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High nitrate levels in groundwater must be reduced as per legal guidelines to ensure that water is suitable for human consumption. Techniques such as ion exchange, reverse osmosis, electrodialysis, and biological denitrification have proven to be successful on an industrial scale. Unfortunately, the catalytic reduction of nitrate to nitrogen with hydrogen has not been commercialized despite 30 years of research. The main problem is that the reduction produces nitrite and ammonium byproducts above the permitted legal levels, making water unsuitable for human consumption. This research postulates that thermodynamics, rather than kinetics, limits achieving the desired selectivity. We use a thermodynamic approach, known as the attainable region, to compare the theoretical analysis to the experimental data. The graphical results show why it is so difficult to avoid the production of byproducts.