Ages of core sediments were dated using a combination of physical and chemical markers. Pre-reservoir sediment, which was identified in a core by the presence of coarse sand and pebbles, was at a depth of 27 centimeters. Depths of core subsamples were correlated with the 137 Cs content of the subsamples. In the Context of What We Know Concentrations of metals, organochlorine compounds, and PAHs in Lorence Creek Lake sediments change over time and reflect the historical concentrations of those constituents in the water that has entered the lake since its construction. The source of the arsenic during the 1960s is unknown, although a possible source is pesticides containing arsenic.
Organic arsenic is the active ingredient in some pesticides, mostly weed killers.
Mercury can enter the air from the burning of coal and waste and from manufacturing plants. It can enter water and soil at waste-disposal sites. The increasing trend in zinc concentrations probably is associated with increasing urbanization in the watershed upstream of the lake because zinc has many commercial uses. DDT breaks down to DDD and DDE, which also are toxic and resistant to further chemical decomposition. Detection of DDD and DDE in the upper parts of the core indicates that DDT breakdown products are continuing to enter Lorence Creek Lake, probably with soils eroding from the watershed upstream of the lake.