Flux ratios of H₂¹⁶O lines of the same upper level in the data compared to slab models (top); and H₂¹⁶O/H₂¹⁸O ratio in the data and slab models (bottom). The properties of the transitions can be found in Table 2. The dotted black vertical line indicates the upper limit of the column density based on the 1σ level of the spectrum and the nondetection of the H₂¹⁸O line. The x-axis shows the column density of H₂¹⁶O. In the bottom panel, H₂¹⁶O/H₂¹⁸O ratio of 550 is assumed.