The vertical distributions of C1 in the pore waters of the sediments and in the water column of Onondaga Lake, New York, are documented and used to estimate the diffusion flux of C1 across the sediment-water interface, and establish the reduction in lake concentrations that has occurred as a result of the closure of an industrial source of C1. Despite the industrial enrichment of the water column with C1 over the last 100 yr, gradients within the upper sediments toward the overlying water of 6.9 to 8.5 ร 10 -2 mg cm -3 cm ~ exist in the deep basins of the lake. These upwards gradients are apparently a manifestation of the salt deposits that underlie the region. The estimated diffusion flux from the sediments to the water column, 3.2 3< 106 kg yr 1, represents a small percentage of the external load during the operation of the industry (0.4%) and after its closure (l.6%). The C1 concentration has decreased approximately 70% in response to an approximately equal percent reduction in external loading, as a result of closure of the industry. This supports the position that the lake's concentrations largely reflect external loading and lake flushing associated with surface inputs.