Abstract
Water suppliers face major challenges such as climate change and population growth. To prepare for the future, detailed knowledge of water resources is needed. In southern Germany, the state water supplier Zweckverband Landeswasserversorgung provides 3 million people with drinking water obtained from a complex karst and alluvial aquifer system and the river Danube. In this study, a combination of different tracing techniques was used with the goal of a multi-scale characterization of the aquifer system and to gain additional knowledge about groundwater flow toward the extraction wells in the Danube Valley. For the small-scale characterization, selected groundwater monitoring wells were examined using single-borehole dilution tests. With these tests, a wide range of flow behavior could be documented, including fast outflow within just a few hours in wells with good connection to the aquifer, but also durations of many weeks in low-permeability formations. Vertical flow, caused by multiple flow horizons or uprising groundwater, was detected in 40% of the tested wells. A regional multi-tracer test with three injections was used to investigate the aquifer on a large scale. For the highly karstified connection between a swallow hole and a spring group, high flow velocities of around 80 m/h could be documented. Exceptionally delayed arrivals, 250 and 307 days after the injection, respectively showing maximum velocities of 0.44 and 0.39 m/h, were observed in an area where low-permeability sediments overlay the karst conduits. With the chosen methods, a distinct heterogeneity caused by the geological setting could be documented on both scales.
Kurzfassung
Wasserversorger stehen vor großen Herausforderungen wie Klimawandel und Bevölkerungswachstum. Um für die Zukunft gerüstet zu sein, ist ein detailliertes Verständnis der Wasserressourcen erforderlich. In Süddeutschland versorgt der Zweckverband Landeswasserversorgung 3 Millionen Menschen mit Trinkwasser, das aus einem komplexen Aquifersystem aus einem Karst- und einem alluvialen Grundwasserleiter sowie der Donau gewonnen wird. In dieser Studie wurde eine Kombination verschiedener Tracermethoden eingesetzt, mit dem Ziel, das Aquifersystem auf verschiedenen Maßstäben zu charakterisieren und zusätzliche Erkenntnisse über den Grundwasserzustrom der Entnahmebrunnen im Donauried zu gewinnen. Für die kleinmaßstäbliche Charakterisierung wurden ausgewählte Grundwassermessstellen mit Einzel-Bohrloch-Verdünnungsversuchen (Single-borehole dilution tests, SBDT) untersucht. Damit konnte ein breites Spektrum an verschiedenem Fließverhalten dokumentiert werden, von schnellem Abstrom innerhalb weniger Stunden in Grundwassermessstellen mit guter Anbindung an den Aquifer, bis hin zu Versuchsdauern von mehreren Wochen in gering durchlässigen Formationen. Vertikale Strömungen, hervorgerufen durch mehrere Fließhorizonte sowie auf- oder absteigendes Grundwasser, wurden in 40 % der untersuchten Grundwassermessstellen festgestellt. Zur großmaßstäblichen Untersuchung des Grundwasserleiters wurde ein kombinierter regionaler Markierungsversuch mit drei Eingaben durchgeführt. Für die stark verkarstete Verbindung zwischen einer Bachschwinde und einer Quellgruppe konnten hohe Fließgeschwindigkeiten von rund 80 m/h dokumentiert werden. In einem Bereich, in dem gering durchlässige Sedimente die Karströhren überlagern, wurden 250 bzw. 307 Tage nach der Eingabe extrem verzögerte Erstnachweise mit Maximalgeschwindigkeiten von 0,44 bzw. 0,39 m/h beobachtet. Mit den gewählten Methoden konnte auf beiden Maßstäben eine ausgeprägte, durch die geologischen Gegebenheiten bedingte, Heterogenität dokumentiert werden.
Résumé
Les producteurs d’eau sont confrontés à des défis majeurs tels que le changement climatique et l’accroissement de la population. Pour préparer l’avenir, une connaissance détaillée des ressources en eau est nécessaire. Dans le sud de l’Allemagne, le producteur d’eau public « Zweckverband Landeswasserversorgung » fournit 3 millions d’habitants avec de l’eau potable prélevée d’un système aquifère complexe karstique et alluvial et du fleuve Danube. Dans cette étude, une combinaison de différentes techniques de traçage est mise en œuvre dans le but d’une caractérisation multi-échelle du système aquifère et d’obtenir de nouvelles connaissances relatives aux écoulements souterrains en direction des puits de production dans la vallée du Danube. Pour une caractérisation à l’échelle locale, une sélection de puits de surveillance a été étudiée en ayant recours à des tests de dilution simple en forage. Avec ces tests, une large gamme de type d’écoulements a pu être documentée, incluant des flux rapides de quelques heures dans les puits bien connectés à l’aquifère, mais également des durées atteignant plusieurs semaines dans les formations peu perméables. Un écoulement vertical, causé par de multiples niveaux d’écoulement ou par des eaux souterraines remontantes, a été détecté dans 40% des puits testés. Un essai de multi-traçage comprenant trois injections a été mis en œuvre pour investiguer l’aquifère à l’échelle régionale. Pour la connexion très karstifiée entre une perte et un groupe de sources, des vitesses d’écoulement élevées, autour de 80 m/h, ont été observées. Des arrivées exceptionnellement retardées, 250 et 307 jours après l’injection, démontrant des vitesses maximales de 0.44 et 0.39 m/h respectivement, ont été observées dans la zone où des sédiments de faible perméabilité recouvrent les conduits karstiques. La documentation d’une hétérogénéité associées au contexte géologique a pu être réalisée aux deux échelles, en utilisant les méthodes retenues.
Resumen
Los responsables del abastecimiento de agua se enfrentan a grandes retos, como el cambio climático y el crecimiento de la población. Para afrontar el futuro, es necesario un conocimiento detallado de los recursos hídricos. En el sur de Alemania, el organismo estatal de suministro de agua Zweckverband Landeswasserversorgung abastece a 3 millones de personas con agua potable obtenida de un complejo sistema acuífero kárstico y aluvial y del río Danubio. En este estudio se utilizó una combinación de diferentes técnicas de trazadores con el objetivo de realizar una caracterización a escala múltiple del sistema acuífero y obtener conocimientos adicionales sobre el flujo de agua subterránea hacia los pozos de extracción en el valle del Danubio. Para la caracterización a pequeña escala, se examinaron pozos de monitoreo de aguas subterráneas seleccionados utilizando pruebas de dilución de un solo pozo. Con estas pruebas se pudo documentar una amplia gama de comportamientos de flujo, incluyendo un rápido flujo de salida en pocas horas en pozos con buena conexión al acuífero, pero también duraciones de muchas semanas en formaciones de baja permeabilidad. El flujo vertical, causado por múltiples horizontes de flujo o por aguas subterráneas ascendentes, se detectó en el 40% de los pozos ensayados. Para investigar el acuífero a gran escala se utilizó una prueba regional de trazadores múltiples con tres inyecciones. En el caso de la conexión altamente karstificada entre un pozo de agua y un grupo de manantiales, se pudieron documentar altas velocidades de flujo de alrededor de 80 m/h. En una zona en la que los sedimentos de baja permeabilidad se superponen a los conductos kársticos, se observaron llegadas excepcionalmente retrasadas, 250 y 307 días después de la inyección, con velocidades máximas de 0.44 y 0.39 m/h respectivamente. Con los métodos elegidos se pudo documentar una clara heterogeneidad causada por el entorno geológico en ambas escalas.
摘要
供水面临着气候变化和人口增长等重大的挑战。为未来做准备, 需要详细了解水资源。在德国南部, 州供水公司 Zweckverband Landeswasserversorgung 从复杂岩溶和冲积含水层系统以及多瑙河中获得了300 万人饮用的水源。在这项研究中, 联合利用不同的示踪技术, 目的是对含水层系统进行多尺度表征, 并获得有关流向多瑙河谷开采井的地下水流的更多知识。对于小尺度表征, 使用单井稀释试验检查选定的地下水监测井。通过这些测试, 可以记录各种流动行为, 包括与含水层连接良好的井中仅几个小时的快速流出, 以及在低渗透地层中持续数周的流动。在 40% 的测试井中检测到由多个流动层位或上升的地下水引起的垂向流动。使用三次注入的区域多示踪试验对含水层进行了大尺度调查。对于溶沟和泉群之间的高度岩溶化联系, 记录到大约 80 m/h 的高流速。在覆盖岩溶导管的低渗透沉积物区域, 观察到在注入后 250 天和 307 天到达异常延迟, 最大速度分别为 0.44 和 0.39 m/h。使用所选择的方法,可在两个尺度上发现由地质环境引起的显著的异质性。
Resumo
Fornecedores de água encontram grandes desafios como mudanças climáticas e crescimento populacional. Para preparar para o futuro, é necessário conhecimento detalhado dos recursos hídricos. No sul da Alemanha, o fornecedor estadual de água Zweckverband Landeswasserversorgung fornece água potável a 3 milhões de pessoas, a partir de um sistema aquífero aluvial e cárstico complexo e do rio Danúbio. Neste estudo, uma combinação de diferentes técnicas de traçadores foi utilizada com o objetivo de uma caracterização multiescala do sistema aquífero e para adquirir conhecimentos adicionais sobre o fluxo das águas subterrâneas para os poços de extração no vale do Danúbio. Para a caracterização em pequena escala, poços de monitoramento selecionados foram examinados utilizando testes de dissolução em poço único. Com esses testes, uma grande variedade de comportamento do fluxo foi documentada, incluindo a saída rápida em apenas algumas horas dos poços com boa conexão com o aquífero, mas também durações de muitas semanas em formações de baixa permeabilidade. Fluxo vertical, causado por múltiplos horizontes de fluxo ou água subterrânea ascendente, foi detectado em 40% dos poços testados. Um teste regional de multitraçadores com três injeções foi usado para investigar o aquífero em larga escala. Para as conexões fortemente carstificadas entre uma dolina e um grupo de nascentes, altas velocidades de fluxo ao redor de 80 m/h foram registradas. Chegadas excepcionalmente tardias, de 250 e 307 dias após a injeção, mostrando respectivamente velocidades de 0.44 e 0.39 m/h, foram observadas em uma área em que sedimentos de baixa permeabilidade estão sobrepostos aos condutos cársticos. Com os métodos escolhidos, uma heterogeneidade distinta causada pelas condições geológicas pôde ser documentada em ambas as escalas.
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Introduction
Providing high-quality drinking water is one of the major challenges of increasing severity that every country or region in the world has to face. In particular, these challenges include the effects of climate change, such as the increasing severity and number of extreme weather events like droughts and heavy rain events, and the growing population, leading to growing stress on the global water supply (Bates et al. 2008; Delpla et al. 2009; Hanjra and Qureshi 2010; Wheeler and von Braun 2013; Olmstead 2014; Stevanović 2019). Apart from supplying the population with drinking and process water, the availability of a substantial amount of water is also crucial for the prosperity and economic development of a region (Olmstead 2014; World Bank 2016).
In southern Germany, water availability is generally sufficient; however, especially at the beginning of the twentieth century, water supply was problematic in some regions, e.g. the Swabian Alb. This historically and culturally important karst landscape (Goldscheider 2019), which is also a UNESCO Global Geopark, is characterized by a distinct karstification of the Jurassic limestones, leading to a lack of surface waters. Also, since the groundwater level is deep below the surface, inhabitants of the Swabian Alb mostly had to rely on dammed ponds with poor quality as their water source (Zweckverband Landeswasserversorgung 2012).
Also in the middle Neckar region, water supply has been critical due to problems with water availability and quality, caused by an enormous population increase and fecal contamination. In the course of industrialization, the population in Stuttgart grew from 119,000 to 253,000 between 1882 and 1907, and the supply of water could barely keep up with the demand. The development of new water resources was inevitable, especially considering that the region is a center of commerce and industry (Zweckverband Landeswasserversorgung 2012). Ideas to provide Stuttgart with water from Lake Constance, the northern Black Forest, or the Neckar Valley were discarded. Finally, the Danube Valley was chosen due to its high quality and rich groundwater resources, fed by inflow from the large Jurassic karst aquifer of the eastern Swabian Alb. Finally, the state water supplier Zweckverband Landeswasserversorgung (LW) was founded in 1912 and a long-distance water supply system for Stuttgart and the eastern Swabian Alb was established. Gradually, more cities and communities joined the association, so that today the economically important triangle, between Stuttgart, Ulm and Aalen, is supplied by this state water supplier (Zweckverband Landeswasserversorgung 2012).
Nowadays, high-quality drinking water for approximately 3 million people in Baden-Wuerttemberg is provided with a maximum capacity of 5,200 L/s. Most of the water is gained through more than 200 extraction wells in the Danube Valley, with a total maximum extraction rate of 2,500 L/s. Additionally, the state water supplier extracts water from the river Danube, on average 1,100 L/s, and the captured Buchbrunnen spring, with up to 800 L/s. For periods of peak demand, deep karst extraction wells near the village of Burgberg can be used to provide an extra 500 L/s. In recent years, the average amount of distributed water is around 100 MCM/a (million cubic meters per year, Zweckverband Landeswasserversorgung 2021). Approximately 50% of this water is obtained from groundwater originating in the karst aquifer, making the state water supplier Zweckverband Landeswasserversorgung comparable to the largest karst water supplier in Europe, the Vienna water supplier, which uses multiple karst springs to provide water for 1.7 million citizens (Stevanović 2019).
Currently, Zweckverband Landeswasserversorgung is facing challenges such as climate change, extreme weather events (Haakh 2019), and increasing nitrate concentrations caused by intense agriculture within the groundwater protection area (Haakh 2018). To address and prepare for these tasks, a better knowledge of the catchment area, which extends over the eastern Swabian Alb and parts of the Danube Valley, is required. A project was launched with the aim of a multi-scale characterization of the complex aquifer system and the flow toward the extraction wells. Due to the distinct heterogeneity of karst, it is very difficult to characterize and represent a karst system in detail with numerical models alone. Here, a combination of tracing techniques on different scales was chosen to identify processes in the catchment, beginning with the western part.
The main objectives of this study were (1) to examine groundwater flow on a small scale by conducting single-borehole dilution tests (SBDT) in groundwater monitoring wells (GMW) focusing on identification of flow horizons, outflow behavior and potential vertical flow, (2) to evaluate selected GMWs with SBDT with regard to their suitability as injection points for a large-scale tracer test, (3) to identify hydraulic connections, flow velocities, and subcatchments on a regional scale with a multi-tracer test, focusing on groundwater flow from areas with potential contamination risk towards the extraction wells in the Danube Valley, and (4) to evaluate the obtained results of the regional tracer tests with regard to inflow to the extraction wells and the existing protection concept. The overall goal of this study is to add to the existing knowledge about the aquifer system and groundwater flow on small- and regional scale, in order to prepare for future challenges.
Materials and methods
Study site
Groundwater protection area Donauried-Hürbe
The groundwater protection area of the state water supply, at more than 510 km2, is one of the largest in Germany. It is located in eastern Baden-Wuerttemberg and was established for the extraction wells in the Danube Valley. A total of 204 wells, subdivided into six well fields, extract water either from the Quaternary alluvial aquifer, which is fed by the Jurassic karst aquifer, or directly from the karst aquifer. Each well field has its own pumping station that pumps the extracted water towards the waterworks. The recharge area of the extraction wells is not perfectly represented by the groundwater protection area, since the European watershed (Fig. 1) crosses the area in the northwest (Schloz et al. 2007). The ground elevation drops from about 700 m asl in the area of the watershed to 450 m asl in the Danube Valley.
Geology
From a geological point of view, the water protection area can be divided into two sectors: the Swabian Alb, which is composed of Jurassic limestones and marls, and the alluvial plain of the Danube Valley (Donauried), where the limestones are overlain by Oligocene, Miocene, and Quaternary sediments (Schloz et al. 2007).
The Jurassic formations in the Swabian Alb have thicknesses of up to 400 m and dip gently toward the southeast. In the study area, bedded facies and reef limestones occur, with the latter showing the strongest karstification (Schloz et al. 2007). In the northwestern part of the groundwater monitoring area, Oligocene and Miocene molasse deposits on top of the limestones are few or entirely absent. Toward the southeast, the molasse covering increases with increasing thickness. In the area of the Danube Valley, the limestones are covered almost entirely, with thicknesses up to 90 m (Schloz et al. 2007). In the Danube Valley, the molasse is overlain by Quaternary gravels and sands with thicknesses of up to 11 m. The youngest units are mostly redistributed deposits and river sediments covered by clay, silt, peat or other organic sediments (Udluft et al. 2000; Schloz et al. 2007).
Hydrogeology
The limestones form a large-volume karst aquifer that is underlain by the low-permeability lower Kimmeridgian marls, which form the base of the karst in the catchment (Fig. 2). The tracer tests in the adjacent areas demonstrate a high degree of heterogeneity with flow velocities of more than 100 m/h, indicating highly karstified zones, mainly toward springs, but low velocities and long travel times in zones with low permeabilities (Kolokotronis et al. 2002). Groundwater recharge is mainly attributed to diffuse percolation of precipitation through shallow soils and epikarst, or small watercourses sinking into swallow holes; the water then flows towards the southeast (Kolokotronis et al. 2002). A large quantity of the karst groundwater transits into the alluvial aquifer, consisting of the Quaternary gravels and sands in the Daube Valley, in zones where the low-permeability molasse sediments (k = 10−6 – 10−5 m/s) were eroded, e.g. in the northern part of the Danube Valley where the gravels lie directly on top of the Jurassic limestones, or show just minor thicknesses. Additionally, karst groundwater can rise into the alluvial aquifer through fractures in the molasse layer (Kolokotronis et al. 2002). Zones with ascending karst groundwater can be localized via temperature anomalies or chemical analyses (Udluft et al. 2000).
In the northwestern part of the groundwater protection area, there is only one large spring, the Lone spring, which discharges 240 L/s on average but up to 3,200 L/s during high-flow conditions (Schloz et al. 2007). Based on age dating with tritium, the water of the Lone spring shows an average residence time of 22 years (Kolokotronis et al. 2002). The spring feeds the Lone River, whose valley crosses the catchment area from west to east. However, due to several swallow holes, most of the river bed is dry; water reaches the Hürbe River only after strong rainfall or during snowmelt. Only a few kilometers beyond the Lone spring and the segments beyond the inflow from the wastewater treatment plants are continuously water-bearing.
The Nau springs, the most important springs in the catchment, are located in the town of Langenau at the border between the Swabian Alb and Danube Valley. A large number of sources, summarized as eight spring groups distributed along the west–east axis through the town, partially discharge the karst groundwater flowing toward the southeast. Discharges of the individual spring groups vary between 5 and more than 200 L/s, and the total accounts for an average of 1,230 L/s (Schloz et al. 2007). The westernmost spring group, Nauursprung spring, was also dated using tritium. This dating investigation showed a mean residence time of 47 years (Kolokotronis et al. 2002), but it did not include any information on the fast-flowing components toward the spring.
In the area surrounding the catchment, numerous tracer tests have been conducted in the past. Yet only one large-scale test with an observed breakthrough was performed investigating the inflow of the Nau springs and the extraction wells in the Danube Valley. A total of 25 kg of uranine was injected in an active swallow hole southwest of Gerstetten on 2 December 1969 and detected at Nauursprung spring. According to Kolokotronis et al. (2002), two values for the maximum velocity can be found, 352 and 823 m/h. Since comparable tests in the surrounding area show maxima between 23 and 167 m/h, Kolokotronis et al. (2002) assess the obtained velocities as questionable.
Single-borehole dilution test methods
Single-borehole dilution test methods are based on the injection of tracer, e.g. fluorescence dyes, NaCl, or heated water, in the saturated zone of a borehole or well, followed by the observation of outflow with measurements of multiple concentration profiles (Brouyère et al. 2008; Maurice et al. 2011; Banks et al. 2014; Libby and Robbins 2014; Read et al. 2014; Poulsen et al. 2019; Fahrmeier et al. 2021). SBDTs can be conducted as uniform injections over the entire saturated length, delivering results for the whole well or borehole. For detailed information of one specific depth or the investigation of vertical flow, point injections can be used (Maurice et al. 2011).
Most uniform injections were performed using fine-grained NaCl and a permeable injection bag (PIB) to achieve a homogeneous tracer concentration in the well (Fahrmeier et al. 2021). Others were conducted using the hosepipe method, which involves lowering a hosepipe into the well, filling it with tracer solution, and then pulling it out with a constant speed to obtain a uniform injection (West and Odling 2007; Maurice et al. 2011). Point injections were conducted with a newly developed injection probe that contains tracer solution and is opened by dropping a weight down the line (Fahrmeier et al. 2022). After all injections, multiple profiles of the electrical conductivity were measured in time intervals that were chosen depending on the recorded changes. The method by which the data were further analyzed is described in Fahrmeier et al. (2021).
Between 2016 and 2021, 17 different GMWs in the groundwater protection area were tested using SBDTs. Thirteen of them are karst GMWs covering depth ranges between 9 and 122 m, with the longest saturated length being 51 m. The other four are alluvial GMWs located in the Danube Valley with depths up to 16 m. A total of 51 SBDTs were conducted, 41 of which were uniform injections (28 in karst wells, 13 in alluvial wells), mostly conducted with the PIB method. The other 10 were point injections in karst wells (3) and alluvial wells (7).
Goals of the SBDTs were to identify possible injection points for the regional tracer, to determine groundwater flow through and within monitoring wells, to evaluate the connections to the aquifer, and to detect in- and outflow horizons, as well as vertical flow. Also, with results from multiple wells, the objective was to develop an overall hydraulic characterization of the aquifer and its flow conditions.
Regional multi-tracer test
Within the scope of this study, possible tracer injection points, swallow holes and GMWs were identified within the catchment area. To test the GMW connections to the aquifer, uniform SBDTs were conducted in selected wells. Besides the flow conditions, the proximity to areas or industry with an increased contamination risk was taken into account. For the multi-tracer test, three injection points representing the western inflow were chosen (Fig. 1).
In all, 16 kg of uranine was injected into GMW 7313 on 11 October 2017. This site was chosen due to its large distance to the extraction wells, with the goal to characterize flow from distant parts of the catchment area. Two SBDTs conducted in the well before the tracer test showed an outflow zone between the water level (60–65 m depth) and a depth of 70 m (all depths refer to the respective well cap); below this zone the well showed just minor outflow. To cut off the inactive part of the well, a hydraulic packer was installed at a depth of 70 m before injecting the uranine using a hosepipe. After removing the packer 2 weeks later, a measurement of a concentration profile with a borehole field fluorometer showed only minimal uranine concentrations below 70 m, indicating that the packer had functioned correctly.
Also on 11 October 2017, 95 kg of sodium naphthionate was injected in the Lone swallow hole north of the village Bernstadt. Just 500 m upstream of the swallow hole, a wastewater treatment plant discharges its water into the Lone, which poses a potential risk. In the event of a technical problem in the treatment process, contaminants can be transported into the aquifer within less than 1 h. Due to several beaver dams, the swallow hole was not active, but in consultation with the local authorities, the dams were opened slightly, resulting in a steady infiltration for several hours.
On 12 October 2017, 14 kg of eosin was injected in GMW 7721. This particular GMW was chosen due to its location close to a hazardous materials storage unit and an old landfill site. Also, with a saturated length of approximately 50 m, it covers a large depth range and shows a fast outflow over the entire length. For this reason, the tracer was distributed throughout the well by moving the hosepipe up and down during the injection.
A total of 72 sampling points, including springs, surface water, pumping stations, extraction wells, alluvial GMWs and karst GMWs, were monitored using water samples (taken manually and automated), activated charcoal adapters, and field fluorometers. Especially due to the sodium naphthionate injection into a natural swallow hole, the sampling was designed to cover potential fast flow velocities towards the Nau springs and in particular the westernmost Nauursprung spring, where one of the fluorometers was placed. The other was installed to monitor a deep artesian karst well that is used by the state water supplier. The sampling is still ongoing, but the intervals have been adapted several times since the injections. Water samples, as well as the activated charcoal adapters, are analyzed in the laboratory using a fluorescence spectrometer LS-55 from PerkinElmer (Waltham, USA).
With linear distances between injection points and the extraction wells in the Danube Valley of up to 18.5 km, the regional multi-tracer test covers a very large area. Tracer tests spanning comparable distances were undertaken by Kogovšek and Petric (2004); Petrič et al. (2018) and Fronzi et al. (2020).
Results and discussion
Results of single-borehole dilution tests
With the SBDTs, a wide range of different flow behaviors and outflow times could be documented for the GMWs in the protection area. In Fig. 3, three examples of SBDT results from karst GMWs are displayed. Figure 3a shows the results of a point injection in GMW 7950, where a previous uniform injection indicated a good connection to the aquifer and fast outflow. The assumed vertical flow was verified with the point injection showing a clear upward movement of the tracer plume. Based on both tests, an inflow at a depth of 63 m and an outflow at around 39 m were identified. Since the decrease of salt amount during the point injection is negligible until the plume reaches the top (approx. 0.6 h), no other significant outflows are present in between. The SBDTs in GMW 7950 showed that already in the higher parts of the catchment, multiple conduit levels with different hydraulic heads exist. In this case, the higher hydraulic head in the deeper flow horizon results in upward flow within the well.
Figure 3b shows normalized concentration profiles of a uniform-injection SBDT in GMW 7945. Based on the fast decrease of NaCl concentration, the major flow horizon was identified at a depth of 36 m. After 5.25 h, the remaining salt amount was around 23% of the injected mass, indicating that the well overall shows a good connection to the aquifer. Slower outflow in the lower part was indicated by remaining NaCl concentrations 20 h after the injection. GMW 7929 (Fig. 3c) is located close to the sodium naphthionate injection point. With a uniform injection, the fastest decrease was identified at a depth of 20 m. Compared to the other two wells, the overall outflow is significantly slower. Two days after the injection, concentrations in the upper part almost reached the background; however, in the lower part, still (only changes the meaning of the sentence. I want to explain that in the lower part the decrease (or outflow) is slower and so 40% of the initial salt amount is still detectable, while in the upper part almost no concentrations were measured, due to a higher outflow) around 40% of the initial concentrations were measured.
With all SBDTs a broad range of behaviors could be documented for the wells in the western part of the catchment. Half-times, the time when 50% of the tracer has flowed out of the respective well, vary between 10 min and 22 days. The longest test was monitored for more than 34 days. While the alluvial wells generally showed a faster outflow than the karst wells, some of the latter, e.g. GMW 7733 or GMW 7721, also showed an extremely good connection to the aquifer and a fast decrease in tracer amount.
Forty percent of the tested wells showed vertical flow (Fig. 4), which was mostly expected in the karst aquifer, but out of the 13 karst GMWs, only two showed an upward movement and only one showed downward flow. In the upper part of the catchment, which is the main recharge area, downward movement as part of the regional flow system was assumed. However, with the upward movement in GMW 7950, only the existence of multiple conduit levels could be documented. GMW 7721 showed a combination of vertical and horizontal flow, which fits the regional model, but might also be induced by local conditions, e.g. a stream that infiltrates into the aquifer close to the well. The upward flow in GMW 7932 suggests that karst groundwater rises in the area before the karst aquifer is overlain by the low-permeable molasse and flows towards the Nau springs or directly into the alluvial aquifer.
With regard to the alluvium, usually no distinct vertical flow would be assumed in a shallow and homogenous alluvial aquifer. However, due to the special hydrogeological setting in the groundwater protection area, with karst groundwater ascending into the alluvial aquifer, vertical flow was documented in all tested alluvial GMWs. Consequently, the two wells with upward movement are most likely located in areas with uprising karst groundwater, while downward flow in the other wells is induced by compensatory movement.
Depth-dependent outflow differences were documented in multiple wells, e.g. GMW 7313, which shows a good connection in the upper part, but in the lower part, increased NaCl concentrations were still measured 34 days after the injection. Also, wells close to each other showed different behavior. GMWs 7733, 7932 and 7933 form a triangle with side lengths between 50 and 60 m. Despite these small distances, each well shows a different flow behavior. GMW 7733 is well-connected to the aquifer with the major flow horizon at a depth of 29 m and no vertical flow. GMW 7932 has approximately the same outflow horizon, but also an inflow near the bottom and a resulting vertical upward movement. GMW 7933 is poorly connected to the aquifer and shows no vertical flow component. Regarding both depth and distance, these distinct differences can be explained by the characteristic heterogeneity of karst aquifers that, in this case, could be nicely documented on a small scale using borehole dilution tests.
Regional multi-tracer test
With the selected monitoring network, all three injected tracers could be detected. While sodium naphthionate and eosin arrived at the Nau springs, uranine concentrations were measured in three karst GMWs southeast of the injection well; however, no tracer has been observed in the extraction wells so far.
Ninety hours after the injection, sodium naphthionate was detected in a water sample from monitoring point NU1, which covers the whole group of sources at Nauursprung spring. With the high-resolution data of the field fluorometer, the first arrival could be determined at around 80 h (Table 1). At 114.5 h after the injection, sodium naphthionate was also detected at NU2, an additional sampling point upstream of NU1. This leads to the conclusion that multiple sources must exist in the area of Nauursprung spring which can also be seen based on the breakthrough curves (BTC; Fig. 5). While a high conformity of water samples and field fluorometer data is given, the latter shows three peaks. While the first peak, around 100 h after the injection, represents the first arrival at the spring group, the second peak, after ca. 115 h, can be explained through the high concentrations arriving at NU2. Due to the larger interval, this behavior is not visible in the water sample data. The same applies for the third peak after approximately 135 h.
For Nauursprung spring, a recovery rate of around 0.03% was calculated. Since no further sodium naphthionate concentrations were detected, additional flow paths have to exist, leading towards deeper layers of the aquifer with long residence times. This explains that no other sampling point showed sodium naphthionate concentrations. Also, due to the enormous groundwater volume in the aquifer system, dilution can occur to an extent such that the concentration is below the limit of detection.
Regarding the eosin, transport towards the southeast and especially to the deep artesian karst wells was expected on the basis of groundwater-level contour lines. However, after 3 days, eosin was detected in GMW 7939, which is poorly connected to the aquifer, as shown by a SBDT (Fig. 4) and geophysical borehole logging, and located east–northeast of the injection point. At 250 days after the injection, the tracer arrived at Nauursprung spring (NU1) and after 307 days at Öchslesmühlen spring (OMS), both of which are located in a similar direction. The descriptive parameters obtained from the BTCs measured at the two springs and GMW 7939 are summarized in Table 2.
The time gap between the detection in the GMW and the springs can be explained by the geological setting in this area. Following the injection, eosin was transported towards the springs through a karst conduit that is blocked by low-permeability sediments, either molasse or Quaternary clays and silt, before the springs. This leads to an infiltration of eosin into the limestone matrix and also into GMW 7939. As the second effect of the low-permeability sediments, the further transport towards the springs is decelerated, explaining the exceptionally long travel times until the first detection and the still ongoing breakthrough.
The eosin BTCs documented at NU1 and OMS are characterized by low and strongly fluctuating concentrations (Fig. 6) and are a result of the specific setting. Nauursprung spring covers a large catchment with different flow times, as proven by the fast arrival of sodium naphthionate; a similar situation can be assumed for the nearby OMS. This leads to varying discharge conditions and variable dilution before the tracer reaches the sampling points. A heterogenic catchment also explains why a correlation between precipitation and tracer concentrations is not possible. Additionally, minor tracer degradation via ultraviolet radiation can contribute to the fluctuations, since the eosin is exposed to sunlight between the direct sources and the sampling points. However, due to changing flow conditions and weather, this effect cannot be quantified.
Uranine concentrations were detected in GMWs 7950, 7945 and 7929, all of them located in the expected flow direction, 62 days after the injection (Table 3). During the following sampling, only GMW 7945 still showed marginal uranine concentrations. Since the dilution tests showed fast groundwater flow, especially for GMWs 7950 and 7945, it is possible that they are connected to the conduit system that transports the uranine towards the southeast. The delayed detections are most likely due to low flow velocities between the injection well and the karst conduit, but then fast transport within the conduit system.
A future detection of uranine at the springs or in the extraction wells cannot yet be ruled out. The long distance in combination with the possibility of low-permeability sediments blocking the conduits could lead to an extremely delayed arrival at the springs or wells. In addition, low-flow conditions were predominant since the injections, resulting in very low hydraulic gradients. All results of the multi-tracer test until December 2021 are summarized in Fig. 7.
The breakthrough curves documented at NU1 and OMS were modeled to obtain transport parameters. Due to the fluctuating concentrations, especially of the eosin breakthrough, a robust approach based on the advection-dispersion equation of Kreft and Zuber (1978) was chosen; the modeled curves are shown in Figs. 5 and 6, the parameters in Table 4.
For sodium naphthionate, the modeling resulted in characteristic longitudinal dispersions for limestones of the Swabian Alb: 1,970 m2/h for the water samples and between 492 and 1,165 m2/h for the different peaks of the field fluorometer. The eosin values until December 2021, 54 m2/h for NU1 and 38 m2/h for OMS, are amongst the lowest recorded in this formation, the same applies for the documented flow velocities (Fig. 8). Regarding the relation of distance and dispersivity (Fig. 9), all calculated values are within the expectable order of magnitude, only the dispersivity of the sodium naphthionate breakthroughs are a little below average.
The large-scale tracer test conducted within the scope of this project covered large distances, up to 18.5 km between injection and the extraction wells in the Danube Valley, and showed exceptional long travel times. Comparable results were observed in the artesian karst aquifer below Stuttgart and in the Unica catchment in Slovenia, both showing breakthrough durations of more than 500 days (Goldscheider et al. 2003; Goldscheider 2008; Kogovsek and Petric 2014; Petrič et al. 2018).
With the breakthroughs at Nauursprung spring, two different behaviors could be observed. The fast arrival and the high flow velocities resulting from the sodium naphthionate injection confirm a well-developed karst conduit which must open directly into the pond. Despite a smaller distance, the eosin arrived significantly later due to low-permeability sediments blocking the conduits, resulting in a more diffuse transport pathway towards the springs. A schematic illustration of this setting is shown in Fig. 10.
Conclusions
Within this study, a complex karst and alluvial aquifer system with supraregional importance for water supply was characterized using a combination of different tracing techniques. For small-scale results, a total of 51 single-borehole dilution tests were performed in 17 groundwater monitoring wells. A regional multi-tracer test was conducted to investigate groundwater flow on a larger scale. The main conclusions are:
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The tested groundwater monitoring wells showed a wide range of results regarding connection to the aquifer and outflow behavior. Vertical flow was detected in 40% of the wells, partially caused by uprising karst groundwater in the Danube Valley. The duration of tracer outflow varied from a few hours for wells with a good connection to the aquifer and active flow horizons to more than 34 days for wells in low-permeability formations. Also, depth-dependent differences within single wells were documented.
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Several groundwater monitoring wells with a good connection to the aquifer were identified as possible injection points for large-scale tracer tests using borehole dilution tests.
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With the large-scale tracer test, variable flow systems and different hydraulic connections were documented in the study area. High karstification between a swallow hole and Nauursprung spring leads to a fast tracer breakthrough with maximum velocities of around 80 m/h. In contrast, due to low-permeability sediments, another connection to the same spring showed an exceptionally delayed arrival, with a first detection after 250 days and a maximum velocity of 0.44 m/h.
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Despite the high extraction rates, no tracer concentrations were detected in the extraction wells in the Danube Valley. This confirms long residence times and also the effectiveness of the existing protection concept; however, the hydraulic situation and the low-flow conditions must be considered for all results.
Overall, the chosen methods have proven to be applicable for an examination of a complex aquifer system. In the groundwater protection area Donauried-Hürbe, a distinct heterogeneity caused by the geological setting could be documented.
Since eosin concentrations are still being measured at the springs, more than 4 years after the injection, the sampling for the multi-tracer test is still ongoing in order to document the breakthrough completely and also to maybe detect tracer in the extraction wells. To complement the results of the western part, the project will be continued in the eastern part of the groundwater protection area with several SBDTs and a second multi-tracer test with shorter distances to the extraction wells in the Danube Valley.
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Acknowledgements
Special thanks go to Prof. Dr. Frieder Haakh, Dr. Martin Emmert, Rainer Scheck, Dr. Beatrix Wandelt and the personnel of the Zweckverband Landeswasserversorgung workshop. We would also like to thank all students involved during the project: Alexander Albrecht, Felix Allgaier, Philip Engel, Lennard Fromm, Sonja Hilpert, Alexander Jenett, Johanna Lundin, Samuel Mentz, and Yasin Öztürk. Further thanks to Prof. Dr. Arthur Palmer (USA) for checking the original manuscript.
Funding
Open Access funding enabled and organized by Projekt DEAL. The authors would like to thank Zweckverband Landeswasserversorgung (state water supply) for funding and supporting this work.
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Fahrmeier, N., Frank, S., Goeppert, N. et al. Multi-scale characterization of a complex karst and alluvial aquifer system in southern Germany using a combination of different tracer methods. Hydrogeol J 30, 1863–1875 (2022). https://doi.org/10.1007/s10040-022-02514-4
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DOI: https://doi.org/10.1007/s10040-022-02514-4