牧区河岸潜流带硝酸盐氮和氨氮浓度对水文过程的响应机制

doi:10.13866/j.azr.2023.06.09

摘要/Abstract

摘要：

在牧区和灌溉农业区，大量含氮的畜禽排泄物和氮肥从土壤进入地表水和地下水，是流域面源污染的主要来源。河岸潜流带是削减氮素污染负荷的有效屏障，厘清河岸潜流带对氮素的迁移转化和去除作用是控制流域氮素污染的关键。本研究选取位于典型草原牧区的锡林河上游河段，开展了夏汛期河水与河岸地下水的水位、氨盐（NH₄⁺）和硝酸盐（NO₃^-）浓度，以及相关环境因子的连续监测，并利用FEFLOW建立了河岸潜流带水流和氮素溶质反应运移模型。利用实测数据拟合的模型能够准确再现河岸潜流带水位和两种主要氮素浓度的动态变化。结果表明：（1）夏汛期河岸带氮素污染风险较高，河岸带NH₄⁺浓度从降雨前的0.2 mg·L^-1升高到降雨后的7.23 mg·L^-1，NO₃^-浓度从1 mg·L^-1升高到8.27 mg·L^-1。（2）实测和模拟结果均显示潜流带中氮素动态与降雨、地表水-地下水交换等水文过程密切相关，且NH₄⁺和NO₃^-浓度对暴雨事件的响应机制不同。（3）降雨期间，流动性较强的NO₃^-在淋滤作用下从河水和地表入渗进入河岸带，导致浓度显著升高。同时，降雨事件加强了河水-地下水的交换作用，通过控制营养物质的输入影响氮素生物地球化学循环，从而调节河岸潜流带NH₄⁺和NO₃^-浓度的变化。本研究初步揭示了牧区河岸带对于氮素的水文和生物地球化学过程的缓冲作用机制，为牧区氮素污染控制提供了科学参考。

关键词: 河岸潜流带, 氮素运移, 水文过程, 地下水模拟, 锡林河

Abstract:

In pastoral and irrigated agricultural areas, nitrogen-containing livestock, poultry manure, and nitrogen fertilizers can enter the surface water and groundwater from the soil, and this is the main source of non-point source pollution in basins. The riparian hyporheic zone acts as an effective barrier to reduce the nitrogen pollution load. Understanding the mechanisms of the migration, transformation, and removal of nitrogen in riparian hyporheic zones is key to controlling nitrogen pollution in the whole basin. In this study, an upper reach of the Xilin River, located in typical pastoral areas, was selected and its water levels, ammonia (NH₄⁺) and nitrate (NO₃^-) concentrations, as well as the related environmental factors of the river water and riparian groundwater during the summer flood season, were continuously monitored. Based on the high-solution measurements, a water flow and nitrogen reactive transport model of the riparian hyporheic zones was established using FEFLOW. The model fitted using the measured data was found to accurately reproduce the water level dynamics and two main nitrogen concentrations in the riparian hyporheic zone. The results indicate that there is a high risk of nitrogen pollution in the riparian zones during the summer flood season. The NH₄⁺ concentration in the riparian zones can increase from 0.2 mg·L^-1 before rainfall events to 7.23 mg·L^-1 after rainfall events, and the NO₃^- concentration can increase from 1 mg·L^-1 to 8.27 mg·L^-1. Both measured and simulated results show that the nitrogen dynamics in the hyporheic zone are closely related to hydrological processes such as rainfall events and groundwater-surface water exchange. During rainfall events, NO₃^- with high mobility was found to infiltrate from the river and the ground surface into the riparian zone due to the leaching effect, resulting in a significant increase in the concentration. Meanwhile, the groundwater-river water exchange enhanced by rainfall events can further regulate NO₃^- and NH₄⁺ concentrations in the riparian hyporheic zone by controlling the input of nutrients and affecting the biogeochemical nitrogen cycles. This study preliminarily reveals the buffering mechanisms of pastoral riparian zones in the hydrological and biogeochemical processes involving nitrogen and provides scientific references for the nitrogen pollution control in pastoral areas.

Key words: riparian hyporheic zone, nitrogen transport, hydrological process, groundwater simulation, Xilin River

薛栋元, 胡海珠, 张锦宁, 任嘉伟. 牧区河岸潜流带硝酸盐氮和氨氮浓度对水文过程的响应机制[J]. 干旱区研究, 2023, 40(6): 937-948.

XUE Dongyuan, HU Haizhu, ZHANG Jinning, REN Jiawei. Response mechanisms of nitrate and ammonia nitrogen concentrations to hydrological processes in the riparian hyporheic zone of pastoral areas[J]. Arid Zone Research, 2023, 40(6): 937-948.

图/表 10

图1

图2

表1

模型参数取值"

模型参数	给水度 /(m³·h^-1)	渗透系数 /(m·d^-1)	纵向弥散度/m	横向弥散度/m	分子扩散系数/(m²·s^-1)	一级反应常数/d^-1
模型参数	给水度 /(m³·h^-1)	渗透系数 /(m·d^-1)	纵向弥散度/m	横向弥散度/m	分子扩散系数/(m²·s^-1)	$N O 3 -$	$N H 4 +$
参数值	0.1	1.75~3.25	10	1	1×10^-9	0.79	3.81
取值依据	[35]	实测	[36]	[36]	[37]	[38-39]	[38-39]

表1

图3

图4

图5

图6

表2

模型参数敏感性分析结果"

参数增量	$N O 3 -$ 浓度相对于基本情景的减小量/(mg·L^-1)			$N H 4 +$ 浓度相对于基本情景的减小量/(mg·L^-1)
参数增量	10％	20％	40％	10％	20％	40％
纵向弥散度α_L	0.011	0.029	0.067	0.007	0.013	0.035
横向弥散度α_T	0.001	0.003	0.007	0.001	0.001	0.002
分子扩散系数D₀	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001
$N O 3 -$ 衰减系数K₁	1.029	1.410	2.642	-	-	-
$N H 4 +$ 衰减系数K₂	-	-	-	0.467	0.683	1.131

表2

图7

图8

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