During a 5-month field test, active remediation of a benzene, toluene, ethylbenzene, xylene (BTEX)-contaminated aquifer was initiated by injecting water with varying amounts of KNO3. The experiment was performed prior to selecting bioremediation for full-scale cleanup, particularly to evaluate the competing reaction of nitrate with hydrocarbons and reduced sulfur components. The nitrate oxidized sulfides that had precipitated earlier as a result of the natural degradation of BTEX with SO42- from groundwater. When the sulfides were exhausted, BTEX degradation was enhanced by nitrate. A hydrogeochemical model with kinetic oxidation reactions for Fe(II), FeS and BTEX by nitrate was developed to calculate the observed concentration patterns along a flow line in the aquifer. The rates for the kinetic model were based on published kinetic reaction equations for oxidation with oxygen. Nitrate was introduced in the equations in the same form as oxygen, with a premultiplier added to fit the observed concentration changes in the aquifer. The oxidation of Fe(II) with nitrate in the aquifer was 4 times slower than the abiological oxidation reaction with oxygen in water. Similar rates were found for oxidation of FeS with nitrate as for FeS2 with oxygen, but the specific surface area of FeS in the aquifer was larger. The reaction rate for degradation of BTEX compounds was about 107 times faster than for natural organic matter. BTEX release from pools in the aquifer was modeled with a linear driving force equation in which the pollutant/water interfacial area was linked to the mass of BTEX. The release rate and the denitrification rate were used to calculate the initial amounts of BTEX at the start of the KNO3 injection. This study shows that an assessment of the efficiency of nitrate addition for stimulating bioremediation has to consider possible reactions of nitrate with reduced sulfur components and ferrous iron.