Increasing interest in the use of the deep subsurface for disposal of industrial waste requires that both the practitioner and the governmental regulatory body be assured that injection is not harming the environment.

There are three principal areas of interest in monitoring subsurface injection systems: (1) the well, (2) the surface equipment, and (3) the subsurface.

Minimum monitoring function for the well requires measurement of wellhead injection pressure and of injection tube-casing annulus pressure; definition of corrosive effects of the waste on the well materials; in some cases, bottomhole monitoring of injection pressure; and the location of a conductor-insulator interface.

Monitoring of the surface equipment should include records of the injection-pump discharge pressure, the rate and cumulative measurement of injected volume, injecta temperature and quality, and the corrosive-erosive effects of the injected stream upon the materials of construction.

Because the real purpose of the monitoring process is to establish that the waste is going where it is intended to go--and remaining there--an examination of the subsurface takes on special importance. The requirements will vary depending on the geographic location, the properties of the waste, the subsurface geology, and the design and construction of the disposal well itself. An occasional monitoring requirement is the drilling of one or more wells to the disposal formation to obtain pressure data and, perhaps, fluid samples. Although there is some purpose for monitor wells of this type where relatively shallow formations are used for disposal, the use of such wells to obtain measurements in deep aquifers may not serve a purpose commensurate with the expense and possible hazards tha may result.