Description of the HADES piezometers used for the study of in situ Boom Clay pore water chemistry

The Boom Clay in Belgium is studied as one of the potential host formations for the geological disposal of high-level and/or long-lived radioactive waste. Within this frame, the pore water chemistry of the Boom Clay has been studied for more than 30 years with the main objective to better understand in situ geochemical conditions of the clay, both in undisturbed and in geochemically disturbed conditions. In situ pore water extraction from Boom Clay is realised by using piezometers which are installed in the Boom Clay host rock around the HADES Underground Research Facility. A major update of the description of twelve important piezometers, specifically used for Boom Clay pore water sampling in the HADES Underground Research Facility, is the subject of this reference document. This update includes amongst other activities a profound research in historical documents and maps, a compilation of the most important chronological piezometer data and an in-depth technical survey of the borehole drilling activities, the construction and the installation of these twelve piezometers. Besides, forpractical use a HADES local reference coordination system, that is integrally based on the Belgian Lambert 72 geological reference system, was developed and is described in detail. As a result of these combined research efforts to realize this report, the data in this report and its annexes represent the most accurate available information of the twelve piezometers. Information or data found in other reports that contradict with the updated information or data recorded in this reference report should be considered as erratic. It is noteworthy to mention that some erratic data might even have accumulated in many previous documents, reports or technical notes. We first explain some general hydraulic theoretical aspects that are related to Boom Clay pore water sampling using piezometers. These basic piezometer hydraulics can be used to help when organising a Boom Clay pore water sampling campaign, to evaluate hydraulic responses after induced geological perturbations or to determine fundamental hydraulic characteristics of the Boom Clay rock in its vicinity In the next sections, we explain the general methodology of borehole drilling, construction and installation of the piezometers. We give an explanation of which standard nomenclature is used. Then we discuss the Lambert 72 and the new derived local reference coordination system. We present an overview of the locations of the twelve different piezometers. Each individual piezometer is described in more detail by attempting to answer the questions like: What was the (initial) purpose of the piezometer? Where is it located inside HADES? What is its orientation? How many filters are present and what are their dimensions? … . Furthermore, a description of the different pore water sampling techniques is given. The technical fiches of the piezometers summarize its major technical and hydraulic characteristics and can be used as first quick reference document. The technical drawings of the experimental hard ware are given in the annexes. For the piezometers foreseen with more advanced experimental set-ups, sampling protocols are added in the annexes.