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Home All issues Volume 79 (2024) Sci. Tech. Energ. Transition, 79 (2024) 48 Full HTML
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Issue
Sci. Tech. Energ. Transition
Volume 79, 2024
Article Number 48
Number of page(s) 15
DOI https://doi.org/10.2516/stet/2024043
Published online 07 August 2024

© The Author(s), published by EDP Sciences, 2024

Licence Creative CommonsThis is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1 Introduction

The reduction of carbon dioxide emissions by 2050 requires the rapid development of alternative clean energy industries [1]. One such alternative is using hydrogen (H2) produced from renewable resources, producing pure water as the only waste product [2–6].

Manufactured hydrogen is generated by a number of mechanisms such as gasification of coal (black hydrogen), steam reforming of methane (grey hydrogen), pyrolysis of methane (turquoise hydrogen) and electrolysis of water (green, yellow, or pink depending on the power source) [7, 8]. In such cases, the hydrogen acts as an energy carrier for the original energy source. Natural hydrogen is directly produced from a set of geological mechanisms including radiolysis, organic activities, tectonics, and mineral alterations, and is thus a primary energy source. Natural hydrogen has an estimated full lifecycle greenhouse gas intensity of one-third that of green hydrogen [9] making it advantageous for the environment, but also more energy efficient than other forms of hydrogen.

With recent natural H2 discoveries in Mali [10], France [11], and South Australia [12], exploration has gained strong interest internationally [10, 13–15]. However, natural H2 exploration is in its infancy and little data is available. Past petroleum wells rarely recorded dihydrogen gas concentrations, making this new field of research challenging.

Detecting dihydrogen surface micro seeps may be an indicator of deeper accumulations, however, yet there are no standard procedures to sample H2 at the surface. It is therefore critical to standardize exploration techniques for soil gas sampling in the field [16]. Point sampling for natural H2 in soils has been documented by several authors [17–19], but continuous and long-term studies of natural H2 seeps are limited to two sites in the Sao Francisco Basin in Brazil [20–22].

Variable H2 gas emissions were measured from the soil in and around sub-circular depressions in Brazil. These sub-circular depressions may appear as salt lakes and are thought to relate to the occurrence of H2 in the subsurface [14, 21, 23, 24].

No studies to date have continuously monitored natural H2 seepage in Australia or have combined long-term H2 with other trace gases to better constrain the potential source. Additionally, prior to this work, long-term autonomous monitoring has been focussed in and around areas with sub-circular depressions; thus, significantly more research is required to enable efficient natural H2 exploration and production.

We here present the first long-term monitoring of a natural dihydrogen seep in Australia. In a novel approach, carbon dioxide (CO2) and hydrogen sulphide (H2S) were monitored together with H2, to determine whether associated trace gases could assist in identifying the geologic source. The Yilgarn Craton site represents the first natural H2 seep identified on the flood plain of an ephemeral stream, as opposed to in and around sub-circular depressions.

We identify the scale and variability of the natural H2 seepage and discuss potential endogenic, environmental, and climatic controls at the site named FF4 and the implications for exploration. This work therefore advances the natural H2 energy industry and aids in the development of a low-carbon energy future.

2 Study location and geological context

2.1 Yilgarn Craton

The study area is located on private property on the Yilgarn Craton, in the southwest of the state of Western Australia (Fig. 1a) where bedrock comprises extensive Archean granitoid and greenstone terrains [25–27]. The greenstones comprise metamorphosed basalts, gabbros, chlorite schists, and serpentinites and are interbedded with metamorphosed sediments and banded iron formations [28, 29]. They form arcuate belts of metamorphosed sedimentary and mafic volcanic rocks that lie between the granitoids at the surface and are interpreted to extend to depths of up to 5 km below the study location [30, 31]. Granitoids include granites, tonalites, granodiorites, and Proterozoic sedimentary rocks that overlie parts of the Craton [32, 33]. The Yilgarn Craton comprises an imbricated middle crust, which was subjected to subsequent crustal spreading, reworking, and injection of late plutons via rejuvenated listric faults (Fig. 1d). These reworked faults, imaged on deep seismic data, sole out near the Moho boundary and offer potential deep-seated pathways for fluid migration through the crust [30].

thumbnail Figure 1

Shows (a): Yilgarn Craton and FF4 Sample location (30°55′39″S, 116°35′35″E). (b) FF4 site interpreted bedrock geology (c) site morphology of ephemeral stream and salt-affected flood plains. Google Earth Image modified after Maxar Technologies, and (d) Results of a regional point soil gas sampling program that identified elevated H2 concentrations at the FF4 site.

The geomorphology of the study area comprises an ephemeral stream in the centre of an elongated depression, with a wide rim of low-lying salt-affected flat areas for 500 m to 2 km on either side (Fig. 1c). The stream is slow-flowing and meanders seasonally across the salt pan, causing yearly variations in standing water and soil saturation. The groundwater at the FF4 site was acidic (pH 5.75) and highly saline (77,000 ppm), consistent with previous groundwater studies in the western Wongan Hills locality [34].

Point soil gas monitoring was performed at 15 locations in the area in January 2022, six of the locations were drilled within a 1 km radius including the FF4 site (Fig. 1d). The initial FF4 H2 concentration measured during point sampling was 800 ppm [35] and the decision was made to conduct more detailed autonomous monitoring at that site in order to prove that H2 was emitting continuously, and not simply the result of drill bit cataclasis [36], and to better understand the variations in natural H2 emissions.

3 Methods

3.1 Boreholes

Three boreholes were drilled for the autonomous monitoring to a depth of between 80 cm and 100 cm initially with a 40 mm diameter auger drill bit used to minimize artifact H2 generation via drill bit metamorphism [36]. Capped PVC tube liners were placed into each borehole comprising a 20 mm diameter PVC pipe. The lower 20 cm of the liner was perforated with 5 mm holes (1 per 2.5 cm2 spacing). During soil gas measurements, 3 mm PVC tubing was inserted through the tube liner into the perforated interval to draw gas from the base of the hole (80–100 cm below the surface).

3.2 Gas detection

3.2.1 Point sampling

Gas was detected during the initial point sampling survey by a handheld iBrid MX6. The iBrid MX6 gas detector was fitted with H2, CO2, methane (CH4) and H2S sensors. Measurement ranges and precisions of the sensors are shown in Table 1. The iBrid MX6 is fitted with an internal pump that pumps gases across the sensors at a rate of 300 ml per minute and was calibrated prior to each survey period.

Table 1

Measurement ranges and precision ranges of gas detectors used in this study.

3.2.2 Autonomous sampling

Two stationary autonomous multi-gas detectors (Axiom Sensing; WHALI & EVA, origin: Australia) were used in this study.

The pumped autonomous logging unit (Axiom Sensing; WHALI, origin: Australia) was equipped with H2, CO2, and H2S sensors, temperature, and pressure sensors, and a peristaltic pump capable of pumping gas vapors at 300 ml per minute, similar to the iBrid MX6. The range of the H2 detector was 0–2000 ppm, with a precision of 10 ppm (Table 1). Automatic measurements were made for 3 min either every 15 or 30 min.

The autonomous unit was deployed at the FF4 site for 5 h on 20th February 2022, 7 days from the 23rd to the 30th of April 2022, and 7 days from the 9th to the 16th of February 2023.

The pumped autonomous WHALI unit shut down after five hours on the first day of operation (20th February 2022) due to groundwater intrusion. A water management system was subsequently added and used during April 2022 and February 2023 to evacuate and dispose of groundwater drawn through the system while pumping soil vapors to the sensors (Fig. 2).

thumbnail Figure 2

WHALI Autonomous Water Management System.

In April 2022 the FF4H site was selected for the autonomous sampling, located 8 m up-dip from the original FF4 site, which was inundated with water. An initial H2 concentration of 47 ppm was measured at FF4H with the handheld MX6 gas meter.

The site was inaccessible from July to November, and upon returning in November 2022, measured trace gas concentrations in the air space above the standing water in the boreholes were consistent with air, with no H2 detected [35].

In February 2023 the original FF4 borehole was chosen for the autonomous sampling.

A second autonomous, passive measurement system, i.e. non-pumped (Axiom Sensing; EVA, origin: Australia,) was also deployed at the FF4 site during February 2023 approximately 80 cm away from the WHALI unit to compare the results of the two systems. The passive system was not available prior to that time.

As both autonomous units sit above ground, and electrochemical sensors are sensitive to temperature variations, all H2 concentrations (cH2) and hydrogen sulphide concentrations (cH2S) results displayed herein represent temperature-corrected concentrations according to the manufacturer’s specifications [40]. The following polynomial equation has been applied:(1)

x = Raw concentration

y = Temperature corrected concentration

z = Measured temperature at sensor

Figure 3 shows the results of test measurements in air (H2 < 1), which were recorded for two minutes every 15 min over a four-day period, where the temperature at the sensor ranged from 8.5 °C to 41 °C. The raw H2 concentrations represent the maximum recording during each two-minute sampling period. Equation (1) was applied to generate the temperature-corrected H2 concentration. This shows the extent of the sensor reaction due to temperature variation at a peak of 41 °C was 12 ppm, which corrected to 2.5 ppm. As the accuracy of the sensor in the autonomous units is 10 ppm, <10 ppm is insignificant. Therefore, the cH2 measured can be effectively corrected with the temperature correction equation. The CO2 sensors are infra-red sensors with internal thermometers and self-calibration, and thus CO2 concentrations do not require environmental correction.

thumbnail Figure 3

Sensor reaction to temperature (red) while measuring air, showing raw H2 concentration measurement (grey) and temperature-corrected H2 concentration measurement (blue). The dotted line shows the lower accuracy of the sensor of 10 ppm.

4 Monitoring results

4.1 H2 concentration

Normal cH2 in air is <1 ppm, therefore, cH2 > 1 ppm indicates H2 seepage. However, as the autonomous monitoring unit had an accuracy of 10 ppm, for this study we consider >10 ppm as significant. Here cH2 >> 10 ppm was measured at FF4 during all three monitoring periods (February 2022, April 2022, and February 2023 (Fig. 4).

thumbnail Figure 4

Gas concentrations of H2, hydrogen sulphide, and carbon dioxide plus atmospheric pressure and temperature measured at the sensor from the FF4 site (a) WHALI – 20th of February 2022, installed immediately after drilling of the FF4 hole (b) WHALI – 23rd of April to the 30th of April 2022 (c) WHALI – 9th to the 16th of February 2023 and (d) EVA – 9th to the 16th of February 2023.

The amplitude of the H2 concentrations varied strongly between monitoring periods, despite similar temperature and pressure conditions in the summers (February) of 2022 and 2023 (Table 2). In February 2022 a maximum cH2 of 493 ppm was measured (average 326 ppm), while the highest cH2 for April 2022 was 330 ppm (average 62 ppm) and the peak cH2 was 52 ppm (average 26 ppm) for February 2023 (WHALI). The peak cH2 measured by the EVA system in February 2023 was 180 ppm, but the average (4 ppm) was insignificant.

Table 2

Peak gas concentrations and atmospheric conditions measured during the three monitoring periods in the WHALI autonomous unit.

A diurnal bell-shaped trend was established during monitoring in April 2022 and February 2023, with the highest cH2 measured in the middle part of the day and the lowest cH2 in the evenings.

4.2 Carbon dioxide concentrations

The carbon dioxide concentrations (cCO2) measured across the monitoring periods ranged from 442 to 15,558 ppm (Table 2), indicating valid soil gas measurements. In February 2022, very high and stable cCO2 was measured across the limited monitoring period. For the first week of April 2022 monitoring period, the cCO2 concentrations were high, irregular, and gradually increasing. A more regular oscillating pattern was observed in the second half of the April 2022 monitoring period and in most of February 2023.

cCO2 peaked on the EVA unit in February 2023 at 7570 ppm an hour after installation along with a cH2 of 16 ppm. Other than the two cH2 peaks on the 13th of February 2023, concentrations did not reach those levels again. This may have been due to soil gas readings reaching the sensors before the borehole became inundated with groundwater.

4.3 Hydrogen sulphide concentrations

Hydrogen Sulphide concentrations (cH2S) were insignificant, measuring below the 0.6 ppm precision of the H2S sensors (Table 1, Table 2). This result varies from the headspace gas analysis conducted at the site between November 2022 and February 2023, where cH2S up to 31 ppm (average 3.4 ppm) was detected with vigorous agitation of the groundwater samples [35]. Hydrogen sulphide is slightly soluble in water, but solubility decreases with increasing salinity [42]. This suggests that gaseous H2S may be held within the groundwater, with degassing requiring vigorous agitation, rather than the gentle water movement created through the WHALI water management system.

4.4 Methane concentrations

Methane was measured during initial point sampling at the site in early February 2022 but was not measured by the autonomous units during continuous monitoring. Five measurements were taken using the handheld iBrid MX6 at the FF4 site on the 7th and 8th of February 2022 and measured methane concentrations from 0 to 4500 ppm. Headspace gas analysis of groundwater samples at this site between November 2022 and February 2023 measured methane concentrations from 0 to 6500 ppm [35].

4.5 WHALI pumped vs EVA passive autonomous unit

The average cH2 for the EVA system was approximately one-quarter to one-fifth of the cH2 measured by the WHALI system at the same time. Because the WHALI system is pumped, it draws a sample of 600–900 ml of soil gases over the sensors, which is derived from the area around the borehole.

Alternatively, the EVA system relies on whatever volume of soil gases migrate up the borehole to the sensor. If there is standing water in the borehole, soil gases must break through the interfacial tension of groundwater to rise to the sensors at the surface.

cCO2 measured by the pumped WHALI unit was approximately ten times that measured the passive EVA unit. The exception to this was one hour after both units were emplaced on the 9th of February 2023, when measurements were similar. This may have been due to there being no water saturation in the borehole for the initial part of that day. Again, the main difference is likely to be the pumping. As CO2 is significantly heavier than H2, a larger concentration differential is required for the gas to migrate up the borehole, compared to lighter H2.

4.6 Groundwater volumes

We measured groundwater volumes extracted from the WHALI unit over the April 2022 and February 2023 autonomous monitoring periods. The total groundwater extracted during the seven-day monitoring periods in April 2022 and February 2023, was 20.3 L and 30.1 L, respectively (Table 2). While a greater total volume of water was extracted in February 2023, the largest volume of water extracted during a single monitoring cycle occurred in April 2022. Thus, groundwater levels fluctuated during April 2022, whereas by February 2023 the groundwater saturation was consistently high.

4.7 Gas samples

Gas Samples were collected from the FF4 site during August 2023 and April 2024, in order to confirm the H2 gas emitting from the boreholes, as detected by the handheld gas sensors and autonomous sensors. The samples confirmed cH2 in soil gas from the FF4 site. The maximum cH2 measured from gas samples was 300 ppm (August 2023) and 530 ppm (April 2024) (Fig. 5).

thumbnail Figure 5

Gas Chromatogram of sample FF4 from 19th April 2024. cH2 measured 530 ppm and cCO2 measured 1160 ppm.

5 Discussion

5.1 Soil gas transport mechanisms

Diffusion and advective pressure pumping are the two fundamental processes governing the gas exchange between soils and the atmosphere [43–46]. Diffusion occurs as gases naturally move from areas of high concentration to low concentration, driven by concentration gradients [47, 48]. This process is slow but operates over short distances, allowing gases to permeate through the soil matrix. The diffusion of gases through soil is influenced by physical parameters such as the density of the gases; atmospheric pressure; total porosity; soil moisture; permeability and fractures facilitating preferential flow [46, 47, 49].

Advective pressure pumping involves the movement of gases due to pressure differentials within the soil. This can be caused by wind, temperature and atmospheric pressure gradients, leading to the displacement of gases through soil pores [43, 45, 50, 51].

5.2 Climatic and environmental controls

5.2.1 Temperature

Diurnal H2 emissions occurred at FF4, where cH2 fluctuated in a bell-shaped pattern, peaking in the middle of the day, and falling to low levels during the cooler evenings (Fig. 4). While cH2 and temperature are correlative on a daily scale, Figure 6a shows that there was no overall significant relationship between cH2 and temperature measured at the sensor.

thumbnail Figure 6

All data from autonomous monitoring periods April 2022 and February 2023 (a) cH2 vs temperature (b) cCO2 vs temperature (c) cH2 vs pressure and (d) cCO2 vs pressure.

Soil temperatures influence diffusive gas movements through soil and fluctuate both with depth and seasonally due to factors such as solar radiation, air temperature, soil moisture content, and the thermal properties of the soil itself [52, 53]. The influence of air temperature and solar radiation on soil temperature is highest close to the surface, and diminishes with increasing depth [53]. Cheng et al. [52] demonstrated modest seasonal variation in soil temperature at 80 cm in the low-latitude plateau of China (15–23 °C range), but minimal diurnal variation. Zeng et al. [54] found, that in arid climates, diurnal temperature variations can penetrate soils down to 1 m depth, but the hourly temperature gradient variation was low (0–0.5 °C).

The soil gas in this study was drawn from 80 cm to 1 m below the surface, where soil temperatures would be expected to vary seasonally, but not significantly on an hourly basis. Therefore, we can assume that in the summer and autumn months, the soil temperature is higher, enabling increased diffusive flow of all gases. However, soil temperature cannot account for the diurnal flux of cH2 observed in all cases or the diurnal pattern observed in CO2 following the precipitation event on the 26th and 27th of April 2022.

The average ambient temperatures, average cH2 and average cCO2 were all highest during the February 2022 monitoring. February 2023 had the next highest temperatures and average cCO2, but the lowest average cH2. This suggests a difference in the main transport mechanisms of CO2 and H2 through soils. The average CO2 concentrations appear to follow the seasonal variations in soil temperature, suggesting the primary mechanism of CO2 transport may be diffusion.

While the diurnal H2 variations appear to trend with temperature on a daily time frame, this relationship appears to be correlative rather than causal (Fig. 6a).

The diurnal patterns observed at FF4 are consistent with those documented at the Sao Francisco site in Brazil [22]. In that case, the authors suggested that while there was a correlation between temperature and cH2, it was coincidental, as the temperature did not vary significantly below 0.5 m depth. Rather, they postulated a relationship between atmospheric pressure and near-surface gas transport with a lag between the decreasing atmospheric pressure and subsequent increase in cH2 [20, 22].

5.2.2 Pressure

Overall, cH2 and cCO2 varied substantially across pressure conditions (Fig. 6c & d), suggesting that absolute atmospheric pressure is not a predictor of the amplitude of the cH2 or cCO2. This suggests that an alternative control, such as advection from deep in the sub-surface, or bacterial activity controls the amount of cH2 in the soil. However short-term relationships were seen and are discussed further here.

On the 13th of February 2023, a large pressure drop was observed on both the WHALI (Fig. 4c) and EVA (Fig. 4d) units. Following the pressure drop, two large H2 peaks were measured on the EVA system (178 ppm and 180 ppm) and corresponded to the highest cH2 measurement on the WHALI system (52 ppm) for February 2023. The difference in reaction by the two systems is deduced to be the difference in sampling methods. The passive EVA system does not draw water but relies on passive flux. We propose the large pulse was the result of the H2 gas pressure building up before overcoming the interfacial tension and forcing through the groundwater to the sensor [55]. In contrast, the pumped WHALI system consistently detected the elevated cH2 over several hours, dispersed within the groundwater, by agitating the sample through the water management system (Fig. 7).

thumbnail Figure 7

cH2 for the WHALI and EVA autonomous logging units for the February 2023 monitoring period, with atmospheric pressure. Dotted line shows the lower accuracy limit of the H2 sensors.

cH2 exhibited diurnal flux throughout all periods of monitoring at the FF4 site. cCO2 followed a subdued diurnal flux pattern after the rainfall event of the 26th to 27th of April, and throughout the February 2023 monitoring period (on both the pumped WHALI unit and the passive EVA unit). The diurnal pattern is consistent with measured daily drops in pressure (Fig. 4). A similar diurnal flux pattern of cH2 has been documented in Brazil [20, 22, 56, 57], and diurnal CO2 flux is well known, although for the latter samples depths are usually shallower and therefore more affected by ambient temperatures [58–63].

In both the April 2022 and February 2023 campaigns, small dips in pressure coincided with the peak diurnal H2 concentration (Fig. 8).

thumbnail Figure 8

Temperature (upper squares) and Pressure (lower circles) measured at the site FF4 from the 24th to 29th April 2022. Both graphs are coloured by cH2 (ppm) where blue is high and red is low.

The process of pressure pumping (changing barometric pressure causing upward and downward mass flow of soil gases) as a mechanism for soil gas transport is well known [22, 43–47, 50, 61, 63–65]. Gases of different densities react differently to pressure pumping [46]. High density gases such as CO2 are less sensitive to pressure-induced advection, whereas low-density gases such as Helium, are highly reactive to pressure-induced advection [43, 44, 46, 64].

We propose that the strong diurnal flux observed in the cH2 trend is the result of the low density di-hydrogen molecule reacting to barometric pressure pumping similar to the behaviour of Helium in soil gas [43, 64]. Figure 8 shows temperature and pressure plots for the April 2022 monitoring period, with both plots coloured according to cH2. As pressure falls in response to ambient temperature increase, the H2 concentrations increase. The asymmetry of the H2 concentration with temperature confirms the flow of gas through the subsurface is controlled by pressure, but the daily barometric pressure change is related to temperature, hence the appearance of cH2 to trend with ambient temperature.

Thus, while neither pressure or temperature control the absolute H2 soil gas concentrations in the soil, atmospheric pressure appears to enable the movement of H2 to the surface on a diurnal basis through pressure pumping.

5.2.3 Groundwater levels and soil moisture

Groundwater volumes extracted by the WHALI water management system were measured in April 2022 (20.3 L) and February 2023 (30.1 L). The highest cH2 measured at FF4 in this study occurred during February 2022, when no water had yet inundated the borehole. Following this, the next highest concentrations occurred during April 2022 when there was moderate groundwater extraction. During February 2023, H2 concentrations were consistently and significantly lower than the previous monitoring periods while overall water volume extraction exceeded the April 2022 monitoring period by 48%.

Water levels in boreholes varied substantially between campaigns due to the higher levels of rainfall in the preceding months (Fig. 9). Rainfall was measured by the Bureau of Meteorology at the Wongan Hills Weather Station #08137, located 13 km northeast of FF4 [34]. High rainfall in the second half of 2022 led to high groundwater levels [34], and saturated salt lakes in the region throughout the summer of 2023 (Fig. 10). There was a strong negative correlation between the six months rainfall before each monitoring period (BOM, 2023), and cH2 (Fig. 9). Thus, when no water was initially observed in the FF4 borehole (summer season 2022), peak cH2 was 493 ppm; however, as the groundwater at the FF4 site rose following significant rainfall the H2 concentrations reduced.

thumbnail Figure 9

(a) Rainfall preceding the monitoring periods at FF4, compared to the highest cH2 from the WHALI autonomous monitoring unit during the monitoring periods Feb 2022, April 2022 and February 2023, and cH2 measured via gas chromatography from gas samples taken at the FF4 site. (b) Rainfall preceding the monitoring periods at FF4, compared to the highest cH2 from the WHALI autonomous monitoring unit and measured from gas samples. Pearsons coefficient is higher for the cH2 vs previous 6 months rainfall compared to cH2 vs previous 3 months rainfall.
thumbnail Figure 10

A nearby ephemeral salt lake shows very little standing water was present during (a) January 2022 compared with (b) February 2023.

In Sao Francisco, Brazil, the centres of circular depressions, which were described as flood zones, measured lower H2 emissions to surface than the surrounding dry areas [21].

The reason for the apparent inhibition of cH2 flow in moist soils may be due to either higher bacterial consumption in groundwater, or the physical inhibition of H2 diffusion.

The interfacial tension between H2 and H2O is high at ambient conditions, and diffusivity at surface conditions of dihydrogen in water is close to zero [55]. Therefore, porous water-wet rocks, and even water itself, can form an impermeable seal for H2 gas [10, 55, 66].

Experimental studies by [67] showed significantly lower H2 diffiusion rates through water and brine saturated sedimentary rocks as opposed to dry samples, and higher diffusion rates through sandstones vs claytones. A similar relationship can be expected in soils, where diffusion rates will be lower in brine saturated samples, and indeed is supported by the results of this study. [68]

Davies et al. [35] demonstrated that high concentrations (up to 1684 ppm) of natural H2 was trapped in groundwater and degassed into the headspace of a measurement vessel when agitated vigorously. In this study, the WHALI water management system (Fig. 2), provided significantly less agitation and cH2 concentrations measured were lower. The high levels of H2 liberated during headspace gas analysis by [35] suggest that physical inhibition, rather than bacterial consumption, is responsible for the reduced H2 soil gas emissions measured at the FF4 site during periods of high groundwater.

We propose that during periods of low groundwater levels, H2 can migrate through the subsurface via feeder systems such as fault zones into the dry soil where it flows to the surface. Conversely, when groundwater levels rise, H2 diffusion into surface soils may be impeded by water at the terminus of the conduit fault, trapping H2 in groundwater and preventing its fast movement to the surface.

Soil moisture is known to impact the movement gases through soils as it reduces the porosity [46, 47, 69–71] and CO2 emissions are lowest when groundwater levels are closest to the surface [72].

While soil moisture was not monitored in this study, we use rainfall as a proxy. During the April 2022 monitoring period, CO2 emissions were high and generally increasing for the first three days. cCO2 dropped dramatically when the rainfall occurred on the 26th of April, and thereafter adopted a subtle diurnal pattern similar to cH2. We surmise that rainfall increased soil moisture, which decreased the soil temperature and free air porosity, leading to a drop in diffusive CO2 flux. The diurnal pattern which follows suggests that pressure pumping may be the dominant mechanism of transport for heavier gases only in moist soils [46, 63].

This study demonstrated that groundwater impeded the natural H2 flow to the surface. Such inhibition may lead to false negative readings in soil gas surveys after high rainfall and in low-lying areas. Alternative exploration methods should be considered in such wet areas.

5.3 Natural hydrogen source

Understanding the source rocks for natural H2 can de-risk exploration, by focussing efforts on areas with the most conducive geology. This study aimed to advance understanding through continuous monitoring of multiple gases at the surface. Gas analysis techniques such as isotopic analysis may help to characterize the natural H2 source rocks and are planned for future studies. While isotopic analysis was outside of the scope of the existing study, we may still postulate the most likely sources with the continuous monitoring data available. Five natural H2 generation mechanisms are possible at the FF4 site, namely radiolysis, water interactions with iron-bearing minerals, mantle flux, gas release from fluid inclusions, and/or bacterial activities [17, 73–75].

5.3.1 Bacterial source

Bacterial community abundance has been shown to be at its highest in the 5–10 cm zone below surface and reduce at depth [76–78]. The soil gas measured during this study was taken from 80 cm to 1 m below surface, at which depth microbial abundance is low [76, 78].

CO2 and H2S are classical by-products of bacterial fermentation [72, 79–83]. If the H2 was produced by bacteria, elevated concentrations of cH2S and cCO2 would be expected, and correlate with cH2. Such patterns were not observed at FF4 where cH2 and cCO2 did not correlate (Table 2), and cH2S was <0.4 ppm during autonomous monitoring.

While further bacterial soil studies are required to verify the microbial influence on the H2 emissions, the depth of sampling combined with the observed trace gas mixtures suggest that bacterial fermentation is not the most likely source of H2 at the FF4 site.

5.3.2 Iron-rich minerals and water interactions

The interaction between iron-rich minerals and water, potentially generates H2 in the ultra-mafic greenstone belt adjacent to FF4, which projects several kilometres into the subsurface [30, 31]. Geochronological data suggest deformation of the Yilgarn Craton greenstone belts around 2.8–2.6 billion years ago [84–86]. However, it remains uncertain whether mafic minerals of this age actively produce H2 or have fully reacted. Similarly, the permeability of these formations and their ability to allow water penetration crucial for H2 generation, as well as the release of H2 to adjacent formations, are unknown.

Studies indicate gas vents above ophiolite belts in various regions produce H2 created by serpentinization, along with high methane volumes [87–89]. However, methane measured at FF4 comprises less than 0.65% of soil gas volume [35].

Serpentinization and low-temperature iron oxidation typically produce highly alkaline fluids with a pH of 9–12 [89–91], and consistent generation of H2 gas occurs at pH 8.5 and above [92]. As the groundwater at FF4 is acidic (pH = 5.75), the H2 generating system would need to be separated from the FF4 groundwater via a perched water table, which seems unlikely as it would impede H2 migration into the groundwater system. For these reasons, serpentinization is not the preferred mechanism for the H2 produced at the FF4 site.

5.3.3 Mantle flux

The volatile elements of the earth’s mantle are predominantly H2, carbon, nitrogen, noble gases and sulphur [93]. Those elements are vented as volcanic gases; water vapor, CO2, SO2, CO, H2S and H2 at the surface [94–96]. The acidic groundwater at FF4 is consistent with a deep mantle-derived fluid source [96], into which abundant CO2 is dissolved [97].

CO2 constitutes ~10–49 mol% of volcanic gas emissions [94, 98]. However, the cCO2 at FF4 were significantly lower (Table 2), ranging from 442 to 15,558 ppm (0.04–1.56 vol%); consistent with cCO2 in natural soils [99].

Based on these observations, it appears that mantle flux is not the most likely source for the FF4 H2 seep; however, a full-scale geo-chemical analysis should be conducted before dismissing this potential source mechanism including isotopic analysis of CO2 and noble gases including helium. Such analysis was outside the scope of the current study but is planned for future work.

5.3.4 Fluid inclusion release

H2 gas is known to have been trapped in Precambrian granites worldwide, and the Archean granitoids of South and Western Australia have some of the highest H2 concentrations measured [75, 100].

H2 may be released from fluid inclusions by heating to ≥600 °C or by mechanical crushing [100, 101]. On the Yilgarn Craton, extensional faulting extends 30–35 km into the lower crust [30, 102]. These faults may provide conduits for migration of gases deep in the crust. Magneto-telluric surveying of the Yilgarn Craton shows zones of high conductivity stretching from the Moho to the near surface, supporting the theory of fluid migration through faults [103, 104].

Heat flow modelling predicts a geothermal gradient of ~18 ± 2 °C km−1 over the Yilgarn Craton [105], such that granitoids in the lower crust at 30 km depth exist at temperatures of ~565 ± 60 °C. At such temperatures, H2 may exsolve from fluid inclusions in the lower crust, contributing a volumetrically significant amount of H2 into the subsurface.

The juxtaposition of listric faults extending into the lower crust and the endogenic conditions necessary for the release of H2 from fluid inclusions into these migration conduits provides the ideal geological conditions for the H2 source at the FF4 site.

5.3.5 Radiolysis

In radiolysis, H2 is produced by dissociation of groundwater molecules due to radiation from the decay of radioactive isotopes [100, 106–110].

Granites and granitoids have the highest radiogenic heat flow of all rocks, due to their relative abundance of radioactive isotopes (40K, 232Th, 235U, and 238U) [111]. Global studies show that Australia has some of the most radiogenic granites in the world, averaging at 3.53 μW/m3, and that the Archean granites of Western Australia are anomalously high, producing up to 9.73 μW/m3 [111]. The basement rocks of the Wongan Hills area are well documented through mineral drilling and surface mapping, and comprise granites, granodiorites, tonalites and monzonites [30, 32, 86, 112].

Water radiolysis also produces hydronium ions resulting in the formation of temporary acidic pH spikes in the irradiated groundwater [113]. Flowing groundwaters may retain an acidic signature, consistent with the low pH of the groundwaters at the FF4 site.

The Archean basement lithologies combined with the acidity of the groundwater support radiolysis as a potential source for the natural H2 observed at FF4.

6 Conclusions and implications

Natural H2 has the potential to revolutionize the energy industry by enabling zero-carbon emissions from a natural fuel source. However, exploration of natural H2 accumulations remains limited. Thus, we present the first long-term, multi-gas monitoring of a natural H2 seep in Australia.

H2 emissions occurred in significant concentrations, varied in amplitude across seasons and occurred as diurnal emissions with irregular high-concentration pulses. We interpret that H2 micro-seepage was controlled by pressure pumping and soil moisture. The large pulses are interpreted to be the result of the H2 gas pressure overcoming the interfacial tension of groundwater resulting in a sudden release.

H2 emissions were inhibited by long-term high groundwater levels, which indicates that groundwater at ambient temperatures and pressures can form a barrier to the upward flow of H2 gas through the soil profile and inhibit the soil gas H2 readings at the surface.

This work has significant implications for natural H2 exploration using soil gas monitoring techniques. False negatives are possible and likely in areas subject to inundation. When conducting soil gas sampling it is preferable to operate during the dry season and groundwater levels need to be considered, particularly in low-lying areas. Novel exploration methods will be required in wet environments.

Our confirmation of natural H2 seepage on the Yilgarn Craton indicates active H2 generation in the subsurface. The fundamental information on natural H2 seepage provided by this study de-risks H2 exploration in Western Australia, thus aiding in the implementation of a hydrogen economy.

Acknowledgments

The WHALI and EVA units providing autonomous monitoring measurements were developed by Axiom Sensing Pty Ltd and used with permission from Axiom Sensing Pty Ltd. The authors are grateful to Hayden Long, Ted Atkinson, Alex Vouyoucalos, Brent Spiers and Michael Atkinson for their assistance in the field. The FF4 site is located on private property and the authors would like to thank Mr M.Brennan and later Mr R. Fields for allowing access to the site over three years.

Funding

This research is jointly supported by the CSIRO iPhD Scholarship in collaboration with Gold Hydrogen Ltd. The sponsors played no role in the design, analysis, or reporting of this study.

We would also like to thank the Australian Research Council for financial support (ARC grant DP220102907), and Gehyra Flux Pty Ltd for financial support (G1006788).

Conflicts of interest

The authors declare no conflicts of interest.

Author contribution statement

All authors have read and agreed to the published version of the manuscript. K.D. was responsible for the project scope, design, and fieldwork. K.D. and E.F. were jointly responsible for writing the paper. S.I., A.K., L.E., and A.G. together were responsible for project conceptualization and editing of the manuscript.

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All Tables

Table 1

Measurement ranges and precision ranges of gas detectors used in this study.

In the text
Table 2

Peak gas concentrations and atmospheric conditions measured during the three monitoring periods in the WHALI autonomous unit.

In the text

All Figures

thumbnail Figure 1

Shows (a): Yilgarn Craton and FF4 Sample location (30°55′39″S, 116°35′35″E). (b) FF4 site interpreted bedrock geology (c) site morphology of ephemeral stream and salt-affected flood plains. Google Earth Image modified after Maxar Technologies, and (d) Results of a regional point soil gas sampling program that identified elevated H2 concentrations at the FF4 site.
In the text
thumbnail Figure 2

WHALI Autonomous Water Management System.
In the text
thumbnail Figure 3

Sensor reaction to temperature (red) while measuring air, showing raw H2 concentration measurement (grey) and temperature-corrected H2 concentration measurement (blue). The dotted line shows the lower accuracy of the sensor of 10 ppm.
In the text
thumbnail Figure 4

Gas concentrations of H2, hydrogen sulphide, and carbon dioxide plus atmospheric pressure and temperature measured at the sensor from the FF4 site (a) WHALI – 20th of February 2022, installed immediately after drilling of the FF4 hole (b) WHALI – 23rd of April to the 30th of April 2022 (c) WHALI – 9th to the 16th of February 2023 and (d) EVA – 9th to the 16th of February 2023.
In the text
thumbnail Figure 5

Gas Chromatogram of sample FF4 from 19th April 2024. cH2 measured 530 ppm and cCO2 measured 1160 ppm.
In the text
thumbnail Figure 6

All data from autonomous monitoring periods April 2022 and February 2023 (a) cH2 vs temperature (b) cCO2 vs temperature (c) cH2 vs pressure and (d) cCO2 vs pressure.
In the text
thumbnail Figure 7

cH2 for the WHALI and EVA autonomous logging units for the February 2023 monitoring period, with atmospheric pressure. Dotted line shows the lower accuracy limit of the H2 sensors.
In the text
thumbnail Figure 8

Temperature (upper squares) and Pressure (lower circles) measured at the site FF4 from the 24th to 29th April 2022. Both graphs are coloured by cH2 (ppm) where blue is high and red is low.
In the text
thumbnail Figure 9

(a) Rainfall preceding the monitoring periods at FF4, compared to the highest cH2 from the WHALI autonomous monitoring unit during the monitoring periods Feb 2022, April 2022 and February 2023, and cH2 measured via gas chromatography from gas samples taken at the FF4 site. (b) Rainfall preceding the monitoring periods at FF4, compared to the highest cH2 from the WHALI autonomous monitoring unit and measured from gas samples. Pearsons coefficient is higher for the cH2 vs previous 6 months rainfall compared to cH2 vs previous 3 months rainfall.
In the text
thumbnail Figure 10

A nearby ephemeral salt lake shows very little standing water was present during (a) January 2022 compared with (b) February 2023.
In the text

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