Groundwater-surface water interaction in urban lowland catchments: Water quality dynamics in the city of Amsterdam

This research unravels the role of an overlooked source of nutrients in coastal low-lying urban settings: groundwater. In such hydrogeological conditions, for which the Amsterdam region serves as an example, drainage for urbanization has promoted the seepage of nutrient-rich groundwater into surface water. Groundwater is in these settings likely to outcompete other well-known sources such as urban runoff, agricultural inputs, treatment plant effluents, and leaking sewage systems. The water quality dynamics in such regions are determined by the variable interaction process between groundwater and surface water over space and time. Overload of nutrients causes surface water quality deterioration, e.g. low clarity, smelly water, fish kill, and loss of biodiversity due to the excessive growth of algae and plants. In some cases, harmful algae blooms even poses a serious risk to the health of humans, pets, and city wildlife. Substantial efforts have been made worldwide in abating nutrient overabundance in the natural and artificial water bodies. However, the formation of an effective strategy is seriously hampered by a lack of knowledge of the water quality dynamics and the underlying hydrological and hydrogeochemical processes. This research encompassed a novel multidisciplinary approach of studying the water quality dynamics over space and time, combining the regional hydrogeology, long term surface water and groundwater quality grab sampling data, and state-of-the-art high-resolution water quality monitoring technology. The time scales ranged from thousands of years of sedimentary geological conditions to temporal variations of water quality within an hour. We found that groundwater is the dominant source of nutrients in the study area, compromising the compliance of the water quality status required by the Water Framework Directive. The influences of the nutrient-rich groundwater on surface water quality were intensified by the installation of urban rain and groundwater drainage systems, and by maintaining low water levels by pumping which redistributes the nutrients from groundwater into the surrounding catchments. The flow paths of rain and groundwater were shortened due to the installation of the drainage systems bypassing the purification capacity of the subsurface redox transition zone. The mixing between the anoxic nutrient-rich groundwater and oxic rain water in the open water system performed as the major hydrological process determining the nutrient dynamics in the study catchment. Superposed on that, primary production was found to be the key process regulating N dynamics in spring and summer. Moreover, the sediment-water interface of the shallow surface waterways was found to fixate P during the growing season and releasing P when temperature dropped in the late autumn and winter. Based on this research, we recommended a change from the typical surveillance monitoring based management to a dynamic process-based management, aiming to mitigate the negative side effects of urbanization based on a better understanding of the governing processes.