Integrated assessment of chemical stressors and ecological impact in mixed land use stream systems

The increasing human population and development pressure during the last century has motivated land use changes of importance on a global scale. Urban expansion and increasing agricultural production have thus created a wide range of pressures, which in particular affect the freshwater bodies such as streams, as they are highly connected to their catchment through their draining system.

The pressures include hydromorphological alterations, as well as diffuse chemical sources (e.g. geogenic, agricultural activities) and point sources (e.g. wastewater outlets, contaminated sites). The degradation of these mixed land use stream systems causes critical changes and thus jeopardizes the health of the stream ecosystems.

The various chemical sources result in a highly diverse group of chemical stressors leading to a decrease in the chemical quality of the different stream compartments (i.e. stream water, hyporheic zone and bed sediment). These compartment(s) will be impacted differently by the various chemicals present in the system, depending on e.g. the stressor’s pathway to the stream, their physico-chemical properties, and controlling hydrological and biogeochemical processes.

The resulting impairment of the different stream compartments thus comprises both temporal and spatial variation. Despite the growing understanding of the complexity, approaches for a holistic risk assessment of the potential impacts in the three stream compartments of a mixed land use stream system are still missing.

To investigate and improve the understanding of the presence of multiple chemical stressors and any related ecological impacts in such a system, Grindsted stream was chosen as the study site for this PhD project. The groundwater-fed stream is situated in a mixed land use catchment hosting both urban and agricultural activities, including contaminated sites.

To determine potential impacts, the chemical quality of both organic (i.e. pharmaceuticals, gasoline constituents, chlorinated solvents, and pesticides) and inorganic (i.e. metals, general water chemistry and macroions) compounds was assessed in all three stream compartments. To evaluate the magnitude of the sources, a combination of three established approaches was employed: contaminant mass discharge, toxic potential and threshold values.

To subsequently account for potential ecological impacts, benthic invertebrate communities (both macro- and meiofauna) were characterized to enable a full coverage of the quality of all three stream compartments. Possible links between the chemical quality of the individual compartments and the ecological stream quality were then explored by using multivariate statistical analyses.

The evaluation of the chemical quality in the three stream compartments revealed a substantial influence on both stream water and hyporheic zone from the diffuse metal sources (aluminum, barium, copper, lead, nickel, zinc) of both geogenic and anthropogenic origin in the catchment. The release of metals (particularly copper, nickel, zinc) was additionally enhanced by acidification of the noncalcareous aquifer.

Moreover, the approach combining an evaluation of the contaminant mass discharge of the known anthropogenic point sources in the catchment together with the in-stream contaminant mass discharge showed to be an effective tool to both display their mutual importance and to reveal “new” sources. It further demonstrated the importance of contaminated sites as a potential noteworthy source to continuously impact the chemical stream quality (> ½ tonne per year of organic xenobiotics).

An assessment of the chemical patterns (similarities) along the investigated stream corridor made it possible to link the chemical quality to a detected ecoresponse in the invertebrate communities. This study thus demonstrated significant ecological impact resulting from the additional chemical stress of the inflow of a contaminated groundwater plume.

The mechanism for this impact indicated that this was not caused solely by the presence of organic xenobiotics, but also by the strongly reduced redox conditions (e.g. high concentrations of dissolved iron and manganese) and secondary effects (e.g. high concentrations of dissolved arsenic), as a result of the organic degradation (e.g. benzene, toluene, ethylbenzene, and xylene) in the plume.

The ecological impact was detected predominantly in the organisms living in the upper bed sediment. The sensitivity was especially pronounced in the meioinvertebrate community, which could be a promising tool to standardize the characterization of the ecological quality of streams considering their ubiquitous distribution throughout all ecoregions.

The methodology developed here, applying a holistic evaluation of both the chemical and ecological stream quality, thus demonstrates the importance for future risk assessments to include multiple compounds (i.e. organic and inorganic chemical stressors) and stream compartments to locate key sources and risk drivers.

The approaches and findings in this thesis could truly be helpful for management and future remediation of mixed land use stream systems.