The researchers studied how treated sewage effluent affects nitrogen levels in a desert stream. They found that nitrification increases downstream, with nitrate levels peaking at specific distances. Ammonia and total nitrogen levels decrease as the effluent flows downstream. The rate of nitrification is influenced by the distance the effluent travels and the stream's physical characteristics. The effluent released into the stream may contribute to high nitrate levels in the groundwater near Tucson.