Environmental Science: Processes & Impacts blog

Degradation by sunlight is an important sink for black carbon in surface waters. This study by researchers at University of Michigan and Old Dominion University in Norfolk, Virginia demonstrates how partial degradation of condensed aromatic compounds is the primary mechanism for this process.

The predicted frequency of wildfire activity during the 21^st century will lead to increased production of black carbon (BC) in the environment. The recalcitrant nature of BC means it displays a relatively long residence time in soils, thus constituting a key sink for atmospheric CO₂.

Abiotic and microbial degradation of condensed aromatics of BC in soils can lead to transfer of water-soluble BC components to surface waters, as well as particulate BC. This will expose both particulate and dissolved BC components to degradation by sunlight.  The condensed aromatic component of BC in surface waters can be photo-oxidized either completely, forming CO₂ or partially, forming compounds no longer be detected as BC.

Identifying the contribution of complete and partial BC photodegradation is key to understanding the extent and speed to which CO₂ is returned to the carbon cycle. To date no study has investigated the relative importance of these two competing reaction pathways in surface waters. This study by Collin Ward and co-workers represents the first quantitative assessment of partial vs. complete photoxidation of BC in dissolved and particulate phases.

Previously, degradation of condensed aromatics has investigated using FT-ICR-MS or detection of molecular markers such as benzene polycarboxylic acids (BPCAs). However this approach cannot distinguish between complete and partial oxidation.

In this study, aqueous suspensions of particulate BC (BC-p) and solutions of dissolved BC (BC-d) were prepared from charring of arctic biomass, mimicking BC produced from wildfires. These were exposed to natural sunlight in a 17 hour exposure experiment. Complete and partial oxidation were quantified by monitoring CO₂ production and O₂ consumption using a dissolved inorganic carbon (DIC) analyser and membrane inlet mass spectrometer (MIMS) respectively.

Complementing this approach, the study also investigated the shift in chemical/optical properties of BC during exposure using UV-Vis absorbance, fluorescence spectroscopy and FT-ICR-MS. Results were assessed relative to dark control tests.

Results indicate BC-d in surface waters was disproportionately more susceptible to photooxidation than BCp, based on O₂ consumption and CO₂ production during exposure. It was shown that 8-13% of the C in BC-d was photo-mineralized to CO₂ while 68-91% was partially oxidized to compounds that remained in solution.

Following exposure to sunlight the majority of identified molecules in the mass spectra were aliphatics (85%) with smaller amounts of condensed aromatics (10%) and aromatics (5%), while in dark controls, most identified compounds were condensed aromatics, suggesting these compounds are being partially oxidised primarily. Indeed, 89% of the condensed aromatics identified in dark controls were absent from the mass spectrum following exposure.

Furthermore, the average O/C of aromatics was shown to decrease for aromatics and aliphatics but increase for condensed aromatics remaining after photooxidation, suggesting condensed aromatics are more susceptible to oxidation.

These results strongly suggest sunlight is a key sink for BC in surface waters and that condensed aromatics are the class of compound primarily involved in these photo-reactions. However, the primary reaction pathway involved partial oxidation to less aromatic photoproducts with unknown susceptibility to further degradation. The authors highlight that there is a need to further investigate the reactivity of BC photoproducts to understand the role of sunlight as a sink for BC in the wider geochemical system.

To downloads the full article for free* click the link below:

Insights into the complete and partial photooxidation of black carbon in surface waters

Collin P. Ward, Rachel L. Sleighter, Patrick G. Hatcher, and Rose M. Cory
DOI: 10.1039/c3em00597f

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The presence of cattle in close proximity to water courses can influence pollution levels in both direct and in-direct means. This study by researchers at Lancaster University reveals how these impacts can be best understood using simultaneous monitoring of both physical movement of cattle and water quality parameters.

Livestock farming in close proximity to streams can influence water quality in numerous ways. For example enhanced bank-side erosion and removal of vegetation can result in mobilization of sediments into water courses and increase turbidity. However, uncertainty exists regarding the extent to which the physical movement of cattle into and within the stream bed influences sediment resuspension and contribute to water quality degradation.

Previous studies assessing in-stream cattle impacts have focussed on short-term controlled events and often do not differentiate between bank-side and in-stream activity. The irregular behaviour of cattle means there is a need for high–frequency and simultaneous monitoring of cattle in-stream movement and water quality parameters over  sufficiently long periods.  This investigation by Julie Terry and co-workers at the Lancaster Environment Centre, UK attempts to assess the temporal relationship between physical in-stream activity of cattle and the level water quality.

This study investigated the physical movement of cattle in a stream and the impacts on suspended solid concentration (SSC) and observing a ‘signal response’ in an unfenced stretch of river in Cumbria, north-west England over a 4 month period. This involved the use of high resolution monitoring data derived using motion capture camera surveillance, taking over 31,000 images. At the same time, high resolution water quality data including turbidity (converted to SSC), water level and flow rate were also monitored.

It was shown that for the days when cattle were grazing nearby, of the instances that SSC exceeded 25 mg/l (the Freshwater Fish Directive guidance threshold), 58% could be attributed to the presence of cattle. However, only 3.6% of total sediment load in the stream was directly caused by cattle in-stream activity. Flow is still the main factor influencing total sediment transported, with cattle contributing a much smaller proportion. No relationship was noted between SSC response and absolute number of cattle feet in the water, attributed to the episodic nature of cattle-related sediment disturbances and the variability in stream flow.

Hysteresis analysis (modelling the relationship between concentration levels and discharge) demonstrated a lag time between the presence of cows in the stream and a critical amount of their contribution to SSC  with a reoccurring ‘first-flush’ of sediment created from non-cattle activity.  It was indicted, however that cattle can also provide considerable contribution indirectly e.g. due to erosion of stream banks.

Don’t let this work go in one ear and out the udder. It demonstrates the impact cattle can have on water quality in unprotected and unmonitored stretches of river and indicates the need for best practice measures for livestock management need to be adhered to closely in order to reduce maintain the ecological status of water courses. The authors have demonstrated the need for high-resolution monitoring data is to best understand and mitigate against these dangers and also outline directions for further work to compliment these results.

To access the full HOT Article for free*, click the link below:

Temporal dynamics between cattle in-stream presence and suspended solids in a headwater catchment
Julie A. Terry, Clare McW.H. Benskin, Emma F. Eastoe and Philip M. Haygarth
Environ. Sci.: Processes Impacts, 2014,
DOI: 10.1039/C3EM00686G

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