Groundwater bodies (horizonts)

In order to manage the groundwater, the Flemish subsurface is divided into several three-dimensional units: the groundwater bodies. Within these bodies of groundwater, environmental objectives are assessed and measures are imposed if necessary. The definition of groundwater bodies is mandatory in the European Water Framework Directive 2000/60/EC, which defines a body of groundwater as ‘a separate body of water in one or more aquifers’. The classification of the subsurface into different groundwater bodies is mainly based on the physical characteristics of the groundwater reservoirs and on the regional groundwater flow. The sequence of aquifers (sand, gravel, chalk, solid rock,...) and regionally occurring non-aquifers (clay,...) and the prevention of clear barriers to the groundwater flow (thick clay layers, fractures, groundwater separations, highly draining rivers, salination limits, etc.) are the main starting points. The delimitation of groundwater bodies was therefore based on the Hydrogeological Coding of the Underground of Flanders (HCOV coding) and the classification of the subsurface of Flanders into groundwater systems. The groundwater bodies are three-dimensional and can occur side by side and above each other. In total, there are 42 groundwater bodies. They are very different from each other: * surface area from 50 km² to 6 000 km^2 * thickness from 20 m to 1 000 m * Kh from 0.0000005 to 2 300 m/day * from sweet to salt * made up mainly of limestone, sand, gravel, peat, silt,... * *... The groundwater bodies have both a name and a code. The code is structured as follows: The naming of a groundwater body is always based on the HCOV code of the main aquifer. Each groundwater body has also received a meaningful code ‘GWS_HCOV_GWL_NR’. The code consists of an abbreviation of the groundwater system in which the groundwater body is located (e.g. CFS, Central Flemish System), followed by the HCOV code, which corresponds to a main aquifer layer (e.g. 0600 stands for the Ledo-Paniselian-Brussels Aquifer system). Then the abbreviation ‘GWL’ is added, after which a serial number NR indicates the further spatial classification of the aquifer in different regions. Finally, in some cases the letters ‘s’ and ‘m’ were added, indicating that a groundwater body was split into a part that can be located on the one hand in Scheldt district or on the other in the Meuse district. To help clarify the three-dimensional aspect of the groundwater bodies, they were split into horizons, i.e. the order in which groundwater bodies occur in depth. In a given place, each groundwater body occurs maximum in one horizon and there is a maximum of one groundwater body in each horizon. “1” means that the groundwater body appears at the top, “2” that it occurs as a second groundwater body, “3” as a third,... Conversely, there are 4 groundwater bodies above a groundwater body with horizon “5”.为开展地下水管理工作，佛兰德斯（Flemish）地下区域被划分为多个三维单元——地下水含水层体（groundwater bodies）。在这些地下水含水层体范围内，需开展环境目标评估，并在必要时实施管控措施。地下水含水层体的定义是欧盟《2000/60/EC水框架指令》（European Water Framework Directive 2000/60/EC）的强制要求，该指令将地下水含水层体定义为“分布于一个或多个含水层中的独立水体”。 地下区域划分为不同地下水含水层体的主要依据，是地下水储层的物理特性与区域地下水流动规律。核心划分依据包括：含水层序列（砂、砾石、白垩、基岩等）与区域非含水层（黏土等）的分布，以及对地下水流动造成明显阻隔的因素（厚黏土层、断裂带、地下水分隔带、强径流河流、盐化界限等）。因此，地下水含水层体的划定基于佛兰德斯地下水文地质编码系统（Hydrogeological Coding of the Underground of Flanders, 简称HCOV编码），以及佛兰德斯地下区域向地下水系统的分类结果。 地下水含水层体均为三维结构，可横向并列分布或纵向叠置。目前总计包含42个地下水含水层体，彼此间差异显著： - 表面积介于50 km²至6000 km²之间 - 厚度介于20 m至1000 m之间 - 水平渗透系数（Kh）介于0.0000005至2300 m/天之间 - 水质从淡水到咸水不等 - 主要由石灰岩、砂、砾石、泥炭、粉土等构成 每个地下水含水层体均配有名称与编码，编码规则如下： 地下水含水层体的命名始终以主含水层的HCOV编码为依据。此外，每个地下水含水层体还被赋予了兼具业务含义的编码"GWS_HCOV_GWL_NR"。该编码的结构为：首先为该含水层体所属地下水系统的缩写（例如CFS，即中央佛兰德斯系统（Central Flemish System）），随后为对应主含水层层位的HCOV编码（例如0600代表Ledo-Paniselian-Brussels含水层系统）；接着添加缩写"GWL"，之后通过序列号NR对该含水层在不同区域的空间分布进行进一步分类；在部分场景下，还会追加字母“s”与“m”，用于标识该地下水含水层体可分别分布于斯海尔德特（Scheldt）流域或默兹（Meuse）流域。 为明确地下水含水层体的三维空间属性，需将其按深度层序划分为不同层位（horizons），即地下水含水层体在垂向上的出现顺序。在同一地点，每个地下水含水层体最多仅出现在一个层位中，且每个层位最多仅包含一个地下水含水层体。其中，数字“1”代表该含水层体位于最上层，“2”代表其为第二层含水层体，“3”代表第三层，依此类推。反之，若某一地下水含水层体的层位编号为“5”，则其上方共存在4个地下水含水层体。