Evidence for groundwater mixing at Freeling Spring Group, South Australia
- 133 Downloads
Water sampling at springs that are a part of the Freeling Spring Group, South Australia, was used along with electrical resistivity imaging (ERI) data to evaluate the sources and pathways for groundwater to the springs and to find evidence of mixing between the Great Artesian Basin (GAB) aquifer system (Algebuckina Sandstone, Cadna-owie Formation and lateral equivalents) and waters from the adjacent mountain block basement (MB) aquifer. Five springs and a well were used to evaluate spring chemistry; multi-electrode resistivity data were collected along three orientations over the Freeling Spring site. The resistivity data indicate three independent electrically conductive curvilinear features connected to the spring. These features are evidence of mixing at the spring vent similar to what would be predicted from traditional hydraulic flownets. The chemistry of the spring water samples indicates that the water emanating from the Freeling Spring Group is a mixture of waters from both the GAB and the MB aquifers, supporting the geophysical evidence. The data suggest mixing occurs along a fracture in the body of the MB and porous media flow in the GAB beds, but the system is dominated by the GAB flow, which provides approximately 90% of the discharge.
KeywordsAustralia Fractured rocks Geophysical methods Groundwater mixing Electrical resistivity imaging
Mise en évidence d’un mélange d’eaux souterraines aux sources Freeling dans le sud de l’Australie
A partir d’un échantillonnage d’eau sur des sources appartenant au groupe des sources Freeling au sud de l’Australie, et de données d’imagerie de résistivité électrique (IRE) l’origine et les trajets souterrains de l’eau souterraine vers les sources ont été évalués, et le mélange des eaux du système aquifère du grand bassin artésien (GBA) (grès d’Algebuckina, formation de Cadna-owie et équivalents latéraux) avec celles de l’aquifère de socle montagneux voisin (SM) a été mis en évidence. Cinq sources et un puits ont été utilisés pour évaluer la chimie des sources; des données de résistivité multi-électrode ont été rassemblées le long de trois orientations au-dessus de l’emplacement des sources Freeling. Les données de résistivité indiquent trois directions conductrices curvilignes, électriquement indépendantes, connectées aux sources. Ces caractéristiques sont la preuve qu’un mélange s’effectue à l’émergence de la source comme cela pourrait être attendu d’un réseau d’écoulement hydraulique traditionnel. La chimie des échantillons d’eau des sources indique que l’eau émanant du groupe des sources de Freeling est un mélange des eaux du GBA et de celles des aquifères du SM, confirmant les éléments géophysiques. Les données suggèrent que le mélange se produit à partir d’une fracture dans la masse du SM et d’un écoulement en milieu poreux le long des lits du GBA, mais le système est dominé par l’écoulement dans le GBA, qui fournit approximativement 90% du débit.
Evidencias de mezcla de aguas subterráneas en el Freeling Spring Group, Australia del Sur
El muestreo de agua en manantiales que forman parte del Freeling Spring Group, South Australia, se utilizó junto con datos de imágenes de resistividad eléctrica (ERI) para evaluar las fuentes y las trayectorias de acceso de las aguas subterráneas a los manantiales y para encontrar evidencias de mezcla entre el sistema acuífero de la Great Artesian Basin (Algebuckina Sandstone, Cadna-owie Formation y equivalentes laterales) y las aguas del acuífero deL basamento adyacente en el bloque montañoso (MB). Se utilizaron cinco manantiales y un pozo para evaluar la química de los manantiales; los datos de resistividad de múltiples electrodos se recolectaron a lo largo de tres orientaciones sobre el sitio de Freeling Spring. Los datos de resistividad indican tres características curvilíneas conductoras de electricidad independientes conectadas al manantial. Estas características son evidencias de que la mezcla en el conducto del manantial es similar a la que se predeciría a partir de las tradicionales redes de flujo subterráneo. La química de las muestras de agua de manantial indica que el agua que emana del Freeling Spring Group es una mezcla de aguas de los acuíferos GAB y MB, que apoyan la evidencia geofísica. Los datos sugieren que la mezcla ocurre a lo largo de una fractura en el cuerpo del MB y el flujo de medios porosos en los estratos del GAB, pero el sistema está dominado por el flujo en el GAB, que proporciona aproximadamente el 90% de la descarga.
利用南澳大利亚Freeling泉群部分泉水取样和电阻率成像(ERI)数据, 评估了地下水到泉水的来源和途径, 找到了大自流盆地(GAB)含水层系统(Algebuckina砂岩, Cadna-owie地层和横向等价物)和邻近山区基岩(MB)含水层之间水混合的证据。本文对5口泉水和1口水井进行了化学分析, 并收集了Freeling泉点上三个方向的多电极电阻率数据。电阻率数据显示了与水井相连的三个独立的导电曲线特征。这些特征证明了泉水排泄点的混合作用, 类似于利用传统水力流网进行预测分析。泉水水样化学分析结果表明, Freeling泉群的水来自GAB含水层和MB含水层水的混合, 与地球物理证据相符。数据表明, 混合作用主要发生在沿MB体的一条裂隙和GAB含水层中的多孔介质流动中, 但以GAB水流为主, 它占了约90%的排泄量。
Evidências para a mistura de águas subterrâneas no Grupo Freeling Spring, Austrália Meridional
A amostragem de água em nascentes que faz parte do Grupo Freeling Spring, sul da Austrália, foi usada junto com os dados de imagem de resistividade elétrica (IRE) para avaliar as fontes e os caminhos das águas subterrâneas para as nascentes e para encontrar evidências de mistura entre a Grande Bacia Artesiana (GAB) (Arenito Algebuckina, Formação Cadna-owie e equivalentes laterais) e águas do aquífero adjacente do embasamento de blocos montanhosos (BM). Foram utilizadas cinco nascentes e um poço para avaliar a química da nascente; dados de resistividade de vários eletrodos foram coletados ao longo de três orientações no site da Nascente Freeling. Os dados de resistividade indicam três características curvilíneas independentes eletricamente condutoras conectadas ao poço. Essas características são uma evidência de mistura no respiradouro da nascente, semelhante ao que seria previsto pelas redes de fluxo tradicionais. A química das amostras de água da nascente indica que a água que emana do Grupo Freeling Spring é uma mistura de águas dos aquíferos GBA e BM, suportando as evidências geofísicas. Os dados sugerem que a mistura ocorre ao longo de uma fratura no corpo dos BM e do fluxo de mídia porosa nos leitos do GBA, mas o sistema é dominado pelo fluxo da GBA, que fornece aproximadamente 90% da descarga.
Previous work on evaluating mixing at springs has usually taken a chemical approach and evaluated major ions, isotopes, or both (Lyons et al. 1995; Uliana and Sharp 2001; Wang et al. 2006). If the mixing is a complex blend of waters, statistical or multitracer approaches are utilized to determine endmember water composition and their relative proportions (Plummer et al. 2001; Crossey et al. 2006; Doctor et al. 2006; Moore et al. 2009). In addition, geophysical approaches are utilized to find flow paths and to understand the underlying structure of the system. These approaches utilize seismic or electrical methods to determine preferential pathways and find hidden structures but may not observe mixing phenomenon (Halihan et al. 2013). Integrated hydrogeophysical approaches are useful when evaluating spring mixing problems (Povinec et al. 2006; Gary 2010; Petitta et al. 2015). These approaches utilize the geophysical data to look at the geometry of the flow system and the chemistry to test flow hypotheses.
This hydrogeophysical investigation incorporated both geophysical and geochemical data with previously known data from literature, including structural and hydrogeologic mapping, in order to better understand flow paths and source water(s) to the Freeling Spring Group. The integrated hydrogeophysical data can potentially quantify flow paths and source water(s) by using geophysical data to infer flow paths and geochemical data to calibrate geophysical data, and to evaluate mixing models based on end member source waters. The integration of these datasets was used to provide an improved interpretation of spring water sources and distinguish the poorly understood flow paths and source water(s) for the Freeling Spring Group.
Freeling Spring Group is located at the Peake Overland Telegraph Station ruins on the Oodnadatta Track, South Australia, and has been suggested as a possible mixing zone between GAB waters and waters from the granitic basement (Aldam and Kuang 1988). The springs and the ruins of the nearby former overland telegraph station are a focus for tourism. The springs are considered to be sacred sites as well as important natural resources by the local indigenous people. The springs’ habitats represent vital fragile ecosystems that have adapted to modern conditions of flow and water chemistry. These habitats include not just permanent wetland water sources for aquatic species but phreatophytes living near the springs and migrating species using the springs to cross through the arid area where it exists.
In the event of multiple groundwater sources mixing or converging, these flows and flowpaths must be understood in order to maintain spring flow and spring water chemistry in the area and to secure the ecosystem (Fensham et al. 2005; Keppel et al. 2011). The loss of one or more of these source waters may impact the site ecosystem and animals depending on the springs during droughts (Greenslade et al. 1985; Ponder 2002). With concerns over pressure heads decreasing in the GAB due to groundwater use and natural long-term resource decline, it is important to understand both the flow mechanics and source water(s) for the springs to allow informed management decisions to be made for these systems (Love et al. 2013).
Geology of Freeling Spring Group
Hydrogeology of Freeling Spring Group
Researchers studying springs of the GAB have developed a set of nomenclature related to the geometry of the springs in the aquifer. Freeling Spring Group is a “spring group” in the sense that it is a cluster of springs at one location in a single area. Due to the scarcity of water in the Outback, the terms “spring” and “seep” have been used to describe any amount of water appearing at the surface from depth. This can include a clump of reeds or a single plant that is dependent on the groundwater source but does not occur in a traditional spring wetland habitat. The “spring vent” is the surface discharge point of a spring. Many GAB springs also have a “spring tail” which is water that exits the spring vent and discharges over the surface, usually supporting a spring wetland. Spring tails are often considered to be surface runoff from the primary spring vent, but recent work has shown that additional small springs may appear in linear spring tails, adding additional discharge and mixing processes to the surface flows and acting as a line source of groundwater over a fault zone (Halihan et al. 2013). These additional sources of native groundwater in the tails are apparent in geophysical and geochemical data but are not apparent from visual inspection making water sampling to determine water evolution complex (Halihan et al. 2013; Keppel et al. 2012; Keppel 2013).
The Freeling Spring Group consists of multiple springs and seeps with the largest spring in the southern portion of the complex being Freeling Spring (spring No. EFS001) located 75 m to the southeast of the Peake Telegraph Station ruins. Photos of the site can be seen in Keppel et al. (2019). The largest discharge spring (Freeling Spring, EFS001) has the highest flow location from a diffuse area in a zone of high reeds (Phragmites australis). The Freeling Spring primary spring tail is located in a lineament perpendicular to the Kingston Fault and is likely composed of a number of small springs discharging along the fault orientation as it occurs near the fault intersection (Karlstrom et al. 2013; Keppel et al. 2019). The spring tail for Freeling Spring is smaller and oriented to the east of the spring along the cross-section line A–A′ (Figs. 2 and 3). It is not the larger downgradient spring tail originating further northeast in the spring group (Fig. 2).
Over 130 locations in the Freeling Springs Group have been defined as springs or seeps. The majority of the springs are located within 500 m of the Kingston Fault, in a northeast trending line that decreases in elevation away from the fault (Fig. 2). There is a line of individual springs along the Kingston Fault; these are individual springs on top of the travertine to the east and downgradient of the fault. The spring group has sufficient discharge to support a diverse ecosystem including a range of flora and fauna. Flow rates measured for the Freeling Spring Group as part of this investigation range from just seepage to over 61 L/min. Groundwater potentiometric levels at the Freeling Spring Group are 5–25 m above the land surface (Love et al. 2013; Fig. 1). The potentiometric surface in the granitic areas is poorly characterized due to limited available monitoring locations.
Published literature indicates that the Freeling Spring Group is a discharge feature of the GAB (Habermehl 1986; Aldam and Kuang 1988, 1989), but the area’s flow mechanics are poorly understood and need further investigation (Aldam and Kuang 1988, 1989). The Freeling Spring Group’s setting would allow for a convergent flow system setting along the Kingston Fault, (Aldam and Kuang 1989; Rogers and Freeman 1996) but little discussion has been included in the literature regarding the potential contribution from the Peake and Denison Inliers.
Freeling Spring Group conceptual mixing models
Geochemical analysis data used for the mixing models and mixing model results. Specific conductance of water or fluid electrical conductivity of water is denoted as EC
Site name (site ID number)
UTM location (m)
Measured chemistry (mmol/L)
Alk (pH 4.5)
Parameters normalized to chloride
Percent mountain block water - linear mixing model
The lateral migration of water away from Kingston Fault also has a number of possible configurations (Sampson et al. 2012). One possibility includes water moving upward along the fault zone with shallow subsurface water migrating away from the fault, down the topographic gradient to the north and east. This would result in the entire spring group discharging waters similar to the fault zone waters. Another possibility is that water is sourced east of the fault zone by waters moving upward from the GAB near the Kingston Fault through additional faults in the Bulldog Shale confining unit, like many other springs in the area (Keppel et al. 2011; Halihan et al. 2013). This would result in springs of the GAB having chemistry of GAB fluids and would potentially be different from the fault fluid chemistry. These issues of spring sources may become further complicated in the channels that form the spring tails. Along the channel for discharge from Freeling Spring, located orthogonal to the Kingston Fault and east of Freeling Spring, surface water flows toward the east, away from the fault, while subsurface flow is potentially originating in the channel in additional upwelling springs (Halihan et al. 2013). Surface water along the spring tail can be traced from the main Freeling Spring (EFS001); at the same time, individual subsurface flow springs inside the tail can be found and isolated from the surface flow. This may result in more complex mixing of spring waters.
Based on an interpretation of seismic data, Aldam and Kuang (1988) hypothesized that the source of fluid along the Kingston Fault could be a mixture of MB (Fig. 3, arrow MB) and GAB waters (Fig. 3, arrows C and A). This should be evident in both the electrical resistivity imaging data and the geochemical data. If the springs are only sourced from the MB, it should be evident in the geochemistry, even if it is ambiguous in the geophysical data, as a small fracture in the granite may not be a significant electrical signature. If the springs are only sourced from the GAB aquifer, it should be evident in both datasets as the electrical signature of the sandstone beds should be distinctive regardless of whether the fluids they contain provide significant contrast. Additional evidence of a potential MB source is the presence of the former Peake Telegraph Station at the spring which may have been located due to the availability of fresher water from a granitic source.
For the lateral and/or vertical transport east of the fault, waters are hypothesized to be a mixture of fault waters from the GAB or MB moving up the Kingston Fault (Fig. 3, arrow KF), then down the topographic gradient to the northeast (Fig. 3, arrow T). This hypothesis should be supported by both geochemical and geophysical evidence. If the spring discharge east of the Kingston fault is purely GAB fluids from depth, the geophysical data may not be clear as a lack of mixing would not alter the electrical signature. The geochemistry data would be clear with regard to the water source and a lack of evaporation along the surficial flowpath (Fig. 3, arrow T).
A hydrogeophysical investigation was conducted to test the conceptual models for the springs. Three multi-electrode surface resistivity lines or electrical resistivity images (ERI) were obtained over Freeling Springs and geochemical samples were collected from four spring vents (Fig. 4). Mixing models were evaluated using the geochemical data with endmembers from a GAB well and an MB spring. The methods described include resistivity data acquisition and processing, geochemical sampling and analyses, and geochemical mixing models. These data were evaluated in a context of regional potentiometric data (Fig. 1; Sampson et al. 2012).
Electrical resistivity survey
Four water samples were collected along the ERI lines and around the Peake Telegraph Station during December 2010 (Dailey 2001). The Freeling Spring Group water sampling points (Fig. 4) were Freeling Spring (EFS001), and three samples at individual discharging spring vents away from the main spring (EFS017, EFS071, and EFS088). Samples were collected from locations that appeared to be the first surface occurrence of groundwater at the locations, but sample chemistry from these vents is variable in composition as reactions happen immediately upon the water reaching the surface (Keppel et al. 2012). Available regional chemical data included a MB spring water sample (MB002) and a nearby well in the GAB (well ID 6041-36; Fig. 4).
All water samples were filtered through a 0.45-μm filter during collection, or in the laboratory for one sample that was turbid. The samples were collected and stored in HDPE bottles that were unacidified for anions and acidified to a pH <2 with high purity HNO3 for cations and metals with no headspace in the sample bottles. The samples were cooled on ice in the field and transported to the laboratory where they were stored at 4 °C, for storage in Australia. The samples were not refrigerated for 10 days while in transit from Australia to Oklahoma (USA) for analysis.
Temperature, electrical conductivity (EC), pH and dissolved oxygen (DO) were measured using a calibrated Yellow Springs Instrument (YSI) multiprobe prior to sample collection. Alkalinity was measured by acid titration in the field. Water analyses were conducted at Oklahoma State University School of Geology geochemical laboratories, Stillwater, Oklahoma. Major anions (chloride, bromide, nitrate and sulfate) and major cations (sodium, potassium, ammonium, magnesium and calcium) were measured using a Dionex ICS 3000 ion chromatograph. Regional end member water samples for MB and GAB spring fluids were analyzed by the Commonwealth Scientific and Industrial Research Organization (CSIRO) in Adelaide, South Australia.
Mixing model analyses
Each ion was evaluated separately for mixing as the surface processes that may be affecting some data were not clear. A normalized EC was also evaluated, due to the conservative nature of chloride and the varying values for EC seen across the site using the ERI data (Uliana and Sharp 2001; Table 1). Other normalized parameters were tested for mixing percentages as well (Genereux and Pringle 1997). These analyses were attempting to confirm the geophysical results of a flowpath from the mountain block side of the fault (Fig. 5).
Results of this study, including geophysical data (ERI), geochemical data (cations, anions, electrical conductance (EC) and temperature) and mixing models are reported in the following subsections. The geophysical results affect the interpretation of the chemical data and are presented first.
Electrical resistivity image (ERI) results
Survey line PK255 results
ERI survey line PK255 (Fig. 5b) is perpendicular to the Kingston Fault with the fault lying at the approximate location of Freeling Spring. The line runs approximately along and close to a joint orthogonal to the Kingston Fault, apparent from the topography to the west and the flow of the spring tail to the east. West of the fault, the line runs along an east–west valley in the granite, and east of the fault, the line runs along the primary tail for Freeling Spring (EFS001). The west side of the Kingston Fault shows resistive zones ranging from 5 Ω-m (green in Fig. 5) to over 200 Ω-m (gray), with the majority of the granitic domain having a resistivity greater than 15 Ω-m. There is a conductive vertical feature on the west side of the fault ranging from 5 to 15 Ω-m (green), extending from the bottom of the domain approximately 100 m west of the fault to an area just east of the Freeling Spring vent. The feature mimics the pattern of the conductor between 15 and 100 Ω-m (orange) that extends from the edge of the dataset at 100 m laterally and angles upward towards Freeling Spring.
The east side of the fault over the GAB shows resistive zones ranging from 0.1 Ω-m (blue) to over 200 Ω-m (gray); with the majority of the east side having a resistivity less than 30 Ω-m. Two prominent conductive features are a 0.1–15 Ω-m resistive zone along the top of the east side of the fault, and one vertical feature coming from the bottom of the image approximately 100 m east of the fault. The trend of the conductive features mimics the pattern in the cross sectional interpretation of the site (Fig. 3). The tail of the spring corresponds with the strongest conductor (<5 Ω-m in blue) from the shallow lateral conductor.
Survey line PK345 results
ERI survey line PK345 (Fig. 5a) is along the approximate location of the Kingston Fault and shows less resistive zones ranging from 0.3 Ω-m (blue) to 84 Ω-m (orange/white) along the line, with the top 50 m of the image and the central portion being predominately 0–15 Ω-m. Electrically conductive areas below 5 Ω-m are constrained to the upper portions of the image. Although the conductive portion in the center of the image may represent an area of upwelling, the image may also depart from the fault zone and thus have higher resistivity due to the location of the line instead of fluid changes. The 5-15 Ω-m (green) zone does complement the same zone beneath Freeling Spring in image PK255 with the same resistivity range.
Geochemical analysis results
The regional water sample for the Peake and Dennison Inliers MB sample (Spring 2: MB002) has an EC of 1,295 μS/cm, a chloride value of 4.2 mmol/L and a sodium value of 5.4 mmol/L (Table 1). The regional GAB is represented by a sample from the Centenary Well (6041-36) with an EC value of 3,971 μS/cm, a chloride value of 27.9 mmol/L and sodium value of 35.9 mmol/L (Table 1). The water samples for the Freeling Spring Group have electrical conductivities ranging from 3,952 to 4,785 μS/cm, chloride values ranging from 25.7 to 29.6 mmol/L and sodium values ranging from 34.8 to 41.4 mmol/L (Dailey 2001).
Mixing model analyses
The mixing models were analyzed for source water potential for the Freeling Spring Group (Table 1). The normalized EC model is detailed followed by the other mixing models.
Fluid conductivity mixing model
The MB spring sample has an EC value of 1,295 μS/cm, while the GAB well EC value is 3,971 μS/cm and the Freeling Spring Group samples range from 3,952 to 4,785 μS/cm (Table 1). The Freeling Spring Group samples’ EC mixing-model values range from 1.3 to 11.7% MB fluids (88.3–98.7% GAB fluids). The model indicates that the mixing percentage increases to the north of the primary spring vent, consistent with the lack of a high bulk electrical conductivity (low bulk resistivity <5 Ω-m) path to the north in the ERI data (Fig. 5a).
Other mixing models
Other parameters used to evaluate mixing often include sodium, but normalized sodium parameters were identical across the springs and they all had higher than normalized sodium values at the endmember locations. Thus, they are not used for the analysis. The other parameters indicated between 0.3 and 13.5% MB fluids exiting from the springs. Normalized potassium provides the smallest variability in mixing estimates with an average estimate of 10.4% MB waters (89.6% GAB waters) exiting the springs (Table 1). The average of all models is 6.7% MB fluids exiting the springs, or 93.4% GAB waters. The average values increased from the primary vent similar to what was seen in the fluid conductivity mixing model implying that MB water was migrating north to the east of the fault.
Relative to literature values for unsaturated rocks, resistivity values are lower than expected over the Freeling Spring Group (Eberhart-Phillips et al. 1995). Over 50% of the dataset have resistivity values under 15 Ω-m, which are interpreted as the result of fresh to brackish fluids in the underlying formations. This is based on the comparison between the measured fluid EC values at the springs along the ERI lines, and ERI bulk resistivity at those locations. The west side of the Kingston Fault over the Wirriecurrie Granite (MB) would be expected to have resistivity values exceeding 1,000 Ω-m if no significant fluid were present and the rock was unfractured (Palacky 1987; Fig. 5b). The mean value is less than 500 Ω-m which is interpreted as evidence for the presence of fluids in fractures in the granite. The east side of the images over the GAB is predominately under 15 Ω-m, this implies the sandstone is fully saturated with fresh to brackish fluid.
ERI survey line PK255 (Fig. 5b) may be inferred as basement rocks (MB) on the west side of Kingston Fault and the GAB (saturated sandstone) on the east side. PK255 is interpreted as demonstrating three fluid flow paths coming from depth (marked with arrows), one from the MB (5–15 Ω-m vertical green feature) and two from the GAB (1–15 Ω-m vertical and horizontal blue and green features) discharging at Freeling Spring (Fig. 5b). The 5–15 Ω-m vertical conductor to the west of the fault starts approximately 100 m to the west of the fault at the bottom of the dataset and has a distinct curve at an elevation of approximately 0 m relative to the Australian Height datum (AHD) when it heads nearly vertically in the image. As the Kingston Fault is thought to be near vertical, this is interpreted to be a MB flowpath instead of a feature of the fault damage in the rock body. The similar pattern of the conductive feature to the conductive (orange) and resistive (gray) trends on the west side of the fault suggest that the conductive trend is following the pattern of the rest of the rock body. As the zone is in an area of known deformation, this could be a folding pattern or other pattern in the lithology, but no drilling data are available to test the dataset further. The correlation between the interpreted MB flowpath and the granitic portions of the dataset as well as the correlation with the same range of resistivity in PK345 in the fault zone suggest a distinct conductive signal of fluid migration as opposed to a noise feature or simply part of the Kingston Fault damage zone.
Based on geological mapping of the area, the less resistive (blue) values centered at approximately 75 m AHD of the east side of PK255 may be inferred to be the Cadna-owie Formation (GAB) aquifer flowing to Freeling Spring beneath the spring tail. The lower fluid path on the east side of the Kingston Fault may be inferred to be generated by the fluids of the Algebuckina Sandstone (GAB; Fig. 5b). The uppermost layer in the shallow surface east of the Kingston Fault is interpreted as fractured Bulldog Shale or alluvium that has fluids moving upward similar to imaging conducted on other springs exiting the Bulldog Shale (Halihan et al. 2013).
ERI survey line PK345 (Fig. 5a) along the Kingston Fault does not have ERI values normally associated with an undeformed granite. The values seen under the fault imply the electrical properties of the fault is dominated by fluid signatures. This helps support the idea of water coming up from depth along the fault zone and discharging along the surface. The more resistive fluids at the 5–15-Ω-m range are inferred to come from the MB system as it coincides with the values in the perpendicular line at PK255. The shallower more conductive features below 5 Ω-m are interpreted as fluids from the GAB Cadna-owie sandstone (Fig. 5a). If drilling were possible in this area, a borehole into the fault or to the west of the fault would assist in strengthening the interpretation.
Mixing models and geochemical analysis
The mixing models show the Freeling Spring Group samples lie between the MB and the GAB samples with a small noticeable contribution from the MB system. These data would suggest MB water is coming to the fault from the Wirricurrie Granite and feeding the springs along the fault and on the travertine platform in the Freeling Spring tail. This also suggests the GAB provides the most influential water sources on the springs but they are likely influenced by contributions from MB water. If this influence was greater in the past, it may be the cause of the siting of the telegraph station on the site in 1870 compared with other potential more brackish spring locations. Additional monitoring of the springs over time may also provide insight over mixing percentages over time.
Due to the remote site location, the choice of endmember water samples is limited. The authors have a range of experience in the area to select locations that appeared representative of the respective aquifer while being located as close as possible to the springs. Other investigations of springs in the area found degassing to strongly affect chemistry with an increase in pH and a decrease in alkalinity (Keppel et al. 2012). While this study’s results see this increase in pH, a strong decrease in alkalinity in the samples is not evident. While the system is called a spring group, there is also uncertainty as to whether the vents that were sampled were all related to each other on a single subsurface flowpath. Additionally, the vent locations were commonly vegetated, and finding the exact vent from the groundwater system has uncertainty. Subsurface investigations using borings would aid at improving the understanding of the springs, but cultural factors limit this approach.
Although the geophysical and geochemical methods agree that multiple waters are exiting the Freeling Spring Group, it is difficult to interpret more than two waters contributing to the spring in the chemical data. The chemical mixing models show a clear contribution of MB fluid exiting the spring group, in agreement with the ERI data showing a more electrically conductive interpreted pathway leading to the Kingston Fault from the west in PK255 relative to two conductors east of the fault in the GAB leading up to Freeling Spring (Fig. 5b). One consistent interpretation is that the two electrically conductive pathways to the east are the two distinct layers in the GAB: the lower Algebuckina Sandstone and the upper Cadna-owie Formation (Fig. 5b). These electrical conductors, possibly having two distinct chemistries, could be mixing in proportions not seen in the regional GAB samples used for the mixing model evaluation. So while the chemistry data have some scatter but indicate a fluid mixing at the springs from the MB waters, the geophysical data suggest the major GAB fluid contribution comes from two fluids of different conductances and thus somewhat varying chemistry. This interpretation appears to satisfy all of the datasets, and illustrates why the chemical analysis can determine that at least two waters are mixing while the geophysical data indicate three waters are actually mixing at the springs.
Considering the remote location and the required sampling limitations at the site, further investigation was not possible at this time. Further clarification of the chemistry could be achieved with targeted borings on both sides of the fault in an effort to obtain independent samples of the three potential water sources to the spring. This could eliminate the issue of mixed and/or reacted chemical samples caused by evaporative processes, microbial processes, precipitation of minerals, and potential reactions caused by the vegetation at the spring vents (Keppel et al. 2012). Lithologic analysis of new borings and additional seismic data could provide further information on the fault structure controlling the location of the springs and the mixing processes that are interpreted. While this additional data would provide more insight into the mixing process, the historical use of the site and the existing hydrogeologic, geophysical, and geochemical datasets still indicate that mixing of fresher waters from the MB side of the fault have an influence on the springs.
The Freeling Spring Group offers a unique, although remote, study site which allows an evaluation of the boundary of two aquifers of differing lithologies, fluids from the Peake and Dennison Inliers MB aquifer, and fluids from the Great Artesian Basin aquifer. This boundary, the Kingston Fault, is apparent in electrical resistivity images and is supported by the geochemical signatures of local and regional waters. The ERI results indicate converging waters with potentially three waters mixing under and around Freeling Spring. Geochemical analysis results, including simple mixing models, indicate at least two distinct water types around the Freeling Spring Group.
Geological mapping, vegetation, geochemical analysis and ERI all provide evidence of flow along the fault. The location of the fault is easily recognizable. The ERI data indicate there are fluids flowing in the fault, as the majority of the fault is underlain by low resistivity values associated with rock saturated by fresh to brackish waters in the fault plane and the orthogonal dataset illustrates a completely different electrical geometry. The geochemical signatures of Freeling Spring directly over the fault indicate that the spring is a mixture of approximately 10% MB and 90% GAB waters, and therefore not entirely fed by the GAB.
The MB waters influence the springs in the group, including springs east of the Kingston Fault. What is uncertain is whether that the balance changes significantly over time, as the MB aquifer has a higher topography and a history of settlement around these springs in the past. While it is difficult to determine if the system was fresher during the 1800s, additional investigations may determine how much the relative contributions of each aquifer may vary over time.
Dr. Eliot Atekwana and CSIRO is thanked for geochemical data.
The authors wish to thank the Australian National Water Commission, Flinders University, and Oklahoma State University for funding.
Compliance with ethical standards
Conflict of interest
It is here noted that Dr. Halihan has a managed conflict of interest with regard to ERI developments.
- Aldam R, Kuang KS (1988) An investigation of structures controlling discharge of spring waters in the South Western GAB. Department of Mines and Energy, Adelaide, South AustraliaGoogle Scholar
- Aldam R, Kuang KS (1989) An investigation of structures controlling natural discharge of artesian waters in the southwestern Great Artesian Basin. Q Geol Notes South Aust 109:2–9Google Scholar
- Crossey LJ, Fischer TP, Patchett PJ, Karlstrom KE, Hilton DR, Newell DL, Huntoon P, Reynolds AC, De Leeuw GA (2006) Dissected hydrologic system at the Grand Canyon: interaction between deeply derived fluids and plateau aquifer waters in modern springs and travertine. Geology 34(1):25–28. https://doi.org/10.1130/G22057.1 CrossRefGoogle Scholar
- Dailey MK (2001) Hydrogeophysical evidence for ground water mixing at Freeling Spring Group, South Australia, MSc Thesis, Oklahoma State University, Stillwater, OKGoogle Scholar
- Doctor DH, Alexander EC Jr, Petrič M, Kogovšek J, Urbanc J, Lojen S, Stichler W (2006) Quantification of karst aquifer discharge components during storm events through end-member mixing analysis using natural chemistry and stable isotopes as tracers. Hydrogeol J 14(7):1171–1191. https://doi.org/10.1007/s10040-006-0031-6 CrossRefGoogle Scholar
- Fensham R, Fairfax RJ, Ponder WF (2005) Recovery plan for ‘The community of native species dependent on natural discharge of groundwater from the Great Artesian Basin 2006-2010’. Australian Government, CanberraGoogle Scholar
- Gary MO (2010) Karst hydrogeology and speleogenesis of Sistema Zacatón, Tamaulipas, Mexico. PhD Thesis, University of Texas, Austin, TX, 114 pp. AMCS Bull 21, Association for Mexican Cave Studies, Austin, TXGoogle Scholar
- Greenslade J, Joseph L, Reeves A (eds) (1985) South Australia’s Mound Springs. Nature Conservation Society of South Australia, AdelaideGoogle Scholar
- Habermehl MA (1980) The Great Artesian Basin, Australia. BMR J Geol Geophys 5:29Google Scholar
- Habermehl MA (1986) Regional groundwater movement, hydrochemistry and hydrocarbon migration in the Eromanga Basin. In: Gravestock DI, Moore PS, Pitt GM (eds) Contributions to the geology and hydrocarbon potential of the Eromanga Basin. Geological Society of Australia, Hornsby, NSW, pp 353–376Google Scholar
- Harvey FE, Ayers JF, Gosselin DC (2007) Ground water dependence of endangered ecosystems: Nebraska’s eastern saline wetlands. Papers in Natural Resources 131. https://www.ncbi.nlm.nih.gov/pubmed/17973752. Accessed November 2019
- Karlstrom KE, Keppel MN, Love AJ, Crossey L (2013) Structural and tectonic history, chap 4. In: Keppel MN, Love AJ, Karlstrom KE, Priestley S (eds) Hydrogeological framework of the western margin of the Great Artesian Basin, Australia. National Water Commission, CanberraGoogle Scholar
- Keppel MN (2013) The geology and hydrochemistry of calcareous mound spring wetland environments in the Lake Eyre South region, Great Artesian Basin, South Australia. PhD Thesis, Flinders University, Adelaide, South AustraliaGoogle Scholar
- Keppel MN, Karlstom K, Crossey L, Love AJ, Priestley S (2019) Evidence for intra-plate seismicity from spring-carbonate mound springs in the Kati Thanda–Lake Eyre region, South Australia: implications for groundwater discharge from the Great Artesian Basin. Hydrogeol J. https://doi.org/10.1007/s10040-019-02049-1
- Love AJ, Wohling D, Fulton S, Rousseau-Gueutin P, DeRitter S (eds) (2013) Allocating water and maintaining springs in the Great Artesian Basin, vol II: groundwater recharge, hydrodynamics and hydrochemistry of the Western Great Artesian Basin. National Water Commission, CanberraGoogle Scholar
- Palacky GL (1987) Resistivity characteristics of geologic targets. In: Nabighian MN (ed) Electromagnetic methods in applied geophysics: theory, vol 1. SEG, Tulsa, OK, pp 53–129Google Scholar
- Petitta M, Caschetto M, Galassi DM, Aravena R (2015) Dual-flow in karst aquifers toward a steady discharge spring (Presciano, central Italy): influences on a subsurface groundwater dependent ecosystem and on changes related to post-earthquake hydrodynamics. Environ Earth Sci 73(6):2609–2625. https://doi.org/10.1007/s12665-014-3440-1 CrossRefGoogle Scholar
- Ponder WF (2002) Desert Springs of the Australian Great Artesian Basin. In: Sada DW, Sharpe SE (eds) Spring-fed wetlands: important scientific and cultural resources of the Intermountain Region, Las Vegas, USA. In: Conference proceedings, Las Vegas, NV, May 2002, DHS Publ. 41210, Desert Research Institute, Reno, NVGoogle Scholar
- Povinec PP, Aggarwal PK, Aureli A, Burnett WC, Kontar EA, Kulkarni KM, Moore WS, Rajar R, Taniguchi M, Comanducci JF, Cusimano G (2006) Characterisation of submarine groundwater discharge offshore south-eastern Sicily. J Environ Radioact 89(1):81–101. https://doi.org/10.1016/j.jenvrad.2006.03.008 CrossRefGoogle Scholar
- Rogers PA, Freeman PJ (1996) Explanatory notes for the Warrina geological map. Department of Mines and Energy, Adelaide, South AustraliaGoogle Scholar
- Sampson L, Wohling D, Jensen-Schmidt B, Fulton S (2012) South Australia and Northern Territory Great Artesian Basin (Eromanga Basin) hydrogeological map, part 2. Department of Environment, Water and Natural Resources, South Australian Government, AdelaideGoogle Scholar
Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.