Source and enrichment mechanism of ammonium in shallow confined aquifer in the west of Dongting Plain
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摘要: 洞庭湖平原西部地区浅层承压含水层是当地主要的地下水开采层，却面临严重的水质型缺水问题，其中以铵氮异常最为典型，但目前对于其来源和富集机制的认识十分薄弱。以洞庭湖平原西部为研究区，沿区域地下水流方向对地下水样品进行水文地球化学分析，旨在查明地下水中铵氮的来源，揭示地下水流动对铵氮富集的控制机理。结果表明：NH4-N质量浓度为0.05~16.75 mg/L，且与DOC、HCO3-、As、Fe2+、Mn、P质量浓度呈现较好正相关性；而高质量浓度的NH4-N对应着很低质量浓度的Cl-、SO42-、NO3-和很低的Cl/Br比值，可以推测浅层承压水中的铵氮主要由天然有机质矿化作用产生，而非人为输入。沿着地下水流向，NH4-N和As、Fe2+、Mn质量浓度均显著升高，说明由于水流越来越滞缓，含水介质颗粒越来越细，沉积物有机质越来越富集，含氮有机质矿化作用逐渐增强，使得NH4-N质量浓度逐渐升高，并形成了还原性逐渐增强的地下水环境，相关地球化学过程产生的还原性组分（砷、铁、锰等）也逐渐富集。本研究进一步丰富了地下水原生铵氮的成因理论，可为当地的供水安全保障提供理论基础。Abstract: Shallow confined aquifer is the main groundwater exploitation layer in the West of Dongting Plain, but it is faced with serious water shortage owing to worse water quality, among which ammonium anomaly is the most typical.However, its source and enrichment mechanism has been poorly understood at present.Taking the west of Dongting Plain as the study area, the hydrogeochemical analysis of groundwater samples along the direction of regional groundwater flow was carried out to find out the source of ammonium in groundwater and reveal the controlling mechanism of groundwater flow to the enrichment of ammonium.The results showed that the concentration of NH4-N was 0.05~16.75 mg/L, and had good positive correlations with DOC, HCO3-, As, Fe2+, Mn and P, while the high concentration of NH4-N corresponded to very low concentrations of Cl-, SO42-, NO3-, and very low Cl/Br ratio, it can be speculated that ammonium in shallow confined aquifer was produced by the mineralization of natural organic matter rather than anthropogenic input.Along the groundwater flow direction, the concentrations of NH4-N, As, Fe2+ and Mn increased significantly, indicating that with the more sluggsih groundwater flowed, the particles of water-bearing media were becoming finer and finer, and the organic matter in sediments was more and more enriched, thus the mineralization of nitrogen-bearing organic matter was gradually enhanced and the concentration of NH4-N increased gradually, forming a gradually reduced groundwater environment.As a result, the reductive components (arsenic, iron, manganese, etc) produced from related geochemical processes were also gradually enriched.The study further enriches the genetic theory of geogenic ammonium in groundwater and provides theoretical basis for the safety and security of local water supply.
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图 1 研究区位置及地下水采样点分布图
Figure 1. Location of the study area and distribution of groundwater sampling points
图 2 研究区水文地质平面图(a)和水文地质剖面图(b) (据文献[19]修改)
Figure 2. Hydrogeological plan (a), and hydrogeological profile (b) in the study area
图 4 各离子组分浓度与铵氮的相关性图
Figure 4. Relationship between NH4-N concentration and concentration of each ion component
图 5 铵氮及其相关组分沿地下水流动路径一的质量浓度变化
Figure 5. Change of the concentration of ammonium and related components along the groundwater flow path 1
图 6 铵氮及其相关组分沿地下水流动路径二的质量浓度变化
Figure 6. Change of the concentration of ammonium and related components along the groundwater flow path 2
图 7 研究区地下水铵氮及其相关组分迁移概念模型图
Figure 7. Conceptual model of the migration of ammonium and related components in the study area
表 1 洞庭湖平原第四系地层岩性
Table 1. Lithology Quaternary strata in the Dongting Plain
地层代号 地层名称 埋藏深度/m 主要岩性 水文地质特征 Qh dal 洞庭湖组河湖相沉积 20~25 灰色、灰褐色砂质黏土、含钙质黏土 浅层孔隙潜水含水岩组由第四系全新统组成，广泛分布于湖区平原及湘江、资江、沅江、澧水4条水系的河漫滩上，厚度为5~20 m Qh dl 洞庭湖组湖沼相沉积 50~55 灰色、灰褐色、灰黑色、棕褐色砂质黏土，粉砂、细砂黏土，含钙质粉砂质黏土和淤泥。底部偶含细砾 浅层孔隙承压含水层由上更新统、中更新统组成，厚度为54~150 m。含水量大，易于开采，是区内开采量最大的含水层 Qp3 xs 下蜀组 55~60 黄褐色、棕黄色、灰黄色含铁锰质结核和薄膜的砂质黏土、粉砂质黏土、黏土、粉砂，局部含淤泥质和炭化植物残骸，底部偶发育细砾层 Qp2 mw 马王堆组 130~150 砂砾层、含砾砂层，偶夹黏土 Qp2 b 白沙井组 200~210 冲湖积形成的棕黄色、棕红色砾层、砂层及黏土层 深层孔隙－裂隙承压含水岩组在区内分布广泛，含水介质在水平方向和垂向上都存在很大差异 Qp2 x 新开铺组 280~290 河湖交互相砾石层与黄色砂砾层、粗砾层及泥砾砂互层 Qp1 m 汨罗组 400~410 河湖交互相碎屑黏土 
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表 2 地下水水化学指标统计
Table 2. Statistics of groundwater hydrochemical indexes
水化指标 18S-25G 18S-26G 18S-27G 18S-28G 18S-29G 18S-30G 18S-31G 18S-32G 18S-33G 18S-34G 18S-35G 18S-36G 18S-39G 18S-40G 18S-41G 18S-42G 18S-43G 最大值 最小值 平均值 变异系数 pH 7.32 7.70 7.33 7.16 7.66 7.26 7.32 7.08 7.15 7.19 6.83 6.66 6.76 7.13 7.24 7.31 7.52 7.70 6.66 7.21 0.04 EC/(μS·cm-1) 435 453 492 396 406 407 481 445 440 508 637 600 759 582 396 399 458 759.00 396.00 487.88 0.21 Eh/mV -88.1 -54.9 -73.4 -25.0 -47.4 -79.3 -73.1 -88.0 -73.6 -90.5 -104 -84.7 -96.4 -88.2 -50.4 -72.1 -42.4 -25.00 -104 -72.40 -0.30 NH4-N 0.20 0.31 0.12 0.05 0.14 0.07 1.07 0.75 0.65 2.25 16.75 11.10 7.50 0.70 0.40 0.14 0.14 16.75 0.05 2.49 1.91 Fe2+ 2.25 0.72 0.77 0.58 0.61 0.83 1.57 1.95 1.70 5.45 11.10 20.20 9.70 1.60 2.05 0.74 0.74 20.20 0.58 3.68 1.44 Ca2+ 64.60 50.60 59.10 42.60 57.90 67.80 69.80 61.70 64.50 64.40 77.40 70.40 102 62.90 51.70 59.20 67.80 102 42.60 64.40 0.20 Mg2+ 14.10 18.50 18.40 14.80 15.90 12.10 19.70 18.70 16.30 22.20 21.20 25.60 37.00 24.00 15.90 14.50 18.70 37.00 12.10 19.30 0.30 Na+ 34.90 47.60 51.50 38.10 23.90 26.30 32.40 32.80 28.90 35.30 20.60 16.50 35.60 47.60 35.70 32.80 34.60 51.50 16.50 33.80 0.27 K+ 0.20 0.32 0.21 0.33 0.50 0.28 0.16 0.15 0.26 0.27 0.74 0.61 0.73 0.21 0.15 0.43 0.34 0.74 0.15 0.35 0.56 Cl- 35.90 1.12 3.86 2.19 4.20 1.37 0.68 0.86 1.29 1.30 3.93 0.80 1.09 3.42 0.81 0.76 0.72 35.90 0.72 3.80 2.21 NO3- ρB/(mg·L-1) 0.58 0.55 0.55 0 1.03 0.56 0.55 0.57 0.54 0.56 0.56 0.60 0.59 1.03 0.60 0 0.59 1.03 0 0.53 0.43 SO42- 2.72 0 1.32 2.31 3.45 1.06 0 0 0 0 0 0 0 0 0 0.95 1.01 3.45 0 0.75 1.47 HCO3- 277 366 410 314 321 332 389 356 347 399 460 454 617 425 320 333 377 617 277 382.00 0.21 DOC 2.91 1.84 3.32 6.78 2.52 2.09 5.47 4.15 2.71 1.58 6.58 5.22 5.15 1.35 1.58 1.11 1.39 6.78 1.11 3.28 0.58 Fe 1.01 0.39 0.76 0.37 0.20 0.80 1.47 2.03 1.72 4.86 15.90 20.80 11.10 1.57 0.85 0.90 0.33 20.80 0.20 3.83 1.60 Mn 0.49 0.09 0.30 0.15 0.91 0.32 0.26 0.21 0.17 0.33 0.36 1.73 0.68 0.30 0.24 0.13 0.10 1.73 0.09 0.40 1.02 P 0.22 0.07 0.06 0.12 0.10 0.09 0.53 0.74 0.51 1.27 3.45 1.02 2.17 0.86 0.26 0.06 0.05 3.45 0.06 0.68 1.34 As ρB/(μg·L-1) 1.47 0.72 2.34 0.27 4.56 1.01 19.60 8.87 2.33 3.65 26.60 33.20 9.33 3.13 2.62 0.73 0.79 33.20 0.27 7.13 1.39 S2- 13.00 3.00 0 4.00 2.00 10 5.00 7.00 3.00 9.00 19.00 0 22.00 2.00 13.00 13.00 13.00 22.00 0 8.12 0.81 井深/m 36 30 30 40 58 35 40 25 53 31 38 30 40 25 30 19 40 58 19 35.29 0.04
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表 3 世界范围内已报道的天然高铵氮地下水
Table 3. Natural high concentration of ammonium groundwater reported worldwide
地点 深度/m 最高铵氮质量浓度/(mg·L-1) 水文地质背景 来源 中国珠江三角洲 20~50 390 海岸带更新世砂质含水层 文献[10] 意大利费拉拉 8~10 45 海岸带泛滥平原，第四系古沼泽相泥炭层 文献[11] 越南红河三角洲 50~90 100 河口三角洲平原，第四系泥炭层 文献[12] 孟加拉盆地 28~45 约23 全新世三角洲平原下伏含水层 文献[25] 中国江汉平原 20~30 14.1 冲湖积平原，第四系松散沉积物 文献[27] 美国密歇根 16~19 13.5 冰碛相含水层，全新世湖相淤泥层 文献[29] 澳大利亚珀斯 约20 17.6 潮汐河口第四系砂-黏土夹层 文献[30] 墨西哥北部 < 20 19.2 第四系弱透水层富黏土，咸水 文献[31]
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