The small difference observed in the groundwater NO3-N
concentrations between S and A in 2002 (Fig. 5) is noteworthy. This
may be caused by the improving in sewage composition over the
last 5 years. The increase in hazardous compounds in sewage was
accompanied by the reduction in wheat output that we observed in
our investigation in 2007. The NO3-N concentration in the
groundwater significantly increased due to the combination of
the two factors: (1) the nitrate uptake by wheat decreased with the
reduction of wheat crops, as shown above; and simultaneously (2)
more nitrate was introduced through sewage irrigation.