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Better constraining silica-enthalpy mixing models in a setting of two separate (karst and non-karst) dilution regimes

Modèles de mélange silice-enthalpie mieux contraints dans un contexte de deux régimes de dilution distincts (karstiques et non karstiques)

Mejores modelos de mezcla sílice - entalpía en un entorno de dos regímenes de dilución separados (kársticos y no kársticos)

在一组两个(岩溶和非岩溶)不同的稀释环境下更好约束二氧化硅-热焓的混合模型

Modelos de mistura de melhor constrição de entalpia de sílica em um cenário de dois regimes de diluição separados (cárstico e não cárstico)

Abstract

It would be possible to elucidate an ambiguity associated with the “silica-enthalpy mixing models” geothermometric technique, provided that an independent estimate was obtained concerning the enthalpy and the dissolved SiO2 content of the hot end-member involved in a mixture between two liquid-phase groundwaters. This would allow one to ascertain whether or not, prior to mixing with the cold parent-water, the hot end-member had undergone a boiling process. The specificities of the Băile Herculane geothermal system (South Carpathians, Romania) provides an opportunity to solve this uncertainty, in contrast to the majority of the so-far investigated natural settings. Advantage has been taken, in this respect, of the fact that a hot brackish Na-Ca-Cl-type parent-water was found to be involved at this site in two separate mixing settings: one which involved only a diluting fluid that was, itself, chloride-rich, and another one which additionally involved, as a third end-member, karst freshwater. By exploiting the two distinct, and converging, linear mixing trends (obtained from the conservatively-behaving natural tracers Cl and Na+, as well as reasonably tight correlations between the chloride concentration and the contents of both heat and SiO2), the concerned hot brackish end-member contents of heat (enthalpy) and of conservative solutes were quite accurately assessed. This result enabled independent validation of the “no steam loss” hypothesis, out of various alternative scenarios proposed by the “silica-enthalpy mixing models” technique.

Résumé

Il serait possible d’élucider une ambiguïté associée à la technique de géothermométrie des “modèles de mélange silice-enthalpie”, à condition qu’une estimation indépendante ait été obtenue concernant l’enthalpie et la teneur en SiO2 dissoute du membre de l’extrémité chaude impliqué dans un mélange entre deux eaux souterraines en phase liquide. Cela permettrait de déterminer si, avant le mélange l’eau froide d’origine, le membre de l’extrémité chaude avait subi un processus d’ébullition. Les spécificités du système géothermal de Băile Herculane (Sud des Carpathes, Roumanie) donnent l’occasion de lever cette incertitude, contrairement à la majorité des milieux naturels étudiés à ce jour. L’avantage a été pris à cet égard, du fait qu’une eau chaude originelle saumâtre de type Na-Ca-Cl a été impliquée sur ce site dans deux conditions distinctes de mélange: une impliquant seulement un fluide de dilution qui était, lui-même, riche en chlorures, et une autre impliquant de plus, en tant que troisième membre, une eau douce karstique. En exploitant les deux tendances de mélange linéaire distinctes, et convergentes (obtenues à partir des traceurs naturels conservatifs Cl et Na+, ainsi que de raisonnablement bonnes corrélations entre les concentrations des chlorures et les teneurs de la chaleur et de SiO2), les teneurs en chaleur (enthalpie) de l’élément de l’extrémité chaud et saumâtre et des solutés conservatifs ont été évaluées de façon très précise. Ce résultat a permis de valider de manière indépendants l’hypothèse de “non perte de vapeur”, à partir de divers scénarios alternatifs proposé par la technique des “modèles de mélange silice-enthalpie”. This result enabled independent validation of the “no steam loss” hypothesis, out of various alternative scenarios proposed by the “silica-enthalpy mixing models” technique.

Resumen

Sería posible dilucidar una ambigüedad asociada con la técnica geotermométrica en los “modelos de mezcla de sílica-entalpía”, siempre que se obtuviera una estimación independiente con respecto a la entalpía y el contenido de SiO2 disuelto del miembro del extremo caliente involucrado en una mezcla entre dos fases líquidas de las aguas subterráneas. Esto permitiría determinar si, antes de mezclarse con el agua original fría, el miembro del extremo caliente se había sometido o no a un proceso de ebullición. Las especificidades del sistema geotérmico de Băile Herculane (South Carpathians, Romania) brindan la oportunidad de resolver esta incertidumbre, en contraste con la mayoría de los escenarios naturales hasta ahora investigados. Se ha aprovechado, a este respecto, el hecho de que se encontró un agua salobre caliente de tipo Na-Ca-Cl en este sitio en dos configuraciones de mezcla separadas: una que involucraba solo un fluido diluyente que era en sí mismo, rico en cloruro, y otro que adicionalmente involucró, como tercer miembro extremo, agua dulce kárstica. Al aprovechar las dos tendencias de mezcla lineales distintas, y convergentes (obtenidas por trazadores naturales, Cl and Na+, de comportamiento conservador, así como por correlaciones razonablemente ajustadas entre la concentración de cloruro y el contenido tanto de calor como de SiO2), los contenidos de calor (entalpía) del extremo salobre caliente y de los solutos conservadores se evaluaron con bastante precisión. Este resultado permitió la validación independiente de la hipótesis “sin pérdida de vapor”, de varios escenarios alternativos propuestos por la técnica de “modelos de mezcla sílice-entalpía”.

摘要

如果能获取两个液相地下水混合物中有关热端员的热焓和溶解二氧化硅含量独立估算值, 那么,就有可能阐明与“二氧化硅-热焓的混合模型”地热温度技术相关的含糊情况。这将使人们能够确定与冷的母水混合之前热端员是否经历了沸腾过程。相对于大多数目前调查研究的自然背景来说,(罗马尼亚喀尔巴阡山南部)Băile Herculane地热系统的特异性为解决这个不确定性提供了机会。在这方面占有优势,发现在两个不同的混合背景下都有热的微咸Na-Ca-Cl-类型母水,这为研究提供了优势:一种背景只涉及稀释的液体,这种液体本身富含氯化物,另一种背景还涉及作为第三端员的岩溶水。通过找到两个明显的及会聚的线性趋势(从中规中矩的天然示踪剂Cl 和 Na+,及氯化物含量与热量和SiO2含量之间的合理严格对比中获取),就可以准确地评价有关热量(热焓)和保守溶质的热微咸端员含量。这个结果能够使人们在“二氧化硅-热焓混合模型”技术提出的众多的可选择的方案中独立确认“无蒸汽损失”假设。

Resumo

Seria possível elucidar uma ambiguidade associada à técnica geotermométrica de “modelos de mistura de entalpia de sílica”, desde que uma estimativa independente fosse obtida com relação à entalpia e ao conteúdo de SiO2 dissolvido do membro final envolvido em uma mistura entre águas subterrâneas de duas fases líquidas. Isso permite determinar se é ou não, antes de se misturar com a fonte de água fria, o elemento final quente que tivesse sofrido um processo de ebulição. As especificidades do sistema geotérmico Băile Herculane (Cárpatos do Sul, Romênia) oferecem uma oportunidade para resolver essa incerteza, em contraste com a maioria dos cenários naturais investigados até agora. Nesse sentido, foi tirada vantagem do fato de ter sido encontrada uma fonte de água quente salobra do tipo Na-Ca-Cl para ser envolvida neste local em duas configurações de mistura separadas: uma que envolveu apenas um fluido de diluição que era, ela própria, rica em cloreto; e outra que envolveu, adicionalmente, como terceiro membro final, água doce cárstica. Ao explorar as duas tendências distintas, e convergentes, de mistura linear (obtidas dos traçadores naturais de comportamento conservador Cl e Na+), bem como correlações estreitas entre a concentração de cloreto e o conteúdo de muito calor e SiO2), os conteúdos do membro final quente salobro do calor (entalpia) e dos solutos conservativos foram avaliados com bastante precisão. Esse resultado permitiu a validação independente da hipótese “sem perda de vapor”, a partir de vários cenários alternativos propostos pela técnica de “modelos de mistura de entalpia de sílica”.

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Acknowledgements

We gratefully acknowledge the dedicated support provided by Elisabeta Primejdie and Socrate Bucur during the field operations. Special thanks go to Floarea Răducă for her continuing assistance in the laboratory work. We are also grateful to Ioan Berbeleac, Florina Chitea, Cristina Fianu, Ioan Seghedi and Mihai Tatu for helping us to enlarge our necessary database of scientific publications. We feel deeply indebted to two anonymous reviewers, whose insightful suggestions greatly contributed to improving the original version of our manuscript. Also, we greatly appreciate the support of Martin Appold, Susanne Schemann, Sue Duncan and the other members of the Hydrogeology Journal editorial team.

Funding

This work was supported by a grant from the Romanian National Authority for Scientific Research, CNDI-UEFISCDI, project number PN-II-PT-PCCA-2011-3.1-1619 (contract No. 48/2012).

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Mitrofan, H., Marin, C., Povară, I. et al. Better constraining silica-enthalpy mixing models in a setting of two separate (karst and non-karst) dilution regimes. Hydrogeol J 27, 291–304 (2019). https://doi.org/10.1007/s10040-018-1846-7

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Keywords

  • Thermal conditions
  • Hydrochemistry
  • Silica geothermometers
  • Karst
  • Romania