Harmful contaminants such as chromium (Cr+6), and TCE carl be removed from groundwater by reactions with reduced subsurface sediments. Establishing an in situ Fe(II) barrier through the reduction of soil-bound Fe(III) to Fe(II) by injecting a sodium dithionite (Na2S2O4) solution is studied. Critical to this problem is the possible formation and expansion of a zone around die injection, where all the soil-bound Fe(III) is reduced to Fe(II). Different reaction models apply inside and outside of this zone so that a determination of this moving boundary is a fundamental part of the solution. The complete analytic solution to this problem was used to develop optimal process parameters, such as injection rate and operational time, that maximize the radius of the Fe(III)-reduced zone when a given mass of sodium dithionite is injected at a well. When a large reduction [>63% of initially, present Fe(III)] is desired, the results indicate that it is better to use a low flow rate to form a Fe(III)-free zone around the injection. The opposite is true for smaller reductions (<63%), so that a faster injection rate that avoids the formation of the Fc(III)-free zone yields a larger reduction zone.