Use of hydroacoustic measurements to characterize bottom sediments and guide sampling and remediation of organic contaminants in lake sediments.

Sampling of bed sediment for contamination characterization is often limited by the heterogeneity in sediment properties and distribution. In this study, we explored the use of hydroacoustic measurements to characterize sediment properties and guide sediment sampling in a small lake contaminated by organochlorine pesticides (OCPs) and PCBs. A dual frequency hydroacoustic survey was conducted to characterize sediment properties, distribution, and thickness in McGrath Lake, near Ventura, CA. Based upon these results, sediment core samples were collected from 15 sites on the lake, and sectioned into 20 cm intervals for sediment characterization and analysis of OCPs and PCBs. Very high concentrations of total DDT and total chlordane were found in the sediments, with mean values of 919 and 34.9 ng g(-1), respectively. Concentrations of OCPs were highest at 60-80 cm depth near the inflow at the north end of the lake. Total PCB concentrations were much lower (mean concentration of 4.5 ng g(-1)). Using the hydroacoustic and chemical data, it was estimated that nearly 30,000 m(3) of DDT- and chlordane-contaminated sediment (above effects range median values) was present in the uppermost 1.2 m of sediment in the lake. A hydroacoustic survey can be a valuable tool used to delineate sediment distribution in a lake, identify areas with deeper organic sediment where hydrophobic contaminants would likely be found, and guide sampling. Sampling and chemical analyses are nonetheless needed to quantify contaminant levels in bottom sediments. When combined with hydroacoustic measurements, this approach can reasonably estimate the distributions and volumes of contaminated sediment important in the development of remediation strategies.

Characterization of bottom sediments in lakes using hydroacoustic methods and comparison with laboratory measurements.

The acoustical properties of bottom sediments in two lakes were shown to be strongly correlated with clay content, organic C and total N concentrations, and other important sediment properties. The fractal dimension of the bottom echo was more strongly correlated with sediment physical and chemical properties than energy-based measures. The fractal dimension was also related to rates of PO4-P and NH4-N release from intact sediment cores and sediment oxygen demand. Measurements made at 430-kHz were more sensitive to differences in sediment properties than 201- or 38-kHz. Hydroacoustic measurements allow rapid assessment of properties important in lake restoration and water resource management.