Transport, fate and risk assessment of groundwater contaminants discharging to a stream: novem approaches and current understanding

Abstract

Contaminants such as chlorinated solvents, pharmaceuticals and pesticides, as well as emerging micropollutants are released to streams from multiple point and diffuse sources. Sustainable management of water resources requires assessment of multiple contamination sources within a watershed in order to assess their direct impact on water quality. Determination of flow paths and groundwater fluxes are essential for evaluating the transport, fate and potential impact of contamination discharging to streams. This implies that investigators have the tools to evaluate the governing parameters, including an appreciation of the scale of variability, as well as conceptual and numerical models that incorporate the various mechanisms affecting flow, transport and fate. A major multidisciplinary field-scale investigation of the Grindsted stream area was carried out in 2012-2018 to develop the scientific basis for conducting risk assessments for contaminated sites impacting groundwater and stream water. The contamination originating from a former pharmaceutical industry discharges into a multilayered aquifer system and a downgradient stream. The groundwater plume contains high concentrations of pharmaceutical compounds, chlorinated ethenes and benzene. The contamination in the source area has been depleted and today the main challenge is related with the complex plume discharging into the stream. The main impact zone is located within a mixed land-use stream system, comprising urban areas and agricultural production, with multiple chemical stressors impacting the stream corridor.Our overall aim of the field investigations was to (i) advance our understanding of transport and fate of chlorinated ethenes and pharmaceutical compounds in a multilayered aquifer-stream system; (ii) test the applicability of different methods for mapping groundwater flow and pollution as it enters a stream at a complex site; (iii) perform a risk assessment of the stream using the contaminant mass discharge approach; and (IV) assess the stream’s chemical and ecological status in a multiple stressor context.The study included development of geological and hydrogeological models, geophysical measurements, numerical modeling of the flow and transport, mapping of the contaminant plume at various scales, and detailed field investigations at the main entry point of the plume into the stream. We quantified the flow, contaminant mass discharge and attenuation of the plume at the groundwater-surface interface using different approaches and innovative tools. The transport and fate of pharmaceutical compounds in the plume were additionally assessed by statistical methods, analyzing the historical development and the current state of the plume and a literature review. Finally, the stream system was risk assessed by chemical and ecological methods in relation to other potential sources impacting the streams.Our current understanding of the attenuation processes for key contaminants in the aquifer, the hyporheic zone and stream, the value of the innovative field methods applied, and the benefit of the contaminant mass discharge for risk assessment will be discussed in the presentation