The Hydrological and Sediment Impacts of Forest Activities under differing Geographical and Soil Settings in Ireland

Ensuring the availability of clean and natural sources of water is essential to supporting all life on Earth. Key to this is understanding pollution sources and receptors within catchments, that ultimately facilitates the design and implementation of policy and practices to mitigate against water quality deterioration. While studies have been carried out relating forest operations to erosion and stream sediment loading, there exists a deficit of information that relates these issues to Irish settings, where both climate and soil characteristics may differ significantly from those reported in the literature. This field-based study utilised high-frequency measurements of stream suspended sediment concentration (SSC) across five Irish forests in peats, peaty mineral and mineral soils. Clearfelling, windrowing and reforestation operations were scheduled in the study sites such that baseline, operation and post-operation datasets would be produced in the lifetime of the project. Turbidity was used as a proxy for suspended sediment concentration (SSC), which, in combination with measures of SSC, allows for the establishment of SSC-turbidity ratings at monitoring stations of study sites, from which time- series SSC datasets were derived. It was intended that pre- and post-operation comparison at upstream and downstream sites, separated by impacted areas, would be undertaken, underpinning a Before-After-Control-Impact (BACI) type methodology that accounted for any natural or pre-existing differences between the monitored sites, and thus informing the actual impact of the forestry operation. However, a range of issues and challenges were encountered. Issues with forest operation licencing, which resulted in deferral of a number of the planned forestry operations beyond the timescale of this project limited significantly the scope of data that was collected and resulted in only pre-operation data being collected in four of the five study sites. Very significant challenges were also encountered in data collection where continuous issues with the deployed turbidity sensors compromised the establishment of reliable data at the study sites, resulting in many data gaps, but also in very ‘noisy’ data. To address this, a data cleansing algorithm, following that of Yousif et al. (2022) was developed and applied to collected data. The algorithm relied on other input data, and where these data were available, the algorithm was shown to be effective in ‘smoothing’ anomalies in the observations and reducing the number of outliers present in the datasets. Mean turbidity values in the monitored period ranged between 1.25 NTU at downstream monitoring station at a peaty podzol site, GDN2, to 6.59 NTU at a downstream station on an upland peat forest, CGD3. The highest turbidity value recorded was 1,796 NTU from the peatland forest, CGD3, and this was associated with a significant rainfall event. The lowest values for ‘maximum turbidities’ were recorded in peaty podzol and gley soils, where maximum NTU values were 325 NTU at GRN2 and 343 NTU at DRY2. There is also evidence of sediment storage between the upstream and downstream stations at some study sites. Sediment dynamics in the peaty CGD site were monitored. Suspended sediment concentrations during rainfall events peaked at 32.30 mg/L and 38.71 mg/L at CGD1 (upstream) and CGD3 (downstream), respectively. These values were found to be within the normal range for pre- forest operation peat forests in Ireland. The relationship between turbidity and suspended sediment concentration at this site was, however, weak. Regression analysis established that the relationship between SSC and turbidity, as well as SSC and level, was weak. As such estimations for SSC throughout the study period varied and were inaccurate when analysed. This research, while limited by numerous constraints over its three-year period, is important for understanding and managing forest operations.