Intensive rice agriculture deteriorates the quality of shallow groundwater in a typical agricultural catchment in subtropical central China

High nitrogen (N) concentrations in rural domestic water supplies have been attributed to excessive agricultural N leaching into shallow groundwater systems; therefore, it is important to determine the impact of agriculture (e.g., rice production) on groundwater quality. To understand the impact of agricultural land use on the N concentrations in the shallow groundwater in subtropical central China, a large observation program was established to observe ammonium-N (NH₄-N), nitrate-N (NO₃-N), and total N (TN) concentrations in 161 groundwater observation wells from April 2010 to November 2012. The results indicated that the median values of NH₄-N, NO₃-N, and TN concentrations in the groundwater were 0.15, 0.39, and 1.38 mg N L⁻¹, respectively. A total of 36.3 % of the water samples were categorized as NH₄-N pollution, and only a small portion of the samples were categorized as NO₃-N pollution, based on the Chinese Environmental Quality Standards for Groundwater of GB/T 14848-93 (General Administration of Quality Supervision of China, 1993). These results indicated of moderate groundwater NH₄-N pollution, which was mainly attributed to intensive rice agriculture with great N fertilizer application rates in the catchment. In addition, tea and vegetable fields showed higher groundwater NO₃-N and TN concentrations than other agricultural land use types. The factorial correspondence analysis (FCA) suggested that the flooded agricultural land use types (e.g., single-rice and double-rice) had potential to impose NH₄-N pollution, particularly in the soil exhausting season during from July to October. And, the great N fertilizer application rates could lead to a worse NO₃-N and TN pollution in shallow groundwater. Hence, to protect groundwater quality and minimize NH₄-N pollution, managing optimal fertilizer application and applying appropriate agricultural land use types should be implemented in the region.