Bromine and bromide in rainfall, cloud, stream and groundwater in the Plynlimon area of mid-Wales

Abstract. Bromine in rainfall, cloud water, stream water and groundwater were measured in the upper River Severn catchments at Plynlimon in mid-Wales using two analytical procedures for bromide determination: a colorimetric method and, later, ion chromatography. Discrepancies between the methods indicated that the colorimetric method was measuring more than just bromide so it was "dissolved labile bromine" (DLBr) (Neal et al., 2007). This paper reports the variation in DLBr and bromide across the Plynlimon region and assesses the differences between these two measures. The average DLBr concentration in the streams ranges between 18.3 and 27.8 µg l⁻¹ compared with a rainfall average concentration of 15.6 µg l⁻¹. The corresponding bromide concentrations in the streams are 13.8–18.6 µg l⁻¹ and 13.2 µg l⁻¹ in rainfall. For cloud water, throughfall and stemflow, DLBr concentrations are an order of magnitude higher than in rainfall and the concentrations are strongly correlated with chloride. Average values of bromide in rainfall and cloud water are similar to those of DLBr and are strongly correlated with chloride: unfortunately, no bromide measurements were taken in throughfall and stemflow. These results indicate that marine aerosol inputs are high and that the DLBr in these inputs is dominated by bromide. For cloud water, DLBr was correlated with dissolved organic carbon (DOC) as well as with chloride; hence, there may well be an organic-associated component. Within the streams draining the forested areas, DLBr increases with time as DOC increases. DLBr concentrations in the streams are correlated with DOC in most cases and, to a lesser degree, with chloride. However, for bromide there is no correlation with DOC, but a weak correlation with chloride remains and the gradient is higher than for the DLBr case. Increasing concentrations of both DLBr and DOC may well reflect the wetting up of the catchments as transpiration diminshes as the trees age as well as through felling. In groundwater, only DLBr has been measured, and its concentration is similar to that for the streams. However, in the groundwater zone, DLBr is poorly correlated with DOC concentrations, in which DOC is largely converted to CO₂. Atmospheric input–stream output flux estimates indicate a net uptake of bromide by the catchment of around 30%, matched by a corresponding release of DOC-associated bromine.