Groundwater flow analysis using different geothermal constraints: The case study of Acqui Terme area, northwestern Italy

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Abstract

We review some analytical techniques that use underground thermal data as tracers of groundwater flow. These techniques allow the evaluation of the Darcy velocity in shallow aquifers of mid-low permeability and the evaluation of heat gain/loss by conduction in deeper aquifers. Examples of application are then given for the Acqui Terme hydrothermal system, located in the Tertiary Piedmont Basin (northwestern Italy). The analysis of borehole temperatures allowed the inference of the hydraulic features of the sedimentary cover of the hydrothermal system. The results show the presence of a relatively weak flow, with upward and horizontal components, only in conglomerates occurring at the base of the marly impermeable cover. The analysis of the heat transported in the deep parts of the hydrothermal system was approached by splitting the water path into different sections, each with given shape, slope and hydraulic properties. The recharge area is situated in the upland, south of the discharge area. Meteoric water initially descends and then flows horizontally within the fractured metamorphic basement of the basin, heating by conduction. Finally, from a reservoir positioned at intermediate depths, hot water reaches rapidly the surface through a sub-vertical fault. This scheme of deep water flow is constrained by the regional surface heat flow and the local geothermal gradient, and it is consistent with data of rock–water equilibrium temperature.

Research Highlights

► We review some techniques which use underground thermal data as tracers of groundwater flow. ► Examples of application are given for the Acqui Terme hydrothermal system (northwestern Italy). ► Deep water flow is constrained by means of the regional heat flow and the local geothermal gradient.

Introduction

The study of groundwater flow and heat transport mechanisms is vital in understanding and exploring geothermal resources. Within any aquifer, the analysis of the thermal effects of groundwater flow should be carried out on a wide variety of scales and quantitative descriptions may become a difficult task. For these reasons, several strategies have been developed to explore the heat transport associated with groundwater flow, which involve the use of more or less sophisticated models. A common approach is to apply numerical groundwater flow models that are constrained by borehole data, including measurements of temperature, head, hydraulic conductivity and tracer concentrations, in addition to geologic information (e.g., Haenel et al., 1988, Beck et al., 1989, Anderson and Woessner, 1992, Swanson and Bahr, 2004). However, water and heat transfer analytical solutions are still a benchmark for the formulation of more complex numerical simulations.

This paper reviews some analytical techniques for the study of underground heat and water flows, focusing on those of particular interest in the investigations of hydrothermal systems. As a case study, these techniques are applied to the hydrothermal system of Acqui Terme located in the Tertiary Piedmont Basin (Fig. 1). This basin, placed at the suture between the Alps and the Apennine belts (northwestern Italy), consists of a thick Oligo-Miocenic sedimentary cover of marls and embedded sandstone layers. There is evidence of a positive thermal anomaly centred on the Acqui Terme district, where numerous thermal springs with a total flow of more than 15 l s 1 give a thermal yield of 3 MW (Pasquale et al., 1986, Chiozzi et al., 1998). The main hot spring has maximum temperature of 70 °C. The regional context of the basin is non-volcanic and of normal surface heat flow. The water flow path, the heating mechanism and the structure of the hydrothermal system is still poorly known (Bortolami et al., 1983, Verdoya et al., 1999, Verdoya et al., 2008).

We first attempt to deduce information on the hydraulic features of the sedimentary cover of the Acqui Terme hydrothermal system from borehole thermal data. The heat transfer of a deep groundwater flow through fractured medium is then discussed. This approach is used to formulate a model of the groundwater flow path in the metamorphic basement of the basin. The discharge temperature at the main spring, the local geothermal gradient and the regional surface heat flow are considered as fundamental constraints to assess the relative scale of groundwater flow.

Section snippets

Hydrothermal regime of the sedimentary cover

Temperature–depth data from saturated porous layers contain quantitative information on groundwater flow (see, e.g., Anderson, 2005, Pasquale et al., 2010). We show how such information can be extracted by means of some analytical solution for heat and water flow. These solutions, generally used to study aquifers of mid-low permeability, are then applied in fully saturated porous layers of the sedimentary cover of the Acqui Terme hydrothermal system.

Deep aquifer

In this section, we review some analytical solutions and show how they provide a quantitative tool for analyzing the water temperature along the deep flow path of the Acqui Terme hydrothermal system. Beneath the sedimentary cover of the Tertiary Piedmont Basin, the presence of an aquifer in the fractured uppermost part of the Palaeozoic–Mesozoic (Alpine) metamorphic basement is suggested by Pasquale et al., 1986, Chiozzi et al., 1998, on the basis of stratigraphic and thermal data. This aquifer

Discussion

The results of remove Darcy velocity inferred for the sedimentary cover of the investigated hydrothermal system can be, in principle, affected by different error sources. In all the proposed expressions, uncertainties in thermal properties are not a major concern for the estimation of thermo-hydraulic parameters. Due to both instrumental errors and vertical variations of lithology, a maximum uncertainty of about 10% can be considered for the bulk thermal conductivity. This uncertainty yields a

Conclusions

Despite the possible limitations, the proposed approach shows that underground temperatures are a natural and sensitive tracer of subsurface flows. The regional surface heat flow and the local geothermal gradient provide independent constraints on the depth of groundwater flow. Together with rock–water equilibrium temperature data, they allow the assessment of the relative scale of groundwater flow. Water flow in aquifer is complicated by preferential fracture flow paths, matrix diffusion,

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