Background/Objectives: Waterway sediment at the Berry's Creek Study Area (BCSA) consists of recent estuarine deposits overlying discontinuous Holocene Epoch alluvial deposits and varved clay, which represents depositional sequences of glacial Lake Hackensack of the Pleistocene Epoch.At the BCSA, remedial investigation data suggest that the vast majority of COPC mass is present in the estuarine deposits (generally identified as "soft sediment"). As such, the volume of soft sediment present throughout the system is of interest to both site characterization and the evaluation of potential remedial options.
Geologically-influenced topographical variability in the Pleistocene sediment surface in turn influences the thickness of overlying estuarine sediment. Local and regional variations in soft sediment thickness lead to variations in soft sediment volume; thus, an accurate, precise, spatially-robust dataset of soft sediment thickness measurements was needed to support RI/FS analyses.
Approach/Activities: Soft sediment is easily penetrated by coring and probing devices, including those advanced by hand, distinguishing it from the more consolidated, underlying Holocene or Pleistocene deposits. Using a re-purposed Geoprobe® Large-Bore Soil Sampler, the soft sediment thickness was measured in primary waterways, tributaries, and on mudflats throughout the BCSA (the "SPR" method). Probes were advanced through the soft sediment to refusal, considered representative of a transition in sediment material from soft sediment to underlying Holocene or Pleistocene deposits. The sampler also allowed for the collection of material from the refusal depth, allowing for the positive confirmation of a transition in sediment material and thus further supporting the accuracy of the thickness measurement.
SPR measurements were compared to alternative thickness measurements derived as secondary data from other BCSA programs, including sub-bottom profiling (SBP), SBP calibration cores, sediment characterization cores, and sediment probing previously performed for different studies. Comparisons focused on co-locations or near-co-locations between SPR locations and alternative methods, with pairings based on distance and sediment morphologies.
Results/Lessons Learned: The development of a method specifically designed to measure soft sediment thicknesses allowed for the cost-effective collection of a dataset sufficient to delineate longitudinal trends and calculate volume estimates (over 360 discrete locations), while also achieving sufficient precision to characterize complex and unexpected profiles. The method allowed for in-field verification and recovered underlying deposits exhibiting textures distinct from those of the overlying soft sediment. SPR thicknesses compared favorably to soft sediment interpretations in COPC cores. SPR thicknesses differed from those of SBP, SBP calibration cores, and previous probing. Differences in the comparisons were explained by identifying differences in data quality objectives or test methods that would not affect the alternative source's primary objective. These comparative analyses further supported the decision to utilize the SPR data as the preferred source of soft sediment thickness data for the RI/FS.