3.5. Irrigation water resource impact on NO3-N concentrations in
groundwater
Between 2002 and 2007, we observed a significant difference in
groundwater NO3-N concentrations in wheat–maize rotation fields
when irrigation resource changed from alternative irrigation with
well and sewage water (A) to well water (W) or sewage (S) (Fig. 5).
In 2007, both S and A resulted in an increase in NO3-N
concentrations in shallow groundwater (22.0 mg L1 for S and
7.74 mg L1 for A), whereas W demonstrated a decrease of
6.37 mg L1. This suggests that there was a significant nitrate
contribution from the irrigation water to the groundwater and
supports the common argument that wastewater irrigation has the
potential to accelerate the increase of groundwater nitrate levels,
which is also consistent with the findings of Tang et al. (2004).