January 2012
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617 Reads
Pant-y Llyn is Wales’ and Britain’s only recorded Turlough. It is a key feature of Cernydd Carmel SSSI and SAC and a groundwater dependent terrestrial ecosystem (GWDTE) that has been identified as being at risk of significant damage from enriched groundwater input. This investigation is a collaborative project between hydrogeologists from Environment Agency Wales (EAW) and ecologists from the Countryside Council for Wales (CCW). The investigation was undertaken to fulfil requirements detailed by CCW in their Special Sites Actions Database, which is a list of actions required to bring protected sites (SACs and SSSIs) into favourable status. The main aims of the investigation are to: · Improve our conceptual and hydrological understanding of the site in the context of the hydrogeology of the surrounding area; · Review the water chemistry of Pant-y-Llyn and the surrounding groundwater, identifying any potential nutrient issues; · Incorporate recent data for water quality and quantity into CCW core management plan and objectives; Using continuous monitoring equipment, the hydrology of the site has been measured. Due to instrument security issues the hydrology of the bottom 1.5m of the turlough could not be recorded. We have also compiled all relevant water quality data for the site. In 2005 CCW assessed Pant-y-Llyn as in “unfavourable – unclassified” condition, based on limited water quality data. The Habitats Directive requires all SAC designated sites to be in favourable condition by 2015. Here we update the 2005 condition assessment with detailed hydrological data and much improved water quality data. There is a clear relationship between rainfall, groundwater levels and water level fluctuations within Pant-y-Llyn. The maximum fill level is just over 3m depth above the estavelle. Water level peaks lagged by between 1 and 8 days behind heavy rainfall events. Recharge is more rapid during wetter months where the water table is higher. The empty and filling cycle of Pant-y-Llyn is also reflected in the distribution and type of vegetation. Previous tracer data suggests that the groundwater supply comes from a fault-bounded karstic limestone block catchment within the immediate vicinity of Pant-y-Llyn rather than the main karst system. However groundwater hydrographs within the Glanwenlais karst area are very similar to water levels at Pant-y-Llyn. Further detailed work is recommended to define a better groundwater catchment for the turlough, so that future management decisions can be taken from the best possible knowledge base. A hydrological model capable of predicting water level from rainfall and evapotranspiration records was developed for Pant-y-Llyn. The hydrological response of the turlough, represented as a reservoir within the model, was controlled by inflow and outflow relationships derived from analysis of the turlough water budget. Long-term modelling of Pant-y-Llyn water levels highlighted the control the man-made overflow pipe exerts on the hydrological regime of the turlough. The hydrological behaviour of Pant-y-Llyn was compared with that of recorded Irish turloughs. This found that Pant-y-Llyn was a relatively small, shallow turlough that lay towards the end of the turlough flooding continuum characterised by uni-modal, long-duration flooding. Water chemistry at Pant-y-Llyn is typical of a high alkalinity system. The water in Pant-y-Llyn is less mineralised than groundwater in the area, reflecting surface water or rapid groundwater inputs. The importance of the surface water catchment should be considered. The most significant water quality risk within the surface water catchment is the adjacent road which may contribute surface water drainage. Nutrient levels at Pant-y-Llyn are low, but there are unexplained spikes in nutrients and chlorophyll that require further investigation in combination with water level and / or rainfall data. The site was phosphate limited on all sampling occasions. Based on the revised dataset we have proposed revised water chemistry targets for the turlough. Current biological monitoring is inadequate and both invertebrate and vegetation surveys are required. Hydrologically, Pant-y-Llyn is considered to be meeting existing performance indicators for emptying and filling. Water chemistry gives some cause for concern but further monitoring is required to establish whether episodic poor water quality is linked to pollution or to natural variation. There are no concerns with regard to vegetation zonation although current data is limited to aerial photography. Based on the above we conclude that Pant-y-Llyn turlough is in favourable condition, with low confidence. There are several risk factors that might affect the long-term viability of the site including a resumption of quarrying and its effects on hydrology; the effects of road drainage; scrub invasion; pollution of groundwater from domestic sewage treatment, and climate change.