WRRCTR No.146 Wastewater Injection Well Problems, Processes, and Standards

Abstract

Wastewater disposal into injection wells almost inevitably results in aquifer clogging and reduced injection capacity. In Hawaii, clogging has produced well overflows resulting in numerous public health, legal, and financial problems. Near-well processes, their relationship to clogging, and technology appropriate under Hawaiian hydrogeologic conditions for dealing with clogging were examined in this study. The first phase involved monitoring of functioning injection well systems on Oahu and pointed out deficiencies in the areas of (1) site selection, (2) well design and construction, (3) injection well testing, (4) effluent quality control, (5) injection well monitoring and maintenance, and (6) injection well redevelopment. Monitoring of existing injection sites furthermore pointed out the need for construction of experimental injection well systems where more detailed studies could be made of injection clogging mechanisms. The second phase involved construction and operation of experimental injection well systems to investigate problems of injection well siting, design, testing, and redevelopment. Results show the need for selection of high initial injection capacity sites, standardization of injection testing to predict maintainable injection capacity, and well redevelopment. The third phase involved examination of near-well clogging processes and their implications for injection well operation and redevelopment. Results show that filtration of suspended solids is not a long-term cause of clogging as is generally cited in the literature. It is probably a short-term cause at the start-up of effluent injection. As injection continues, however, the microbial biomass becomes established and biodegrades the injected organic particulates. During the same period denitrifying bacteria become sufficient1y established to produce significant amounts of nitrogen gas, which in turn produces a gas-bound zone about 0.5 to 1 m out in the injection stratum as revealed by the injection head gradient. Initially, most of the head loss is immediately adjacent to the well, but after several weeks it shifts to a region over ½ m from the well. With continued injection the nitrogen gas-bound zone slowly extends itself farther out into the injection stratum. Superimposed on this, but its effects masked in part by the gas binding, is dissolution of the carbonate porous medium.