Dna Tracer Transport Through Porous Media-The Effect Of Dna Length And Adsorption

Artificial tracer testing is an effective technique to identify fluid flow pathways and characterize subsurface hydrological properties. Synthetic DNA tracers, available in virtually unlimited number of unique variations, enable multiwell tracer testing and have the potential to improve the characterization of groundwater flowpaths enormously. This study investigated the effect of DNA length (i.e., number of base pairs) and adsorption on DNA tracer transport via well-controlled laboratory experiments. Nine unique synthetic double-stranded DNA tracers were designed with varying lengths (90-200 base pairs) and flowed with reference solute tracers (bromide and lithium) through columns packed with glass beads or Ottawa sand. The peak arrivals of DNA tracers were earlier than bromide in the glass-bead column test, yet significantly retarded in the sand column test. In a given porous medium, DNA length did not have noticeable effects on peak arrival or tracer dispersion. However, DNA tracer recovery increased with increasing DNA length in the glass-bead column test but decreased with increasing DNA length in the sand column test, likely resulting from the corresponding trends in partition coefficients as revealed by tracer transport modeling. Such reversed trends in different porous media likely resulted from the different likelihoods for longer DNA to have multisegment attachment during transport. Our findings suggest that both DNA length and the surface properties of the porous medium have notable effects on DNA tracer transport and hence need to be considered carefully in future tracer test designs in terms of site selection, DNA sequence design and data interpretation.