A modification of freeze-core technology for collecting granular fluvial sediment samples

The presence of coarse grained sediment can potentially reduce the effectiveness of conventional sampling methods in recovering fluvial sediments. A modification to freeze-core technology was used to collect fine grained sediments for the purpose of characterizing the extent of heavy metal contamination in fluvial deposits that contain significant amounts of coarse-grained material such as cobbles and boulders. This material greatly reduces the effectiveness of conventional sampling methods. This method uses 2.5-cm and 2.9-cm diameter by 30-cm long finned samplers that were mechanically- and hand-driven into the ground. The sediment is frozen to the outside of the sampler by injecting liquid CO2 into the sampler and the fins protect the sample from coarse material upon removal. Field and laboratory testing of the method was completed to establish a methodology and assess possible cross contamination of the sediment layers during the driving of the sampler. The testing indicated that for any sampling location, two offset boreholes with sampling intervals of 0.6 m should be used and samples should be collected from the lowest section of the sampler to reduce vertical cross contamination to within acceptable amounts. The field testing indicated that the optimum injection time for liquid CO2 was equal to or less than 1.5 min to optimize liquid CO 2 usage and sample size. The removal of the sampler from coarse material may ultimately limit sample size when CO2 times are increased. The larger sampler had a much higher success rate in recovering samples over 10 gm with a recovery rate of 80% compared to 52% for the smaller sampler.