Hydrochemical data responds at a much slower rate to changes in groundwater conditions than does the propagation of hydraulic pressure, and therefore may provide more insight to groundwater flow paths. In low rank coal measures, where gas is biogenic, it is important to understand the fluid-rock and microbial interactions that affect the spatial and temporal distribution of groundwater composition. Pressure data may not reflect true groundwater conditions pre-anthropogenic influence, nor does it provide information on the main drivers of groundwater composition, actual aquifer behaviour or even prove groundwater flow.

This study uses a process-based approach to interpret a combination of tracer (³⁶Cl, ¹⁴C, ⁸⁷Sr/⁸⁶Sr, ¹⁸O/¹⁶O) and hydrochemical data obtained from coal seam gas production wells to identify the main geochemical processes and thus controls on the groundwater composition in different coal seam producing areas of the Walloon Subgroup, Surat Basin, Australia. This is arguably one of the largest coal seam gas producing regions in the world. Tracer data measured in this study show that the Walloon Subgroup behaves as a stagnant aquitard, as indicated by the almost total loss of cosmogenic tracers over relatively short groundwater flow distances (~15 km), suggestive of very low ground water flow velocities. The range of ³⁶Cl is 9.0 to 23.8 (x 10⁻¹⁵) while the ³⁶Cl values across the Undulla anticline in the eastern edge of the basin, are essentially the same (12.2–14.7) within analytical error. It is argued that these isotopic values represent secular equilibrium for the Walloon Subgroup. Radiometric carbon (¹⁴C) levels across all three production areas (Roma, Undulla Nose, Kogan Nose) are also too low (range = 0.12–1.95 pMC) for viable field interpretation largely owing to the long residence time of the groundwater and the local activity of methanogens. Groundwater flow velocity was estimated to be <0.1 m/y, which is significantly less than the 0.7 m/y recently reported for the underlying Hutton Sandstone.

As a result of the low groundwater flow velocities, trends in geochemistry are visible only in production regions proximal to the subcrop. At flow distances greater than 10–15 km from subcrop, several low-temperature interactions (cation exchange, silicate weathering, matrix diffusion and hyperfiltration) start to influence groundwater composition. Shallow subsurface chemical and microbial reactions may initially dominate the geochemical composition of the meteoric groundwater, but this is then overprinted by the actions of sulfate reducers and methanogens, resulting in groundwater with the typical geochemical characteristics similar to other coal bed methane groundwater in basins across the world (low SO₄, Ca, Mg and high HCO₃, Na, Cl). As distance and depth increase further, low temperature fluid-rock interactions then begin to influence the groundwater composition.

This holistic, process-based approach applied to a combination of cosmogenic and stable isotopes, and standard hydrochemical data interpreted against basin lithology has enabled a more comprehensive picture on the behaviour of the groundwater of the Walloon Subgroup and is applicable to the study of other sedimentary basins.

与水压传播相比，水化学数据对地下水条件变化的响应速度要慢得多，因此可以更深入地了解地下水流动路径。在低阶煤措施中，气体是生物成因的，了解影响地下水成分时空分布的流体-岩石和微生物相互作用非常重要。压力数据可能无法反映人为影响之前的真实地下水状况，也无法提供有关地下水成分、实际含水层行为的主要驱动因素的信息，甚至无法证明地下水流量。

本研究使用基于过程的方法来解释示踪剂 ( ³⁶ Cl, ¹⁴ C, ⁸⁷ Sr/ ⁸⁶ Sr, ¹⁸ O/ ¹⁶O) 和从煤层气生产井获得的水化学数据，以确定主要地球化学过程，从而控制澳大利亚苏拉特盆地瓦隆亚群不同煤层产区的地下水成分。这里可以说是世界上最大的煤层气产区之一。在这项研究中测量的示踪剂数据表明，瓦隆亚群表现为一个停滞的透水层，正如在相对较短的地下水流距离（~15 公里）内宇宙源示踪剂几乎完全丢失所表明的那样，这表明地下水流速非常低。³⁶ Cl的范围是 9.0 到 23.8 (x 10 ^-15 ) 而³⁶盆地东部边缘 Undulla 背斜的 Cl 值在分析误差范围内基本相同 (12.2-14.7)。有人认为，这些同位素值代表了瓦隆亚群的长期平衡。所有三个生产区（罗马、Undulla Nose、Kogan Nose）的放射性碳 ( ¹⁴ C) 水平也太低（范围 = 0.12–1.95 pMC），无法进行可行的现场解释，这主要是由于地下水和当地的长时间停留产甲烷菌的活性。地下水流速估计为 <0.1 m/y，远低于最近报道的下伏 Hutton 砂岩的 0.7 m/y。

由于地下水流速低，地球化学趋势仅在靠近亚作物的生产区可见。在距离亚作物超过 10-15 公里的流动距离处，几种低温相互作用（阳离子交换、硅酸盐风化、基质扩散和超滤）开始影响地下水成分。浅层地下化学和微生物反应最初可能主导大气地下水的地球化学成分，但随后被硫酸盐还原剂和产甲烷菌的作用叠加，导致地下水具有典型的地球化学特征，类似于整个盆地中的其他煤层甲烷地下水。世界（低 SO ₄、Ca、Mg 和高 HCO ₃、钠、氯）。随着距离和深度的进一步增加，低温流体-岩石相互作用开始影响地下水成分。

这种基于过程的整体方法应用于宇宙成因和稳定同位素的组合，以及根据盆地岩性解释的标准水化学数据，能够更全面地了解瓦隆亚群地下水的行为，并适用于其他沉积物的研究。盆地。