Evidence of spatio‐temporal variations in contaminants discharging to a peri‐urban stream

Xenobiotic organic compounds can be discharged from contaminated groundwater inflow and may seep into streams from multiple pathways with very different dynamics, some not fully understood. In this study, we investigated the spatio‐temporal variation of chlorinated ethenes discharging from a former industrial site (with two main contaminant sources, A and B) into a stream system in a heterogeneous clay till setting in eastern Denmark. The investigated reach and near‐stream surroundings are representative of peri‐urban settings, with a mix of high channel alteration and more natural stream environment. We therefore propose an approach for risk assessing impacts arising from such complex contamination patterns, accounting for potential spatio‐temporal fluctuations and presence of multiple pathways. Our study revealed substantial variations in pathway contributions and overall contaminant mass discharge to the stream. Variable contaminant contributions arising from both groundwater seepage and urban drains were identified in the channelized part of the north stream, primarily from source A. Furthermore, variations in the hyporheic and shallow groundwater flows were found to enhance contaminant transport from source B. These processes result in an increase of the overall mass of contaminant discharged, correlating with the channels' flow. Thus, an in‐stream control plane approach was found to be an effective method for integrating multiple and variable discharge contributions quantitatively, although information on specific contaminant sources is lost. This study highlights the complexity and variability of contaminant fluxes occurring at the interface between groundwater and peri‐urban streams, and calls for the consideration of these variations when designing monitoring programmes and remedial actions for contaminated sites with the potential to impact streams.