HCOV_0400, Event area of the Oligocene Aquifer system

Flanders is composed of an alternation of aquifers (sand, gravel, chalk, rock, etc.) and regionally occurring non-aquifers (e.g. clay). The sequence of these aquifers and aquitards has its own coding in Flanders: hydrogeological Coding of the Subsoil of Flanders (HCOV coding). The HCOV coding is composed of main hydrogeological units, sub-units and basic units. The main unit brings together a sequence of geological layers which have broadly the same hydrogeological characteristics and thus form a whole. This hydrogeological main unit comprises the complete aquifer system between the final layers of the Boomse clay (Formation of Boom) and the Bartoon clay (Maldegem Formation). This is a sequence of aquifers, whether or not in contact with each other, separated by non-continuous clay layers of limited size. This main unit consists, inter alia, of the layers belonging to the Formations of Bilzen, Borgloon, Saint Huibrechts-Hern and Zelzate.弗兰德斯地区的地层由含水层（aquifer）（砂、砾石、白垩、岩石等）与局部分布的非含水层（non-aquifer）（如黏土）交替分布。这类含水层与弱透水层（aquitard）的序列在弗兰德斯地区拥有专属编码体系：弗兰德斯地下水文地质编码（Hydrogeological Coding of the Subsoil of Flanders，HCOV）。HCOV编码体系由水文地质主单元、亚单元及基本单元构成。其中，水文地质主单元汇集了一系列水文地质特征大体一致的地质层位，共同构成一个完整的水文地质整体。该主单元的范围覆盖博姆黏土（博姆组）最顶层地层与巴通黏土（马尔德赫姆组）之间的完整含水层系统，包含相互接触或互不接触的含水层序列，各含水层之间被规模有限的非连续黏土层分隔。该主单元尤其涵盖比尔岑组、博尔洛恩组、圣伊夫雷茨-埃尔恩组以及泽尔扎特组所属的地层。