<p>Choushui River alluvial fan is an important agricultural area with complicated agricultural cropping patterns in central-western Taiwan. Groundwater has long been regarded as an alternative water source because of the lack of sufficient surface water. Nitrogen loading from intense application of chemical fertilizer has been identified as a major source of non-point source pollution of shallow groundwater in this area. To evaluate the effectiveness of rational fertilization policy promoted by governmental agency on nitrogen pollution control since the beginning of the 21st century, the currently status (averaged from 2018 to 2020) and variation trend of ammonium-N and nitrate-N concentrations recorded from 38 monitoring wells were determined. High level of ammonium-N concentrations appeared not only in double-rice cropping area, but also in rotational cropping area, which dominate the nitrogen pollution in shallow groundwater currently. Mann-Kendall’s trend test revealed that upward trends only exhibited in 4 and 6 wells for ammonium-N and nitrate-N, respectively. In contrast, downward trends occurred in 15 and 25 wells for ammonium-N and nitrate-N. However, the further analysis combined with the Theil-Sen slope estimation and magnitude classification indicating that most of the downward trends should be classified into stable conditions due to the relatively small variation magnitudes. Nevertheless, the promotion of rational fertilization has achieved a preliminary goal for mitigating the nitrogen pollution in shallow aquifer. As the current situation of ammonium-N contamination in shallow groundwater of the study area is still severe, relevant agricultural non-point source pollution control measures should continue to be vigorously promoted for a long time.</p><p>Keywords: Groundwater, Ammonium-N, Nitrate-N, Mann-Kendall’s trend test</p>

The characteristics of nitrogen contamination of shallow groundwater were evaluated through current status analysis and trend detection of ammonium–N and nitrate–N concentrations under various cropping patterns to assess the effectiveness of rational fertilization in the Choushui River alluvial fan, central Western Taiwan. The influence of cropping patterns on both ammonium–N and nitrate–N contamination associated with redox conditions/dissolved oxygen (DO) in shallow groundwater was also discussed in this study. The analysis revealed that shallow groundwater beneath double rice cropping and rotational cropping regions is still characterized by high ammonium–N concentration despite rational fertilization promotion. However, very few monitoring wells showed an upward trend of ammonium–N/nitrate–N concentrations, indicating that shallow groundwater is not further deteriorated by nitrogen pollution in most parts of the study area. Therefore, the remediation of nitrogen contaminated groundwater will be a long-term process and more effort must be invested. Moreover, the strict redox conditions defined by a single DO threshold value may not account for groundwater nitrogen pollution in the study area. It is difficult to determine the redox conditions and predominant nitrogen pollution patterns of shallow groundwater purely from cropping patterns. Instead, contamination may have resulted from an integrated process governed by several other factors. Tracing the potential sources of nitrogen pollution and establishing a more integral monitoring network should be implemented to formulate a more comprehensive nitrogen pollution control strategy in this area.