Dissolved iodine in rainfall, cloud, stream and groundwater in the Plynlimon area of mid-Wales

Abstract. Iodine concentrations in rainfall, cloud water, throughfall, stemflow, stream water and groundwater are described for the upper River Severn Plynlimon catchments in mid-Wales. Iodine concentrations average 1.55 µg l⁻¹ in rainfall while the corresponding averages of iodine concentrations in cloud water, throughfall and stemflow iodine concentrations are three to eight times higher. For the atmospheric inputs, there are both marine and anthropogenic sources while, for stemflow and throughfall, humic substances may be a dominant component. The average concentrations in the streams and groundwaters vary between 1.00 to 4.22 µg l⁻¹ and 0.57 to 8.51 µg l⁻¹, respectively. The higher averages are for waters associated with gley soils that are high in dissolved organic carbon (DOC). For the streams draining forested catchments, iodine concentrations have increased over time, probably due to wetting up of the soils because of reduction in transpiration as the trees are maturing and felling activities. The increases vary between 0.025±0.003 and 0.044±0.010 µg l⁻¹ per year and are associated partly with an increase in DOC concentration: adjusted for the effect of DOC, the increases range between 0.013±0.003 and 0.023±0.009 µg l⁻¹ per year. For the moorland areas, no such trend is observed but the value is lower. For most of the Plynlimon catchments, a net uptake of iodine of around 40% is lower for streams draining gley soils with even a net export for the smaller streams. Within the streams, iodine concentrations are highly correlated with DOC and there is an annual cycle of iodine and DOC concentration change, with maxima between July and November. For the groundwaters, a loss of iodine and DOC relative to the streams probably reflects within catchment losses in the lower inorganic soils and bedrock areas. However, in one case, groundwater iodine concentrations were relatively high and this coincided with high DOC concentrations.