| Hilly valley plain is the most concentrative landform for intensive agriculture and rural life in Chinese mountainous areas.And phreatic water is the most important source of water quality and drinking water in the hilly valley plain.Thus,under the conditions of agricultural intensification,it is of great significance to study and control the agricultural non-point source pollution in phreatic water.This study took the Changle River watershed,a typical hilly valley plain in the southeastern coastal area of China,as the research object.We arranged 12 sampling points in the study area and determined their chemical measurements monthly for three consecutive years.Based on the results of water quality evaluation by comprehensive index method,this study analyzed the seasonal variations and spatial distributions of nitrogen and phosphorus pollution in phreatic water of the hilly valley plain.The area occupation ratios of different land use patterns were extracted in the areas of different radius from 50 to 1500 m around each sampling site to investigate the correlation relationships between different land use patterns and the corresponding groundwater nitrogen and phosphorus concentrations by multiple regression analysis.The main findings are as follows:(1)The chemical properties(i.e.TN?NO3--N?N4+-N?TP?CODMn and pH)of the phreatic water in hilly valley plain of Changle River watershed was determined monthly for three years and their water quality was evaluated by comprehensive index method.The results showed nine,two and one of twelve sampling points had met Class?,?,V water standards,respectively.The main problem was that the concentrations of nitrate nitrogen(NO3--N)and ammonium nitrogen(NH4+-N)met Class V standards of the national groundwater and total phosphorus(TP)met Class IV standards of the national surface water in some sampling points.These results suggested that the quality of the phreatic water in this area was generally in good condition,whereas part of it showed obvious trends of nitrogen and phosphorus pollution and eutrophication.(2)The average concentrations in three years of NO3--N,NH4+-N,TN in the Changle River watershed were 1.326(±0.618)mg L-1,0.434(±0.158)mg·L-1,2.717(±1.614)mg·L-1,respectively.The nitrogen forms of phreatic water were mainly consisted of NO3--N,accounting for 49%,followed by organic nitrogen(34%)and NH4+-N(16%).The concentrations of TN and NO3--N were the highest in 2015 with the largest annual rainfall.The monthly average concentrations of NO3--N reached the highest value while the concentration of NH4+-N reached the lowest value in June.The monthly average concentration of NO3--N in phreatic water showed a significant correlation with the monthly average rainfall(r=0.790**)and was much higher in wet season than in dry season.On the contrary,NH4+-N was mainly diluted by heavy rainfall,and its concentration in dry season was slightly higher than in wet season.Additionally,there was a significant correlation between TN and NO3--N concentrations(r=0.635'),so the concentration of TN was higher in wet season than in dry season.(3)The average concentrations in three years of TP in the Changle River watershed were 0.154(±0.083)mg·L-1.Average annual concentration of TP was the highest in 2015.The monthly average concentration of TP reached the highest value in June.The concentration of TP was lower under short-term heavy rainfall conditions and increased under long-term heavy rainfall conditions,and it was slightly higher in the wet season than in dry season.(4)The results of Kriging spatial interpolation analysis showed that TN,NO3--N and NH4+-N had strong spatial autocorrelation with nugget coefficients 0.014%,1.52%and 0.37%,respectively.TP spatial autocorrelation was moderate and the nugget coefficient was 55.6%.The concentrations ofNO3--N and TN were similar in the spatial distribution which had the highest concentration value at the No.9 sampling point mainly consisting of gardens(vegetables and seedlings)and cultivated lands(rice and rape)and decreased with distance.The spatial distribution of NH4+-N and TP presented the characteristics that the concentrations decreased from upstream in southwestern region of the watershed to the downstream in eastern region,and were higher near the water area(river and pond)·(5)The correlation analysis between nitrogen and phosphorus concentrations and land use patterns in phreatic water of Changle River watershed showed that the concentration of nutrients in phreatic water not only depended on the pattern of land use at the sampling point but also related to different patterns of land use within a certain range.The most closely range of land use effects related to the concentration of NO3--N and TP was about 1000 m,while the optimal range between TN and NH4'-N concentrations and land use patterns was about 1250 m.Through multivariate regression analysis,multiple regression linear equations between nitrogen and phosphorus concentrations and ratios of different land use pattern areas were established within the optimal research radius.The results showed that the garden land had the greatest impact on the concentrations of NO3--N and TN,for positive correlation;water area and cultivated land were also positively related to the concentration of NH4+-N,and the influence of water area was greater than that of cultivated land;the woodland mainly had negative correlation influence on the concentration of NH4+-N.Cultivated land and water area had a greater influence on the TP concentration of phreatic water as TP concentration values were relatively higher near these areas.Finally,based on the analysis results,the suggestions and measures for prevention and control of non-point source nitrogen and phosphorus pollution in underground phreatic water were put forward. |
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