Using Size Fractionation and Pb Isotopes to Study Pb Transport in the Waters of an Organic-Rich Upland Catchment

Processes controlling Pb release from a small organic-rich upland catchment in northeast Scotland were investigated via measurement of Pb concentrations and ²⁰⁶Pb/²⁰⁷Pb ratios in rainwater, throughflow, surface flow, and receiving streamwaters under storm and baseflow conditions. For this catchment, the output of Pb via streams was only 2.0 ± 1.2 kg year^-1 (11.4 ± 6.8 g ha^-1 year^-1), much lower than the input of 7.5 ± 2.0 kg year^-1 (42.6 ± 11.4 g ha^-1 year^-1), and so the catchment is still a sink for anthropogenic Pb. Most (68−87%) of the output, however, occurred under storm conditions. Size fractionation revealed that 50−60% was in large particulate form (>25 μm) with a ²⁰⁶Pb/²⁰⁷Pb ratio of ∼1.16, similar to that of the surface soils. Some 30−40% of the storm Pb output was associated with dissolved organic matter in the <0.45 μm fraction and had a lower ²⁰⁶Pb/²⁰⁷Pb ratio of ∼1.14, close to the value obtained for near-surface throughflow. Future extreme weather conditions such as prolonged dry or wet periods will increase transport of Pb to receiving waters. Although particulate forms could then rapidly be removed under low flow conditions, Pb associated with dissolved organic matter will persist longer in aquatic systems and may also be more bioavailable.