Groundwater-driven temperature changes at thermal springs in response to recent glaciation: Bormio hydrothermal system, Central Italian Alps

Abstract

Thermal springs are widespread in the European Alps, with hundreds of geothermal sites known and exploited. The thermal circulation and fluid outflows were examined in the area around Bormio (Central Italian Alps), where ten geothermal springs discharge from dolomite bodies located close to the regional Zebrù thrust. Water is heated in deep circulation systems and upwells vigorously at a temperature of about 40 °C. Heat and fluid transport is explored by steady and transient three-dimensional finite-element simulations taking into account the effect of the last glaciation, which in the study area was recognized to end around 11,000–12,000 years ago. The full regional model (ca. 700 km²) is discretized with a highly refined triangular finite-element planar grid. Numerical simulations suggest a reactivation of the system following the end of the Last Glacial Maximum. Results correctly simulate the observed discharge rate of ca. 2,400 L/min and the spring temperatures after ca. 13,000 years from deglaciation, and show a complete cooling of the aquifer within a period of approximately 50,000 years. Groundwater flow and temperature patterns suggest that thermal water flows through a deep system crossing both sedimentary and metamorphic lithotypes along a fracture network associated with the thrust system. This example gives insights into the influences of deep alpine structures and glaciations on groundwater circulation that control the development of many hydrothermal systems not necessarily associated with convective heat flow.

Résumé

Les sources thermales sont répandues dans les Alpes Européennes, avec des centaines de sites géothermaux connus et exploités. La circulation thermale et les émergences du fluide ont été étudiées dans la zone autour de Bormio (Alpes Centrales Italiennes), où dix sources géothermales drainent les corps dolomitiques proches de la zone de chevauchement régional du Zebrù. L’eau s’échauffe dans les systèmes de circulation profonds et émerge puissamment à une température proche de 40° C. Le transfert de chaleur et de fluide est étudié grâce à des simulations en régime permanent et transitoire par éléments finis tridimensionnels, prenant en compte l’effet de la dernière glaciation, dont il est admis que dans la zone d’étude elle se termine il y a environ 11,000–12,000 ans. Le modèle régional complet (environ 700 km²) est discrétisé par une grille plan à haute définition aux éléments finis triangulaires. Les simulations numériques suggèrent une réactivation du système survenant à la fin du Dernier Maximum Glaciaire. Les résultats simulent correctement le débit de décharge observé d’à peu près 2,400 L/min et la température des sources 13,000 années après la déglaciation, et montrent un refroidissement complet de l’aquifère pendant une période d’environ 50,000 ans. Les modèles de température et d’écoulement d’eau souterraine suggèrent que l’eau thermale s’écoule à travers un système profond recoupant à la fois des lithotypes sédimentaires et métamorphiques le long d’un réseau de fractures associé au système de chevauchement. Cet exemple apporte une connaissance poussée de l’influence des structures alpines profondes et des glaciations sur la circulation d’eau souterraine qui contrôle le développement de nombreux systèmes hydrothermaux, pas nécessairement associés à un flux de chaleur convectif.

Resumen

Los manantiales termales están muy extendidos en los Alpes europeos, con cientos de sitios geotérmicos conocidos y explotados. La circulación térmica y los flujos de fluidos se examinaron en la zona de Bormio (Alpes italianos centrales), donde diez manantiales geotermales descargan de cuerpos dolomíticos situados cerca del corrimiento regional de Zebrù. El agua se calienta en sistemas de circulación profunda y se eleva vigorosamente a una temperatura de aproximadamente 40 °C. El calor y el transporte de fluidos se exploran mediante simulaciones de elementos finitos tridimensionales, estables y transitorios, teniendo en cuenta el efecto de la última glaciación, que en el área de estudio fue reconocida como terminada hace alrededor de 11,000–12,000 años. El modelo regional completo (aproximadamente 700 km²) se discretiza con una cuadrícula planar triangular de elementos finitos altamente refinada. Las simulaciones numéricas sugieren una reactivación del sistema después del final del último máximo glacial. Los resultados simulan correctamente la tasa de descarga observada de aproximadamente 2,400 L/min y las temperaturas del manantial después de aproximadamente 13,000 años a partir de la desglaciación, y muestran un enfriamiento completo del acuífero en un período de aproximadamente 50,000 años. Los patrones de flujo y de temperatura del agua subterránea sugieren que el agua termal fluye a través de un sistema profundo que cruza los litotipos sedimentarios y metamórficos a lo largo de una red de fracturas asociada con el sistema de corrimiento. Este ejemplo da una idea de las influencias de estructuras alpinas profundas y de las glaciaciones en la circulación del agua subterránea que controla el desarrollo de muchos sistemas hidrotermales no necesariamente asociados con el flujo convectivo de calor.

摘要

热泉广泛分布于欧洲阿尔卑斯山脉,数百个热泉非常知名并且得到开发。在博尔米奥(意大利中部的阿尔卑斯山脉)周边地区对热循环和流体流出物进行了检查,在这里有十个热泉从位于区域的Zebrù冲断层附近的白云岩体向外排泄。水在深部循环系统中被加热,向外喷涌的温度达40 °C 。考虑到本地区大约11,000–12,000年前终结的最后的冰川作用,通过稳定和瞬时三维有限元模拟探索了热量和流体传输。完全区域模型(700 km²)离散成高度精细的三角有限元平面网格。数值模拟显示,在最后大冰期结束后系统有一个再活跃期。结果正确地模拟了观测到的大约2,400 L/min 的排泄量,及冰期结束后大约13,000年后的泉温度,显示出在大约50,000年的时间内含水层完全冷却。地下水流和温度模式显示,热水流经深部系统,穿过沿与冲断层相关的断裂网络的沉积岩类和变质岩类。这个实例使人们了解到深部阿尔卑斯山脉结构和冰川作用对对地下水循环的影响,而地下水循环控制着许多不一定和传递性热流相关的水热系统的发育。

Resumo

Nascentes termais são disseminadas nos Alpes Europeus, com centenas de localidades geotermais conhecidos e explorados. A circulação termal e o fluxo de saída foram examinados ao redor de Bormio (Alpes Italianos Centrais), onde dez nascentes geotermais de corpos de dolomita foram localizadas próximas ao thrust regional Zebrù. A água é aquecida em sistemas de circulação profunda e eleva-se vigorosamente a uma temperatura por volta de 40°C. O transporte térmico e de fluidos é explorado por simulações estáveis e de elementos finitos tridimensionais transientes levando em consideração o efeito da última glaciação, que na área de estudo reconhece-se ter terminado por volta 11,000–12,000 anos atrás. O modelo regional total (cerca de 700 km²) é discretizado a partir de uma malha plana triangular de elementos finitos altamente refinada. Simulações numéricas sugerem uma reativação do sistema seguindo o final do Último Máximo Glacial. Os resultados simularam corretamente a taxa de descarga por volta de 2,400 L/min uto e a temperatura da água depois de cerca de 13,000 anos da glaciação, e demonstra um resfriamento completo do aquífero no período de aproximadamente 50,000 anos. O fluxo de águas subterrâneas e os padrões de temperatura sugerem que o fluxo de águas termais permeia através de sistemas profundos atravessando ambos os tipos litológicos sedimentar e metamórfico pela rede de fraturas associado ao sistema de empurramento (thrust). Esse exemplo nos mostra entendimentos das influências das estruturas alpinas profundas e glaciações na circulação das águas subterrâneas que controlam o desenvolvimento de muitos sistemas hidrotermais não necessariamente associados com o fluxo de calor convectivo.

Appendix

FEFLOW solves the following set of governing equations in saturated porous media:

Fluid mass conservation:

$$ S\frac{\partial \varphi}{\partial \mathrm{t}}+\operatorname{div}\left(\mathbf{q}\right)=0 $$

(1)

Darcy’s law:

$$ \mathbf{q}=-\mathbf{K}\left(\operatorname{grad}\left(\varphi \right)+\frac{\rho_{\mathrm{f}}-{\rho}_{0\mathrm{f}}}{\rho_{0\mathrm{f}}}\mathbf{u}\right) $$

(2)

Energy balance equation:

$$ \frac{\partial }{\partial t}\left\{\left[{\varphi \rho}_{\mathrm{f}}{c}_{\mathrm{f}}+\left(1-\varphi \right){\rho}_{\mathrm{s}}{c}_{\mathrm{s}}\right] T\right\}+\operatorname{div}\left({\rho}_{\mathrm{f}}{c}_{\mathrm{f}} T\mathbf{q}\right)-\operatorname{div}\left[\uplambda \mathrm{grad}(T)\right]=0 $$

(3)

In the equation of fluid mass conservation (Eq.1 ), S is the specific storage, φ is the hydraulic head and q is the Darcy velocity defining the specific discharge of the fluid.

In the Darcy’s law (Eq. 2) K is the hydraulic conductivity tensor, u the gravitational unit vector and $\frac{\rho_{\mathrm{f}}-{\rho}_{0\mathrm{f}}}{\rho_{0\mathrm{f}}}\mathbf{u}$ is the buoyancy force induced by density variation (ρ _0f is the reference value of the fluid density ρ _f).

In the energy balance equation for the fluid and the porous medium (Eq. 3), c _f and c _s denote the heat capacity of the fluid and the solid, respectively, T is the temperature and λ is the thermal conductivity of the saturated porous medium.

The flow and transport equations (Eqs. 2 and 3) are non-linear and strongly coupled since temperature controls the hydraulic conductivity tensor K, the fluid density and dynamic viscosity, as expressed by the following constitutive and phenomenological relation:

$$ \mathbf{K}=\frac{\mathbf{k}{\boldsymbol{\rho}}_{0\mathrm{f}} g}{{\boldsymbol{\mu}}_{\mathrm{f}}\left( C, T\right)} $$

(4)

In Eq. 4, K is the hydraulic conductivity tensor, k is the permeability tensor, g is the gravitational acceleration and μ _f(C, T) takes into account the fluid viscosity effects due to temperature and concentration variations.

The EOS for the fluid density is written as:

$$ {\rho}_{\mathrm{f}}={\rho}_{0\mathrm{f}}\left[1-\overline{\beta}\left( T, p\right)\left( T-{T}_0\right)+\overline{\gamma}\left( T, p\right)\left( p-{p}_0\right)\right] $$

(5)

The fluid density in the single liquid phase is expressed in terms of reference values for density, temperature and pressure (ρ ₀, T ₀ and p ₀). $\overline{\beta}\left( T, p\right)$is the coefficient of thermal expansion and$\kern0.5em \overline{\gamma}\left( T, p\right)$ is the coefficient of compressibility. The polynomial expressions used to fit the coefficients in a wide range of temperature (0 ≤ T ≤ 350 °C) and pressure (p _sat ≤ p ≤ 100 MPa) are given in Magri et al. (2009).

The fluid viscosity is calculated with the following function as shown in WASY (2002).

$$ \frac{\mu (T)}{\mu \left({T}_0\right)}=\frac{1+0.70603\times {\varsigma}_{{\mathrm{T}}_0}-0.04832\times {\varsigma}_{{\mathrm{T}}_0}^3}{1+0.70603\times \varsigma -0.04823\times {\varsigma}^3};\varsigma =\frac{T-150}{100};{\varsigma}_{{\mathrm{T}}_0}=\frac{T_0-150}{100} $$

(6)