Ivana La Licata

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.

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.

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%.

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.

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.

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

In the coming decades, the effects of expected climate change will affect key aspects of our society including quantity and quality of available water resources. It is believed that water resources will play a growing important role in Europe and in particular in the countries south of the Alps, where climate change could hit harder (EEA, 2014). Without appropriate interventions, water will turn from a good hearty undervalued to a scarce and precious resource, whose distribution will be less and less equitable, resulting in growing conflicts for its use. As already recognized by the European directives (2000/60/EC and 2006/118/EC), the theme of quantitative management of water resources is therefore a strategic issue that can allow the reduction of the risks and costs associated with climate change. Integrated management of both surface and groundwater for sustainable use nowadays represents more and more a key concern in environmental policies and water management. The goal of this work is to assess the effects on groundwater storage of a new management of irrigation waters. The highly variable availability of water resources during the hydrological season could be particularly troublesome in the management of irrigation systems, since they need water in seasons (spring-summer) when usually its availability has a depletion. Storage of groundwater in aquifers in colder seasons could be used to soften irrigation systems water deficit in drier ones. Specifically, the practice of "winter irrigation", i.e. the use of the main irrigation channels to recharge aquifers during the colder seasons, could increase water resource availability for irrigation during the drier growing seasons. A pilot groundwater flow transient model (MODFLOW2000, Harbaugh et al., 2000) with a domain of about 255 km2 was implemented in the north zone of Lodi Province, including a large part of the Consorzio Muzza irrigation district. The model was then run to estimate change in groundwater resource availability in two main scenarios: in the first scenario the storage in the shallow aquifer was computed in the colder period (October–March), when irrigation is not occurring; for this case, it was used measured monthly rainfall, average temperature for the period 2004-2013 (representing actual climate conditions) and an estimations of water volume used for irrigation. In the second scenario, an integrated management approach of irrigation and groundwater was tested, adopting "winter irrigation". Both scenarios want to assess if water storage in aquifers in colder periods could represent an additional water volume useful to mitigate possible climate changes effects on crops