Atmospheric deposition of heavy metals to the Severn Estuary

Abstract

The atmospheric inputs to the Severn Estuary of the metals cadmium, copper, chromium, nickel, lead, zinc and aluminium were determined by collecting total deposition samples from eleven sites within and around the estuary. Samples were collected in NILU RS1 total deposition sampling devices biweekly over a period of fifteen months and analysed at Wessex Water Plc's Saltford Laboratory for metal content. The sodium and chloride content of the samples were also analysed for evidence of incursion of sea-spray to the samples (Maritime Effect) and hence recycling of metals. An array of sampling devices was also placed at Northwick Landfill sits to test for this phenomenen. The total metal input to the Severn Estuary was determined by interpolation (using linear, Log10 Log*, and square root methods) of the point deposition data across the water surface by means of isoplething. The area between isopleths was calculated and a mean metal input derived. On the basis of the data transformations the most reliable estimates for metal inputs (kgday*) were: Cd - 0.84, Cu - 8.64-8.68, Cr - 1.93-1.94, Ni - 2.75-2.80, Pb - 62.8, Zn - 96.07 and A1 - 0.31. In comparison to earlier estimates (1978/9) these results show that there has been considerable decline in aerial metal inputs to the Severn Estuary by one to two orders of magnitude. The decline was attributed to improved methodologies as well as a real decrese in metal inputs. Recent reports have also indicated that the overall water quality of the Severn Estuary has improved. The Avonmouth area was identified as an important source area for all metals although significant, secondary sources of Cr and Ni appear to exist in the outer estuary that emanated from South Wales. There was an absence of clear seasonal variation in metal deposition although there was marked fluctuation between sampling periods indicating either variable emission rates or meteorological conditions. The deposition of metal also appeared to be dominated by wet processes. There was no conclusive evidence for the existence of the Maritime Effect although more research is needed into this phenomenen in the Severn Estuary.