ENVIRONMENTAL HISTORY OF A 13,000 YEAR SEDIMENT RECORD FROM WASHINGTON STATE’S MOST CONTAMINATED LAKE: WAUGHOP LAKE, PIERCE COUNTY, WASHINGTON

Waughop Lake, located near Tacoma, WA, is currently experiencing severe eutrophication and toxic algal blooms that have led to closures of the lake. The lake has high phosphorous levels, and has been designated the most contaminated lake in western WA. Anthropogenic sources of nutrient-loading in the lake over the past century include animal waste disposal, and septic system leakage. Unsuccessful remediation actions in 2008 included treatment with calcium hydroxide and algaecide. To investigate the origins of the current problems and the options for future remediation we collected a 655 cm sediment core from the center of the lake. The aim of our study was to reconstruct the environmental history of the lake over the past 8000+ years, focusing on changes in nutrient loading and productivity, and on atmospheric heavy metal deposition over the past ~150 years. Our core penetrated the 7,700 BP Mt. Mazama ash layer at 420 cm depth, indicating Waughop Lake is well over 8,000 years old. Extrapolated sedimentation rates suggest a basal age close to 13,000 years. We dated the upper 100 cm of the core using ²¹⁰Pb, and calculated a current sedimentation rate of ~6000 g/m²/yr. Concentrations of heavy metals are dramatically higher above 80 cm depth as a result of emissions from the ASARCO smelter beginning ~1900. Average heavy metal concentrations in the upper 80 cm are 5x to 20x higher than background levels (As: 6.08 to 31.8 ppm, Cu: 7.7 to 61.7 ppm, Fe: 880 to 5014.7 ppm, Mn: 39.6 to 169.6 ppm, and Pb: 10.4 to 200.3 ppm). C/N ratios (weight %) of the organic fraction of the sediments is relatively uniform (~12) from 0 to 500 cm depth, with higher C/N ratios in the lowest ~155 cm of the core (14-16) and decreasing C/N ratios in the upper ~80 cm. The change in C/N ratios with depth indicates a relatively higher contribution of terrestrial plant material (C/N>20) versus algae (C/N<10) during the early lake history. Approximately 80 years ago (100 cm) there is a step-wise shift in the δ¹⁵N values of sediment organics, which likely reflects the input of animal waste and/or human sewage to the lake system. While these pollution sources have been attenuated, future remediation efforts may focus on removal of the top 100 cm of nutrient-rich, contaminated lake sediment.