The History Of Human Impacts In The Clear Lake Watershed (California) As Deduced From Lake Sediment Cores *

120We have raised sediment cores to investigate multiple stresses on Clear Lake, CA over the past 250 years. Earlier work suggested the hypothesis that the use of heavy earthmoving equipment was responsible for erosion, mercury, and habitat loss stresses. Such stresses would have first become significant about 1925 to 1930. The cores are about 2.5 m long and span 200 to 300 years of the lake’s history. We present the results for our, as yet, most thoroughly analyzed core. ²¹⁰Pb dating yields an estimated 1.2 cm yr ¹ average sedimentation rate for this core. Total (primarily inorganic) mercury and a number of other parameters were measured at 5-cm intervals down the core. Nearly all parameters show major changes at depths of 75 to 80 cm, corresponding to an estimated date of 1927. Organic matter, total carbon, water content, and total nitrogen all show significant decreases above this depth. A peak in inorganic deposition rate and minimum values for percent water is present at a depth corresponding to about 1971. Inorganic mercury concentrations show major increases in concentration (roughly tenfold) above the 1927 horizon. There is also a smaller uptick in total mercury at 145 to 150 cm deep in the core. This horizon is beyond ²¹⁰Pb dating capabilities, but most likely represents the early episodes of mercury mining which started in 1873 at the Sulphur Bank Mercury Mine located on the lake’s eastern shore. Peak total mercury levels occur at an estimated date of 196I (last mining was in I957), and a modest decline has occurred since. Interestingly, the first 75 years of European settlement in the Clear Lake basin (including the most productive years of Sulphur Bank Mercury Mine) appeared to have had barely detectable effects on core properties despite considerable presence after the 1870s. Changes since 1925 are much more dramatic. The large increase in mercury beginning about 1927 corresponds to the use of heavy equipment to exploit the ore deposit at Sulphur Bank Mine with open pit methods. The increase in inorganic sediment load during the last 75 years is substantial in this core, but is not replicated in other cores. Increases in sulfate and/or acidity loading from the mine may be responsible for the dramatic changes seen in the upper 75 to 80 cm of the core.