How big a threat to ground water quality do those sorts of shallow injection wells really pose?
The degree of risk to ground water varies from quite high to very low, depending upon things like the geologic setting, well construction and operation, volume and quality of waste fluids, likelihood of accidental spills, etc. For example, state and local governments permit the installation of approved large septic systems that adequately treat and dispose of sanitary waste fluids underground. However, a floor drain or shop sink in a business that would periodically drain small amounts of commercial or industrial fluids through a septic system could slowly spread contaminants underground and lead to ground water pollution. An accidental, one-time spill into a drain could also cause contamination. Business owners and operators need to consider their liability in these kinds of situations and ensure that their underground discharges do not cause ground water contamination which could be costly to clean-up.
