A biogeochemical mass balance of 210Po and 210Pb in an oligotrophic lake with seasonally anoxic hypolimnion

The mass balances of ²¹⁰Po and ²¹⁰Pb were determined for Bickford Pond, Massachusetts. Activities of these nuclides at various depths within the water column, in sediments, in streams flowing into and out of the lake and in precipitation were measured at approximately monthly intervals for a period of one year. Streamwater contained about 10 dpm/100 kg of ²¹⁰Pb, mostly filtrable and showed little variation with changing flow. ²¹⁰Pb in the streams is believed to be derived mostly from ²²²Rn that enters groundwater via alpha-recoil from aquifer matrix materials. It is therefore not meteoric in origin and represents a local source to the lake. Surface inflow was equally important as precipitation as a source of ²¹⁰Pb to the lake. Only half of total ²¹⁰Pb input was trapped in sediments; the rest was lost from the lake as outflow. The ²¹⁰Pb removal times via scavenging and sedimentation both averaged close to 40 days and ²¹⁰Pb scavenging was almost an order of magnitude greater than that of ²¹⁰Po.