Ursula S. McKnight

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 372u202fkg/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 558u202fkg/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.

Linking ecological health to co-occurring organic and inorganic chemical stressors in a groundwater-fed stream system

Freshwaters are among the most endangered ecosystems worldwide, due predominantly to excessive anthropogenic practices compromising the future provisioning of ecosystem services. Despite increased awareness of the role of multiple stressors in accounting for ecological degradation in mixed land-use stream systems, risk assessment approaches applicable in field settings are still required. This study provides a first indication for ecological consequences of the interaction of organic and inorganic chemical stressors, not typically evaluated together, which may provide a missing link enabling the reconnection of chemical and ecological findings. Specifically, impaired ecological conditions - represented by lower abundance of meiobenthic individuals - were observed in the hyporheic zone where a contaminant groundwater plume discharged to the stream. These zones were characterized by high xenobiotic organic concentrations, and strongly reduced groundwater (e.g. elevated dissolved iron and arsenic) linked to the dissolution of iron hydroxides (iron reduction) caused by the degradation of xenobiotic compounds in the plume. Further research is still needed to separate whether impact is driven by a combined effect of organic and inorganic stressors impacting the ecological communities, or whether the conditions - when present simultaneously - are responsible for enabling a specific chemical stressors availability (e.g. trace metals), and thus toxicity, along the study stream. Regardless, these findings suggest that benthic meioinvertebrates are promising indicators for supporting biological assessments of stream systems to sufficiently represent impacts resulting from the co-occurrence of stressors in different stream compartments. Importantly, identification of the governing circumstances is crucial for revealing key patterns and impact drivers that may be needed in correctly prioritizing stressor impacts in these systems. This study further highlights the importance of stream-aquifer interfaces for investigating chemical stressor effects in multiple stressor systems. This will require holistic approaches for linking contaminant hydrogeology and eco(toxico)logy in order to positively influence the sustainable management of water resources globally.

Application of new point measurement device to quantify groundwater-surface water interactions

The streambed point velocity probe (SBPVP) measures in situ groundwater velocities at the groundwater-surface water interface without reliance on hydraulic conductivity, porosity, or hydraulic gradient information. The tool operates on the basis of a mini-tracer test that occurs on the probe surface. The SBPVP was used in a meander of the Grindsted Å (stream), Denmark, to determine the distribution of flow through the streambed. These data were used to calculate the contaminant mass discharge of chlorinated ethenes into the stream. SBPVP data were compared with velocities estimated from hydraulic head and temperature gradient data collected at similar scales. Spatial relationships of water flow through the streambed were found to be similar by all three methods, and indicated a heterogeneous pattern of groundwater-surface water exchange. The magnitudes of estimated flow varied to a greater degree. It was found that pollutants enter the stream in localized regions of high flow which do not always correspond to the locations of highest pollutant concentration. The results show the combined influence of flow and concentration on contaminant discharge and illustrate the advantages of adopting a flux-based approach to risk assessment at the groundwater-surface water interface. Chlorinated ethene mass discharges, expressed in PCE equivalents, were determined to be up to 444u202fkg/yr (with SBPVP data) which compared well with independent estimates of mass discharge up to 438u202fkg/yr (with mini-piezometer data from the streambed) and up to 372u202fkg/yr crossing a control plane on the streambank (as determined in a previous, independent study).