State Key Laboratory of Biogeology and Environmental Geology & MOE Key Laboratory of Groundwater Circulation and Environmental Evolution, China University of Geosciences, Beijing 100083, PR China; School of Water Resources and Environment, China University of Geosciences, Beijing 100083, PR China.
Geology Institute of China Chemical Geology and Mine Bureau, Beijing 100028, China.
Sci Total Environ. 2023 Jun 1;875:162635. doi: 10.1016/j.scitotenv.2023.162635. Epub 2023 Mar 6.
Concerns have been raised on the deterioration of groundwater quality associated with anthropogenic impacts such as oil extraction and overuse of fertilizers. However, it is still difficult to identify groundwater chemistry/pollution and driving forces in regional scale since both natural and anthropogenic factors are spatially complex. This study, combining self-organizing map (SOM, combined with K-means algorithm) and principal component analysis (PCA), attempted to characterize the spatial variability and driving factors of shallow groundwater hydrochemistry in Yan'an area of Northwest China where diverse land use types (e.g., various oil production sites and agriculture lands) coexist. Based on the major and trace elements (e.g., Ba, Sr, Br, Li) and total petroleum hydrocarbons (TPH), groundwater samples were classified into four clusters with obvious geographical and hydrochemical characteristics by using SOM - K-means clustering: heavily oil-contaminated groundwater (Cluster 1), slightly oil-contaminated groundwater (Cluster 2), least-polluted groundwater (Cluster 3) and NO contaminated groundwater (Cluster 4). Noteworthily, Cluster 1, located in a river valley with long-term oil exploitation, had the highest levels of TPH and potentially toxic elements (Ba, Sr). Multivariate analysis combined with ion ratios analysis were used to determine the causes of these clusters. The results revealed that the hydrochemical compositions in Cluster 1 were mainly caused by the oil-related produced water intrusion into the upper aquifer. The elevated NO concentrations in Cluster 4 were induced by agricultural activities. Water-rock interactions (e.g., carbonate as well as silicate dissolution and precipitation) also shaped the chemical constituents of groundwater in clusters 2, 3, and 4. In addition, SO-related processes (redox, precipitation of sulfate minerals) also affected groundwater chemical compositions in Cluster 1. This work provides the insight into the driving factors of groundwater chemistry and pollution which could contribute to groundwater sustainable management and protection in this area and other oil extraction areas.
人们对与人为影响（如石油开采和过度使用化肥）相关的地下水水质恶化表示担忧。然而，由于自然和人为因素在空间上都很复杂，因此仍然难以确定区域尺度上的地下水化学/污染和驱动力。本研究结合自组织映射（SOM，结合 K-均值算法）和主成分分析（PCA），试图描述中国西北部延安地区浅层地下水水化学的空间变异性和驱动因素，该地区存在多种土地利用类型（如各种采油场地和农业用地）共存。基于主要和微量元素（例如 Ba、Sr、Br、Li）和总石油烃（TPH），通过使用 SOM-K-均值聚类，地下水样本分为四个具有明显地理和水化学特征的聚类：重度石油污染地下水（聚类 1）、轻度石油污染地下水（聚类 2）、污染最少的地下水（聚类 3）和无污染地下水（聚类 4）。值得注意的是，位于长期石油开采的河谷中的聚类 1，其 TPH 和潜在有毒元素（Ba、Sr）含量最高。多元分析结合离子比分析用于确定这些聚类的原因。结果表明，聚类 1 的水化学成分主要是由于与石油相关的生产水侵入上层含水层所致。聚类 4 中升高的 NO 浓度是由农业活动引起的。水-岩相互作用（如碳酸盐以及硅酸盐的溶解和沉淀）也塑造了聚类 2、3 和 4 中地下水的化学成分。此外，与 SO 相关的过程（氧化还原、硫酸盐矿物的沉淀）也影响了聚类 1 中的地下水化学组成。这项工作深入了解了地下水化学和污染的驱动因素，这有助于该地区和其他采油区地下水的可持续管理和保护。
