Nutrient modelling and a nutrient budget for Llangorse Lake Final Report

Llangorse Lake is the largest natural lake in South Wales. There have been concerns about eutrophication problems here for many years. The problem is believed to have been caused by high nutrient loads entering the lake from the surrounding catchment. This study aimed to determine the size and main sources of those loads. The phosphorus (P) and nitrate (NO3-N) load to Llangorse Lake was found to be approximately 2 tonnes P y 1 and 74 tonnes NO3-N y 1 (which is equivalent to 1.5 g P m-2 y-1 and 53 g NO3 N m-2 y-1). Most of the P and NO3-N loads were found to be entering the lake from only two of the inflow streams (i.e. those draining to Sites 2 and 6). These accounted for 85 per cent of the annual P load and 82 per cent of the annual NO3-N load to the lake. The hydrology of the Llangorse catchment appears to be strongly affected by groundwater. This is best demonstrated at Site 6, where the surface water catchment upstream of the site accounts for only 36 per cent of the lake catchment but contributes 67 per cent of annual hydraulic load. This, and the fact that the flow at Site 6 is almost double that which can be accounted for by rainfall, suggests that there is considerable groundwater flow in this area. If the streams that flow into the lake have significant input from groundwater, this will affect their hydrology and chemistry. As this groundwater may enter the drainage system from an area beyond the boundary of the surface water catchment, this has serious implications for catchment management aimed at reducing the nutrient loads to the lake. Some of the inflow streams showed evidence of occasional point source pollution in very wet weather. This suggests that there is a need, in some places at least, to control point sources of pollution that leak or overflow during heavy rainfall. However, the evidence suggests that most sources of nutrients within the catchment are diffuse sources. Most of the published literature concerning the eutrophication and recovery