HIGH RESOLUTION VERTICAL PROFILING OF CONTAMINANTS TO CHARACTERIZE A COMPLEX TRIASSIC SEDIMENTARY HYDROGEOLOGIC SYSTEM

A large, former state-owned East German chemical park, with production dating back to 1937, has contaminated the subsurface. These contaminants migrate through a variably lithified sequence of fractured, Triassic (~248-250 Ma), interbedded clay/shale and sand/sandstone of fluvio-lacustrine origin. Conventional characterization techniques, applied on site since the 1980s, have indicated that contamination by volatile organic compounds (VOCs) exists in the source zone as DNAPL and in a dissolved phase plume stretching hundreds of metres down gradient. A suite of high resolution core and borehole methods, known as The Discrete Fracture Network (DFN) Field Approach, was applied at two cored holes: UoG1, 42.5 m deep, in the source zone; and UoG2, 68 m deep, 500 m down-gradient.

The lithologies and fractures observed in each core were logged in detail, indicating strong small-scale heterogeneity caused by alternating layers of sand, silt and clay, with varying degrees of cementation. The strategy then involved sampling at a fine scale to investigate the effect of the variability on the distribution of contaminants. In total, 338 rock samples were taken from the two cores for VOC analysis. In addition to standard down-hole geophysical logs, innovative logs such as FLUTe™ transmissivity profiles and Active Line Source (ALS) temperature logs in FLUTe™ liners were collected at each location. Together, the data were used to design FLUTe™ multilevel systems (MLS), allowing for groundwater quality sampling and head monitoring in depth-discrete intervals.

The contaminants in the system act as a tracer of past processes, allowing us to generate a process-based hydrogeological site conceptual model. Vertical profiles of the rock core VOC concentrations, sampled at a resolution high enough to address the strong vertical heterogeneity, identified small-scale contaminant migration units at a scale that would have been missed by multilevel systems normally considered high-resolution. Though the vertical scale of MLS monitoring is coarser than the scale of the contaminant migration units, the shapes of hydraulic head profiles helped guide as to which units exhibit low vertical bulk hydraulic conductivity (K) and act as aquitards.