Further studies on the distribution of 137Cs in British coastal waters—I. Irish Sea

Abstract

The concentrations of 137Cs, discharged to the northeast Irish Sea from the British Nuclear Fuels Limited (BNFL) Windscale Works, Cumbria, have been measured in the Irish Sea and its approaches during 1970 to 1978. Seawater labelled by 137Cs has been used to study the variability of local and distant dispersion and to estimate residence times and volume transports. In the immediate vicinity of the outfall a release rate of 1 Ci d −1 produces a mean annual concentration of 4.2 pCi l −1 (2.6 to 6.3 pCi l −1) with an approximate 30-fold reduction to 0.15 pCi l −1 in the northern exit to the Irish Sea and at least a 300-fold reduction at the southern entrance. The release rate ranged from 60 to 400 Ci d −1 (annual average) during the study period. The main flow patterns are deduced and show a substantial change before and after 1976. Using estimated inventories of the radionuclide in various areas, the rate of input into the system, and a simple exponential model, the residence ‘half time’ of water east of a line south from Scotland through the Isle of Man to Anglesey is estimated to be 200 days. The estimate for the whole northern Irish Sea was 1 year between 1970 and 1976, with a sharp reduction thereafter. The change is confirmed by estimating the volume transport through the North Channel from data from the Stranraer-Larne, Fishguard-Rosslare, and Swansea-Cork ferries. The mean flow through the Irish Sea for the period October 1971 to May 1978 was 5 km 3 d −1 with mean flows of 3.4 km 3 d −1 prior to January 1976 and 7.9 km 3 d −1 thereafter to May 1978. Transit times from the discharge point to the North Channel have also been estimated from the change in concentration of the radionuclides 137 Cs( t 1 2 = 30.1 y) and 134 Cs( t 1 2 = 2.1 y) and from serial correlation of rates of discharge and concentrations of 137Cs at distance.