Assessing the long-term effectiveness of nitrogen management for groundwater protection in the agricultural crop production sector in Wallonia, Belgium.

Current nitrogen management programs within the agricultural crop production sector aim at optimizing crop productivity while minimizing environmental externalities, in particular groundwater contamination with nitrates. However, the effectiveness of these programs has been varied, with many studies indicating mixed or minimal results. Understanding the drivers of nitrate concentration in groundwater and its change is crucial for evaluating nitrogen regulations and guiding policy and management in the agricultural sector. In this context, our study focused on assessing the effectiveness of the sustainable nitrogen management program for agriculture in Wallonia (PGDA), Belgium, on groundwater protection against nitrate contamination. We analysed nitrate concentration time series over the period 2002–2020 from 36 locations across four groundwater bodies within the Walloon nitrate vulnerable zones, situated in the agricultural belt. To capture the extent and dynamics of nitrate pollution, we developed and applied six indicators, providing a detailed view of both the current state and temporal trends of nitrate levels. Additionally, we computed spatially-explicit variables for each monitoring point to describe potential nitrate sources and their migration potential towards groundwater, and we examined their explanatory power in relation to the six nitrate pollution indicators. Our findings indicate a modest overall improvement in average nitrate concentrations post-PGDA implementation. However, a closer examination at the individual site level reveals encouraging trends, with some locations showing pronounced decreased nitrate levels and with a decline in the average rate of change in nitrate concentration in 2020 indicating a slowdown in the rate of increase (or an acceleration in the rate of decrease) compared to 2002. Our results also underscore a complex array of factors influencing nitrate pollution and trends, with land use patterns and aquifer characteristics identified as key determinants. The study suggests that the absence of desired changes in certain areas could be attributed to a time lag between the introduction of regulatory measures and the observable impact on groundwater quality. This research highlights the intricate relationship between environmental regulation, land use, and groundwater quality, emphasizing the need for continued monitoring and adaptive management strategies to effectively address nitrate pollution in groundwater. [ABSTRACT FROM AUTHOR]