Bedrock fracturing

The clay-based grouting technique uses clay slurries as a base for grout solutions. These solutions are injected into bedrock fracture systems to inhibit or eliminate groundwater flow through these pathways. The clay slurries may also be used as a base for slurry wall construction. 

The results of the multiple nonreactive tracer study showed that secondary contaminant sources may form within the bedrock matrix, and that the importance of this source increases with continued contaminant discharge through the bedrock fracture network. This may be particularly important for reactive contaminants such as radionuclides, where matrix diffusion can enhance solute retardation by many orders of magnitude. 

DNAPL located at the bottom of an aquifer serves as a source of pollution to groundwater flowing past. Considering the low solubility of many chlorinated solvents and the unacceptability of their presence at concentrations higher than a few parts per billion, it is apparent that even a relatively small volume of DNAPL can contaminate an immense volume of groundwater unfortunately, centuries or even millennia may be required for the ultimate removal of DNAPL from an aquifer under natural conditions. DNAPL also commonly will enter channels, fractures, holes, and cracks in an underlying aquitard or aquiclude, thereby sinking even farther (see Fig. 3-26). No satisfactory technique has yet been developed to locate DNAPL in bedrock fractures, much less to predict its movement. 

Plutonium is transported by the groundwater in fractures in the rock (usually <1 mm wide). A typical groundwater velocity (vw) at >100 m depth in Swedish bedrock is 0.1 tn/y. The fractures are filled with crushed, weathered, clayish minerals, which have a high capacity to sorb the plutonium. Assuming instantaneous and reversible reactions, the sorption will cause the plutonium to move considerably slower (with velocity vn) than the groundwater. The ratio between these two velocities is referred to as the retention factor (RF), defined by... 

Two assumptions about the surface have been made to determine the effect of natural attenuation on the contaminated groundwater. First, despite the fractured nature of the bedrock, it has been assumed that the subsurface is homogeneous so as to facilitate the evaluation. Second, the potential for reduction in TCE concentrations has been assessed using a hydrogeologic model in which the fact that the cap would reduce existing leachate production by 75% is taken into account. This model is assumed to predict that the concentration of TCE in the groundwater would be reduced to an excess cancer risk level of 28 pg/L in 60 yr and an excess cancer risk level of 5 pg/L, approximately equal to the MCL, in approximately 100 yr. 

Hydraulic conductivity is one of the characteristic properties of a soil relating to water flow. The movement of water in soil depends on the soil structure, in particular its porosity and pore size distribution. A soil containing more void space usually has a higher permeability. Most consolidated bedrocks are low in permeability. However, rock fractures could create a path for water movement. 

Above the water table, groundwater can also occur in perched aquifer conditions. In these instances, groundwater occurs in relatively permeable soil that is suspended over a relatively low permeability layer of limited lateral extent and thickness at some elevation above the water table. Perched groundwater occurrences are common within the vadose zone high-permeability zones overlie low-permeability zones of limited lateral extent in unconsolidated deposits. However, perched conditions can also occur within low-permeability units overlying zones of higher permeability in both unconsolidated and consolidated deposits. In the latter case, for example, a siltstone or clay stone overlies jointed and fractured bedrock such that groundwater presence reflects the inability of the water to drain at a rate that exceeds replenishment from above. 

The results of indirect and direct investigations were constantly validated against one another and reinterpreted where necessary on the basis of reciprocal results, and field / laboratory data, regarding both information in the sub-surface (e.g., state and degree of fracturing of bedrock, lithological variations, groundwater table level, salinity, seawater intrusion) and its contamination (e.g., presence of LNAPL). 

Lipfert, G., Reeve, A.S., Sidle, W.C. and Marvinney, R. (2006) Geochemical patterns of arsenic-enriched ground water in fractured, crystalline bedrock, Northport, Maine, USA. Applied Geochemistry, 21(3), 528-45. 

We should keep in mind that the Hyde Park landfill site, sitting in fractured bedrock only 0.6 km from the Niagara River gorge, is widely reported to contain in the order of 1000 kg of 2378-TeCDD, although an authoritative confirmation of this information was not found. If true, this is a great deal more than has already been flushed into Lake Ontario, considering that 2378-TeCDD is a small percentage of the inventory of 5800 kg total PCDD/Fs in Lake Ontario sediments. 

Lipfert, G., Sidle, W. C., Reeve, A. S., Ayuso, R. A., and Boyce, A. (2007). High arsenic concentrations and enriched sulfur and oxygen isotopes in a fractured-bedrock ground-water system. Chem. Geol. 242, 385-399. 

Hydrological and Geochemical Processes Controlling the Fate and Transport of Contaminants in Fractured Bedrock... 

FIGURE 1.3 (See color insert) Pictorial example of bedrock core obtained near the experimental site showing the interbedded fractured shale (black) and limestone (white/gray) that dominates the saturated zone. 

Tracer breakthrough profiles within the matrix regime confirmed that the dissolved gases and Br were slowly moving into the bedrock matrix and at different rates. Concentration profiles of the three tracers 6 m from the source and 0.8 m into the matrix relative to the fracture zone are shown in Figure 1.5(a). The movement of He and Ne into and from the matrix was more rapid than Br, which is consistent with the larger molecular diffusion coefficients for the dissolved gases relative to Br. These results support the notion that matrix diffusion contributed to the overall physical nonequilibrium process that controls solute transport in bedrock at this site (Maloszewski and Zuber, 1990, 1993). 

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