| The Three Gorges reservoir is the most distinguished hydraulic engineering construction, the range of reservoir area includes22districts and counties which are from Yichang in Hubei Province to Jiangjin in Chongqing. For a long time, people are very concerned about the water environment of the Three Gorges reservoir. As country pays more efforts on the environment pollution control in the Three Gorges reservoir and its upstream watershed, point pollution has be under controlled gradually, however, the problem of non-point source pollution is increasingly highlighted. The Three Gorges reservoir area is given priority to with agricultural production, application of quantity of fertilizer and the unreasonable utilization of land lead to more serious agricultural non-point source pollution, which poses great threat to the water environment of reservoir. The agricultural non-point source pollution of Three Gorges reservoir area has more occurred conditions, broader source of pollutant and larger potential load. To understand nitrogen and phosphorus concentration and discharge load for reservoir water environment impact,alleviate the pressure of the environment,reduce the agriculture non-point source pollution and protect the ecological environment, we choosed the small Wang Jiagou watershed which was given priority to crop production (rice/corn-pickle) as the research object in the Three Gorges reservoir area,by monitoring the ditch runoff water and shallow groundwater (Wells) for three years and by high frequency sampling (a instrument that can automatically sample)runoff water in the outlet of watershed every day for one year, we explored the dynamic changes of water quality and runoff discharge of nitrogen and phosphorus. The targets of this study were:(1) to understand water quality and grasp the dynamic changes rule of water nitrogen and phosphorus concentrations;(2) to grasp the dynamic change rule of nitrogen and phosphorus and estimate nitrogen and phosphorus discharge load of the runoff in the outlet of watershed. The main research results were listed as follows:1. The water quality of watershedIn the three years of monitoring period,477water samples were collected, including surface water (ditches runoff water) was325, and shallow groundwater (Wells) was152. The number of samples whose total nitrogen exceeded V water quality standard (GB3838-2002) accounted for88.1%of all water samples, among which, the exceeding V water quality standard rate of ditches runoff water was80%. The exceeding V water quality standard rate of total phosphorus and ammonia nitrogen was9.8%and1.2%, respectively. The number of all well water samples whose nitrate nitrogen concentration was more than national drinking water hygiene standards (10mgN/L) was86, and the exceeding standard rate was as high as55%. The results showed that total nitrogen and nitrate nitrogen pollution was very serious, the total phosphorus and ammonia nitrogen pollution was lighter in the watershed.2. Dynamic changes of water nitrogen and phosphorus concentrationsThe total nitrogen concentrations of ditch runoff water ranged from1.37to17.66mgN/L, the average concentrations were6.28mgN/L; the total nitrogen concentrations of well water ranged from7.51to18.55mgN/L, the average concentrations were12mgN/L.TN concentrations of wells were significantly higher than that of ditches runoff water at the same sampling date.The average contribution rate of nitrate nitrogen to total nitrogen was67%and89%in the ditch runoff water and well water, which showed that nitrate nitrogen was the main existing forms of the nitrogen in the water quality.The variation range of total phosphorus concentrations was0.01-0.44mgP/L and0.03-0.63mgP/L in the ditch runoff water and well water, the average concentrations were0.15and0.16mgP/L, respectively, which showed that the total phosphorus concentrations were not significant different.Because of the influence by fertilization activities, concentrations of nitrogen and phosphorus obviously increased in the water quality in the early time of crop sowing of rice/corn crops (the beginning of April and May) and pickle (the end of October to the beginning of November),while it tended to decline in mature harvest.TN and TP concentration of well water and TN and TP concentration of ditches runoff had significant correlations in watershed. Due to the fluctuation that well water nitrogen and phosphorus concentrations was lesser along with the time, by monitoring well water nitrogen and phosphorus concentrations, and then agricultural non-point source pollution were analyzed by the correlation of well water and ditches runoff water, to evaluate reasonably the control effect of agricultural non-point source pollution.3. Nitrogen and phosphorus of discharge characteristics of the runoff in the outlet of watershedIt can be divided into two catchments (A and B) whose spatial distribution pattern of paddy were significant different according to the terrain characteristics in the research watershed, the paddy fields of catchment B all covered in the bottom of catchment, while that of catchment A dispersed in the catchment, In the outlet of A and B catchment, by a high frequency monitoring, it found that the variation range of total nitrogen concentrations were0.39-28.58mgN/L and0.28-20.41mgN/L, and the average concentration of the former (7.40mgN/L) was significantly higher than the latter (4.94mgN/L). It may be related that there existed more rice paddies at the end of B catchment, which had an intercept and purification effect on the nutrients. Runoff total phosphorus concentrations of two catchments were0.0022-2.19mgP/L and0-0.88mgP/L.The difference of averaged concentrations of total phosphorus (0.17and0.056mgP/L) was not obviously.The cumulative discharge loads per unit area of total nitrogen were16.09和5.22kgN/(hm2·a), respectively, total phosphorus were0.21和0.072kgP/(hm2·a) in the outlet of the catchment A and B. The accumulation discharge load per unit area of nitrogen and phosphorus of catchment A was significantly higher than that of catchment B.For the whole watershed, the percentage of runoff nitrogen and phosphorus discharge load in application amount of fertilizer of nitrogen and phosphorus was1.61%and0.13%in one year.If according to the calculation of crop planting season, runoff the loss rate of nitrogen and phosphorus fertilizer in rice/corn season(2.44%and0.36%) was much higher than that in pickle season(1.18%and0.052%).Therefore, the key was to cut nitrogen and phosphorus loss of runoff (particularly nitrogen) in rice/corn season, to reduce the risk of agricultural non-point source pollution to water environment security in the study watershed. |
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