The PAH compounds identified in the Walloons CSG water are likely coal derived, based on their similarity to: known PAH composition in coals [
43,
44], PAHs leached from coals herein (
Table 4 and
Table 5), and PAHs identified in CSG Water of the Powder River Basin [
28]. Naphthalene and phenanthrene are the most water soluble PAHs, likely accounting for their increased presence in CSG waters relative to the higher molecular weight PAHs (see
Table 6). However, the lower molecular weight PAHs detected in the Walloon Coal Measures CSG water (naphthalene, phenanthrene and pyrene) are considered to be less harmful to human health than the higher molecular weight PAHs [
45]. The detected PAHs with 4 and 5 aromatic rings—chrysene, benzo[b]fluoranthene and dibenz[a]anthracene—are suspected human carcinogens [
45,
46], based largely on data from toxicological tests in rodents. The higher molecular weight PAHs, such as benzo[a]pyrene and dibenzo[a,l]pyrene (5 and 6 rings, respectively), are suggested to cause cancer through metabolic activation in cells and formation of stable adducts that damage DNA [
47,
48,
49]. Benzo[a]pyrene (BaP) is the only PAH with a trigger threshold (0.01 µg/L) in Australian drinking water guidelines (ADWG). The ADWG state that insufficient data are available to set a guideline limit for other PAHs (National Health and Medical Research Council, Canberra, ACT, Australia, 2004). Orem
et al. [
28] summarized international PAH guidelines in drinking water according to the United States Environmental Protection Agency (USEPA) and World Health Organization (WHO). Under the ADWG, USEPA and WHO guidelines, no single Walloon CSG well produced water that would exceed the regulatory levels for PAHs. The drinking water guidelines are provided purely as a relative index to the aromatic concentrations in CSG water and do not imply that untreated CSG water is ever used as potable supply in Queensland. Under Australian environmental guidelines [
50], such as those applied to water quality in aquatic ecosystems, only naphthalene has a trigger value (16 µg/L for a 95% species protection level) whereas all higher molecular weight PAHs have insufficient data available to set guideline values.
None of the higher molecular weight, potentially harmful PAHs present at very low concentrations in the CSG water from Walloon wells were present in the holding pond. This is an important consideration when reviewing CSG water quality data from production wells, particularly in the context of ultra-trace organics and also because the holding pond is the feedstock for treatment and beneficial reuse of CSG water. If a small number of wells produce CSG water containing minute amounts of suspected harmful PAHs and the vast majority of wells do not produce CSG water with those compounds above the detection limit, then the PAHs may be further diluted by mixing in the holding ponds. Therefore analysis—particularly of the very insoluble PAH compounds discussed here—may result in nil detection of those compounds from the ponds, despite their presence in water from individual wells. Other processes, such as rainwater input, the binding of PAHs to soils and sediments [
51] and UV degradation [
52,
53] may also reduce PAH concentrations in ponded water.
Orem
et al. [
28] identified PAHs including naphthalene, phenanthrene, fluorene and pyrene in CSG water from a small number of wells targeting the Powder River Basin coals. However, that work more commonly identified functional derivatives of PAH compounds (e.g., alkylated naphthalene, methylpyrene, dimethylphenanthrene) that are not regular water quality analytes in Queensland and often have unknown toxicities. Derivative forms of PAHs are a significant part of the molecular composition of coals [
54] and necessitate higher resolution analysis of QLD CSG water to quantify their occurrence and any potential health risks those compounds may pose.
4.1. Leaching Experiment Discussion
The concentrations of organics leached from lignite were an order of magnitude greater than the bituminous samples (
Figure 2). This concurs with the increased solubilization of lower rank coals relative to higher rank coals as described by some authors [
27,
55]. In aqueous leaching experiments of various coals, Maharaj
et al. [
56] also observed higher concentrations of aromatics in some (but not all) lignite leachates relative to bituminous samples. Naphthalene and phenol are more soluble than the higher molecular weight PAHs (
Table 6 [
57,
58]) and also more likely to be cleaved from the less condensed aromatic framework of a low rank coal [
55]. This is consistent with the increased presence and concentration of lower molecular weight PAHs (and their derivatives) in organic results from CSG wells targeting lignite to sub-bituminous coals of the Powder River Basin [
28].
PAHs detected in Mv-bituminous leachate were of 3, 4 and 5 aromatic ring structures, including pyrene, fluoranthene, chrysene, benzo[a]anthracene, benzo[b+k]fluoranthene and benzo[a]pyrene. Benzo[a]pyrene (BaP) is a known carcinogen with very low solubility and high affinity to remain in the organic phase [
59,
60]. It is detected in the Mv-bituminous leachate results at its threshold concentration in the ADWG (0.01 µg/L). The filtered coal leachates likely contained other dissolved humic material from the coals given the observed yellow/brown colour. The presence of this organic matter may have enhanced the solubility of BaP and other higher molecular weight PAHs [
61].
Table 6.
Aromatic compounds detected in Walloon Coal Measures CSG water and leaching experiments with their physico-chemical properties [
57,
58].
Table 6.
Aromatic compounds detected in Walloon Coal Measures CSG water and leaching experiments with their physico-chemical properties [57,58].
Compound | Molar Mass (g/mol) | Solubility at 25 °C (µg/L) | Log KoW * | No. of Aromatic Rings |
---|
Phenol | 94.1 | 83,000,000 | 1.46 | 1 |
Naphthalene | 128.2 | 31,000 | 3.37 | 2 |
Phenanthrene | 178.2 | 465 | 4.46 | 3 |
Fluoranthene | 202.3 | 260 | 4.9 | 3 |
Pyrene | 202.1 | 133 | 4.88 | 4 |
Benz[a]anthracene | 228.3 | 11 | 5.63 | 4 |
Chrysene | 228.3 | 1.9 | 5.63 | 4 |
Benzo[b]fluoranthene | 252.3 | 2.4 | 6.04 | 4 |
Benzo[k]fluoranthene | 252.3 | 2.4 | 6.21 | 4 |
Benzo[a]pyrene | 252.3 | 3.8 | 6.06 | 5 |
Dibenz[a]anthracene | 278.3 | 0.4 | 6.86 | 5 |
The 3–5 ring PAHs present in the deionized water leachates of bituminous coals were not detected in the synthetic CSG water leachates. PAHs leached in deionized water were exceedingly low and their absence in the synthetic CSG water may relate to the decreased ability of that fluid to leach and dissolve very non-polar substances as ionic strength increases [
62,
63]. The lack of phenol in the bituminous leachates may relate to their increased coal maturity. As coalification increases (and O and H decrease) the concentrations of hydroxylated aromatics are lowered relative to those present in lower rank coals [
44].
4.2. Aromatic Compound Mobilization from Coal
The macromolecular network of coal contains aromatics in two phases. During coalification, resistant plant biopolymers are turned into a progressively denser, aromatic, three-dimensional network [
42]. Within the network structure, a “mobile” phase of smaller molecules is more weakly bonded and can more easily leach to the environment [
64]. The mobility of these compounds makes them of greater human health concern. The PAHs present in the leachates of coal samples most likely represent constituents of the mobile phase.
The leach results suggest that types of compounds leached from coals to deionized water become increasingly aromatic with increasing coal rank. The lowest rank coal leached 1 and 2 ring aromatics at the highest concentrations and the highest rank coal leached 3, 4 and 5 ring aromatics at the lowest concentrations (
Figure 2). Conversely, Stout
et al. [
43] observed more 4–6 ring PAHs in extracts of lignite and sub-bituminous coals compared with greater 2–3 ring PAHs in higher rank coals of the western United States. In addition, those authors showed an increasing concentration of total PAHs in coal with coalification (up to the bituminous rank) (see Figure 4 of [
43]). However, the types of aromatics in the molecular structure of coal do not have a systematic relationship to coal maturity alone. Detailed maceral composition (currently unavailable for these samples) and coal origin is also a major factor [
44,
54]. Laumann
et al. [
54] observed that when considering coals of different origin from multiple basins, 2–3 ring PAHs dominate molecular composition regardless of coal maturity.
Results from previous studies [
43,
54] in which compounds were extracted with organic solvent are not directly comparable to this study, however. Extract composition from natural water will likely differ depending on PAH sorb/desorb behaviour, the ability of coal to act as a geosorbent, PAH solubility in natural waters and the co-elution of PAHs to other dissolved humic material. For example, the very low concentrations of the higher molecular weight (and potentially harmful) 3–5 PAHs detected in the bituminous leachates may be better explained by their very low solubilities in water and higher binding affinities (Log Kow) for organic matter (
Table 6), rather than their relative abundances in coal samples.
Attempting to predict types and concentrations of leached aromatic compounds based purely on coal properties of individual seams may prove unrealistic when trying to correlate these results to the concentration of compounds in water samples from actual CSG production fields. Complications in such modelling would arise when sampling water from open-hole completed wells, where many potentially variable coal seams and their inter-seam sediments are targeted for gas extraction. Also, the inflow of groundwater from aquifers hydraulically connected to the coal seams may dilute any expected compounds, thus site specific hydrogeological conditions must be considered. Where hydraulic fracturing has occurred, organic additives used in injection fluids (see Table 2 of [
3]) may also have the potential to solubilize coal derived hydrocarbons [
29] to CSG waters, particularly where flowback recovery does not appropriately “flush” the formation water. Furthermore, some insoluble aromatic compounds reported in this study occur very close to their detection limits and thus their analytical reproducibility in CSG water may be infrequent.
4.3. Human Health Risk
The first phase of human health risk assessment includes hazard identification and dose response. That is, identification of a substance or situation that has the potential to cause adverse health effects and at what levels of exposure (dose). Risk, as a measure of likelihood of occurrence and severity of outcome, is usually then assessed based on site-specific characterization of pathways of human exposure including acute, cumulative and multiple exposures. Additionally, physiological response to dose may predict the severity of human response or adverse health effects.
This study provides evidence that a minority of Queensland CSG water samples contain some organic compounds recognized as potential health hazards. The significance of these findings however, lies in the relatively small number of samples containing the compounds and the very low concentrations in which they are present. These factors greatly reduce the likelihood of human exposure at doses significant to induce adverse physiological response, at least in short-term exposure scenarios. If present, many potentially harmful coal derived substances can be reduced to negligible levels with common CSG water treatment methods [
4,
26,
31]. Yet more publicly available research employing low detection level (<1 µg/L) analysis of the wide range of potentially dissolved organics in CSG water would assist in refining risk assessments and site-specific treatment methods.
The risk is less clear for hazard accumulation and multiple exposure scenarios and this uncertainty indicates the need for further analysis of both models of organic compound accumulation and migration in ground and surface water before and after treatment and definition of potential pathways of human exposure to these waters. Additionally, future toxicology studies targeting the key compounds identified in this study can clarify the dose-response relationship to adverse health outcomes. A broader understanding of these issues will provide the basis for informed and appropriate industry regulation to best monitor for and militate against release of hazardous substances through CSG production.
4.4. Detection Limits and Reporting
In some instances, the detection level (DL) of some organic compound classes in CSG water may be too high to adequately assess the occurrence of potentially harmful compounds. Aromatic compounds such as BaP have regulatory limits (0.01 µg/L) 50 times less concentrated than the minimum DL of the Bandanna Formation dataset (0.5 µg/L) of this work and the small number of publicly available QLD CSG water organic datasets. Analysis at this level prevented a comparison of dissolved aromatics from Walloon and Bandanna production fields. In addition, relatively high DLs still leave a knowledge gap regarding the occurrence of generally insoluble, but environmentally regulated coal derived organics. BaP was leached from coal in this study at its regulatory limit, thus highlighting the importance of appropriate DLs in CSG water analysis.
Sometimes (even where DLs are sufficiently low) average concentrations of organic compounds across production fields are reported rather than ranges of detected levels. Whilst such reporting is practical for characterizing CSG water quality for legislative requirements and bulk treatment technologies, rare relatively high concentrations of organic compounds detected from individual wells may be ameliorated by the field average. It is therefore best practice to present organic concentrations of CSG water from individual wells in monitoring and regulatory data, as well as production field averages.