Question

Water in an unconfined aquifer at a pressure of 1 atm and a temperature of 250C has a pH of 7.2 and a dissolved oxygen concentration, (O2)aq, of 0.016 mg/L (1.6 x 10-5 g/L). Several reactions relevant to this groundwater system are listed below with the corresponding equilibrium constants:

A. O2(g) = O2(aq) K = 10-2.89 mol/L-atm

B. ¼ O2(g) + H+ + e- = ½ H2O K = 1020.75

C. ½ NO3- + H+ + e- = ½ NO2- + ½ H2O K = 1014.15

D. H2O = H+ + OH- K = 10-14.0

a) Assuming that the redox system is at equilibrium, what is the pe of the groundwater?

b) For this pe and if [NO3 -] = 1.5 x 10-4, what is [NO2 -]?