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<p>The Taipei Basin, Taiwan has been densely populated and highly economically developed in recent decades. Global climate change has led to frequently flooding and drought events in recent years, formulating suitable measures to mitigate climatic disaster has become a crucial issue in this city. The sponge city concept is one of the most important options for disaster mitigation in highly urbanization areas. However, the city is also potentially threatened by soil liquefaction due to its sedimentary geology and increasing groundwater level. High groundwater level might be a key limiting factor in the promotion of sponge city. The aim of this study was to understand the relationship between rainfall and groundwater level and the impacts of cumulative rainfall, depth to groundwater table, and impervious pavement ratio on the rainfall/groundwater level response in study area. The cross-correlation function (CCF) was applied to analyze the correlation between rainfall and groundwater level data obtained from 20 observed wells and nearby rainfall gages during dry and wet seasons from 2012 to 2017. The significance groundwater recharge response can be found in 61% and 37% of the observation wells during the wet and dry seasons, respectively. Compared with the factors such as cumulative rainfall, and depth to groundwater table, the ratio of surface impervious pavement is the primary affecting factor behind the correlation between rainfall and groundwater level response. The analysis results also show the areas with shallow groundwater level, high imperious pavement ratio, and the groundwater level with no significant response to rainfall, are almost overlapped with the middle and high level liquefaction potential areas in this city. Measures such as the application of the sponge city concept to increase infiltration should be carefully reevaluated in this city. The research results can provide a reference for the future development of urban water resources management and disaster mitigation strategies under the challenge of globe climate change.</p>

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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.

Groundwater nitrate-N pollution mainly originates from surface agricultural activities. Integrating spatial information on nitrate-N observations and agricultural land-use data is crucial for identifying groundwater pollution zones. This study used regression kriging (RK) to determine groundwater pollution zones in the Choushui River alluvial fan in Taiwan according to nitrate-N observations and agricultural land uses. Areal ratios of agricultural land-use types within buffering zones were first characterized using geographical information systems. A multivariate linear regression (MLR) model was employed to explore the relationship between groundwater nitrate-N pollution and agricultural land-use types. Then, simple kriging (SK) was adopted to analyze residuals obtained from gaps between nitrate-N observations and MLR predictions; the SK estimates of the residuals with the addition of the MLR predictions served as the RK estimates for groundwater nitrate-N pollution. Finally, groundwater pollution zones were determined according to a specific anthropogenic nitrate-N pollution level. The study results revealed that the “orchard” land-use type positively contributed to groundwater nitrate-N in contract to the “livestock house” and “agricultural facility” land-use types, which were negatively related to groundwater nitrate-N. Moreover, the RK estimates had the ability to characterize the potential pollution source of the orchard land-use type and were suitable for identifying groundwater pollution zones. Therefore, the amount of fertilizer used in the orchards located in groundwater pollution zones must be reduced considerably.