3.1 Case history and description
The main river network area YRD, located in East China, is one of China’s most developed, dynamic, densely populated, and concentrated industrial area. In recent decades, the YRD has grown into an influential world-class metropolitan area and played an important role in China’s economic and social development. In general, the boundary of the YRD varies from different perspectives in terms of its culture, economy, or geography. This paper refers to the area composed of Shanghai, Jiangsu, and Zhejiang provinces.
Since 1970, owing to the strong Shanghai’s industrial base, the cities along the Yangtze River have caught up in the development of non-agricultural industries through rural collective accumulation. In these rural areas, the “five small industries” (small scale steel, machinery, chemical fertilizer, coal, and cement industries) were allowed and started to grow (Wong, 1980). Since then, those towns with more rural industries became ideal places for farmers to work or do business in the YRD. Without exception, the YRD’s rapid industrialization has huge impacts on its natural environment, i.e. water pollution, groundwater levels decline, and soil pollution have become prominent problems. According to the Shanghai Environmental Protection Agency in 2007, Non-point source pollution has become the main factor affecting the stream quality of rural river network area, outpacing industrial point source pollution in the 1990s. Notably, because stream order and catchment boundaries are difficult to delineate in these river networks, the effect of land use on water quality may be quite different than that in other areas (Che et al., 2012; Zhao et al., 2015). Furthermore, the ecological degradation in the YRD is also serious, and the involved issues in terms of land degradation, loss of biodiversity, and serious ecological damage, all have brought serious threats to human survival and sustainable development (Gao et al., 2018).
3.2. Research methodology and Approach
The IR-RRNA project aims to develop effective integrated remediation techniques and equipment for the water environment remediation. It should first clarify the basic information such as the typical pollution in the YRD, the pollution distribution and interactions between water and soil, and the migration and transformation mechanism of pollutants in the water/soil. Accordingly, the literature survey and the typical pollution investigation will be conducted in the RRNA, combined with the collection of the village type and environmental pollution data. A comprehensive analysis will then be conducted to elucidate the distribution of typical pollutants over the RRNA. The scientific principles of ecology, microbiology, and hydrology will be applied to study the process of the pollutant migration and transformation between SW-GW-Soil. Finally, the field pollution survey, experimental methodology, and computer simulation models will be integrated to clarified the migration process of flux between SW/GW, the transformation mechanism of pollutants, and the characteristics of the inner relationship. By this way, we attempt to reveal the regulation principle of surface, soil, and groundwater pollution remediation technology in RRNA.
Based on the above theoretical study, bench-scale and pilot-scale tests will be conducted for polluted surface/groundwater and soil remediation in RRNA. Combining with the theoretical and process analysis, as well as fitness-for-purpose assessment, three key remediation technologies will be formed for three different medium with the advantages of "high efficiency, environmental friendliness, and economy": 1) Aquatic plants and microorganism coupling strengthening remediation technology for surface water; 2) nZVI coupled biochar sustained-release remediation system for groundwater; 3) High efficiency multi-dimensional continuous pollution soil remediation technology using plant-microbial and chemical stabilization. Simultaneously, the outcome of the pilot study will be combined with the theoretical analysis of technology process and pollutant characteristics, to explore the economical, applicable, and easy to hand equipment for the different rural environment remediations. Finally, we will simulate one or two pollution scenarios in RRNA, and systematically study the feasibility of using integrated remediation technology to remediate the contaminated surface/groundwater and soil under different influencing factors, and then establish an efficient and sustainable integrated remediation pilot system.
Considering that the in-situ contaminated environments are complicated and hard to simulate in the laboratory, a field demonstration will be carried out in our project. This field demonstration is characterized by the integrated remediation technique and will be implemented in rural river network areas in Shanghai. In consideration of the natural climate conditions and hydrological characteristics, we will make full use of the spillover effect and carry out integrated remediation in typical pollution sites in RRNA. This platform will take consideration into the complexity of environmental medium and natural biogeochemical processes to form an environmental restoration system that is suitable for the different time and spatial scales, and finally to realize the integrated remediation for contaminated surface/groundwater and soil in RRNA.
3.3. Key technologies and equipment for integrated remediation
The RRNA-remediation project supports the overall target of developing key techniques and devices for rural environmental remediation in RRNA. The selected remediation techniques, such as phytoremediation, microbiological remediation, nZVI/biochar remediation, and chemical stabilization will be combined, regulated, and optimized to effectively restore the polluted water and soil. Afterward, an integrated technical system will be created, including ecological reaction revetment, ecological floating bed, permeable reaction walls, mobile soil leaching device, and plant/roots-microorganism coupling remediation techniques, and ultimately to realize the efficient integrating of the plant, microorganism, and chemical stabilization for the contamination remediation in RRNA (Fig. 6).
River revetment is an important area of the land-water ecotone with comprehensive functions such as safety protection, ecology, and landscape. It also acts as a connection channel between the river ecosystem and the terrestrial ecosystem. However, to accelerate the drainage of rainwater and protect the riverbank from soil erosion, a large number of riverbanks have been cut straight and channelized by constructing revetments in past years (Chen et al., 2016; Yan et al., 2019), resulting in serious damage of the ecological function of these riparian ecosystems. Consideration of improved people’s awareness in ecological and environmental protection as well as the preliminary filed investigation in RRNA, our project put forward an in-situ ecological reaction revetment construction plan. The outcome of this project could facilitate the sustainable circulation of SW/GW and river bank ecological restoration.
Eco-restoration materials for concrete revetment, aquatic plants, and PRBs, are the main constitutes of the ecological reaction revetment. The native aquatic plants with a strong tolerance for pollutants will be selected to fix water pollutants via adsorption, accumulation, and degradation reactions. The plants’ roots further provide a favorable habitat for microbial reproduction and stimulate the microbial proliferation. Microbial consortia can help to improve the water quality and maintain the stability of river slopes. In the laboratory, we will choose one or two native aquatic plants that with good pollution removal capacity, and examine the plant/microbial interaction effect, based on which we attempt to develop an optimized strategy to effectively promote the mass and energy cycle among water/soil and plants/microorganisms.
The PRBs that consist of nZVI/biochar sustained-release materials will be installed parallel to the revetment, in the path of a plume of contaminated surface and groundwater. Compared with the previous vertical installation method, it can dramatically drive down treatment costs and achieve better interception of pollutants in surface water. As the contaminants move through the nZVI/biochar material, the reaction occurs that transforms the contaminants into less harmful (non-toxic) or immobile species. For instance, nitrates will be reduced to N2 and/or NH4+ by nNZVI and the addition of biochar could be favorable for this process, as NO3− can be selectively reduced to N2 instead of NH4+ (Wang et al., 2019). The PRBs are a barrier to the contaminants rather than a barrier to the groundwater. Therefore, PRBs should be designed to be more permeable than the surrounding aquifer materials so that the contaminants are removed as groundwater readily flows through but without significantly altering the groundwater hydrogeology.
Phytoremediation can improve the biological quality of the soil and has been recognized as a benign technology, so it has been selected to our project to degradation, accumulation, or stabilization of contaminants in the polluted aquatic systems. Prior to establishing the demonstration project, we will first select the native plant species that have an extremely high capacity of adsorption of metals, affiliated by the microorganism-based remediation technologies to decompose, transform, and absorb pollutants. For the heavily polluted regions caused by long-term industrial production, the contaminated soil will be moved to a mobile soil washing device (a kind of ex-situ technique), and the contaminants (heavy metals) will be extracted and washed from soils by physical and/or chemical procedures. Meanwhile, a novel ecological floating bed has been proposed in our project that integrates graphene photocatalytic materials, act as a net between ecological floating bed.
The graphene, as a two-dimensional monolayer of sp2-bonded carbon atoms, was used for contaminants removals due to its large specific surface area, good charge transportation, and mechanical strength (Zheng and Kim, 2015). Then, the purification capacity and the stability of the ecological floating bed system can be greatly promoted, which favors flexibly cope with the fluctuation of the water quality of the polluted river.
It should be noted that the integrated remediation system proposed in IR-RRNA project will fully consider the impact of pollution types, pollution levels, and hydrological conditions in different scenarios, and flexibly control the operation of the remediation system. We hope to realize the optimal integration of surface/groundwater/soil remediation in the RRNA in China.