University of California Water Resources Center

The purpose of this research was to assess the effect of motorboat usage on the long-term lead concentration of a multipurpose reservoir, Turlock Lake in Stanislaus County, California. As the study was made during two drought years. it was possible to examine the effects of large volume changes in the lake. Samples were taken at weekly intervals at the inlet canal where water enters the lake, at the outlet and at the boat dock, then lead analyses were made.

An apparent correlation was found to exist between the lead level at the boat dock and the boats per-unit-volume. Also, at the boat dock, the largest lead concentration correlated with the highest boat concentration. The inlet and outlet lead data obtained were treated in terms of a simple plug-flow model. In order to correlate between the change in lead concentration obtained from the plug-flow model and boat concentration, a calculation was made to estimate the number of boats that would have to be launched on the lake to give the observed value of ~(Pb). It was found the increase in the lead level of the water as it passes through the lake is greater, by at least a factor of 15, than would be expected on the basis of the number of boats on the lake.

The lead concentration was found to correlate with lake volume. Experiments were done to find if an alternate source of lead in the lake is the sediment. The mechanism linking lake volume and lead concentration is unclear but is likely to involve migration of lead from the sediment, which was found to contain exchangeable lead, and/or biotic processes.