Simulation of Nitrate Hydrologic Residence Time in a Hierarchical Groundwater Flow System of Sandy Basin and Implications from Management
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摘要:
以小型砂质盆地为例，基于二维Tóth模型探究了硝酸盐在多级次地下水流系统中的水文遗留时间特征.随着入渗强度增大，水流系统从单一区域系统、嵌套系统向单一局部水流系统转化，盆地硝酸盐平均水文遗留时间递减，但较长遗留时间（250~500 a）占比始终最大.排泄区硝酸盐浓度受相邻局部水流系统主导，其发育深度越大，水文遗留时间越长.若不削减输入，排泄区硝酸盐浓度经2~8 a稳定后无法下降，立即停输比逐步停输使硝酸盐提前4~6 a恢复本底值.控制地下水硝酸盐污染需优先管控相邻局部水流系统硝酸盐输入，或通过调控补给强度改变局部水流系统的发育深度，但快速削减地下水硝酸盐输入量无法显著改善管控措施对水质改善的滞后性.
Abstract:This study investigates the nitrate hydrologic residence time (NHRT) in a sandy, hierarchical groundwater flow system based on a 2D Tóth basin. Results show that with increasing groundwater recharge intensity, groundwater flow system evolves from a single regional system to nested systems, and finally to a single local flow system. The mean NHRT within the basin decreases across five distinct flow system patterns, but the proportion of longer-duration NHRT (250-500 a) remains dominant. Nitrate concentrations in discharge zones were primarily governed by adjacent local flow systems. A greater development depth of the local system prolongs NHRT in the discharge zone. Under nitrate input control scenarios, the baseline simulation (no input reduction) shows that concentrations in discharge zones stabilized within 2-8 years and then failed to decline. Compared to a gradual cessation, an immediate cessation of nitrate input accelerates recovery to background levels by only 4-6 years. The findings suggest that effective control of groundwater nitrate pollution requires either prioritizing input reduction in recharge areas of adjacent local flow systems or altering their development depth by regulating recharge intensity. Nevertheless, rapid cuts in nitrate loading do not substantially mitigate the inherent time lag of water quality response to remediation measures.
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图 1 地下水流系统概念模型及硝酸盐水文遗留时间(NHRT)计算方法示意
度量线示意了局部、中间系统流线上标圈硝酸盐质点的水文遗留时间及区域系统流线上标圈地下水质点的滞留时间；右侧为排泄区NHRT的计算方法示意图
Fig. 1. Schematic of the conceptual groundwater flow system and the method for calculating nitrate hydrologic residence time (NHRT)
图 2 输入强度管控情景中地下水硝酸盐输入浓度变化
Fig. 2. Nitrate recharge concentrations to groundwater under three input-intensity control scenarios
图 3 不同水流系统模式盆地流场及氧化还原带分布
A.地下水流场分布；B.硝酸盐浓度分布；a.单一区域水流系统（R）；b.局部-区域二级嵌套水流系统（2L+R）；c.局部-中间-区域三级嵌套水流系统（6L+M+R）；d.局部-中间二级嵌套水流系统（6L+M）；e.单一局部水流系统（6L）；图中数字表示局部水流系统的编号
Fig. 3. Flow field and distribution of redox zones in the basin under different flow patterns
图 4 不同水流系统模式下的硝酸盐水文遗留时间分布
A.空间分布特性；B.硝酸盐水文遗留时间及地下水滞留时间归一化频率直方图；a.单一区域水流系统（R）；b.局部-区域二级嵌套水流系统（2L+R）；c.局部-中间-区域三级嵌套水流系统（6L+M+R）；d.局部-中间二级嵌套水流系统（6L+M）；e.单一局部水流系统（6L）
Fig. 4. Distribution of NHRT under different flow patterns
图 5 不同输入位置下排泄区硝酸盐的穿透曲线
A.仅RZ1输入；B.仅RZ2输入；C.仅RZ3输入；a.单一区域水流系统（R）；b.局部-区域二级嵌套水流系统（2L+R）；c.局部-中间-区域三级嵌套水流系统（6L+M+R）；d.局部-中间二级嵌套水流系统（6L+M）；e.单一局部水流系统（6L）
Fig. 5. Nitrate breakthrough curves in the discharge zone for different input locations
图 6 不同输入强度下排泄区硝酸盐的穿透曲线
A.情景ⅰ“保持现状”；B.情景ⅱ“逐步停止”；C.情景ⅲ“立即停止”；a.单一区域水流系统（R）；b.局部-区域二级嵌套水流系统（2L+R）；c.局部-中间-区域三级嵌套水流系统（6L+I+R）；d.局部-中间二级嵌套水流系统（6L+I）；e.单一局部水流系统（6L）
Fig. 6. Nitrate breakthrough curves in the discharge zone for different input intensities
表 1 模型几何尺寸、水文地质与溶质运移相关参数取值
Table 1. Parameter values for model geometry, hydrogeology and solute transport
参数类别 参数 取值 几何尺寸 盆地宽度L 3 600 m 最低排泄点高程h0 250 m 区域和局部起伏振幅hR、hL 25 m 局部起伏频率k 6 水文物理参数 渗透系数K 0.25 m/d(Domenico and Schwartz, 1997) 孔隙度θ 0.40(Allard and World Health Organization, 1992) 耦合长度尺度l 0.01 m(年庚乾等, 2020) 地球化学参数 初始浓度C0 0 mg/L 纵向弥散度aL 40 m(蒋小伟等, 2012; Gelhar et al., 1992) 横向弥散度aT 4 m(蒋小伟等, 2012; Gelhar et al., 1992) 分子扩散系数D* 1.16×10-9 m2/s(蒋小伟等, 2012) 溶解氧一级反应动力学常数kDO 0.04 a-1 (McMahon et al., 2008) 硝酸盐一级反应动力学常数$ k_{\mathrm{N}{\mathrm{O}}_{3}} $ 0.02 a-1 (McMahon et al., 2008) 
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表 2 各水流系统模式的入渗强度比设置
Table 2. Settings of infiltration intensity ratios for different flow patterns
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表 3 各工况中硝酸盐输入的时空条件
Table 3. Spatiotemporal conditions of nitrate input in the simulation cases
工况 硝酸盐输入位置 硝酸盐输入时间序列$ C_{\mathrm{N}{\mathrm{O}}_{3}, \text{watertable}} $ (t) Case-Ⅰ不同水流系统模式中的硝酸盐水文遗留时间(tNHRT)分布特征 上边界均匀输入(RZ1+RZ2+RZ3) 恒定值100 mg/L Case-Ⅱ不同输入位置下的排泄区硝酸盐水文遗留时间(tNHRT')特征 分别在RZ1/RZ2/RZ3输入 100 mg/L，溶质运移稳定后(85 a)降至0 Case-Ⅲ不同管控强度下的排泄区硝酸盐水文遗留时间(tNHRT')特征 上边界均匀输入(RZ1+RZ2+RZ3) 保持现状/逐步停止/立即停止
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表 4 不同水流系统模式下地下水平均滞留时间和硝酸盐平均水文遗留时间
Table 4. Mean groundwater residence time and mean nitrate hydrologic residence time across different flow patterns
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