A method of in-situ groundwater nitrification, by: extracting groundwater from the ground; adding oxygen to the extracted groundwater; adding nutrients to the extracted groundwater; and injecting the groundwater back into the ground, wherein the oxygen reacts with in-situ nitrifying bacteria to oxidize ammonia to nitrate, and wherein the nutrients encourage the nitrifying bacteria to grow and reproduce. A method of in-situ groundwater de-nitrification, by: extracting the groundwater from the ground; adding carbon to the extracted groundwater; adding nutrients to the extracted groundwater; and injecting the groundwater back into the ground, wherein the carbon reacts with in-situ de-nitrifying bacteria to reduce nitrate to nitrogen gas, and wherein the nutrients encourage the de-nitrifying bacteria to grow and reproduce.