Weihe River, which runs through Guanzhong area, has become a sewage river in Guanzhong area, and a large number of untreated industrial wastewater and domestic sewage are discharged directly, which makes Weihe River suffer greatly. The monitoring data in 2007 showed that Weihe River water was not only impossible to be used as drinking water source, but also seriously threatened as agricultural irrigation water, and aquatic organisms such as fish in the main stream basically disappeared. Weihe River not only basically lost its original function, but also affected the water quality of the Yellow River. See table 1 for the statistical results of sewage outlets and sewage discharge in Guanzhong area.
Table 1 Statistical Table of Sewage Outlet and Pollution Discharge in Guanzhong Area of Weihe River Basin
2. Non-point source pollution
Regional non-point source pollution is mainly related to rainfall-runoff process, land use, vegetation and human activities. The water pollution of Weihe River system mainly comes from point source pollution, that is, a large number of urban industrial and domestic sewage discharges. Because there is no detailed investigation and monitoring of river non-point source pollution, only the main influencing factors that can produce non-point source pollution are analyzed here.
(1) is closely related to soil erosion in this area. After years of treatment, the annual sediment yield is 55.29 million t, accounting for 7.4% of the whole province, and the average erosion modulus for many years is 998t/km2.
(2) Compared with the soil background value, the average contents of organic matter, total nitrogen, available nitrogen, available phosphorus and available potassium in the surface soil of Weihe River Basin are 65438 0.65, 438 0%, 0.08%, 49PPM, 7.4PPM and 65438±0.55 ppm, respectively, which are slightly higher than the provincial average.
(3) It is related to the use of pesticides and fertilizers. According to statistics, the total annual consumption of pesticides in Weihe River Basin is 4728t, with an average consumption of 0.279kg per mu, which is higher than the average consumption of 8 1.2% in the whole province. The total annual consumption of chemical fertilizer is 3410.5 million t, with an average consumption per mu of10.28 kg, which is 70.2% higher than that of the whole province. The average pesticide consumption per mu in Xi is 0.348 kg, followed by Weinan, Xianyang, Baoji and Tongchuan.
(4) It is related to air pollution and acid rain. According to the monitoring, the annual average value of SO2 in seven cities and counties in Weihe River Basin 1995 is 0.085mg/m3, and the daily average value ranges from 0.003 ~ 1. 130, and the over-standard rate is 14. 1%. Only Tongchuan, which exceeds the national second-class standard, appears in winter; During the period of 1995, the pH value of precipitation in Guanzhong area was between 4.35 and 9.35, and the total amount of acid rain collected in the whole year accounted for 2.75% of the total collected water, and acid rain samples accounted for 2.0% of the total rainfall. Acid rain only occurs in Xi and Weinan in Guanzhong.
(5) Related to the utilization of land resources. The unreasonable utilization of land resources such as steep slope reclamation, decreased forest coverage rate and intensified human activities will increase the load of non-point source pollution. At present, the main problems existing in the utilization of land resources in Weihe River Basin are: the total area of forest land and grassland is declining sharply. Due to the decrease of cultivated land, the cultivated land in Guanzhong area is more than 25 degrees 124.94hm2, accounting for 5.8% of the cultivated land area, of which the cultivated land in Baoji area accounts for 17. 1%, and the cultivated land in steep slope accounts for 25% of the water quality of Weihe River system.
Present situation of water quality in Weihe River basin
1. Present situation of river water quality
(1) section selection
Six monitoring sections (Tuoshi, Linjiacun, Xianyang, Gengzhen, hua county and Tongguan Suspension Bridge) and eight monitoring sections (Heiyukou of Heihe River, Weikou of Fenghe River, Wang Du of Bahe River, Zhangjiashan of Jinghe River, Jingcun, Wuqi of Beiluo River, Jiaokou and Zhuangtou) were selected as evaluation sections, and the river length was controlled at 40.
(2) Evaluation index, evaluation standard and evaluation method.
According to 24 indexes in Table 1 of Environmental Quality Standard for Surface Water (GB3838- 2002).
(3) Evaluation results
Beiluohe River system: the length of the assessed river reaches is 54 1.4km, with an average annual water quality of 58.8% of the assessed river reaches, and the length of the river reaches with poor water quality of V accounts for 4 1.2% of the assessed river reaches. The main items exceeding the standard are ammonia nitrogen, biochemical oxygen demand and chemical oxygen demand.
Jinghe River System: The length of the assessed river reaches is 222.5km, with an average annual length of 72.6% of the assessed river reaches with Grade V water quality, and 27.4% with inferior Grade V water quality. Biochemical oxygen demand, chemical oxygen demand, etc.
Weihe river system: the total length of the main and tributaries is 524.7 kilometers, and the average annual river length of Grade I ~ III water quality accounts for 35.0% of the assessed river length; The length of rivers with Grade IV ~ V water quality accounts for 29.4% of the assessed river length; The length of the river with worse water quality than Grade V accounts for 35.6% of the assessed river length; The main items exceeding the standard are ammonia nitrogen, chemical oxygen demand and biochemical oxygen demand.
2. Functional area standard evaluation
Weihe River Basin: In 2007, 22 sections of water functional zones participated in the evaluation, and the river length was evaluated as 1288.6 km.
According to the evaluation times, the average annual compliance rate is 31.8%; The average annual compliance rate of the accumulated river is 14.3%. See Table 2.
Table 2 Statistical Table of Functional Area Evaluation of Weihe River Basin
Among the 22 water functional zones evaluated, there are 6 first-class zones with a river length of 527.6km. According to the evaluation times, the average annual compliance rate is16.7%. The average annual compliance rate of the accumulated river is 13.6%. Excessive items are ammonia nitrogen, chemical oxygen demand and biochemical oxygen demand.
Among the 22 sections of water functional zones evaluated, the secondary zone is 16, and the river length is 76 1.0km. According to the evaluation times, the average annual compliance rate is 25.0%. The average annual compliance rate of accumulated river length is14.7%; Excessive items are ammonia nitrogen, chemical oxygen demand and biochemical oxygen demand.
Cause analysis of pollution
1. The ecological water quantity of the river channel is insufficient.
The precipitation in Weihe River Basin is mostly concentrated in summer and autumn, and 60% of precipitation and 70% of runoff are concentrated in June-65438+10. The self-produced runoff in Guanzhong area of Weihe River Basin is about 7 billion m3/a, and only 2 billion to 3 billion m3/a of water is left after deducting the flood season. The annual surface water intake of Weihe River is about 2 billion to 2.5 billion m3, of which the average annual water intake of Linjiacun Water Diversion Project in Baoji Gorge of Weihe River is 483 million m3, accounting for 49.07% of the river water (see Table 3). Therefore, there is little ecological water in the river at ordinary times, and the dilution and purification ability of the river is low.
Table 3 Water diversion amount of Linjiacun diversion project in Weihe River in recent 10 years
Note: annual water diversion = annual runoff-measured annual runoff downstream of the section.
It can be seen from Table 3 that after 1995, although the annual runoff decreased year by year, the annual water intake did not decrease, and there was an increasing trend, which led to a sharp decline in the ecological water volume of the river and the concentration of pollutants in the water body was not diluted and degraded, thus indicating that the insufficient ecological water volume of the river was an important reason for high pollution after 1995.
2. Irrational use of chemical fertilizers and pesticides
According to the data analysis, the problem of soil nutrient imbalance in Baoji City in Weihe River Basin has always been prominent, with low application of organic fertilizer and large application of nitrogen and phosphorus fertilizers, which has destroyed the soil structure, and the phenomenon of soil hardening, shallow tillage layer and poor water and fertilizer conservation performance has become increasingly serious. Moreover, due to the extensive use of cheap and low-quality nitrogen fertilizer and pesticides, the application ratio of nitrogen, phosphorus and potassium fertilizer is out of balance, and the nitrogen fertilizer dissolves and rushes into the water body, causing pollution.
3. The increase of water consumption leads to the shortage of water resources caused by pollution.
Due to the increase of water consumption, the sewage treatment capacity is seriously insufficient, resulting in water shortage. With the development of economy and the increase of water consumption, the amount of wastewater discharged into rivers is also increasing. According to the general survey and registration report of sewage outlets entering the Yellow River basin in Shaanxi Province, the annual discharge of wastewater from Weihe River is 77 100 million t, which is the main pollution source of water. There is a serious shortage of water resources in Weihe River, the ratio of sewage to diameter is increasing, and the self-purification ability is declining. Urban domestic sewage and industrial wastewater are discharged into Weihe River, which is greater than the environmental capacity of Weihe River itself, making Weihe River almost lose its natural dilution and purification function.
4. Soil erosion
The average sediment transport in Weihe River basin for many years reached 458 million tons, and serious soil erosion led to increased surface pollution. The increasingly serious water pollution not only reduces the use function of water bodies, further exacerbates the contradiction of water shortage, but also seriously threatens the drinking water safety of urban and rural residents and people's health.
Protection countermeasures
1. Ensure the ecological water demand of the river.
From the analysis, it can be seen that the proportion of water diversion in the lower reaches of the Weihe River above Linjiacun is increasing, which leads to the sharp decrease of Weihe River runoff, the loss of environmental capacity and the intensification of pollution. Therefore, agricultural water-saving irrigation can reduce water diversion and increase river runoff, thus improving the water quality of Weihe River and alleviating the pressure of pollution control in the near future. At the same time, the water resources on which Guanzhong Plain depends for survival and development mainly come from the supply of forests and hills in the north and south mountains. Therefore, we should strengthen the construction of ecological environment, build water conservation forests, and increase surface water and groundwater resources in the Weihe River basin to ensure the ecological water demand of the river.
2. Strengthen the control of non-point source pollution
(1) Promote the use of various types of "sustainable agriculture" farming methods, reduce soil erosion and sediment discharged into the Weihe River, thus reducing the amount of N and P in fertilizer flowing into the water body and preventing the Weihe River from eutrophication.
(2) Improve the irrigation methods of farmland, adopt new technologies such as sprinkler irrigation, drip irrigation and infiltration irrigation and mixed irrigation technology of fertilizer and water to save irrigation water and fertilizer and prevent water pollution.
(3) Improve fertilization methods to reduce fertilizer loss.
(4) Strengthen the test of soil and fertilizer, and apply fertilizer scientifically and quantitatively.
(5) Adjust the variety structure of chemical fertilizers, strengthen the development of ecological fertilizers and pesticides, develop and popularize nitrogen and phosphorus fixation technologies for soil and crops, encourage the use of organic fertilizers, and reduce the use of conventional fertilizers and pesticides.
(6) Strengthen afforestation, return farmland to forests, increase surface coverage, implement wetland restoration projects, protect the ecological environment, and avoid soil erosion and fertilizer flowing into water bodies.
3. Vigorously carry out industrial restructuring and pollution control.
The large number, small scale and wide distribution of papermaking enterprises in Weihe River Basin are the main reasons for COD pollution. Therefore:
(1) Through the adjustment of industrial layout, industrial structure and product structure, key polluting enterprises and products with large pollutant emissions will be eliminated.
(2) Strictly control major industrial pollution sources within a time limit. For machinery, electric power and paper industry, sewage must be treated up to standard before it can be discharged.
4. Implement bio-ecological management.
Urban domestic sewage discharged by small and medium-sized towns and villages along the Weihe River is also an important aspect of Weihe River pollution. Therefore, according to the basic situation of small and medium-sized towns and rural areas, we should first consider using wasteland, wasteland, poor land, pits and depressions to build sewage land treatment system and sewage stabilization pond system to control water quality at low cost and effectively. This economical, simple and energy-saving treatment technology, if combined with local ecological agriculture, can realize the harmless and resource utilization of sewage and achieve the purpose of ecological control.
5. Give priority to prevention and combine prevention with control.
To protect the water environment, we should implement the policy of putting prevention first, combining prevention with control, comprehensive management and comprehensive utilization. For new industries that may produce new pollution in the future, we must first strictly control the construction. If the pollution is not serious, prevention should be given priority, and industrial pollutants should be solved in the production process. Second, comprehensive utilization of pollutants; Thirdly, sewage treatment is carried out, and the sewage is discharged after being treated to a certain extent according to the situation of river sections and administrative regions.
6. Total quantity control
Take comprehensive prevention and control measures, organically combine sewage treatment facilities, biological measures and water conservancy measures, rationally use the dilution and self-purification capabilities of various water environments, and change the past discharge concentration control into total control. Combined with various water functional areas and administrative areas, various water bodies in the basin should be monitored in time to gradually and fundamentally solve the pollution.