Sewage Sludge Application Protocol for Quarry Restoration (Catalonia)

Quarry restoration is often limited by the characteristics of the soil or the substrate available. Residual soils and spoils from extraction activities are the most commonly available materials for soil replacement. These substrates are often rich in rock fragments, low in organic matter, and less weathered and poorer in nutrients than the original topsoil.

Table 1.

Aspects to consider before sewage sludge applications in quarry restoration. High scores indicate low aptitude. If one factor is at the limit or the composite score exceeds 12, organic amendment is not recommended.

Moderate amounts of organic amendments like sewage sludge can improve substrate fertility. The aim of organic amendment is to improve soil fertility and to guarantee sufficient vegetation development to prevent soil erosion during the early phases of restoration. Sewage sludge contains nutrients and trace elements essential for plant growth, and the organic matter can act as a soil conditioner to improve the physical properties, such as soil aeration and water-holding capacity. Furthermore, soil microbial respiration increases after sewage sludge applications.

However, excessive sludge may inhibit microbial activity and nutrient mineralization of the amended soil (Chander and Brokes 1993). Salinity and heavy metal content of sludge are factors that affect the microbial activity and nitrification process (Wong and Lai 1996). Additionally, the toxicity of some organic wastes to plants and soil fauna could not be correlated with their pollutant burden but was negatively correlated to the degree of stability of their organic matter (e.g., Ramírez et al. 2008). Furthermore, overfertilization may favor ruderal vegetation establishment rather than indigenous species colonization.

Since sewage sludge and other organic amendments used for mine reclamation are catalogued as residues and included on the European Waste Catalogue and other waste lists, their origin, chemical composition, and pollutant burden are strictly controlled by environmental agencies. A working document on sludge was presented for the European Commission (EC 2000), but it largely addresses fertilizing or agricultural uses.

Since some mined sites are not suitable to receive organic amendments during the reclamation process, we propose an evaluation protocol that represents concerns for safety and environmental hazards as well as logistical considerations (Table 1). For instance, sewage sludge application is not permitted in areas prone to flooding or near watercourses or wells, due to the risk of pollution. Accessibility of the quarry is also important. Roads should be safe and in good condition and not too steep in order to facilitate the transport of the organic amendment. Moreover, the quarry must have enough space to accumulate the organic amendment in safe conditions and to carry out mixing with mine spoils. The availability of machinery for such mixing operations is also important, as are land uses at the quarry site and nearby areas. If the restored site will be visited frequently for something like community gardens, then sewage sludge applications must be avoided. Nearby lawns (high farming utilization) and farmyards (moderate farming utilization) are also problematic. It is also important to avoid applications near inhabited areas because of smell.

All of these parameters should be evaluated before the application. A preliminary elimination of eligible quarries can be undertaken using GIS. For example, sewage sludge application can be quickly ruled out for mine sites located in nitrate-vulnerable zones or close to rivers.

Organic amendment is recommended only when the substrate's total organic content is less than 20 g/kg. In order to avoid overfertilization, salinization, and the harmful effects of toxic elements, a dose calculation protocol has been developed (Alcañiz et al. 2008). This protocol fixes a maximum load of labile organic matter (mainly polysaccharides and proteins) that can be applied to the substrate. This amount is based on earlier experience (Domene et al. 2007, Tarrasón et al. 2008), according to EC50 values obtained from ecotoxicological assays using plants and soil fauna (Figure 1). The total amount of amendment to be applied is calculated from the sewage sludge's total amount of organic matter and its degree of stability and the thickness of...