History of Metal Contamination Recorded in Sacramento-San Joaquin Delta Soil

Sacramento, Calif. – Scientists have traced the history of lead and mercury contamination in tidal wetlands of the Sacramento-San Joaquin Delta, according to a new U.S. Geological Survey article published in Science of the Total Environment.  The study, led by Dr. Judith Drexler, USGS Research Hydrologist, and co-authored by five USGS colleagues, examined lead and mercury concentrations in the region over more than 6,000 years, exploring historical factors that may have contributed to increased contamination levels throughout time.   

Drexler and her team focused their research on two remaining tidal peatlands in the Delta.  These wetlands contain highly organic “peat” soils, which serve as a natural archive of environmental pollution.  The study shows that the Delta was a pristine region for most of its 6,700-year existence; however, since about 1425 CE, it has received both lead and mercury contamination originating from human activities.   

As part of the study, a variety of local sources were examined that may have contributed to the initial uptick in metal pollution at about 1425 CE.  None was a match for the lead isotope composition of the contamination found in the peat.  For this reason, Drexler and her colleagues examined a much larger pool of potential sources, which could have been transported through the atmosphere to the Delta.  The team found strong similarities between the Delta peat samples and some ores from China. This result, together with the fact that atmospheric aerosols from China can be transported to the San Francisco Bay Area in less than two weeks, led Drexler and her team to conclude that smelting in Asia was the likely cause of this first contamination on the West Coast of the United States.

The paper also explores periods of contamination in modern history during which there were major peaks in lead and mercury pollution. For example, the study found the highest levels of lead and mercury pollution during the California Gold Rush period beginning in the 1850s. In addition, the paper tracks the rise and fall of lead and mercury contamination from industrial activities and leaded gasoline during the 20th century. 

“Overall, this study demonstrates that tidal peatlands can be a highly useful archive for establishing the onset of human-caused contamination and chronicling the transition from a pristine to a polluted watershed,” Drexler said.

“A Millennial-Scale Record of Pb and Hg Contamination in Peatlands of the Sacramento-San Joaquin Delta of California, USA” can be read online. To learn more about USGS Water Science, please visit the USGS Water Resources of the United States website or the USGS California Water Science Center website.