Chemical studies of chloride and stable oxygen isotopes in two conifer afforested and moorland sites in the British uplands

Abstract

Chloride concentration variations for three streams in the Crinan Canal region of southwest Scotland, draining heather ( Calluna vulgaris) and immature and mature sitka spruce conifer forest ( Picea sitchensis (Bong.) Carr) are compared with incident rainfall values. Results show the importance, and correspondence, of in-catchment processes for attenuating atmospheric inputs of chloride before its entry to the stream. All three streams exhibit a damped chloride concentration variation compared with rainfall. The chloride chemistries for the three streams are intercorrelated. The weekly data show a net accumulation of chloride into the catchment over the 1 year sampling period. This is associated with an inappropriate stream sampling frequency and/or large yearly fluctuations, in the rainfall input; 80% of the rainfall chloride contribution was introduced during four of the 52 weeks of sampling. The yearly flux of chloride leaving the catchment as stream flow is remarkably similar for all three catchments. The study highlights the need for long and detailed data records from which information can be obtained on chemical budgets and the deficiencies in the measurement of atmospheric inputs of chloride. Although mature conifers are known to capture significant inputs of chemicals to a catchment, not measured by a standard rainfall collector in uplands areas, analogous results are not inferred in the lowland Crinal Canal case. Far more detailed studies are required if a definitive statement is to be made. The chloride results from the Crinan Canal Study are compared with analogous data for streams draining the Hafren Forest at Plynlimon in mid-Wales. Similar degrees of damping are observed and seasonal oscillations occur which do not have the same phase or amplitude. In addition, oxygen isotopic data are presented for the Plynlimon case to highlight a higher degree of damping than for chloride.