Assessing the impact of windfarm-related disturbance on streamwater carbon, phosphorus and nitrogen dynamics: A case study of the Whitelee catchments

Murray, Helen Susan (2012) Assessing the impact of windfarm-related disturbance on streamwater carbon, phosphorus and nitrogen dynamics: A case study of the Whitelee catchments. PhD thesis, University of Glasgow.

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This research examined the impact which onshore windfarms sited on peat-based soils have on streamwater carbon (C), phosphorus (P) and nitrogen (N) dynamics. Significant disturbance to peatland arises through the excavation of borrow pits, construction of access tracks, insertion of turbine bases and associated deforestation during windfarm development – potentially increasing the transfer of C, P and N from terrestrial stores to the stream network. To identify which impacts occur, streamwater samples from nine catchments draining the Whitelee windfarm, Scotland, Europe’s largest onshore windfarm, were collected approximately bi-monthly from October 2007 to September 2010 during and after windfarm construction, building on pre-disturbance data from Waldron et al. (2009). The samples were analysed for dissolved organic carbon (DOC), particulate organic carbon (POC), total organic carbon (TOC), soluble reactive phosphorus (SRP), total phosphorus (TP) and nitrate (NO3-). Time series were constructed and annual streamwater exports were calculated so that inter-catchment and annual differences could be detected.

DOC concentrations ranged from 2.9 mg L-1 to 57.1 mg L-1 and DOC exports ranged from 9.0 g m-2 yr-1 to 42.3 g m-2 yr-1. POC concentrations ranged from 0.09 mg L-1 to 23.4 mg L-1 with POC exports ranging from 0.80 g m-2 yr-1 to 3.93 g m-2 yr-1. DOC exhibited seasonality with maximum concentrations and exports towards the end of each summer. Harmonic regression analysis of the TOC data indicated a slight increase during the maximum phase of the seasonal cycle, coincident with windfarm-related disturbance observed in one catchment only. Regardless of windfarm development, streamwater DOC exports for four peatland-dominated catchments were observed to exceed typical values of C sequestration rate for Scottish peatlands over the four year time series.

SRP concentrations ranged from 1 ug L-1 to 289 ug L-1 and exports from 12 mg m-2 yr-1 to 104 mg m-2 yr-1 with TP concentrations and exports ranging from 2 ug L-1 to 328 ug L-1 and exports from 25 mg m-2 yr-1 to 206 mg m-2 yr-1 respectively. In two catchments where windfarm-related clear-felling and extensive brash mulching were carried out, as much as a tenfold increase was observed from June 2007 in P concentration, coincident with the timings of windfarm-related forestry operations. The water quality status of these two catchments declined from “good” to “moderate” in terms of the Water Framework Directive (2000) UK Technical Advisory Group Environmental Standards for SRP in Rivers, and had still not shown a full recovery after a further two years.

NO3- concentrations ranged from 0.001 mg L-1 to 2.44 mg L-1 and exports from 0.07 g m-2 yr-1 to 1.64 g m-2 yr-1. Harmonic regression analysis indicated a potential impact on streamwater NO3- concentration through a change in the seasonal pattern observed in three catchments during 2007-2008, coinciding with windfarm-related disturbance.

To determine the most likely controls of C, P and N, a geographic information system (GIS) analysis was employed to describe the physiography of each catchment and to quantify the extent of windfarm-related disturbance. Multiple linear regression analysis was performed using median concentration and export for a low-disturbance phase from June 2006 to August 2007, and a maximum-disturbance phase from September 2007 until May 2009, with the catchment characteristics from the GIS analysis to identify potential impacts of windfarm-related disturbance.

The percentage of the catchment which was HOST class 15 and 29, both peat-based soil types, was observed to influence streamwater C, with the proportion of the catchment which was pasture observed to influence streamwater N. Windfarm-related disturbances were also found to control streamwater dynamics. The extent of deforestation was observed to greatly increase streamwater P concentration and that there was a smaller increase in C, with consequent impacts on SRP and POC export. The source of this additional C and P resulted most likely from forestry operations, namely, clear-felling large areas of catchment and extensive brash mulching associated with the windfarm habitat restoration, with new organic material available for decomposition and the potential for erosion of the newly-exposed soil surface. Residual fertiliser used to establish the conifer trees, the reduced vegetation uptake of soil P and P release from the roots of the felled trees are three further sources of P in streamwater. The distance to the nearest disturbance was also found to influence streamwater dynamics.

This research has shown that windfarm-related infrastructure can also influence streamwater chemistry, although exerting a less obvious impact: increasing access track length was correlated with decreasing POC and P concentration causing subsequent decreases in export. This is likely to be caused by the effective use of settlement ponds, flocculation blocks and ditch blocking in order to reduce the amount of particulate matter reaching the stream network and potentially through the adsorption of P by the access track construction material. Windfarm-related disturbance did not influence N dynamics; NO3- concentration was strongly correlated with HOST class 24 and the proportion of the catchment which was pasture, and NO3- exports were influenced by the flow length per catchment area, the percentage of coniferous forest cover and the extent of pasture.

Two recommendations for best practice at future windfarm developments on peatland are made: firstly, monitor C, P and N in streamwater in all catchments before, during and after windfarm construction on C rich soils as part of EIA process in order to ensure that water quality is maintained in rivers, to determine whether discharges from windfarm developments should require to be licensed and to improve the accuracy of C payback time estimates; and secondly, minimise brash-mulching to reduce the impact on streamwater P.

Furthermore, some suggestions for future research were proposed, including: plot studies to determine the best practice mitigation measures for P release in peatland catchments, longitudinal catchment studies, investigation of the relationship between access track construction material and streamwater dynamics, installation of continuous monitoring equipment along with a sampling regime which targets high flow events, examination of the changing stiochiometry, quantification of inorganic C and gaseous forms of C, and measuring catchment C sequestration rates.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: windfarm, Whitelee, carbon, phosphorus, nitrogen, peatland, catchments
Subjects: Q Science > QD Chemistry
G Geography. Anthropology. Recreation > GE Environmental Sciences
Colleges/Schools: College of Science and Engineering > School of Geographical and Earth Sciences
College of Science and Engineering > School of Chemistry
Supervisor's Name: Waldron, Prof. Susan, Flowers, Dr. Hugh and Heal, Dr. Kate
Date of Award: 2012
Depositing User: Miss Helen Murray
Unique ID: glathesis:2012-3343
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 15 May 2012
Last Modified: 10 Dec 2012 14:06

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