Contemporary Flux of Radionuclides to Contaminated Saltmarshes in the Esk Estuary, Cumbria

Abstract

The Esk estuary is approximately 10 km from the marine outfall fromBritish Nuclear Fuels plc (BNFL) Sellafield Site and saltmarshes here havereceived significant quantities of radionuclides as reported in many studiessince 1975. These studies have concentrated on the inventory ofradionuclides in the estuary, but they have not addressed the continualreworking of radionuclides from these deposits. A detailed investigation ofboth the concentration of ¹³⁷Cs in the surface 10 cm and gammaair-kerma dose rates has been made where 120 determinations were made in agrid over 14600 m² of saltmarsh. The surface microtopographyis shown to be important for the continuing deposition of contaminatedsediments to the saltmarsh surface.

This study has concentrated on thedevelopment and the possible application of sediment traps made fromAstroturf (an artificial grass). They were deployed at three sites whichwere representative of the major saltmarsh units in the estuary. The trapswere used to investigate the mobile sediments during a single tide, for aweek, and for a month. The Astroturf provided a reasonable analogue for thesaltmarsh surface and was arranged such that the radionuclide concentrationof the trapped sediment was measured directly by gamma spectrometry.

Sediment deposition rates of between 30 and 240 g m^-2d^-1 were determined for the study sites, and these wereconsistent with earlier studies. Measurement of the radionuclideconcentration of the deposited sediment showed the addition of between 90and 750 Bq ³¹⁷Cs m^-2 d^-1 and 200and 1400 Bq ²⁴¹Am m^-2 d-1. At thedepositional sites over the saltmarsh this would represent an annualaddition of about 90 kBq m^-2 of ¹³⁷Cs and 180kBq m^-2 of ²⁴¹Am.