Examination of near-well clogging processes at two experimental injection sites in Hawaii shows that filtration of suspended solids is not a long-term cause of clogging. While particulate filtration is probably a short-term cause of clogging at the start-up of injection, the injected organics are biodegraded once the microbial biomass becomes established. The injection head gradient determined from monitoring wells shows that most of the initial head loss is immediately adjacent to the well, but that after several weeks it shifts to a region over 0.5 m from the well. Denitrifying bacteria become sufficiently numerous to produce significant amounts of nitrogen gas, which in turn produces a gas-bound zone about 0.5 to 1 m from the well. With continued injection the nitrogen gas-bound zone is slowly extended farther out into the injection stratum. Dissolution of the carbonate aquifer also occurs, but its effects are partially masked by gas binding.