Water-Quality Trends From Lake Cores

Science Center Objects

Sediment cores let us look back in time at the contaminant history of a watershed. Learn about what lake and reservoir sediment cores tell us about trends in metals, organochlorine pesticides, polycyclic aromatic hydrocarbons, and other sediment-related contaminants.

Lake coring in Lake Lanier, Georgia

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. 

Examining sediment core

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



Lake sediment core sample

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.


Processing sediment core samples

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