"Till Birnam Wood remove to Dunsinane": Implications of the Meenbog Peat Slide
Early one autumn morning, part of a forest on the eastern edge of Donegal quite literally upped and wandered off along the Irish border. Whilst this may sound like a scene from a horror film, it really did happen! On 13th November 2020, an area of Meenbog about 100m x 100m and probably about 2m deep, underwent structural failure and burst, releasing a stream of liquid peat that carried rafts of vegetation, including whole trees, down a slope and into the Sruhangarve River (Figure 1a).
Whilst only a few trees were carried more than a couple of hundred metres, much of the peat continued down the Sruhangarve (Figure 1b), into the Mourne Beg River and through to the River Derg, with some likely ending up all the way down the system in Lough Foyle.
Figure 1a&b: The Sruhangarve River showing clear signs of the peat slide - a. (top) a tree carried downstream during the slide has been stranded on rocks in the middle of the river and b. (bottom) exposed peat left on the banks after the initial wave reaches all the way to the Mourne Beg River. (Photos: Phoebe Morton)
By pure chance, the Source to Tap project (which is developing sustainable catchment-scale solutions to protect the health of waterways in the Erne and Derg cross-border catchments; see www.sourcetotap.eu for more information) happened to have a monitoring point on the River Derg, about 37 km downstream of the peat slide, with an autosampler happily taking up water once every 7 hours.
Whilst it appears (according to data from the water treatment works that the monitoring station is powered from) that the first post-peat slide sample was taken about 6 hours after the initial wave of peat reached the monitoring point, the colour of the samples very much indicates that the peat slide was still having a substantial effect on the water colour at this point and for some time after (Figure 2).
The samples from 2.5 days before and over 3 weeks after the peat slide were filtered and analysed for particulate organic carbon (POC; i.e. carbon in the solid matter trapped on the filter paper) and dissolved organic carbon (DOC; i.e. residual carbon left in the filtrate). Rainfall data from the Source to Tap project was measured 16.6 km downstream of the peat slide and discharge (river flow) was measured 28.3 km downstream of the peat slide but could be approximated for the sampling location using water level data recorded there.
Figure 2 The colour of the water samples demonstrate when the peat slide hit the monitoring point (left hand side are samples taken before; bottles progress chronologically from left to right in 7-hourly samples). (Photo: Phoebe Morton)
The rainfall recorded in the 30 days before the peat slide was more than twice that of the same periods in the previous two years and included two days where rainfall exceeded 30mm, just 11 days and two days before the peat slide occurred. Therefore, whilst the specific trigger of the peat slide is currently undefined, it is likely that the high rainfall caused the peat to become supersaturated, which probably contributed to its failure.
Regardless of its cause, the samples taken from the river before and after the peat slide showed a few interesting aspects. Firstly, the DOC concentrations were all in the range that would be expected for rivers in areas surrounded by peatlands, including in the samples taken in the days after the peat slide, and indicated that DOC was largely driven by river flow rates and was unaffected by the peat slide.
Secondly, although the POC concentrations were definitely affected by the peat slide (the sample shortly after the first wave had passed the monitoring point was 346 mg C/L compared to concentrations averaging 1.6 mg C/L before the peat slide), comparison of the POC concentrations to river discharge indicated that the effects of the peat slide were only properly observable in the first nine days.
Nonetheless, even assuming that the peat slide only affected the POC concentrations in the first nine days following the event, that still amounts to an additional 383,882 kg of POC reaching at least as far as the monitoring point, probably much further, and may well have oxidised to carbon dioxide and been released to the atmosphere by the time of writing this. Visually, the peat slide turned the river black, and estimates from local organisations suggest that 100% of the fish in the rivers were killed and that other ecology was seriously affected.
Also, the Derg is a drinking water catchment and this quantity of suspended matter in the water overwhelmed the water treatment works to a point where the intake needed to be turned off for several hours. Thus, whilst the peat slide itself may have only taken a few hours to complete, its effects may be felt for some time.