Regional differentiation of non-point source pollution of agriculture-derived nitrate nitrogen in groundwater in northern China

Nitrate is one of the major agriculture-derived pollutants leaching to groundwater. This paper discusses the contamination process and spatial distribution of nitrate-N concentrations in groundwater in a typical high-yielding area of northern China. On-site monitoring of nitrate-N concentrations in groundwater from two separated regions that had been managed similarly indicated different trends of nitrate-N elevation during the past 4 years: one remained relatively stable while the other one significantly increased. Statistical analyses show that nitrate-N concentrations in groundwater were significantly correlated with the sampling depth of 60 m and deeper and nitrate had diffused to the depth of 150-200 m. Results of surface interpolation analyses based on a large number of groundwater samples indicate that the distribution and pollution patterns of nitrate-N were mainly influenced by groundwater flow in horizontal direction. It was found that some regions were more obviously contaminated than others and a few regions were less polluted or even absent of pollution. The spatial variation in pollution patterns of nitrate-N could be mainly attributed to the diffusion and translocation of nitrate-N in three dimensions after being leached into groundwater. The differentiation of nitrate-N concentration in groundwater resulted in poor correlation between land use types on the ground and nitrate-N concentrations in the underlying water. Nitrate-N pollution of groundwater might be directly related to management practices in situations where relatively stable groundwater tables and negligible water flow existed. Therefore, the observed nitrate-N concentration in groundwater at a specific site could be a static appearance of dynamic distribution of the agriculture-derived nitrate-N within a watershed. The importance of this study is in recognition of the guidelines on managing and controlling agriculture-derived pollution within a watershed-the basic ecologic unit.