Raising awareness on algal toxins—a discussion on potential pathways of microcystins to urban groundwater systems

Abstract Accelerated by urbanization, as well as by agricultural intensification and climate change, increasing nutrient fluxes towards surface waters and increasing water temperatures are increasing the probability for cyanobacterial harmful algal blooms. Significant deterioration of water quality occurs with such events when microcystins (MCs) are released as the most prominent algal toxins. Despite the typical assumption that MCs are removed during percolation, studies have found significant concentrations in groundwater bodies. In this article, advances and challenges associated with monitoring and modeling techniques for characterizing the fate of MCs are discussed. Missing insights in the mechanisms leading to elevated MC concentrations in the subsurface—ultimately limiting the reliability of risk assessments—are identified. An important aspect is the a priori identification of environmental conditions and corresponding events that support bloom formation in surface waters and subsequent transport into groundwater. Challenges associated with required improved monitoring and simulation techniques are formulated. In particular, the role of environmental conditions in urban regions as major drivers for heat and nutrient emissions controlling bloom formation, potentially limiting MC retardation, needs to be further investigated. Based on existing experimental studies on MC reactive transport in porous media, existing monitoring methods need to be refined and combined with suitable modeling approaches. A thorough data analysis can then support efforts for sustainable water management of urban regions under threat by algal toxins.