Tracing groundwater in a fractured arenitic aquifer by DNA-labelled, silica encapsulated nanoparticles in the Northern Apennines (Italy)

A field tracer test using DNA-labelled silica encapsulated nanoparticles was conducted in an arenitic fractured aquifer. The hydrogeological context involves a primary discharge outlet of the Pantano Formation, a shallow marine arenite serving as a crucial groundwater reservoir for the mountainous region of the metropolitan area of Bologna (Northern Apennines, Italy). The aquifer’s characteristics, including calcarenite composition prone to karstification and the enlargement of sedimentary and tectonic discontinuities, were conducive to showcasing the ability of nanoparticles to trace preferential flow pathways within a dual-porosity aquifer. Nanoparticles were introduced into a borehole and retrieved from three sampling points: two spring-boxes and another borehole. The distances between injection and retrieval points ranged from 5–15 m in planar projection and 12–63 m in linear depth. Additionally, a NaCl tracer test with similar boundary conditions was conducted during a borehole dilution test. DNA concentration was determined through quantitative polymerase chain reaction (qPCR) following the dissolution of the silica shell using a buffered fluoride etch solution. Complete breakthrough curves for nanoparticles were observed in all three sampling points, with early detection and nearly symmetrical curves at the two closest points, suggesting minimal dispersion effects. In contrast, salt tracing resulted in more dispersed and delayed breakthroughs. The median effective groundwater velocity from the tracer tests was one order of magnitude higher compared to literature data on siliciclastic turbiditic sandstones in the Northern Apennines. These findings hold significance for implementation of protection measures around the spring head.