Exercise 2
The Long Point septic system plume (Figure 2) has NO3––N of about 50 mg/L, but this nitrate is completely attenuated at a reaction front located about 20 m downgradient of the drainfield. The aquifer contains trace amounts of reduced S (0.02 percent by weight) reflecting the presence of biogenic pyrite (Aravena and Robertson, 1998), and this material contributes to autotrophic denitrification (Equation 14).
The average linear groundwater velocity in this plume zone has been estimated to be 28 m/yr (Robertson, 2008). Assuming that all of the NO3– loss is attributable to autotrophic denitrification utilizing the reduced sulfur compounds, how fast will the nitrate plume advance, based on the rate of S consumption? The aquifer has a dry bulk density of 1.8 g/cm3 and saturated porosity of 0.3.