“Fracking” and Ground Water Contamination

Many are worried that the chemicals in the fluids can find their way back to the surface, especially to surface water aquifers. In the Osborn et al. (2011) study, they show that contamination is a problem, but it occurs close to the surface around well bores. This most probably indicates defective casing or cement in the well. If the contamination was related to fracturing, one would not see this correlation since factures are deep underground and reach out a mile or more away from the well.

The problem with focusing on fracturing rather than the integrity of casing is that it ignores what does go wrong and can be fixed. To be clear, hydraulic fracturing is a procedure conducted on some wells after drilling to fracture the reservoir rock to create permeable flow paths for oil or gas to reach the well-bore. The fractures do not extend to the surface. They are confined to within a few hundred feet of the zone of interest, which is thousands of feet below the surface and below any potable aquifers. Indeed, if a fracture were to extend upward from deep reservoirs, at about 3000 feet of depth, it would turn horizontal, due to the dominant stress in the rock.

Even with this clarification, it may be helpful to establish two particular regulations that would likely improve confidence about safety. First, set casing and cement inspection standards to ensure they are of good quality throughout the active life of the well. Research is needed to develop the best practices and the industry is addressing these issues, but this new regulation would inevitably accelerate that process. Second, set a minimum depth above which fracturing is highly restricted. Wells should probably be 4000 feet deep before fracturing is initiated. Regulations should relate the required depth to the size and pressures of a frac job, details that must be informed in consultation with fracture design experts.

Studies have extensively mapped the extent of fracture growth in both The Barnett Shale in Texas and in the Marcellus Shale in the Appalachian Basin. The Marcellus (which is a focal point of much of the controversy about frac jobs) does show higher fracture growth than the Barnett, but do not show fractures growing to shallower depths than 4500 feet. The deepest freshwater aquifers are generally less than 1000 feet deep. This means that the top of the most extreme fracture growths are still separated from potable aquifers by thousands of feet of rock: generally two-thirds of a mile (Fisher 2012). There is no likelihood of any significant amount of frac water traveling up to contaminate surface waters.

Although huge quantities of water are used to comprise the frac fluid, the relative proportions of potentially contaminating chemicals are quite small. The composition of frac fluids is typically 98—99.5% water. The total chemicals used are extremely diluted once mixed into the overall frac fluid. Of these chemicals, the largest fraction is generally hydrochloric acid (HCl). Of course, pure HCl is quite dangerous if humans are exposed to it. By the time it is mixed in frac fluid, though, its concentration is reduced to about one-eighth of 1 percent. The natural concentration of HCl in your stomach is about 4 times greater. Furthermore, the acid that is in the fluid at least in part will react with and dissolve carbonate material in the rock, which helps neutralize the acid (US DOE 2009).

The various other chemicals are mixed at lower concentrations. Most of them can be found in much higher concentrations in household products, which are spilled on the ground by someone somewhere every day. Even if a portion of the frac fluid somehow found a conduit to the surface (such as an induced fracture intersecting a naturally existing fracture or fault line) and enter the ground water, the frac fluid would be further diluted by the ground water. No contamination of drinking water sources with alien chemicals is desirable, but the impact on water quality would be extremely low.

In assessing risk, the most accepted practice is to multiply the severity of an outcome by the likelihood of that outcome. Therefore, since the severity is low (the chemicals are heavily diluted in the original fluid, which is further diluted by the water source it would enter, and some of the prominent chemicals will react with rock materials and be neutralized) and the likelihood has been demonstrated to be exceedingly low, the risk to human safety from the frac jobs is really too low to measure reasonably. The impact of highly diluted and neutralized chemicals times a likelihood that is clearly less than 1 in 10,000 is exceedingly small. The real risk of cleaning your bathtub, or spraying for ants, or painting your house is much higher. The chemicals are in much higher concentrations and the likelihood of an accidental spill contaminating something that you ingest is probably higher.

Of course, it is also possible for chemicals to be spilled during the transport and mixing of frac fluids. This is a much more serious risk factor. If the pure chemicals are spilled before mixing, they may pose real danger, because they are in thousands of times higher concentrations. It is also more conceivable that fluid spilled on the surface would run off into streams or percolate through the soil and surface rocks to contaminate aquifers. It is, then, very important to take multiple layers of precaution to ensure against chemical spills on the surface. Most oil and gas companies are indeed using multiple layers of containment to prevent any fluid spilled from running off or contaminating ground water. The ground is covered with rubber mats or tarps and dikes are built around the entire well site and around each area where chemicals are being handled.

The need for utmost caution in handling chemicals is true for all large-scale activities that use potentially dangerous chemicals (and almost all chemicals are potentially dangerous in high concentrations). Lots of chemicals are used in the manufacture of photovoltaic cells, and they will be used, transported, and stored in vast quantities if solar cell production takes off at the rate that many people hope. It may well be that this chemical intensive process will result in more chemical spillage than frac jobs. It is also true for all large-scale activities that, regardless how many precautions are taken, some accidents will happen and some chemicals will be spilled. We must seek to engineer carefully to mitigate against spillage.

 
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