Saltwater Intrusion and Freshwater Storage in Sand Sediments along the Coastline: Hydrogeological Investigations and Groundwater Modeling of Nauru Island

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 se

Hydrogeological study and numerical model of groundwater rise mitigation actions effects in the glacial - fluvioglacial territory of Grandate (Como, north Italy)

Water levels rising, Grandate, Italy, transient numerical mode

A Numerical Study on the Impact of Grouting Material on Borehole Heat Exchangers Performance in Aquifers

U-pipesforgroundsourceheatpump(GSHP)installationsaregenerallyinsertedinvertical 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 hydraulicallyimpermeable,sothatgroundwaterflowinsidetheboreholeisinhibited. Theinfluenceof 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 aquiferwitharelevantgroundwaterflow. Here3maincasesforasingleU-pipeinasandyaquiferare compared. InCase1theboreholeisback-filledwiththesurroundingsoilformation,whileathermally enhancedgroutandalowthermalconductivitygroutareconsideredinCase2andCase3respectively. 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

Assessment of groundwater resources: Nauru project 2010 – 2019

Nauru is a small limestone island in the Pacific region where, as in many small islands in the world, the population heavilyrely upon groundwater as primary freshwater resource, in conjunction with rainwater and desalinated water.The Nauru project started in 2010 and led by Politecnico di Milano (http://nauru.como.polimi.it/), consisted on the hydrogeological characterization of the northern part of the island and the implementation of 3 numerical models for: (1) understanding the mechanisms governing groundwater flow and accumulation, (2) assess the adequate sustainable fresh groundwater exploitation in order to prevent saltwater upconing occurrences in the area more suitable for groundwater extraction and (3) to simulate future scenarios based on climate changes and population growth

Borehole Heat Exchangers: heat transfer simulation in the presence of a groundwater flow

The correct design of the Borehole Heat Exchanger is crucial for the operation and the energy performance of a Ground Source Heat Pump. Most design methods and tools are based on the assumption that the ground is a solid medium where conduction is the only heat transfer mechanism. In turn in regions rich in groundwater the groundwater flow influence has to be assessed, by including the convection effects. In this paper a numerical model of a 100 m U-pipe in a saturated porous medium is presented. The model is created adopting MT3DMS coupled to MODFLOW. A Darcy flow is imposed across the medium. The typical operation of a Borehole Heat Exchanger operating both in winter and in summer is simulated for two years, under different groundwater velocities. The energy injected to and extracted from the ground is derived as a function of the Darcy velocity and compared with the purely conductive case. Temperature fields in the ground at key moments are shown and discussed. From both the energy and the aquifer temperature field points of view, the velocity ranges for respectively negligible and relevant influence of the groundwater flow are identified

Hydrogeological study of the glacial-fluvioglacial territory of Grandate (Como, Italy) and stochastical modeling of groundwater rising

On November 2014, the Municipality of Grandate, near Lake Como, had to deal with a great emergency that was caused by the flooding of factory undergrounds. The authors realized a hydrogeological study to understand the causes of groundwater flooding and to prepare a pre-feasibility study concerning possible actions for groundwater control. The hydrogeological structure is rather complex and required time-consuming reconstruction of the conceptual site model. A transient numerical model was developed to analyse the system behaviour in different scenarios. The flow model was calibrated in a steady and unsteady-state using the automatic calibration code Model-Independent Parameter Estimation (PEST). The study demonstrated that the reason for floods was mainly due to the concurrence of three causes: (1) the hydrogeological structure of the area was recognized as a stagnation zone, (2) groundwater rising, and (3) extremely heavy rainfall in 2014. Through the PEST RandPar function, 100 random rainfall scenarios were generated starting from rainfall data for the last 20 years. The model was used to run 100 1-year long simulations considering the probability distribution of recharge related to the 100 randomly generated rainfall scenarios. Through collecting the piezometric heads that resulted from the simulations, monthly probability curves of groundwater exceeding a threshold level were obtained. The results provided an occurrence probability of groundwater level exceeding the underground structures level between 12% and 15%

Assessment of groundwater resources: Nauru project 2010 - 2019

Nauru is a small limestone island in the Pacific region where, as in many small islands in the world, the population heavily rely upon groundwater as primary freshwater resource, in conjunction with rainwater and desalinated water. The Nauru project started in 2010 and led by Politecnico di Milano (nauru.com.polimi.it), consisted on the hydrogeological characterization of the northern part of the island and the implementation of 3 numerical models for: (1) understanding the mechanisms governing groundwater flow and accumulation, (2) assess the adequate sustainable fresh groundwater exploitation in order to prevent saltwater upconing occurrences in the area more suitable for groundwater extraction and (3) to simulate future scenarios based on climate changes and population growth

Borehole Heat Exchangers: how flow velocity influence and dispersion influence heat transfer

The heat pumps coupled to geothermal systems likely to use low enthalpy resources (T<20°C) are gradually spreading, representing one of the most efficient and lower environmental impact technologies for cooling and heating of buildings. Most common geothermal systems are formed by closed loop boreholes (Borehole Heat Exchangers or BHEs) buried into the ground, typically 100 m deep, where a thermal-carrier fluid is circulated into polyethylene U-pipes, extracting heat from the ground in winter and/or injecting heat into the ground in summer. The energy performance of these systems depends on the heat transfer process between the BHEs and the ground. In many applications the ground can be considered as a purely conductive medium: in fact this hypothesis is at the base of the most commercially availabele tools used to design BHEs, such as GHLEPRO or EED (Hellstrom 2001). Therefore some efforts have recently been carried out to include the effects of the presence of a groundwater flow into the BHEs modeling (Diao 2004). In this case the heat is transported not only by conduction but also by advection. To consider this extended problem could change both the correct prediction of the energy performance of the BHEs and their design and also the investigation of the thermal impact, in other words the temperature perturbation produced by the BHEs operation in surrounding aquifer. The aim of this work is the evaluation of these two aspects, varying the rate of groundwater flow velocity and dispersion coefficient using a numerical model realized through Modflow/MT3D (Angelotti 2014), already validated respect to the Moving Line Source (Molina-Giraldo 2011), demonstrating that both advection and dispersion play an important role in the heat transfer