Groundwater seepage, saltwater intrusion and nutrient leaching from a coastal lagoon at Slapton Sands, UK

Ground water seepage through gravel barriers are a significant contributor to the flushing of coastal freshwater lagoons. Alternatively, salt water intrusion into the freshwater lagoon through the barrier may lead to the destruction of its freshwater status. Previous studies into seepage have shown that it is highly dependent on the hydraulic gradient which in turn is dependent on the lagoon water level and tidal elevation levels. Despite the high transmissivity of gravel barriers due to their highly permeable matrix, seepage has received comparatively little attention in the literature next to sand barriers. Sand barriers and mixes of sand and gravel barriers lead to fundamental differences in behaviour. There is a gap in the literature to assist in the management of coastal freshwater lagoons vulnerable to salt water intrusion and nutrient dispersal through gravel barriers. It is shown that that the hydraulic gradient present in the gravel barrier at Slapton Sands was always in a seaward direction except for a significant wave event in January. Conductivity was used as a proxy for salinity but measurements were only taken in June when the gradient was always in a seaward direction. At that time, salinity never penetrated into the barrier as far borehole well 3 and, also, nitrate was shown to transport the length of the barrier but in decreasing concentrations due to fresh water/salt water mixing. The study has demonstrated that the hydraulic gradient is relatively stable in a seaward direction but it is anticipated that further study will build on the work presented here to compile an annual salinity and water elevation level data set, including extreme weather conditions, to analyse salt water intrusion into Slapton barrier over an extended time period.Faculty of Science & Technolog