U-pipes for ground source heat pump (GSHP) installations are generally inserted in vertical boreholes back-filled with pumpable grouts. Grout thermal conductivity is a crucial parameter, dominating the borehole thermal resistance and impacting the heat exchanger efficiency. In order to seal the borehole and prevent leakages of the heat carrier fluid, grouting materials are also hydraulically impermeable, so that groundwater flow inside the borehole is inhibited. The influence of groundwater flow on the borehole heat exchangers (BHE) performance has recently been highlighted by several authors. However groundwater impact and grouting materials influence are usually evaluated separately, disregarding any combined effect. Therefore simulation is used to investigate the role of the thermal and hydraulic conductivities of the grout when the BHE operates in an aquifer with a relevant groundwater flow. Here 3 main cases for a single U-pipe in a sandy aquifer are compared. In Case 1 the borehole is back-filled with the surrounding soil formation, while a thermally enhanced grout and a low thermal conductivity grout are considered in Case 2 and Case 3 respectively. Simulations are carried out maintaining the inlet temperature constant in order to reproduce the yearly operation of the GSHP system. For each of the 3 cases three different groundwater flow velocities are considered. The results show that a high thermal conductivity grout further enhances the effects of a significant groundwater flow. The conditions when neglecting the grout material in the numerical model does not lead to relevant errors are also identified.

Water resources sustainable management is a vital issue for small islands where groundwater is often the only available water resource. Nauru is an isolated and uplifted limestone atoll island located in the Pacific Ocean. Politecnico di Milano performed a feasibility study for the development of sustainable use of groundwater on the island. This paper focuses on the first phase of the study that concerns the conceptual site model development, the hydrogeological characterization and the 2D model implementation. During the project, different activities were performed such as GNSS topographic survey of monitoring wells and groundwater level surveys taking into account tidal fluctuation. This data collection and the analysis of previous studies made it possible to identify the most suitable areas for groundwater sustainable extraction. The characterization findings suggested, unlike previous studies and surveys, the presence of only few drought resilient thin freshwater lenses, taking place in low conductivity sandy deposits, unexpectedly next to the seashore. Thanks to the 2D modeling results, it has been possible to clarify the mechanism that allows the storage of freshwater so close to the sea.

A proper management of fresh groundwater lenses in small islands is required in order to avoid or at least limit uncontrolled saltwater intrusion and guarantee the availability of the resource even during drought occurrences. An accurate estimation of the freshwater volume stored in the subsoil is a key step in the water management decision process. This study focused on understanding the hydrogeological system behaviour and on assessing the sustainable use of the groundwater resource in Nauru Atoll Island (Pacific Ocean). A first phase, concerning the hydrogeological characterization of the island, highlighted the occurrence of few drought-resilient freshwater lenses along the seashore. The second part of the study focused on the characterization of a freshwater lens found in the northern coastal area and identified such area as the most suitable for the development of groundwater infrastructures for water withdrawal. The characterization activities allowed quantifying the freshwater lens thickness and volume in order to assess the capability to satisfy the population water demand. A geo-electrical tomography survey was carried out, and a 3D density-dependent numerical model was implemented in SEAWAT. The model results demonstrated that in small islands freshwater can unexpectedly accumulate underground right along the seashore and not in the centre of the island as is commonly believed. Furthermore, the model can constitute a useful tool to manage the groundwater resources and would allow the design of sustainable groundwater exploitation systems, avoiding saltwater intrusion worsening.

In Ground Source Heat Pump installations the U-shaped pipes are located in boreholes filled with grouts that are enhanced from the thermal conductivity point of view and usually hydraulically impermeable. Here two cases are considered: (1) a numerical finite-difference model of a single U-pipe in a sandy aquifer, implemented using the MT3DMS code, where the grout material surrounding the Borehole Heat Exchanger is not simulated and (2) the same numerical model where, in turn, the presence of the grout is accounted for in a small volume around the Borehole Heat Exchanger, as in real in situ conditions. Simulations were carried out first keeping the heat rate constant, as when simulating a Thermal Response Test, and then maintaining the inlet temperature constant in order to reproduce the yearly operation of the Ground Source Heat Pump system.\ud Then, fitting the numerical ground response with analytical solutions and\ud comparing the two models, the effect of the grout material was assessed in\ud terms of exchanged energy and temperature distribution in the subsoil.\ud Results show that the presence of the grout around the Borehole Heat\ud Exchanger is negligible from the exchanged energy and temperature\ud distribution point of view. Whereas, looking at the Darcy velocities it was\ud noticed that the ones returned by the analytical solutions (that necessarily\ud neglects the presence of the grout) are at least 30% smaller than the\ud velocities implemented in the grouted model. So when comparing the\ud MT3D simulations, or using MT3D for TRT interpretation, the presence of\ud the grout around the BHE can at a first approximation be disregarded in\ud terms of both exchanged energies and temperature distribution in the\ud subsoil. In turn when Darcy velocity, obtained by Thermal Response Test\ud interpretation through analytical solutions (MLS), is used in a numerical\ud model it must be corrected specially for advection-dominated cases