Modelling of groundwater remediation using monitored natural attenuation at a contamination site in Guangzhou
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摘要:
监控自然衰减(MNA)作为一种成本低、不产生二次污染物、对污染场地环境影响较小的地下水污染修复方法, 具有较高的应用价值和发展前景, 值得实践与研究。选取广州某地下水污染场地作为研究区, 评价MNA修复方法的适用性。基于水文地质条件及污染现状分析, 运用地下水数值模拟程序MODFLOW建立了污染场地地下水流模型, 运用污染物迁移数值模拟程序MT3DMS建立了场地污染物迁移模型, 分别模拟了场地地下水流、主要污染物总石油烃(TPH)和重金属镍(Ni)的迁移过程。基于模型, 对比监控自然衰减和抽出处理与监控自然衰减结合的2种方案修复效果。结果表明, TPH和Ni对于Freundlich常数及Freundlich指数变化均较为敏感; TPH的自然衰减效果较好, 采用自然衰减方案, 经过850 d可由初始浓度1.52 mg/L衰减到修复目标值(0.3 mg/L); Ni衰减较慢, 适宜采用结合抽出处理的监控下自然衰减方案, 经过300 d可由初始浓度0.13 mg/L达到修复目标值(0.02 mg/L)。在自然衰减能力较强或地下水流速较缓的条件下, 适宜采用监控自然衰减修复方案; 在自然衰减能力较弱或地下水有显著流动的情况下, 适宜采用结合抽出处理的监控自然衰减修复方案。研究结果对地下水污染修复具有参考价值与借鉴意义。
Abstract:Objective Monitoring natural attenuation (MNA), as a low cost remediation method for groundwater pollution that does not produce secondary pollutants and has little impact on the polluted site environment, has high application value and development prospects, and is worth practicing and studying.
Methods This paper adopts a groundwater-contaminated site in Baiyun District of Guangzhou to assess the applicability of MNA. Based on the analysis of hydrogeological conditions and pollution status, a groundwater numerical simulation program MODFLOW was used to establish a groundwater flow model for contaminated sites. The pollutant migration numerical simulation program MT3DMS was used to establish a pollutant migration model for the site. The migration processes of groundwater flow, main pollutants total petroleum hydrocarbons (TPH), and heavy metal nickel (Ni) were simulated, respectively. Based on the model, the performances of the MNA alone and the MNA combined with pump and treat methods were compared.
Results The results show that TPH and Ni are both sensitive to changes in the Freundlich constant and Freundlich exponent. The TPH shows good natural attenuation effect and can be substantially attenuated by the MNA alone. The concentration of TPH decreased from the initial value of 1.52 mg/L to the target (0.3 mg/L) after 850 days. Ni decay is relatively slow and it is suitable to adopt to a natural attenuation scheme under monitoring combined with the pump & treat method. The concentration of Ni decreased from the initial value of 0.13 mg/L to the target (0.02 mg/L) after 300 days. For a contaminated site with large natural attenuation capacities and/or low groundwater flow velocities, MNA alone is an appropriate remediation strategy. In contrast, for a contaminated site with low natural attenuation capacities and/or high groundwater flow velocities, the MNA combined with pump and treat may be a better remediation strategy.
Conclusion This study provides an appropriate reference for the application of MNA to groundwater pollution remediation.
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图 4 模拟的地下水位与观测水位对比(a)和校正后的地下水流场(b)
Figure 4. Comparison of modelling hydraulic head and observed hydraulic head (a), and the distribution of hydraulic head predicted by the calibrated model (b)
图 5 不同参数条件下(Kf, λ1, a, aL)TPH污染羽随时间变化的空间分布
Kf.Freundich常数(mg·L-1)-a；a.Freundich指数; λ1.溶解相一级反应速度(d-1); aL.纵向弥散度(m)；下同
Figure 5. Changes in the TPH plume with time for different parameters(Kf, λ1, a, aL)
图 6 不同参数条件下(Kf, λ1, a, aL)北区Ni污染羽随时间变化的空间分布
Figure 6. Changes in the Ni (north) plume with time for different parameters(Kf, λ1, a, aL)
图 7 不同纵向弥散度(αL)下南区Ni污染羽随时间变化的空间分布
Figure 7. Changes in the Ni (south) plume with time for different aL
图 8 抽出处理下TPH和Ni污染羽随时间变化的空间分布(左)及污染源(右)穿透曲线
Figure 8. Changes in the TPH and Ni plume with time (left) and breakthrough of the pollution source (right) under pump and treat conditions
表 1 校正后的地下水流模型参数
Table 1. Calibration parameters of the groundwater flow model
参数名称 区域Ⅰ 区域Ⅱ 区域Ⅲ 区域Ⅳ 水平渗透系数K/(m·d-1) 0.26 1.50 1.06 0.007 3 地下水补给量W/(m·d-1) 0.000 67 0.000 97 0.000 67 0.000 67 河流传导系数C/(m·d-1) 0.26 
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表 2 溶质运移模型参数
Table 2. Solute transport model parameters
参数名称 总石油烃(TPH) 镍(Ni) 土壤容重ρb/(kg·m-3) 1 500 孔隙度ne/% 30 纵向弥散系数Dy/(m2·d-1) 10 15 25 5 纵向弥散度aL/m 1(区域Ⅰ) 1.5(区域Ⅱ) 1.5(区域Ⅲ) 0.5(区域Ⅳ) 横向弥散度at/m 0.1(区域Ⅰ) 0.15(区域Ⅱ) 0.15(区域Ⅲ) 0.05(区域Ⅳ) Freundlich常数Kf/(mg·L-1)-a 5×10-6~1×10-3[21] 0.001~0.005[22] Freundlich指数a 0.5~1[21] 1.5~3.5[22] 溶解相一级反应速率λ1/d-1 0.001~0.01[23] 1×10-7~1×10-5[24] 吸附相一级反应速率λ2/d-1 0 0
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表 3 参数敏感度分析取值
Table 3. Parameter sensitivity analysis value
参数名称 污染物 取值1 取值2 取值3 Freundlich常数
Kf/(mg·L-1)-a总石油烃
镍5×10-6
1×10-35×10-4
3×10-31×10-3
5×10-3Freundlich指数a 总石油烃
镍0.50
1.500.75
2.501.00
3.50溶解相一级反应速率
λ1/d-1总石油烃
镍1×10-3
1×10-75×10-3
1×10-61×10-2
1×10-5纵向弥散度aL/m 总石油烃
镍(北部)
镍(南部)0.20
0.20
11
1
55
5
10
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表 4 监控条件下TPH源区域衰减情况
Table 4. Attenuation situation of the TPH source under monitoring
模型参数 衰减比率/% 停留时间/d Kf/(mg·L-1)-a a λ1/d-1 aL/m 150 d 300 d 600 d 0.000 5 0.75 0.005 1.0 24.66 43.44 70.28 800~850
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表 5 监控条件下场地南北部Ni源区域衰减情况
Table 5. Attenuation situation of the Ni source under monitoring
区域 模型参数 衰减比率/% 停留时间/d Kf/(mg·L-1)-a a λ1/d-1 aL/m 500 d 1 000 d 1 500 d 北区 0.003 2.5 1×10-6 1.0 57.61 84.81 94.17 500~550 南区 0.003 2.5 1×10-6 5.0 40.44 79.69 93.13 1 200~1 250
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表 6 抽水井设置
Table 6. Settings of pumping wells
污染物 井类型 井位置 处理时间/d 流量/(m3·d-1) 所需监控时间(原始停留时间)/d TPH 抽水井 污染源中心 0~100 -25 < 100(800~850) 北区 Ni 抽水井 污染区域中轴线 0~100 -20 150~200(500~550) 抽水井 污染区域中轴线 0~200 -20 抽水井 污染区域中轴线 0~100 -20 注水井 污染区域左外侧 0~100 10 南区 Ni 抽水井 污染区域中轴线 0~100 -30 300~350(1 200~1 250) 抽水井 污染区域中轴线 0~100 -50 抽水井 污染区域中轴线 0~100 -40 抽水井 污染羽传播方向 0~100 -30
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