珠江三角洲第四纪各地层生成与存储天然铵能力的探讨与对比

Comparison of the capacity of ammonium generation in the Quaternary formations in the Pearl River Delta

珠江三角洲第四纪底部含水层当中的天异常高然铵,来自于第四纪沉积当中的有机质在厌氧条件下的矿化。珠江三角洲第四纪晚期的沉积序列,主要包括两层海相沉积层(M1和M2),以及两层陆相沉积层(T1和T2)。然而,截至目前,上述地层对天然铵的生成与存储作用,尚不明确。从三角洲的内陆到近岸,选取重点钻孔BJ8、SD1、SD14和MZ4,利用准确的定年数据,以及钻孔剖面各类铵以及总有机碳等数据,探讨与比较了珠江三角洲第四纪各地层生成与储存天然铵的能力。结果表明:全新世时期的海相沉积层(M1),具有高达17.4 g·kg-1的沉积有机质质量分数。对SD14钻孔剖面上高精度的有机碳分析表明,M1、M2层平均有机质质量分数分别为11.7和10.1 g·kg-1。因此,相对于晚更新世时期的海相沉积层(M2)来说,全新世海相沉积层M1具备生成更多铵的能力。对SD14钻孔剖面上各类铵的分析表明,M1和M2总铵质量分数的平均值分别为0.41和0.31 g·kg-1;M1、M2和T2各地层单位面积所储存铵的平均值分别为28.6、11.25和0.34 kg·m-2。而不同地层铵含量的差异,在该研究关注的其他钻孔BJ8,SD1和MZ4也非常明显。因此得出结论,全新世海相沉积层M1,是主要的储铵层,而M2则是次要的储铵层。两个陆相沉积层T1和T2,不论在生铵和储铵的功能上,远远小于两个海相沉积层。在M1层中,铵的量呈现随深度增加而升高的趋势,原因在于铵不断生成累积,并通过扩散作用向下运移。而M2层中铵的含量呈现由上至下递减的趋势,说明M2层中的铵主要来自于上部M1层的扩散,其本身生成铵的量比M1少。M1层在珠江三角洲广泛发育,M2层经过长期的风化和剥蚀,在珠江三角洲许多地方已缺失。珠江三角洲底部含水层中天然高铵的浓度,主要由M1层生成与存储的铵的总量所决定。

The highest ammonium concentration in the confined basal aquifer in the Pearl River Delta （PRD）, China, was found to be 390 mg·L-1. The ammonium is naturally originated from mineralization of sedimentary organic matter under strict anaerobic conditions. Sedimentary sequences of Quaternary age in the PRD are mainly composed of two marine units （M1 and M2） and two terrestrial units （T1 and T2）. This study aimed to investigate the capacity of ammonium generation in the Quaternary formations. Core samples in four key boreholes （BJ8, SD1, SD14 and MZ4） from the inland area to the coast were used to study the hydrogeochemical parameters and the ages of the sediments using deferent dating methods. The results demonstrated that M1 had high contents of sedimentary organic carbon, up to 17.4 g·kg-1. Analysis of sedimentary organic matter on the vertical profile of SD14 showed that the average sedimentary organic matter contents for M1 and M2 were 11.7 and 10.1 g·kg-1, respectively. This indicated the Holocene marine sequence M1 was able to generate larger amount of ammonium than the older marine sequence M2. Various ammonium analyses on the vertical profile of SD14 demonstrated that the average total ammonium contents for M1 and M2 were 0.41 and 0.31 g·kg-1, respectively;the average ammonium contents per unit area for M1, M2 and T2 were 28.6, 11.25 and 0.34 kg·m-2, respectively. Obvious differences of ammonium contents in different sedimentary sequences were also found to exist in other boreholes studied in this research. The above results indicated that M1 was the main sedimentary sequence for ammonium generation and preservation;M2 was the secondary sequence for ammonium generation and preservation. Total available ammonium contents in M1 increased with the depth, and this was mainly caused by ammonium accumulation and diffusion within the marine sediments. On the contrary, Total available ammonium contents in M2 decreased with the depth, indicating that significant amounts of ammonium in M2 were derived from M1. M1 was more extensively developed than M2 in the PRD. Therefore, ammonium concentrations in the confined basal aquifer were controlled mainly by the amounts of ammonium generated and stored within the overlying M1.