Spatio-temporal variability of shallow groundwater quality in a typical agricultural catchment in subtropical central China

Abstract:

Excessive agriculture-sourced N leaching into shallow groundwater has deteriorated the domestic water quality in rural China. To effectively prevent the above environmental contamination issue, it is an essential prerequisite of exploring the spatio-temporal variability (stV) of the groundwater quality. In this study, a large observation program was deployed to observe ammonium-N (NH₄N), nitrate-N (NO₃N) and total N (TN) concentrations in 194 groundwater observation wells (1.5 m deep from soil surface) from April 2010 to November 2012 in the Jinjing river catchment in Hunan Province of China. A logit function was applied to transform NH₄N, NO₃N and TN data for normality; the resultant variables were thus named as NH₄Nt, NO₃Nt and TNt, respectively. A spatio-temporal semivariogram model in a sum-metric form was used to quantify the stV of NH₄Nt, NO₃Nt and TNt. The results indicated that the 33-month means ± standard deviations of the NH₄N, NO₃N and TN concentrations were 0.75±0.10, 1.60±0.19 and 2.99±0.29 mg N L^-1, respectively. NH₄Nt and NO₃Nt exhibited a strong spatio-temporal dependence, while TNt only presented a strong temporal structure. Spatio-temporal ordinary kriging (stOK) was applied to predict the spatio-temporal distributions of NH₄N, NO₃N and TN over the catchment. The cross-validation results indicated that the stOK predictions for NH₄N (r=0.48, RMSE=1.11 mg N L^-1), NO₃N (r=0.46, RMSE=1.21 mg N L^-1) outperformed that for TN (r=0.29, RMSE=2.11 mg N L^-1). Referenced to the Chinese Environmental Quality Standards for Groundwater (GB/T 14848-93), the proportions of areas contaminated by NH₄N, NO₃N and TN in the catchment over a 33-month period were 20.5%, 1.46%, and 5.07%, respectively. Our findings suggested that the Jinjing groundwater was mainly polluted by NH₄N, which is probably attributed to the intensive rice agriculture featured with high urea fertilizer applications in the catchment.