The Potential of DNA-tagged Magnetic Silica Microparticle Tracers for Surface Water Tracer Hydrology

Abstract

Surface water tracing experiments help to identify the flow paths of in-stream mass transport. This provides useful information for developing mitigation measures for contamination, and thereby for solving environmental problems. Salts, fluorescent dyes and isotopes are commonly used tracer substances.
However, it is well known these tracers have several drawbacks, including, for example, a finite number of unique tracers with similar transport properties, which limits the possibility to repeat an experiment due to system memory. Additionally, it is difficult to perform multi-tracer experiments, due to a lack of specificity. Lastly, the detection limit becomes an important constraint when using a tracer in larger water bodies or over longer travel distances, due to the vast dilution. The recently developed silica encapsulated synthetic DNA nanoparticle with a superparamagnetic core (SiDNAMag), can theoretically overcome the aforementioned limitations of existing tracers. SiDNAMag particles can be recovered from a sample by magnetic separation. This limits the influence of water quality on the qPCR analysis and provides an easy
method for up-concentration of samples. However, with the presence of natural colloids, particulate matter,
and river bed sediments, SiDNAMag particles are likely to undergo complex interaction processes in addition to
dispersion and advection. Moreover, little is known about the possible SiDNAMag tracer sinks during transport.
The focus of this work is to identify the transport behaviour of the SiDNAMag tracer in terms of breakthrough
curves (BTC) and hydrodynamic dispersion as well as to study the interactions with the natural environment,
specifically water quality and sediments. This was investigated by comparing SiDNAMag particles to NaCl,
a widely used solute tracer, and silica microparticles. To do so, open channel injection experiments using
SiDNAMag particles were performed in the laboratory in different river water types and with the presence of
sediments at the channel bottom. The resulting BTCs were interpreted with a 1D advection and dispersion
model with transient storage (OTIS), to determine the hydrodynamic dispersion coefficient. Mass recovery
was calculated by integrating the BTC.
The results show that SiDNAMag particles have a reproducible but different transport behaviour than
NaCl solutes. The SiDNAMag particles arrive, peak and return to background concentrations earlier than NaCl
solutes. Furthermore, the interpretation of the data with a 1D advection and dispersionmodel showed that the
dispersion coefficient for SiDNAMag was lower (0.34E-3m2/s – 0.88E-3m2/s) than that of NaCl (0.81E-3m2/s
– 2.31E-3m2/s). Literature on solutes and colloids in the aquatic environment supports the possible difference
in the transport behaviour of solutes and colloids. No relation between the transport behaviour and presence
of bottom-sediments was found. The mass recoveries for SiDNAMag experiments were between 36% and
131%, which could be partly explained by the bottom-sediments which detained between 0.16% and 16.94% of
the SiDNAMag mass during the experiments. No relationship was found between the three natural water types
and the observed mass losses. Additionally, the silica microparticle injection experiment showed the same BTC
characteristics for silica microparticles as SiDNAMag though without mass loss, indicating mass loss occurred
in the lab analysis. Due to the uncertainties in the lab analysis, no accuratemass recovery for the SiDNAMag
tracer could be calculated. Furthermore, the observed scatter in the measurement data could be explained
by the uncertainties in lab analysis as well as possible differences in transport behaviour of solutes and colloids.
In conclusion, the results of the laboratory experiments with the SiDNAMag particle show that it is a
promising hydrological tracer in surface water tracing experiments to trace colloids. Thereby it can be a
valuable tool to gain information on the movement ofmicroparticles, like microplastics and pathogens, in the
natural environment. The next step towards achieving this goal is performing a field experiment, for which
upscaling of the SiDNAMag tracer production is required.