Assessment of carbon and nutrient export from a peatland windfarm construction site

The full extent of a landscape’s resilience to the environmental impact of siting wind-based renewables on peats is currently unknown. This research explores if windfarm construction activities have caused disturbance by investigating; time series of fluvial carbon (C) and nutrient concentrations; constructing aquatic organic C fluxes, before, during and after the windfarm construction period. Additionally, C sequestration rates of peat and nearby lake sediments (Loch Brora) were calculated to provide a historical context to, i) calculated aquatic C fluxes and ii) sediment export from surrounding catchments, considering both a catchment hosting the windfarm construction and one that does not. Furthermore, the effectiveness of a peatland restoration technique, drain-blocking, was assessed as a means of undertaking a whole system approach to assessing the potential impact of the windfarm development, considering how these management strategies can help mitigate potential C losses associated with construction. The research field site was located on the Gordonbush Estate, near Brora, where construction started in July 2010 (the same time this research began) on Scottish and Sothern Energy Renewables (SSER) 35 turbine windfarm. Construction work finished in May 2012 and data collection continued until September 2014. Throughout this period, fieldwork was focussed on storm event sampling (collecting samples for dissolved organic carbon (DOC), particulate organic carbon (POC), total phosphorous (TP), soluble reactive phosphorous (SRP) and total oxidised nitrogen (TON)), collecting peat and lake sediment cores, samples of modern day sediment export and monitoring water table depth in an area where old drainage channels were blocked as part of a peatland restoration initiative. Three river catchments were studied, two affected by windfarm construction activities (GB10 and GB11) and one control site (GB12), DOC concentrations ranged from 1.1 mg l-1 to 48.3 mg l-1, POC from GB11 > GB12 for all determinants. Seasonal patterns in fluvial macronutrient concentrations were observed at Gordonbush: summer maxima and winter minima in DOC and TP concentrations and the opposite trend in TON concentrations. SRP data collected indicates a legacy of forest felling in the Bull Burn Plantation has contributed to increased concentration in the Allt Mhuilin river (GB10) compared to the two other catchments, Allt Smeorail (GB11) and Old Town Burn (GB12) where no forest felling occurred during the data collection period. Differences in DOC and TP concentration in Allt Mhuilin compared to other catchments could also be related to forest felling activities but catchment characteristics such as peat coverage may have also influenced results. For all relevant measures of water quality, macronutrient concentrations from Gordonbush shows studied streams consistently achieved “Good” or “High” status throughout the data collection period. Apart from the legacy of forest felling, a discernible impact of windfarm construction was not observed from macronutrient concentration time series. Calculating annual aquatic C fluxes from studied catchments offered a means of assessing potential impact. Various techniques of estimating fluxes were explored but splitting storm event DOC and POC data based on time of year and whether samples were collected on the rising or falling limbs were concluded to give the best estimates. Calculated fluxes ranged from 3 – 38 g C m-2 yr-1 and DOC consistently accounted for ~90% of total aquatic C export. These values were within limits of other C flux based studies from peatlands but the time series constructed at Gordonbush suggested windfarm construction, between July 2010 and May 2012, may have contributed to an increase in aquatic C export from affected catchments during this time, relative to the control site. Long term C sequestration rates from within the Gordonbush estate were 20-25 g C m-2 yr-1, the same magnitude as aquatic organic C fluxes. However, peat C sequestration was shown to be variable over the last ~9000 years since Scottish peatlands became established, with rates ranging from 10-60 g C m-2 yr-1. Controls on this variation are likely climatic with delivery of moisture influenced by the North Atlantic Oscillation (NAO) a key factor. Calculated lake C sequestration also varied over time, 22-82 g C m-2 yr-1 but an inconclusive radiocarbon dating chronology meant historical comparison of C export dynamics between the, C ‘source’, peatland to the, C ‘sink’, lake was unfortunately not possible. Modern day sedimentary export data showed higher sediment yields from windfarm affected catchment than the control site. Physical characteristics varied considerably between the two catchments so although this observation could not definitively be attributed to a direct windfarm impact, it remains a possibility. Whilst studying and quantifying the impact of drain blocking, manual measurements of water table depth (WTD) ranged between -53 cm to +14 cm in dip-wells and -36 cm to +20 cm in automated logging pressure transducers. The response of WTD throughout both data sets indicates meteorological conditions were more influential as a factor controlling peat hydrology across the site compared to topography. Manual measurements from dip-wells shows the drainage channels investigated (~0.5-0.7 m deep and ~0.5 m wide) had the greatest influence on effect WTD 0-2 m from the main channel but no statistically significant difference was detected in mean WTDs measurements before or after blocking, in relation to distance from the drainage channels themselves or comparisons between drained and un-drained (control) areas. However, data from PTs indicate the net effect of multiple parallel drains can cause water table drawdown at a significant distance, ~ 25 m, from the drainage channel. This is an important finding as methodology used to calculate the C ‘payback time’ of windfarms utilises the lateral drainage extent of peat when turbines bases are excavated. Drain blocking had no obvious effect (either positive or negative) on WTDs however it is acknowledged positive effects can take up to five years, after blocking has taken place, to be observed. Maximum DOC concentrations increased the year after blocking however this result has been recorded at other sites and the exceptionally dry summer of 2013 could have contributed to the noticed increased by promoting more peat oxidation and subsequently DOC production. There was no statistically significant difference between [DOC] collected up and downstream of the drainage channel inputs for samples collected before and after blocking. This suggests drain-blocking has had little impact on the larger site [DOC] signature one year after drain-blocking. However, as discharge from drainage channels was not measured, a potential reduction in overall DOC export could not be fully assessed and this is a highlighted future research need. Combining averages of aquatic organic C fluxes and peat C sequestration rates calculated it is estimated net ecosystem exchange would have to be between -30 to -50 g C m-2 yr-1 for Gordonbush to be classed as a C ‘sink’. If the observed increases in sedimentary export could be attributed to windfarm construction, Loch Brora is unlikely to act as a strong C sink for any potential increased losses as it is estimated ~90% of POC exported is not sequestered on a long-term basis in the lake sediments. It has recently been recommended windfarms should not be developed on peatlands due to the marginal C savings achieved as our future energy mix changes (Smith et al., 2014). However, if similar projects are granted planning permission then findings from this research support the following recommendations: installation of buffer zones around areas of felled forestry to reduce nutrient export into surrounding streams; implementation of a water quality monitoring programme to assess impact of windfarm construction during construction and a period afterwards as it is still unclear from this research if there will be any lasting effects; installation of silt traps to reduce aquatic sediment export and disturbance; limit any high density excavation of drainage channels as the effects of water draw-down could be quite extensive; in addition, blocking all historical drainage channels and retaining as much moisture as possible within, and surrounding, areas of degraded peatland can increase long-term peat C sequestration rates and offset C losses experienced during construction. This research has been funded by SSER, Engineering and Physical Sciences Research Council (EPSRC) and Energy Technology Partnership (ETP). This research has been undertaken and supported at the University of Glasgow within the College of Science and Engineering, specifically aligned to the work of the Carbon Landscapes and Drainage (CLAD) research group headed by Prof. Susan Waldron in the School of Geographical and Earth Sciences. Finally, this research has also been supported in partnership with Stirling University.