Tracers in Groundwater: Use of Microorganisms and Microspheres

TRACERS IN GROUNDWATER: USE OF MICROORGANISMS AND MICROSPHERES bacteriophages (bacteria-specific viruses), often referred to simply as phages, were employed widely as tracers in groundwater applications. A modest number of phage tracer experiments involving groundwater systems were conducted in the 1970s and 1980s. Most notable were transport studies involving the use of an Aerobacter aerogenes phage (9), phage T4 in a gravel aquifer in New Zealand (10), three unspecified phages, having Escherichia coli K12, Serratia marcescens and Enterobacter cloacae as their respective hosts, in a chalk aquifer in the United Kingdom (11,12), phages f2 and P22H5 in a karst aquifer in Greece (13), and MS2 in a sandy aquifer in Arizona (14). A protocol for using phages as groundwater tracers in granular aquifers is depicted in Figure 1. Many of the bacteriophages commonly used as tracers in groundwater applications are coliphages, that is, they have E. coli as their host. Although there are many RNAand DNAcontaining coliphages (15), only a few (e.g., MS2, T4, T7, φX174, and f2) were used in tracer applications involving subsurface media. In addition to the aforementioned list of coliphage tracers, it is noted that the popular tracer phage, PRDI, which can also infect a strain of E. coli, has Salmonella typhimurium as its primary host. Numerous injection and recovery studies were conducted in the 1990s that involved comparing coliphage and conservative solute tracers in a variety of groundwater environments. The tracer test sites included sandy aquifers in Cape Cod, Massachusetts (16-18), and Ontario (19), highly fractured till in Ontario (20), fractured saprolyte in Tennessee (21), a floodplain aquifer in Montana (22), an alluvial aquifer in New Zealand (23,24), and a dune recharge site in the Netherlands (25). In addition to their widespread use with conservative tracers in injection and recovery experiments, coliphages are often used as indicators or tracers of fecal contamination in groundwater environments, including recent groundwater studies in the United States (26,27), Spain (28), Finland (29), and Israel (30). As noted by Edberg and coworkers (3 1) and Leclerc and coworkers (32), there are a number of problems associated with the uses of coliphages as tracers of groundwater contamination and the future of such uses remains unclear. A major problem is the lack of a strong relationship between bacteriophage recovery from groundwater and enteric gastroenteritis outbreaks attributed specifically to enteric viruses. A detailed discussion of the use of coliphages as indicators of human enteric viruses in groundwater is provided by Snowdon and Cliver (15). Recently, various marine phages have been used with conservative solutes as groundwater tracers to understand better the geohydrology in karst (7), heterogeneous (periglacial) granular (7,33), and fractured-granite (33) aquifers in Switzerland. Unlike coliphages and phages of other bacterial inhabitants of mammalian intestinal tracts, marine phages are not a regulatory or water quality concern because their hosts are harmless. Also, the background concentration of marine phages in freshwater aquifers should be zero, even in groundwater contaminated by domestic or agriculture wastes. Comparisons of the transport characteristics of different marine phages with coliphages, indicate that at least one marine phage, RONALD W. HARVEY U.S. Geological Survey Boulder, Colorado