Methane concentrations and depth

Methane concentrations varied significantly with depth within sampled multilevel wells (Fig. S3). Out of 368 groundwater wells with at least two sampled well screens with methane concentrations >0.01 mg L⁻¹, the coefficient of variation, calculated as the standard deviation of the methane concentrations in the well screens present in the well divided by their total mean concentration, was greater than 1.0 in 20% of wells. For this group, 33 of the 76 wells had a mean methane concentration above 1 mg L⁻¹, showing that such variability does not only appear at the low end of the methane concentration range. This vertical spatial variability is much greater than the variability of methane concentrations within single well screens over time (Fig. 2).

Overall, median methane concentrations per depth interval increase with depth down to 100 - 140 mbgs, where they exceed 1 mg L⁻¹. Below 140 mbgs median concentrations gradually drop off again to 0.02 mg L⁻¹ for depths greater than 240 mbgs. Maxima vary between 40 and 100 mg L^-1 down to 140 mbgs, after which they drop off to around 10 and 20 mg L⁻¹. Only the maximum reported single measurement (before averaging concentrations over time in each well screen) of 120 mg L⁻¹ exceeded the theoretical solubility corresponding to the depth of the middle of the well screen and a 100% methane mole fraction (∼96 mg L⁻¹ at 22.3 mbgs). The pattern of decreasing methane concentrations with depth suggests the high concentrations result from local biogenic methanogenesis rather than diffusion from underlying sources of methane, which would yield an overall pattern of increasing methane concentrations with depth.

When isolating the wells that are in the coastal lowlands (n = 369, Fig. 1A), methane concentrations show a distinctly different pattern. Median concentrations are greatest at shallow depths and peak between 40 and 60 mbgs at 4.0 mg L⁻¹, compared to just 0.2 mg L⁻¹ in the same depth interval in the inland areas (Fig. 3). Furthermore, the variability in methane concentrations down to around 100 mbgs is more significant in this area, as shown by the greater interquartile range. However, from 100 to 160 mbgs, both median and 3^rd quartile concentrations are actually smaller in the Holocene part of the country. At this depth interval, third quartile concentrations in the inland areas are around 4 mg L⁻¹. The lower methane concentrations at these depths in the coastal lowlands versus the inland areas can possibly be explained by the generally shallower depth of the fresh-salt interface in this region (Mendizabal et al., Reference Mendizabal, Stuyfzand and Wiersma2011) and hence the prevalence of sulphate, which prevents methanogenesis (Molofsky et al., Reference Molofsky, Richardson, Gorody, Baldassare, Connor, McHugh, Smith, Wylie and Wagner2018). These results give a more nuanced picture of methane concentrations in the country than earlier work by Fortuin and Willemsen (Reference Fortuin and Willemsen2005), who concluded that the highest concentrations occur at shallow depth in the presence of reactive organic matter such as peat. While this is indeed true for the coastal areas, there are other sources of methane in slightly deeper groundwater aquifers in the inland areas, resulting in overall higher methane concentrations there.

Figure 3.

Medians and interquartile range of methane concentrations per depth interval of twenty meters. Results are shown for wells in the part of the country with Holocene marine sediments (blue) and other areas (green), as per the red line in Fig. 1A.

Depositional environment and stratigraphic unit

Methane concentrations in water samples from glacial deposits (1.7 mg L⁻¹) are significantly higher than in sediments with other depositional origins (Fig. 4A). This finding appears consistent with observations in US groundwater, where methane concentrations are also higher in unconsolidated glacial sediments than in unconsolidated non-glacial sediments (McMahon et al., Reference McMahon, Belitz, Barlow and Jurgens2017). However, median concentrations in glacial sediments in the United States (∼0.001 mg L⁻¹) are much lower overall than in the Netherlands. Concentrations in formations of marine and fluvial origin are lower and not significantly different from one another, with median concentrations of 0.21 and 0.31 mg L⁻¹, respectively. Median concentrations in formations with other depositional origins were again statistically significantly lower with a median of just 0.02 mg L⁻¹. Most of the samples in this category are from formations with an aeolian origin, or from ice pushed ridges. These generally very permeable sandy formations serve as important infiltration areas for groundwater in the Netherlands. Hence, the observed low methane concentrations are not surprising, given the predominant Fe-anoxic redox state of the groundwater (Griffioen et al., Reference Griffioen, Vermooten and Janssen2013).

Figure 4.

Box and whisker plots of methane concentrations versus depositional environment (A) and versus Neogene and Quaternary geological units (B), shown in order of increasing age (as per Fig. S1). Concentrations in Holocene (not including the Boxtel Formation) and Paleogene formations are aggregated in plot B and excluded from A. Colours in B indicate the depositional environment as per plot A (Fluvial = Green, Marine = Orange, Glacial = Purple, White = Other). Boxes represent values within the 25^th and 75^th percentile, whiskers the minimum and maximum concentrations. Number labels show the total number of samples per category, capital letters indicate homogeneous subsets following the multiple comparison test procedure.

Multiple comparison tests on the stratigraphic units in which wells were screened revealed a more complex image, with significant differences between 9 homogeneous subsets that are much larger than the differences between the depositional environments (Fig. 4B). The highest median concentrations were observed in the glacial Peelo Formation (4.85 mg L⁻¹), much larger than those in the Drente Formation (0.14 mg L⁻¹), which is the only other glacial formation in the Netherlands. Organic matter content in the Peelo Formation is indeed known to be higher than in the Drente Formation, particularly in the clay deposits that can be more than one hundred meter thick where they have partially infilled deeply incised subglacial tunnels that are not present in the Drente Formation (Griffioen et al., Reference Griffioen, Klaver and Westerhoff2016a). The data show that local methanogenesis in these clays is likely a source of dissolved methane in the Netherlands.

Median methane concentrations in marine deposits are highest in the Late Pleistocene Eem Formation (1.50 mg L⁻¹). Third quartile concentrations in the Eem Formation are particularly high at 49.6 mg L⁻¹, indicating that significant methanogenesis likely occurs in this formation. This corresponds to findings of shallow gas in the Danish subsurface, which were contributed to significant methanogenesis in a comparable Late Pleistocene marine formation formed in glacial basins (Laier et al., Reference Laier, Jørgensen, Buchardt, Cederberg and Kuijpers1992). Median concentrations in Neogene and Early Pleistocene marine deposits were considerably lower (<0.22 mg L⁻¹). Only 5 of 38 total measurements in Paleogene marine formations were above the MRL, and only 1 is larger than 1 mg L⁻¹. This finding also follows from the low observed methane concentrations at depths greater than roughly 200 mbgs (Fig. 3), where the Paleogene formations typically occur in the Netherlands. Similarly, methanogenesis in these formations does not have to be carbon limited but is more likely the result of unfavourable redox conditions due to the presence of SO₄ rich saline groundwater.

The occurrence of methane in fluvial deposits also varies greatly, with median concentrations in the Appelscha Formation (4.65 mg L⁻¹) and to a lesser extent the combined Peize and Waalre Formations (1.10 mg L⁻¹) being significantly larger than in other fluvial deposits. A local methanogenic source is less likely in these formations, given that they are characterised by medium to coarse grain sands and gravel (Griffioen et al., Reference Griffioen, Klaver and Westerhoff2016a) that are not typically associated with high organic matter contents. Since the Appelscha Formation overlies the Peize-Waalre Formation, the occurrence of high concentrations of dissolved methane here is more likely attributable to varying overlying sources, such as the Peelo clays, the Marine Eem Formation or Holocene deposits. One may note that the ice-pushed ridges, which are frequently composed of fluvial sediments from the Urk Formation, have very low methane concentrations, with third quartile concentrations just slightly higher than 0.01 mg L⁻¹.

Surprisingly, the median methane concentration in Holocene deposits is only 0.08 mg L⁻¹. However, the interquartile range is relatively large. The high methane concentration observed at shallow depth in the Holocene marine area (Fig. 3) are therefore likely very local phenomena, linked to reactive carbon in peat of the Nieuwkoop Formation and/or the marine Holocene Naaldwijk Formation.

Spatial position

The spatial distribution of methane concentrations was tested by comparing the average and median concentration in a subset of wells that overlap with certain spatial criteria (Fig. S2) to those that don’t. The biggest difference in average methane concentrations was observed for the subset of wells overlapping with the Holocene Nieuwkoop Formation (10.1 mg L⁻¹), which is the major peat formation in the Netherlands. The difference in median and average methane concentrations for wells in the area with Holocene marine deposits (Fig. 1A) was much smaller (0.5 and 4.7 mg L⁻¹, respectively). This suggests that the Holocene marine deposits might play a smaller role in determining methane concentrations than the presence of peat does. Surprisingly, methane concentrations in wells that intersect the Eem Formation were not significantly elevated (Table 2), in spite of the high observed concentrations in samples directly from the Eem Formation. The discrepancy results from the fact that while 283 samples are from wells that intersect the Eem Formation, only 30 water samples were directly obtained from it (Fig. 4). This highlights a key limitation of using statistical analysis of horizontal spatial relations for determining sources of groundwater methane, which in reality are determined by depth and time dependent processes.

Table 2.

Difference in mean and median values for subgroups of well screens using a criterion with respect to a given threshold distance of or intersecting a certain spatial feature (shown in Fig. S2) that is a potential source of methane in shallow groundwater. P-values are for associated Mann–Whitney U tests

Differences in median concentrations were also high for wells that overlap with known oil and gas reservoirs (7.3 mg L⁻¹). To a lesser extent, median concentrations in groundwater wells in close proximity to gas wells were also substantially higher than those further away, for both cut off distances of 1 (2.9 mg L⁻¹) and 2.5 km (2.7 mg L⁻¹), respectively. Whether this is evidence of a deep, thermogenic source at play in controlling the methane concentrations is questionable however. There is strong overlap between the occurrence of peat and that of oil and gas accumulations in the Netherlands (Fig. S2), which cannot be distinguished between here. Although a dominantly biogenic origin of methane remains the most likely factor controlling methane concentrations in the Netherlands, a local thermogenic source near gas wells or above oil and gas fields in general cannot be ruled out on the basis of methane concentrations alone. Hence, isotopic evidence is required to determine whether a substantial thermogenic component contributes to the elevated concentrations above oil and gas fields and near gas wells. The spatial analysis did reveal that other potential deep sources can be ruled out as major sources of methane in shallow groundwater, as concentrations in groundwater wells near faults were not significantly different, and methane concentrations above the Posidonia and Geverik shales differed less than 1 mg L⁻¹ with those that do not overly these formations.

Shallow aquifers

All but one of the shallow groundwater samples with methane concentrations greater than 1 mg L⁻¹ had a δ¹³C-CH₄ <−60‰, indicative of a biogenic origin, with the exception of the sample collected near the blowout in Sleen (SGW-09). A follow-up study carried out at this location showed that leakage of natural gas has likely been ongoing for several decades. As such, the sample proved to be clear evidence of an anthropogenically opened connection with the deep subsurface (Schout et al., Reference Schout, Hartog, Hassanizadeh and Griffioen2017). The δD-CH₄ values for shallow groundwater samples with biogenic methane in concentrations >1 mg L⁻¹ were around -250‰ or higher, which suggests that CO₂ reduction is the dominant methanogenic pathway (Whiticar, Reference Whiticar1999). This was also found to be the dominant methanogenic process in numerous other groundwater methane baseline studies worldwide (e.g. Humez et al., Reference Humez, Mayer, Ing, Nightingale, Becker, Kingston, Akbilgic and Taylor2016; Kulongoski et al., Reference Kulongoski, McMahon, Land, Wright, Johnson and Landon2018; McIntosh et al., Reference McIntosh, Hamilton, Grasby and Osborn2014; Nicot et al., Reference Nicot, Mickler, Larson, Clara Castro, Darvari, Uhlman and Costley2017).

Five samples from the shallow aquifers had δ¹³C-CH₄ values above −50‰ (Table 3), typically associated with a thermogenic origin. For two of the samples, the combination of δ¹³C-CH₄ and the light alkane ratio (C₁/(C₂ + C₃) even closely resembles that of SGW-09 from the blowout site, which in turn is similar to that of natural gas reservoirs in the Netherlands (Fig. 6A). However, these fives samples all show extreme deuterium enrichment with δD-CH₄ values between 100 and 300‰ (Fig. 6B). Such highly enriched values have previously been associated with fractionation caused by methane oxidation (Etiope et al., Reference Etiope, Baciu and Schoell2011; Schloemer et al., Reference Schloemer, Elbracht, Blumenberg and Illing2016). Rayleigh fractionation modelling with carbon isotope fractionation factors ε_C of 7 and 12 (definitions in Whiticar, Reference Whiticar1999) confirms that this shift in isotopic and molecular composition of the dissolved gasses in these samples can indeed be attributed to methane oxidation (Fig. 6A).

Figure 6.

Methane carbon isotope ratio versus light alkane ratio (A) and hydrogen isotope ratio (B), for gasses found in Dutch shallow groundwater, deep groundwater, geothermal wells and natural gas accumulations. Dashed black lines show hypothetical mixing lines for biogenic and thermogenic gas, with bold labels in A showing the fraction of thermogenic gas in the mixture. Green and orange lines in A depict the calculated effect of anaerobic methane oxidation (AOM) for two carbon fractionation factors (ε_C). Orange line in B depicts isotopic fractionation up to a residual methane fraction of 1% with fractionation factors ε_C = 12 and ε_D = 96. Calculations according to formulations in Whiticar (Reference Whiticar1999).

The process by which these anomalous isotopic values came to be is best illustrated by the analyses of the four samples collected in the multi-level monitoring wells SGW-02 and SGW-16. For example, only the sample collected from the greatest depth in SGW-02 has high methane concentrations (SGW-02d, 36 mg L⁻¹), negligible sulphate concentrations and a clear biogenic isotopic composition. The three other samples collected from the overlying well screens all have trace amounts of methane and high sulphate concentrations (Table 3). The extreme deuterium enrichment in these samples is well explained by isotopic fractionation factors ε_C of 12 and ε_D of 96 (Fig. 6B), that fall within the observed range of fractionation factors for microbial methane consumption (Whiticar, Reference Whiticar1999), leading to an enrichment ratio (ΔD/Δ¹³C) of 8. Using this method, an estimated 99% of the initial methane would have had to be oxidised to reach such extremely enriched values. Given the low CH₄ concentrations (<0.1 mg L⁻¹) this still corresponds to only a minor amount of oxidation in absolute terms. Hence, rather than a thermogenic origin and connection with the deep subsurface, methane in these wells originates from biogenic sources and has subsequently been transformed isotopically as a result of anaerobic oxidation of methane (AOM).

Paleogene and older deep aquifers

In spite of the low methane concentrations and the presence of sulphate in all deep groundwater samples, only sample DGW-09 displays a similar shift in both isotopic and molecular composition that clearly shows the impact of AOM (Fig. 6B). The other samples (except two; see discussion below) lie somewhere on a mixing line, and hence the small methane mass concentrations appear to consist of varying fractions of biogenic and thermogenic methane (Fig. 6A). Measurements of the δ¹³C of ethane and propane corroborate the conclusion of a mixed origin for these samples, as their δ¹³C-C₂H₆ composition resembles that of Dutch natural gas samples closely, and is notably different from the ethane in the shallow groundwater samples with biogenic methane (Fig. 7). This indicates that some upward seepage of thermogenic gas does occur. The ratio between the δ¹³C of methane and ethane, as incorporated in the isotope factor O_(C1/C2) proposed by Cesar et al. (Reference Cesar, Mayer and Humez2021), further suggests that the thermogenic ethane in most of these samples is very early mature. This shows that the source is likely more local than the deeper natural gas reservoirs, which contain more mature thermogenic gas (Fig. S4).

Figure 7.

‘Gas plot’ of carbon isotopic composition versus carbon number for methane (C₁), ethane (C₂) and propane (C₃) in gasses found in Dutch shallow groundwater, deep groundwater and natural gas accumulations. Also shown are potential effects of oxidation and mixing on methane isotopes and ranges of δ¹³C₂H₆ values interpreted to represent either biogenic or thermogenic ethane.

Since the fraction of ethane in thermogenic gas is much larger than in biogenic gas, even mixtures with a small thermogenic component should show a clear thermogenic ethane signature, in conjunction with a mixed isotopic signature of methane which shifts more or less linearly with the fraction of thermogenic methane. Therefore, the lighter ethane signature in the shallow groundwater samples also provides evidence for a lack of even small amounts of thermogenic methane at these depths. This highlights the added value of δ¹³C-C₂H₆ as a tracer for gas migration. However, the amount of sampled ethane is often too small to reliable measure its isotopic composition, which also explains the missing data for a number of samples in this study (Table 3). Increasing the sampled volume could circumvent this issue, however, this may significantly increase sampling efforts depending on its methodology. Furthermore, similar to methane oxidation, preferential oxidation of ethane may also occur in freshwater aquifers (Schout et al., Reference Schout, Hartog, Hassanizadeh and Griffioen2017), which can obscure the interpretation of ethane isotopes.

Two exceptions to a mixed or biogenic origin are the samples from wells DGW-10 and DGW-11 (Fig. 6). DGW-10 is located in the east of the Netherlands and screened at a depth of nearly 700 m (Table 3). It has an isotopic composition that is comparable to that of natural gas reservoirs in the Netherlands and therefore presents clear evidence of upward seepage of thermogenic methane up to at least this depth (Fig. 6B). The C₁/(C₂ + C₃) ratio of 5, slightly lower than that of Dutch natural gas, likely indicates that some oxidation has occurred during or after migration. Sample DGW-11 has an enriched δ¹³C-CH₄ composition (−14‰) that is similar to some of the oxidised shallow samples. However, the combination with its δD-CH₄ value of −314‰ places it outside of the traditionally delineated areas for thermogenic and/or biogenic methane, and actually suggests an abiotic methane origin according to classifications in Milkov and Etiope (Reference Milkov and Etiope2018). While such a finding would present a first for the Dutch subsurface (to the authors’ knowledge), additional evidence for this conclusion is presented by the reversed δ¹³C trend of the light alkanes (δ¹³C-C₁ > δ¹³C-C₂ > δ¹³C-C₃, Fig. 7), which is a known characteristic of abiotic gas (Lollar et al., Reference Lollar, Lacrampe-Couloume, Slater, Ward, Moser, Gihring, Lin and Onstott2006). While no clear evidence is given in the lithological description of the cuttings collected while drilling well DGW-11, intrusions of volcanic rocks are known to be present in the Carboniferous and Permian formations that lie at the base of this well, such as for example the Carboniferous Baarlo Formation (Sissingh, Reference Sissingh2004). In the presence of such intrusions, the low concentration of methane in this sample could be the result of catalytic CH₄ generation. In experiments with serpentinised rocks, this process has been shown to occur down to temperatures as low as 20°C (Etiope and Ionescu, Reference Etiope and Ionescu2015), considerably lower than the estimated 37°C in well DGW-11 (assuming a geothermal gradient of 31°C km⁻¹; Verweij et al., Reference Verweij, Nelskamp, Ten Veen, De Bruin, Geel and Donders2018). Given its unusual composition compared to the gas from all other groundwater samples, additional evidence would be needed to verify the abiotic origin of dissolved methane in DGW-11.

Dissolved gas in the geothermal wells is distinct from that of Dutch gas wells and has a geochemical signature that falls within the range of values associated with mixing of biogenic and thermogenic gasses (Fig. 6A). However, the depths (1640–2625 m, Table 3) and estimated temperature range of these wells is not typically associated with biogenic methane accumulations. Moreover, wells GTW-01 to GTW-04 are completed in the same formations as (depleted) Lower Cretaceous oil reservoirs, where biodegraded oils are known to be present (Griffioen et al., Reference Griffioen, Verweij and Stuurman2016b). Hence, an oil associated origin is more probable. The dissolved gas isotopic and molecular signature confirms this hypothesis, as it closely matches that of oil associated methane (Milkov and Etiope, Reference Milkov and Etiope2018). Additionally, the shift in isotopic and molecular composition observed for GTW-02 and GTW-06 matches the shift that is observed for methanogenic biodegradation of oils (Fig. 6). As these are the two shallowest sampled geothermal wells (with estimated temperatures of 60.8 and 72.9°C, respectively), they fall within the range of temperatures for which microbially mediated biodegradation has been observed (<75°C; Milkov, Reference Milkov2010).

Connectivity deep and shallow subsurface

Shallow groundwater samples were collected near both faults and known gas accumulations, where migration of methane from the deep subsurface is most likely. However, naturally occurring thermogenic methane was only observed below the Neogene and Paleogene clay barriers that form the hydrogeological base in the Netherlands. Although no evidence for gas phase accumulations below these layers were found in this study, this suggests that these geological units form an important barrier to upward gas seepage. Their importance in controlling fluid migration is supported by the δ³⁷Cl data for dissolved chloride. Given the non-reactive nature of chloride in aquatic environments, most natural waters tend to have a δ³⁷Cl close to 0‰ (Eggenkamp, Reference Eggenkamp1994). However, since the diffusion coefficient of ³⁵Cl is slightly larger than that of ³⁷Cl, chloride in groundwater with a lighter ³⁷Cl/³⁵Cl composition (more negative δ³⁷Cl value) has been attributed to diffusion-controlled transport (Beekman et al., Reference Beekman, Eggenkamp and Appelo2011; Desaulniers et al., Reference Desaulniers, Kaufmann, Cherry and Bentley1986).

In this study, a number of samples collected in the south-west of the Netherlands from Paleogene aquifers show very negative δ³⁷Cl values: down to −2.08‰ (Fig. 8). These sandy aquifers are bounded on the top and bottom by low permeable marine clays. They are recharged by fresh rain water infiltrating in neighbouring Belgium, with chloride concentrations increasing from fresh to brackish as the groundwater flows towards the north and west and into the Netherlands (Van Der Kemp et al., Reference Van Der Kemp, Appelo and Walraevens2000). The negative δ³⁷Cl values in these samples therefore point out that transport of solutes is predominantly controlled by diffusion from the under- and overlying clays, rather than advective transport. As chloride concentrations increase further, δ³⁷Cl values increasingly resemble that of the (connate) seawater source in the confining units (0‰), resulting in the observed positive correlation between δ³⁷Cl and Cl.

Figure 8.

δ³⁷Cl chloride stable isotope ratio versus chloride concentration (A) and versus percent Modern Carbon (B), for deep and shallow Dutch groundwater samples. Symbols show the age of the stratigraphic unit. Red dashed line is the chloride concentration of modern seawater.

This positive correlation is also observed for the Cretaceous formation waters (Fig. 8). However, chloride concentrations in these aquifers exceed that of seawater. The elevated chloride concentrations result from dissolution of halite, which has δ³⁷Cl values of around 0‰ (Eggenkamp et al., Reference Eggenkamp, Louvat, Agrinier, Bonifacie, Bekker, Krupenik, Griffioen, Horita, Brocks and Bagheri2019). Subsequent diffusive transport then leads to the observed negative δ³⁷Cl values. Besides the Cretaceous samples, one of the samples from a Paleogene unit in the north eastern corner of the country (DGW-07) also fits this pattern. Here, salinities exceeding that of seawater were indeed shown to result from dissolution of a nearby evaporite diapir (Griffioen et al., Reference Griffioen, Verweij and Stuurman2016b).

Radiocarbon dating shows that groundwater in the Paleogene and older aquifers contains ¹⁴C well above detection limit (0.01–0.1 pMC, Table 3) and a significant fraction of relatively recent groundwater (<40,000 years old) must therefore be present. Significant fractions of modern carbon were also observed in the groundwater samples from the Jurassic, Cretaceous and Carboniferous formations. This confirms that groundwater in these formations is not stagnant, and that some recharge is happening or has happened during the past tens of thousands of years. National-scale groundwater modelling by Valstar and Goorden (Reference Valstar and Goorden2016) indeed pointed out that exchange of groundwater across the Paleogene clay layers is on-going. However, as diffusion appears to be the dominant transport mechanism in these units, significant advective transport of dissolved thermogenic methane from below these layers to the shallow subsurface is unlikely.

Besides the presence of the Paleogene clays, the absence of thermogenic methane in shallow groundwater may also have resulted from a number of other processes that limit its upward migration. First, gas seeping upward through faults that penetrate the sandy Quaternary aquifers that characterise most of the Dutch shallow subsurface would likely become unfocused and hence harder or even impossible to detect (Thielemann et al., Reference Thielemann, Lücke, Schleser and Littke2000). Second, lateral groundwater flow in the aquifers encountered by the migrating gas phase may lead to significant dilution and ultimately stagnation of the upward seeping gas phase, preventing detection in overlying aquifers (Schout et al., Reference Schout, Hartog, Hassanizadeh, Helmig and Griffioen2020). Hence, while no evidence for methane seepage to the shallow groundwater through natural connections with the deep subsurface was encountered, its occurrence cannot be entirely excluded on the basis of this study. Nevertheless, the results of this study do indicate that future observations of thermogenic methane in shallow groundwater are most likely linked to anthropogenic activities.