Mw. Becker et al., Transport and attenuation of carboxylate-modified latex microspheres in fractured rock laboratory and field tracer tests, GROUND WATE, 37(3), 1999, pp. 387-395
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.