Source Analysis of Nitrate Nitrogen in Groundwater Based on Different Modes of Land use

Abstract

Background: The study area is located in the Hebei province Luanxian section along the Luanhe River. The right of Luanhe River is arranged with 3 monitoring sections and is laid with 5 wells in the A-A section and B-B section and 1 well on the floodplain (inside the dike), the distance from the river level is about 10m. The distance between the 4 wells outside the dike is 20m, 50m, 200m and 1500m respectively. 2 wells are arranged on the C-C section, so there are 13 observation wells in the study area. 3# wells and 11# wells have used existing monitoring wells, so 11 wells have been set up actually. The monitoring wells are designed to monitor groundwater level, groundwater quality and isotope analysis. Methods: In the upstream of Luanhe’s A-A section of the No. 4 well near the edge of the woods and downstream of the B-B section of the No. 8 well near the edge of farmland (wheat), each point of soil is sampled to determine the content of nitrate nitrogen and 15N isotope ratio in soil profile. The depth of soil is sampled from surface to groundwater level, the total depth is about 2.5m. Taking about 1kg soil samples at 20cm every time from the ground surface and collect and seal in plastic bags and number. The sampling time was November 2015, March 2016, May 2016 and July 2016, taking 4 times. Results: The analysis showed that higher nitrate content in soil is within 0-60cm, and is same as the depth of fertilizing the crops (including animal manure), so when estimating the proportion of groundwater in nitrate, nitrate content in 0-60cm is assumed as the initial value; which is close to the underground water level in the soil and is regarded as the final value of 60cm, that is, the final value of nitrate into the groundwater, which can estimate the soil nitrate into groundwater ratio. Conclusion: The sources of nitrate in groundwater are complex and related to many factors, such as river water, soil and so on. According to the content of nitrogen and oxygen isotopes, it determines the main source of nitrate nitrogen:soil and Luanhe River. The results showed that the ratio of nitrate to groundwater in the forest soil was 20%~80.8%, and the average value was 56.3%. The proportion of nitrate in the soil in the wheat field was 6.8%~98.2%, with an average value of 48.3%. Nitrate nitrogen from water in proportion of large changes with the seasons, the upper reaches of the Luanhe River (near No. 2 wells) for 0~19.6%; the Luanhe River (near No. 7 wells) for 0~31.5%.