Groundwater Exploitation as Thermal Fluid in Very-Low Enthalpy Geothermal Plants in Coastal Aquifers

Recently, in Italy, the interest for very low enthalpy geothermal resources (T < 20 °C) is growing. This is mainly because, these resources are widely available throughout the country and also unlike the other green energy sources (eg. solar and wind energy), and they do not need to be stored. Among the direct-use of geothermal resources, the open-loop groundwater heat pump (GWHP) system needs particular attention in terms of potential environmental impact. In coastal areas, that are generally densely populated, the installation of GWHP system is particularly appealing because the presence of shallow aquifers. This means significant savings of economic resources in terms of pumping energy and drilling costs. Nevertheless, vast areas of the Italian coastlines, as well as those of other Mediterranean countries, are often affected by seawater intrusion and hence are ruled by restrictive laws aimed to protect the groundwater quality and quantity. In this chapter the environmental impacts, associated with the exploitation of low enthalpy geothermal resources, were assessed. For the purpose, a costal karst area in Southern Italy affected by seawater intrusion was investigated. A detailed characterization of the area was achieved in terms of geological, hydrogeological, geochemical and meteorological parameters. Moreover, the influence of an open-loop geothermal systems on the sea water intrusion was also studied by means of a long-term pumping test. The investigated portion of aquifer was found to have a high hydraulic conductivity, as well as high and fast recharge rates, highlighting a good productivity of aquifer. The temperature of groundwater, reaching over 20 °C near the coast, was particularly useful for direct use especially for the space heating and cooling. The long pumping test, lasted for 16 days, not affected the lowering of the water table that naturally occurs in the dry period. On the contrary, the reinjection of the extracted groundwater into the surface water drainage network partially restored the water table. The test also not detected any quality degradation of groundwater induced by pumping. The quality of groundwater showed that the level of contamination in the investigated area was generally high both because of the presence of urban and industrial pollution and because of the presence of the seawater intrusion. The absence of a strong competition for use of groundwater makes them available for geothermal use. An extensive utilization of natural heat for the space cooling is also justified considering the local climate characteristics of the area that cause a peak of thermal energy demand in summer.