Vinni Rønde

Assessing the chemical contamination dynamics in a mixed land use stream system

Traditionally, the monitoring of streams for chemical and ecological status has been limited to surface water concentrations, where the dominant focus has been on general water quality and the risk for eutrophication. Mixed land use stream systems, comprising urban areas and agricultural production, are challenging to assess with multiple chemical stressors impacting stream corridors. New approaches are urgently needed for identifying relevant sources, pathways and potential impacts for implementation of suitable source management and remedial measures. We developed a method for risk assessing chemical stressors in these systems and applied the approach to a 16-km groundwater-fed stream corridor (Grindsted, Denmark). Three methods were combined: (i) in-stream contaminant mass discharge for source quantification, (ii) Toxic Units and (iii) environmental standards. An evaluation of the chemical quality of all three stream compartments - stream water, hyporheic zone, streambed sediment - made it possible to link chemical stressors to their respective sources and obtain new knowledge about source composition and origin. Moreover, toxic unit estimation and comparison to environmental standards revealed the stream water quality was substantially impaired by both geogenic and diffuse anthropogenic sources of metals along the entire corridor, while the streambed was less impacted. Quantification of the contaminant mass discharge originating from a former pharmaceutical factory revealed that several 100 kgs of chlorinated ethenes and pharmaceutical compounds discharge into the stream every year. The strongly reduced redox conditions in the plume result in high concentrations of dissolved iron and additionally release arsenic, generating the complex contaminant mixture found in the narrow discharge zone. The fingerprint of the plume was observed in the stream several km downgradient, while nutrients, inorganics and pesticides played a minor role for the stream health. The results emphasize that future investigations should include multiple compounds and stream compartments, and highlight the need for holistic approaches when risk assessing these dynamic systems.

Contaminant mass discharge to streams: Comparing direct groundwater velocity measurements and multi-level groundwater sampling with an in-stream approach

Abstract Quantitative knowledge of contaminant mass discharge is useful when assessing the risk posed to streams by point sources. However, due to a multi-directional flow field at the groundwater-surface water interface, reliable estimates close to streams are particularly challenging to obtain. Moreover, since the “true” value of the total contaminant mass discharge across a defined control plane is typically unknown at field sites, it is difficult to assess the accuracy of estimates at the field scale. We estimated the mass discharge of a chlorinated ethene plume entering a low-land Danish stream across a control plane on the stream bank. This was done using multi-level groundwater sampling combined with 1) specific discharge obtained from Darcy’s law, and 2) two different specific discharge fields (a constant equal to the mean, and a varying) obtained from direct groundwater velocity measurements using point velocity probes (PVPs). The methods yielded contaminant mass discharges ranging from 204 to 372 kg/y (PCE equivalents). To help account for the entire contaminant mass, we also quantified the contaminant mass discharge of chlorinated ethenes from measured contaminant concentrations in the stream water and the stream discharge to obtain an in-stream contaminant mass discharge. By subtracting contaminant input upstream from the plume discharge zone, as well as input from two culverts discharging contaminated water to the stream, this estimate was used to calculate a groundwater-borne in-stream contaminant mass discharge of 558 kg/y (PCE equivalents). This method has not previously been explored to access the accuracy of stream bank contaminant mass discharge estimates. Differences between the groundwater-borne in-stream value and those on the stream bank may be due to high concentration zones located in fast flow conduits or missed high concentration zones in the control plane. It is also possible that the groundwater-borne in-stream mass discharge was overestimated due to uncertainty in the input from two discharging culverts. The agreement of these results is sufficient to be of practical usefulness for risk assessment. It is concluded that the combination of direct groundwater velocity measurements and multi-level groundwater sampling can provide a useful extension of Darcy-based methods when quantifying the contaminant mass discharge to streams. Moreover, this study demonstrates the efficacy and usefulness of the in-stream contaminant mass discharge in risk assessment if fully mixed stream water concentration and corresponding stream discharge are available.

Geophysics‐Based Contaminant Mass Discharge Quantification Downgradient of a Landfill and a Former Pharmaceutical Factory

Contaminant mass discharge is a commonly applied tool to evaluate the environmental impact of contaminated sites on water resources. At large contaminated sites with heterogeneous sources, such as landfills, the number of wells available is often not sufficient, leading to a high uncertainty of mass discharge estimates. In this study, we tackle the uncertainty of the contaminant mass discharge due to low sampling densities by interpolating limited water-sample data with the support of surface direct current resistivity and induced polarization geophysical data. The method relies on finding a conceptual link between the bulk conductivity imaged from geophysics and the contaminant concentrations. We investigate the link between (1) imaged bulk and electrical water conductivity, (2) water conductivity and conservative ionic species, (3) water conductivity and redox-sensitive species, (4) water conductivity and semipersistent organic species, and (5) water conductivity and biodegradable organic compounds. The method successfully identify similarities between the distribution of the bulk conductivity and chloride and pharmaceutical compounds in a landfill leachate plume and between the bulk conductivity data and benzene and chlorinated ethenes for a contaminant plume from a former pharmaceutical factory. Contaminant concentrations were interpolated through regression kriging, using geophysical data as the dependent variable. The distribution of concentration determined with the novel method showed a lower mean relative estimation error than the traditional method of kriging only contaminant concentration data. At large sites, the method can improve contaminant mass discharge estimates, especially if surface geophysical measurements are integrated in the site investigation at an early stage.