Degradation and adsorption of synthetic DNA water tracers in environmental matrices

Synthetic DNA tracers are gaining interest as tools for tracking contamination pathways and hydraulic connections in surface water and groundwater systems. However, few quantitative data exist that describe DNA tracer degradation and adsorption in environmental matrices. We undertook laboratory experiments to quantify the degradation of multiple double-stranded DNA tracers in stream water, groundwater, and domestic and dairy-shed effluent, and adsorption to stream sediments, soils, coastal sand aquifer media and alluvial sandy gravel aquifer media. Faster DNA tracer degradation seemed to be associated with high bacterial concentrations in the liquid phase. Overall, the degradation of the 352 base pair (bp) DNA tracers in the aqueous phase was significantly (P = 0.018) slower than that of the 302 bp DNA tracers. Although the tracers' internal amplicon lengths were similar, the longer non-amplified flanking regions of the 352 bp tracers may better protect them from environmental degradation. Thermodynamic analysis suggests that longer flanking regions contribute to greater tracer stability. This finding may explain our previous field observations that 352 bp tracer mass reductions were often lower than 302 bp tracer mass reductions. The 2 sets of DNA tracers did not differ significantly regarding their adsorption to stream sediment-stream water or aquifer media-groundwater mixtures (P > 0.067), but the 352 bp tracers showed significantly less adsorption to soil-effluent mixtures than the 302 bp tracers (P = 0.005). The DNA tracers' adsorption to soil-effluent mixtures was comparatively less than their adsorption to the aquifer media-groundwater and stream sediment-stream water mixtures, suggesting that DNA tracers may compete with like-charged organic matter for adsorption sites. These findings provide insights into the fate of DNA tracers in the environment. The DNA tracers' degradation rate constants determined in this study for a range of environmental conditions could assist the design of future field investigations.