USE OF A CYANOBACTERIUM AS A PARTICULATE TRACER FOR TERRESTRIAL SUBSURFACE APPLICATIONS

Research dealing with particle movement in porous media and in the acq uisition of high quality, uncontaminated subsurface materials requires inexpensive, easily detectable particulate tracers. Cyanobacteria rep resent candidates for such tracers as they can be detected at low conc entrations by fluorescence of chlorophyll, can be readily cultured at relatively low cost in large quantity and are infrequently found in th e saturated subsurface. Aphanocapsa delicatissima, a cyanobacterium is olated from soil and of uniform small size (ca. 1.0 mu m), was tested for characteristics suitable for subsurface studies. In dark incubatio ns, approximately 13% of chlorophyll a was lost after five days, a qua ntity unlikely to be significant during the time periods relevant to t he proposed use for such a tracer. A. delicatissima moved more readily through basalt cores than carboxylated latex microspheres of a simila r size. On average, 37% of the cyanobacteria admitted to the cores wer e retained within the cores whereas under identical conditions 76% of the microspheres were retained. In light of the low detection limits f or chlorophyll and low production costs, cyanobacteria should be consi dered for use as microparticulate tracers in subsurface research. Such tracers would be useful in hydraulic pump tests to determine microbia l transport phenomena through saturated subsurface strata and to ensur e sample integrity of cores used for characterization of subsurface mi crobial populations.