A rescaling perspective on multiple boundary effects and collaborative governance of transboundary water pollution in the Guangdong-Hong Kong-Macao Greater Bay Area: Review and prospects

Transboundary governance is a critical determinant of regional coordinated development. The Guangdong-Hong Kong-Macao Greater Bay Area (GBA), operating under the unique "One Country, Two Systems" framework, is profoundly shaped by multiple intersecting boundaries, i.e., administrative, political, economic, and legal. These overlapping jurisdictional and institutional divides give rise to complex multi-scalar processes and mechanisms in transboundary water pollution governance. Existing environmental policies often exhibit varying degrees of "boundary failure", leading to recurrent and cyclical governance dilemmas that impede effective pollution control and ecological restoration. A review of the literature reveals that relevant research is gradually shifting from viewing boundaries as static entities toward understanding them as dynamic processes that produce socio-spatial heterogeneity. While significant attention has been paid to quantifying the intensity and manifestations of boundary effects, few studies have systematically integrated quantitative and qualitative approaches to dissect the characteristics, underlying mechanisms, and contextual factors of different types of boundary effects. More importantly, the role of rescaling as a theoretical lens remains underutilized, leaving the power dynamics and political-economic negotiations behind transboundary water governance inadequately examined. Furthermore, the intrinsic relationship between transboundary pollution’s boundary effects and the impediments to environmental collaborative governance has yet to be comprehensively investigated. Addressing these gaps, this study conducts a systematic review of the multiple boundary effects influencing transboundary water pollution in the GBA and assesses current research trends and practical needs in collaborative governance. Through the perspective of rescaling, it seeks to clarify a coherent theoretical and analytical framework encompassing boundary effect measurement, macro and micro mechanism analysis, theoretical generalization, and policy formulation. Methodologically, this study advocates a mixed-methods approach that synergistically combines quantitative and qualitative techniques. Proposed tools include: convergence regression equations to quantitatively assess the spatiotemporal distribution of transboundary water pollution boundary effects; a triple difference model to evaluate the macro-level impacts of key governance policies, and rescaling analysis to qualitatively investigate the strategic interactions, bargaining processes, and rescaling tactics among stakeholders in GBA water governance. By integrating these approaches, this study aims to elucidate how different types of boundaries, both tangible and intangible, shape transboundary water pollution outcomes through macro-structural and micro-agential pathways. Ultimately, it seeks to synthesize theoretical insights and practical policy recommendations for enhancing transboundary environmental governance under multi-boundary constraints in the GBA. This study contributes to interdisciplinary fields such as human-environment relationship science by offering a refined conceptual framework and integrated methodology. It also elaborates on the strategic, scholarly, and practical implications of applying a scale-sensitive, boundary-aware approach to transboundary environmental governance, with potential relevance for other mega-urban regions facing similar governance challenges.