Transport and attenuation of carboxylate-modified latex microspheres in fractured rock laboratory and field tracer tests

Understanding colloid transport in ground water is essential to assessing t he migration of colloid-size contaminants, the facilitation of dissolved co ntaminant transport by colloids, in situ bioremediation, and the health ris ks of pathogen contamination in drinking water wells, Much has been learned through laboratory and field-scale colloid tracer tests, but progress has been hampered by a lack of consistent tracer testing methodology at differe nt scales and fluid velocities. This paper presents laboratory and field tr acer tests in fractured rock that use the same type of colloid tracer over an almost three orders-of-magnitude range in scale and fluid velocity Fluor escently-dyed carboxylate-modified latex (CML) microspheres (0.19 to 0.98 m u m diameter) were used as tracers in (1) a naturally fractured tuff sample , (2) a large block of naturally fractured granite, (3) a fractured granite field site, and (4) another fractured granite/schist field site. In all ca ses, the mean transport time of the microspheres was shorter than the solut es, regardless of detection limit. In all but the smallest scale test, only a fraction of the injected microsphere mass was recovered, with the smalle r microspheres being recovered to a greater extent than the larger microsph eres, Using existing theory, we hypothesize that the observed microsphere e arly arrival was due to volume exclusion and attenuation was due to aggrega tion and/or settling during transport. In most tests, microspheres were det ected using flow cytometry, which proved to be an excellent method of analy sis. CML microspheres appear to be useful tracers for fractured rock in for ced gradient and short-term natural gradient tests, but longer residence ti mes may result in small microsphere recoveries.