Water-Quality Trends From Lake Cores

A hydrologic technician collects a core of lake sediment from Lake Lanier, Georgia.  By examining changes in contaminant concentrations from the top of the core (most recent sediment) to the bottom (oldest sediment), contaminant histories can be reconstructed.

Water-quality trends can provide an assessment of the effectiveness of regulatory actions aimed at improving water quality, a warning of water-quality degradation, and an improved understanding of how human activities affect water quality.  

Sediment cores—long tubes of mud—are collected from a lake or reservoir and sliced into thin intervals.  Each slice represents an interval of time. By analyzing the sediments in each slice for the contaminants of interest, changes in the occurrence of contaminants and their use in the watershed can be reconstructed. The approximate date corresponding to deposition of the sediment in each slice is determined by analysis of radionuclides (cesium-137 and lead-210).

USGS studies of reconstructed trends in metals and hydrophobic organic contaminants have shed light on the effectiveness of restrictions on the use of leaded gasoline, DDT, and PCBs, and the effectiveness of the Clean Air Act in reducing concentrations of some heavy metals.  The studies also have identified some contaminants, like polycyclic aromatic hydrocarbons (PAHs), whose concentrations are increasing in urban areas, spurring efforts to identify the source or sources of these upward trends.

Find out more about coal-tar-based sealcoat, a potent source of PAHs to lake and streambed sediment, and related environmental health issues. 

Preparing to cut open a sediment core to release the overlying lake water.

NATIONAL SUMMARIES

• Increased mercury fallout near major U.S. cities (Environ. Pollution, 2012)
• Coal-tar sealcoat largest PAH source to U.S. lakes (Sci. Total Env., 2010) 
• PAH in coal tar sealcoat a national problem (Env. Sci. & Tech., 2009)
• National-scale trends in organics (Env. Sci. & Tech., 2005) 
• National-scale trends in metals (Env. Tox. & Chem., 2006) associated Data Report
• Methods and age dating of cores (USGS SIR 2004-5184)
• Urban trends in PAHs (Env. Sci. & Tech., 2000)
• Rates of decrease in DDT and PCBs (Env. Sci. & Tech., 1998)
• Trends in organochlorine compounds (Env. Sci. & Tech., 1997)

TOPICAL FINDINGS

Lake sediment core sample.  A sediment core can represent from a few to many decades of sediment, depending on the sedimentation rate of the lake or reservoir.

• Evaluating mercury and ²¹⁰Pb atmospheric fallout and focusing to lakes (Env. Sci. & Tech., 2009) 
• Effects of Hurricanes Katrina and Rita on Lake Pontchartrain sediments (Env. Sci. & Tech., 2006)
• Parking lot sealcoat: an urban source of PAHs (Env. Sci. & Tech., 2005) associated fact sheet, data report
• Chemical response to urbanization, New England, USA (Chalmers et al. 2007)
• Contaminant trends in the Mississippi River Basin (Van Metre and Horowitz 2013)
• Reservoir cores versus stream suspended sediments (Env. Sci. & Tech., 2004)
• Monitoring suspended sediment chemistry (Arch. Env. Contam. & Tox., 2003)
• Metal diagenesis in reservoir cores (J. Paleolimnology, 2000)

LOCAL RESULTS

• PAHs decline after ban on coal-tar sealcoat, Austin, TX (Env. Sci. Technol., 2014)
• Chemical response of particle-associated contaminants in aquatic sediments to urbanization in New England (Contam. Hydro., 2007)
• Lead and zinc in greater Atlanta, Georgia (Env. Sci. & Tech., 2000)
• Trends in White Rock Lake, Texas (J. Paleolimnology, 1997)

Sediment cores are sliced into intervals for chemical analysis.  Each slice of sediment represents a "slice" of time.