Gas distributions were then compared with published **data** on the regional hydrology and hydrochemistry, with particular emphasis on groundwater type, geochemical zonation and dissolved gas trends, and then transposed  Part B (left) shows the same chart, zoomed into the shallower horizons of the basin for clarity. Part C (right) highlights the widespread occurrence of CO 2 gas in the vertical profile, making up varying percentages of total gas content. Part D (far right) illustrates CO 2 gas volumes with depth. Parts C and D also show that while CO 2 may make up to 100% of a gas sample, the overall gas volume of this gas in the sequence may be minor. The 'boomerang' CO 2 trend in Part D strongly mirrors the same trend found in the total gas content trend in part A, but at a different depth horizon.