Cyclic variations in conductivity and chloride concentrations in coastal freshwater lagoons, Southwest England

Slapton and Widdicombe Leys are freshwater coastal lagoons situated in South Devon, United Kingdom. They are both separated from the sea by elevated shingle barriers of late Holocene age. Surface water levels of the lagoons are ~4m above Ordnance Datum Newlyn at maximum water level. Cyclic variations in conductivity and chloride were measured in Slapton Ley during two short term (~4 days) automated continuous water quality monitoring surveys. Three competing hypotheses were tested to establish whether increases in salinity measured over high tides were attributed to catchment surface runoff, ground water seepage or seawater ingress. Using chloride as a salt-water indicator, a continuous water quality monitor was deployed in the lagoons, whilst simultaneous flow monitoring and water sampling surveys were undertaken from contributing tributaries to establish any catchment influence at high and low tide over spring and neap tidal ranges. The results showed that increased lagoon discharge and lagoon chloride concentrations at high tide were not attributable to catchment surface water inputs. Mass balances calculated using water sampling and flow surveys provided an estimate of a seawater influx of ~6l s-1 into Slapton Ley and ~0.6l s-1 into Widdicombe Ley. To establish the mechanism for salt-water incursion into the lagoons, five sediment cores were taken from the Slapton Ley catchment and one core from Widdicombe Ley. Results showed that deep saline groundwater is arising from basal sediments within the two Leys at ~3m in depth, suggesting that on a rising tidal cycle seawater is forced up through permeable layers and overlying gyttja into the water column above.