TRANSPORT BEHAVIOR OF GROUNDWATER PROTOZOA AND PROTOZOAN-SIZED MICROSPHERES IN SANDY AQUIFER SEDIMENTS

Transport behaviors of unidentified flagellated protozoa (flagellates) and flagellate-sized carboxylated microspheres es in sandy, organical ly contaminated aquifer sediments, were investigated in a small-scale (1 to 4-m travel distance) natural-gradient tracer test on Cape Cod an d in flow-through columns packed with sieved (0.5- to 1.0-mm grain siz e) aquifer sediments. The minute (average in situ cell size, 2 to 3 la m) flagellates, which are relatively abundant in the Cape Cod aquifer, were isolated from core samples, grown in a grass extract medium, lab eled with hydroethidine (a vital eukaryotic stain), and coinjected int o aquifer sediments along with bromide, a conservative tracer. The 2-m u m flagellates appeared to be near the optimal size for transport, ju dging from flowthrough column experiments involving a polydispersed (0 .7 to 6.2 mu m in diameter) suspension of carboxylated microspheres. H owever, immobilization within the aquifer sediments accounted for a lo g unit reduction over the first meter of travel compared with a log un it reduction over the first 10 m of travel for indigenous, free-living groundwater bacteria in earlier tests. High rates of flagellate immob ilization in the presence of aquifer sediments also was observed in th e laboratory. However, immobilization rates for the laboratory-grown f lagellates (initially 4 to 5 mu m) injected into the aquifer were not constant and decreased noticeably with increasing time and distance of travel. The decrease in propensity for grain surfaces was accompanied by a decrease in cell size, as the flagellates presumably readapted t o aquifer conditions. Retardation and apparent dispersion were general ly at least twofold greater than those observed earlier for indigenous groundwater bacteria but were much closer to those observed for highl y surface active carboxylated latex microspheres. Field and laboratory results suggest that 2-mu m carboxylated microspheres may be useful a s analogs in investigating several abiotic aspects of flagellate trans port behavior in groundwater.