Laboratory investigations of the conservativeness of deuterated water as the artificial tracer for hydrogeological tests.

Deuterated water has been applied in hydrogeological tracer tests in recent years. However, there is a contradiction in the conservativeness of the artificial deuterium (D/H). In this study, what circumstances HDO behaved truly conservatively were investigated through the laboratory-scale sand tank by comparing with the widely used tracer chloride (Cl). In addition, the advection-dispersion equation (ADE) and dual-domain mass transfer (DDMT) equation were employed to describe the breakthrough curves (BTCs) of tracers. Reasons for the non-conservativeness of HDO were discussed comprehensively for the first time. HDO behaved conservatively when transported in the porous media with high permeability, and ADE could describe BTCs successfully. While HDO presented the phenomenon of retardation in media with low permeability, especially in the clay medium. DDMT was more suitable for demonstrating delayed BTCs. Hydrogen bonds between HDO and HO, the isotopic exchange effect, and the dual-domain of the media were used to explain the retardation of HDO. The retardation factor (R = 1.27) was used to describe transporting behaviors of HDO in clay first. The relatively large molecular coefficient of HDO could make the proportion of immobile regions of HDO greater than that of Cl. Tracers tend to be delayed when transported in the media with the larger porosity, smaller hydraulic gradient, and lower permeability. This work provides more considerations for using deuterium as artificial tracers in hydrogeological tests.