C. Knudsen, K. Hinsby, R. Jakobsen, Lars Juul Kjærgård, P. Rasmussen
{"title":"Fingerprinting sources of salinity in a coastal chalk aquifer in Denmark using trace elements","authors":"C. Knudsen, K. Hinsby, R. Jakobsen, Lars Juul Kjærgård, P. Rasmussen","doi":"10.34194/GEUSB.V47.5336","DOIUrl":null,"url":null,"abstract":"Salinity levels above the drinking water standard (>250 mg/l Cl–) are observed at shallow depth in a Maastrichtian chalk aquifer on the island of Falster, south-eastern Denmark. To understand the source of the salt, 63 samples from 12 individual, 1 m, screened intervals between 14 and 26 m b.s. were collected from 1 May to 4 June 2018. The samples were collected during a tracer test to estimate the dual porosity properties of the chalk and were analysed for a wide range of elements. Furthermore, samples from the Baltic Sea and from deeper saline aquifers in the area (40 and 85 m b.s.) were analysed for comparison. The geochemical data were analysed using an unsupervised machine-learning algorithm, self-organising maps, to fingerprint water sources. The water composition in the screened intervals at various stratigraphic levels has specific geochemical fingerprints that are maintained for the first days of pumping and are distinct amongst the different levels. This suggests an evolution in water composition because of reaction with the chalk. Water composition is distinct from both seawater from the nearby Baltic Sea and salty water from deeper levels of the reservoir. Thus, neither up-coning of salty water nor intrusion of seawater caused the elevated salinity levels in the area. The slightly saline composition of groundwater in the shallow aquifer (14–26 m b.s.) is more likely because of incomplete refreshing of the salty connate water in the chalk during the Pleistocene and Holocene. Furthermore, the geochemical fingerprint of salty water from the deeper aquifer at 40 m was similar to water from the Baltic Sea, suggesting a Baltic Sea source for salt in the aquifer at 40 m b.s., c. 100 m from the coast. Statistical analysis based on self-organising maps is an effective tool for interpreting a large number of variables to understand the compositional variation in an aquifer and a useful alternative to linear dimensionality-reduction methods such as principal component analysis. The approach using the multi-element analysis combined with the analysis of self-organising maps may be useful in future studies of groundwater quality.","PeriodicalId":48475,"journal":{"name":"Geus Bulletin","volume":null,"pages":null},"PeriodicalIF":2.0000,"publicationDate":"2021-07-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"2","resultStr":null,"platform":"Semanticscholar","paperid":null,"PeriodicalName":"Geus Bulletin","FirstCategoryId":"89","ListUrlMain":"https://doi.org/10.34194/GEUSB.V47.5336","RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q1","JCRName":"GEOLOGY","Score":null,"Total":0}
引用次数: 2
Abstract
Salinity levels above the drinking water standard (>250 mg/l Cl–) are observed at shallow depth in a Maastrichtian chalk aquifer on the island of Falster, south-eastern Denmark. To understand the source of the salt, 63 samples from 12 individual, 1 m, screened intervals between 14 and 26 m b.s. were collected from 1 May to 4 June 2018. The samples were collected during a tracer test to estimate the dual porosity properties of the chalk and were analysed for a wide range of elements. Furthermore, samples from the Baltic Sea and from deeper saline aquifers in the area (40 and 85 m b.s.) were analysed for comparison. The geochemical data were analysed using an unsupervised machine-learning algorithm, self-organising maps, to fingerprint water sources. The water composition in the screened intervals at various stratigraphic levels has specific geochemical fingerprints that are maintained for the first days of pumping and are distinct amongst the different levels. This suggests an evolution in water composition because of reaction with the chalk. Water composition is distinct from both seawater from the nearby Baltic Sea and salty water from deeper levels of the reservoir. Thus, neither up-coning of salty water nor intrusion of seawater caused the elevated salinity levels in the area. The slightly saline composition of groundwater in the shallow aquifer (14–26 m b.s.) is more likely because of incomplete refreshing of the salty connate water in the chalk during the Pleistocene and Holocene. Furthermore, the geochemical fingerprint of salty water from the deeper aquifer at 40 m was similar to water from the Baltic Sea, suggesting a Baltic Sea source for salt in the aquifer at 40 m b.s., c. 100 m from the coast. Statistical analysis based on self-organising maps is an effective tool for interpreting a large number of variables to understand the compositional variation in an aquifer and a useful alternative to linear dimensionality-reduction methods such as principal component analysis. The approach using the multi-element analysis combined with the analysis of self-organising maps may be useful in future studies of groundwater quality.
在丹麦东南部法尔斯特岛的马斯特里赫特白垩含水层的浅层,观测到盐度高于饮用水标准(>250 mg/l Cl–)。为了了解盐的来源,从2018年5月1日至6月4日,从12个1米的个体中采集了63个样本,筛选间隔在14至26米b.s.之间。在示踪剂测试期间收集样本,以估计白垩的双重孔隙度特性,并对各种元素进行分析。此外,还分析了波罗的海和该地区更深含水层(40和85米b.s.)的样本进行比较。使用无监督机器学习算法,自组织地图,对地球化学数据进行分析,以确定水源的指纹。不同地层水平的筛选层段中的水成分具有特定的地球化学指纹,这些指纹在泵送的头几天保持不变,并且在不同水平之间是不同的。这表明,由于和白垩的反应,水成分发生了变化。水的成分与附近波罗的海的海水和水库深层的盐水都不同。因此,无论是盐水的上升锥还是海水的入侵,都没有导致该地区的盐度水平升高。浅层含水层(14–26 m b.s.)中地下水的微盐成分更有可能是由于更新世和全新世白垩系中含盐原生水的更新不完全。此外,40米深含水层的盐水的地球化学指纹与波罗的海的水相似,表明距离海岸约100米的40米b.s.含水层中的盐水来源于波罗的海。基于自组织地图的统计分析是解释大量变量以了解含水层成分变化的有效工具,也是主成分分析等线性降维方法的有用替代方法。将多元分析与自组织地图分析相结合的方法可能在未来的地下水质量研究中有用。