stolberg-Steinfuhrt wastewater treatment plant is located immediately upstream of Eschweiler in Stolberg and discharges treated wastewater into the Inde river. In the Inde catchment, the average amount of zinc in stormwater collected for treatment is 0.62 – 2.38 t/a and of copper 0.09 – 0.36 t/a [139].
However, the absolute concentrations of trace metals found in the flood sediments in Eschweiler exceed the values reported by Esser et al. [102], who investigated channel sediments from the Inde river. This implies additional pollution sources that are usually not part of the fluvial sediment system. Undercutting and subsequent failure of river banks was a commonly observed result of the flood [88]. Due to the many legacies in the floodplains of the region, this is expected to lead to an intense input of trace metals into the system [4]. Regular flooding washes off unconsolidated sediments which are prone to erosion due to missing vegetation or weathering. Once this sediment is flushed away, no readily available sediment is left, and the discharge might increase further, but the sediment load declines [38]. Extreme flood events cross certain thresholds, which enable the erosion of far larger amounts of sediments, e.g., by lateral erosion of the river bank [140]. As soon as this process starts, large amounts of erodible sediments are exposed to strong currents. In case also coarser grain sizes, such as gravel or bolder get eroded, this might further accelerate erosion.
The flood sediments deposited in Eschweiler are severely enriched in Pb, and concentrations are many times higher than the upper limits of trigger action values. This result is of special concern as no level of Pb exposure is assumed to be safe [130, 141]. A large part of the exposure of the public to Pb results from the resuspension of contaminated soils [70]. This finding is supported by the fact that blood Pb levels in children correlate with weather conditions, e.g., warm and dry weather aggravates soil resuspension [70]. For several American cities, the resuspension of fine materials was found to be the main source of atmospheric Pb [142]. While the acute toxicity of Pb is low, the long-term effects are of worrying relevance [130]. Therefore, the general enrichment of Pb in surface sediments within the Inde catchment is of even more concern than the exposure to the moderately to heavily contaminated flood sediments (Igeo ranges between 1.7 and 2.8). However, floods play a major role in exposure because they induce erosion, redistribution, and the deposition of fine materials in inhabited areas.
Even though absolute concentrations are low, the enrichment of As in flood sediments is of concern for human health. Uptake of As from soil and dust is a well-known phenomenon linked to shortness of breath, cough, and chest pain [19], with long-term exposure being carcinogenic [131]. Similar to Pb, no safe level of As exposure can be specified [131]. However, the Igeo indicates no anthropogenic enrichment for As. Therefore, exposure to As is only due to high background concentrations that already exceed trigger action values (see Fig. 4). Again, flooding is relevant because it enhances public exposure to dust.
The numerical model indicates sediment deposition for large parts of the flooded area, even at the margin of the flooding. Sediment deposition strongly depends on the distance to the channel and objects hindering water flow [26]. Additionally, samples distant from the river channel showed higher amounts of the finest particles. Floods act as mobilization and sorting agents, thereby depositing the finest particles in the outer area of the flooding [26]. Therefore, we assume dust generation to be possibly intense over the whole inundation area, even in areas where only little total sediment was deposited. This is even more true for large areas with sealed surfaces, which are susceptible to erosion due to their smooth surface structure and missing vegetation [46]. Moreover, even areas where coarser sediments were deposited, like, e.g., sandbars, act as source areas for dust [143]. The amount of deposited sediment in the flooding corridor was extreme [96], concordant with our model. In Eschweiler, the uppermost part of the city along the river was especially affected by intense sedimentation. This is because the river valley widens from where the Inde enters the city center of Eschweiler, resulting in reduced flow velocities [26] and because the floodplains are densely populated from here on.
There are generally two paths to consider when looking at exposure to sediments enriched in trace metals. The first one is the inadvertent ingestion of fine particles, which stick to the skin [122]. This is especially problematic for children, who exhibit intense hand-to-mouth activities and have lower tolerances for most trace metals. The second path is the inhalation of fine particles [122], which were resuspended into the atmosphere due to either wind or anthropogenic impact.
Particle size plays a vital role in the exact uptake within the human body because it determines the particle's surface area, which comes in contact with tissues, its ability to stick to the human skin, limits how deep a particle can enter the lungs, and governs the body's capabilities of ejecting the particle [54, 120]. Additionally, larger surface areas lead to higher binding capacities for trace metals and hence higher concentrations within the sediment [114]. As we only did bulk sample analyses, we cannot make any statement on trace metal concentrations within specific size groups. However, several studies found enriched trace metal concentrations in the finest fractions [27, 59, 120–122] or the PM2.5-10 fraction [144, 145], raising concerns about elevated concentrations in the respirable particles. Moreover, finer particles tend to have higher toxic potential because, for finer particles, their chemical composition becomes more and more relevant [146]. However, this trend is not unambiguous, as some studies find higher trace metal concentrations in PM2.5-10 compared to finer particles [81].
Additionally to the flood sediments outdoors, there also needs to be a focus on sediments deposited indoors, where contact with the sediments and their possibly toxic inventory is especially likely. Large amounts of sediment were deposited in the numerous buildings affected by the flooding, especially in the basements. However, the exposure also extends to non-flooded buildings, as transporting exterior sediments into homes, e.g., due to attachment to shoes and clothing [70], is an important pathway for exposure [147]. In addition, sediments attached to people's skin may be a crucial transport vector from outdoors to indoors. Particles < 63 µm with the potential to adhere to the human skin [148, 149] comprise up to 96% of the material in the flood sediment samples collected in Eschweiler. Subsequently, this sediment may get ingested inadvertently. Particles < 2 µm, which account for 6% up to 15% of the flood sediment samples, can even get incorporated into the skin's surface [122]. Therefore, indoor exposure to flood sediment may persist even after buildings have been cleaned of sediment in the first place.
The concentration of airborne PM indoors is higher than outdoors in the aftermath of flooding events [150]. Moreover, indoor dust tends to accumulate trace metals from outdoor and indoor sources [145]. Additionally, resuspended PM10 is proven to be enriched in trace metals compared to settled dust indoors, and resuspension of dust plays a key role in PM levels indoors [145]. Indoor air quality has been shown to be severely deteriorated even months after flooding events and even after full-scale remediation efforts. However, biological particles like mold and fungi have a large share in this [150].
Durational effects are important, as mortality increases for each day with elevated PM10 concentrations [56]. One and a half years later (December 2022), even though cleaning up and rebuilding made progress, there are still marks of fine sediment in the affected areas. In the pavement joints as well as on rough walls, sediments are still trapped in the low-lying parts. Every brushing of these pavements will release some of the material. Regarding the broad deposition of fine sediments enriched in possibly toxic trace metal beyond critical threshold concentrations, there is a significant risk for the people exposed to the flood sediments in Eschweiler. Moreover, there is no level of PM exposure at which no health risk is to be expected [63].
The devastating character of the event led to a broad response in society with a high willingness to support as volunteers. In addition, the police, fire department, technical relief organization, the German Federal Armed Forces, medical specialists, and many other organizations were involved in the work with a high level of deployment [92, 97]. Thereby, not only those directly affected by the flooding are at risk, but everyone who contributed to the rescue, relief, and clean-up efforts. Even at the beginning of 2022, volunteers were still working in flood-affected regions [97].
This risk is also increased if the contaminated sediments come into contact with vulnerable groups of people. For example, children have a generally higher risk when exposed to trace metals [19, 121]. In a broader sense, vulnerability has frequently been shown to be related to several factors like demographics, health, socioeconomic factors, and risk perception [52, 90]. The 2021 July flood disproportionately affected vulnerable groups of people [9]. In the study area, kindergartens, schools, a retirement home, and a hospital were flooded, and thus contaminated sediments were also carried into these buildings.
Limitations due to unknown bioaccessibility
Bioaccessibility describes that part of any substance soluble in the target organ [120]. Hence, following ingestion, it is the gastrointestinal tract where gastric bioaccessibility plays the decisive role, while after inhalation, the respiratory bioaccessibility in the lungs is crucial. These two values can differ significantly [120]. Bioaccessibility depends on a complex interplay of physical and chemical properties of the element, like aggregation, binding to clay minerals, being part of a solid mineral structure, and many more. The preferred determination methods are extraction experiments (commonly used are Artificial lysosomal fluid or Gamble's solution; for details, see [61] and [69]), which is beyond the scope of this paper. Therefore, trace metal concentrations in this paper provide an upper-end estimate of potential toxicity [45]. General predictions on bioaccessibility are impossible, as the share available for uptake in the lungs varies greatly [81]; da Silva et al. [151] found values from 1.1% up to 93% to be bioaccessible. Yet, Luo et al. [122] found significant correlations between trace metal concentrations and human bioaccessibility. For the Inde catchment, a correlation between total trace metal concentrations and the generally bioaccessible fraction is proven [152].
Soluble metals are the main driver of the toxic potential of PM [61] as they can directly diffuse across the lung membrane [63]. High solubilities were proven for Pb, Cu, Cd, As, Sb, Sn, and Ni, which are enriched in the Eschweiler flood sediments [81, 120, 153]. Additionally, there is evidence that the melting process increases the bioaccessibility of metals (Cd and Pb) [120]. This is problematic, as much of the trace metals in the fluvial sediments of the Inde originate from mining legacies [102]. Furthermore, finer particles tend to have higher solubilities [75], which further enhances the risk of the inhalation of trace metal-enriched sediments.
Recommended actions
Considering the high chance of more frequent flooding in the future [3], the widespread deposition of fine flood sediments, and the persistency of trace metals in catchments influenced by former mining activities, we propose the following steps to address the exposure of the public to polluted high flood sediments:
I. Improving flood mapping
Numerical modeling proved to be a helpful tool to assess the areas affected by sediment deposition and might be useful as flood risk mapping needs to be improved to even smaller scales [93, 154]. However, the absolute sediment deposition might be overestimated due to the distinct features of cities, e.g., sewage systems. Flood maps should also include potential pollution sources like legacy sites and industrial areas relevant to pollution so that it becomes visible where protection measurements are necessary and to warn people downstream who might be affected by the polluted sediments. However, in catchments affected by diffuse pollution, like the Inde catchment, it seems more appropriate to educate people about the pollution risks generally. Our results showed that all flood sediment samples were severely enriched in several risk elements and should therefore be treated cautiously.
II. Extending warning mechanisms
In the subsequent evaluation of the flooding event, it became apparent that warnings and awareness within the general public of flood-related risks were very much insufficient [88, 155]. Part of the problem was that the July 2021 flood exceeded all floods the affected people had experienced so far. Therefore, their safety measurements, which had proven reliable during other flood events, were insufficient [155]. Warning messages that include precise information on appropriate behavior significantly positively influence the ability to cope with the situation [156]. However, recommendations on behavior are sparse, with only one-third of media coverage in advance of the July flood dealing with it [156].
III. Improving clean-up activities
Dust clean-up is a difficult, often failed task [149] and should not rely only on purely mechanical methods as they often miss smaller particles [157]. Water flushing can be an effective tool to remove erodible sediments, reduce dust emissions and thereby prevent people's exposure to them [157]. Furthermore, post-flood cleaning has proven efficient in preventing indoor aerosol contamination [8].
IV. Specialized medical support
Thorough control of exposure to airborne particles, education about symptoms, and routine medical assessment build the best approach to prevent health issues [64]. People returning to their homes should be under the care of medical professionals [51]. The options for remediating in the Inde catchment are limited as a high input of diffuse sources has to be expected [158]. Therefore, protecting the endpoint, i.e., people, is the essential tool for mitigation. Educating people in advance instead of solely relying on warnings as soon as the event is predicted or even during it is a key point.
V. Raising awareness
Raising public awareness of the risks due to contaminated flood sediments, as well as educating about suitable safety measurements, is vital because the changes within the landscape due to the July 2021 flood may lead to a modified yet unpredictable response to smaller rainfall events in the future [90, 96]. The intense erosion led to the mobilization of large amounts of fine sediments that can now easily be eroded by smaller events, which consequently will have higher sediment loads [88]. Moreover, a decrease in the return period of similar events is projected by climate models [85, 86, 90], making the reduction of vulnerability and exposure a key task [90].