| Nitrate pollution in groundwater is very widespread in intensive agricultural region. 394 samples from phreatic water wells and 283 samples from confined water wells were collected across Huantai County at the same season of 2002 and 2007, which is representative of high-yield region of North China Plain. The NO-3-N concentration was determined. Geostatistics combined with GIS technique were used to analyze the spatio-temporal variability of groundwater nitrate concentration. The average nitrate concentrations in phreatic water were 8.08 mg·L-1 and 14.68 mg·L-1 in 2002 and 2007 respectively, and that in confined water were 3.87 mg·L-1 and 7.19 mg·L-1 respectively. The spatial correlation distances of nitrate concentrations in confined water for both periods were greater than that in phreatic water. The mapping showed that the areas of phreatic groundwater with high levels of nitrate concentrations (10-15, 15-20, 20-30, and >30 mg·L-1) increased by 13.06%, 14.37%, 12.23%, and 3.85% from 2002 to 2007, while the area with low levels (0-5 and 5-10 mg·L-1) nitrate concentrations were decreased by 28.87% and 14.63% compared with 2002. However, the areas of confined water with nitrate concentrations of 5-10 mg·L-1, 10-15 mg·L-1and 15-20 mg·L-1 were increased by 28.01%, 9.33%, and 0.48% respectively, while the areas of NO-3-N concentration (0-5 mg·L-1) was decreased by 37.82%. The NO-3-N concentration in confined water was significantly negative related to groundwater depth for the two period, we found an increasing trend of NO-3-N concentration in the deeper confined water from 2002 to 2007. |
