Three field-scale infiltration experiments were conducted at a small research basin constructed adjacent to the San Gabriel Coastal Spreading Grounds in the Montebello Forebay of Los Angeles County, California, using bacterial viruses (bacteriophage) MS2 and PRD1 as surrogates for human viruses and bromide as a conservative tracer. The goal of the first field-scale infiltration experiment, done over a 2-day period in August 1997, was to establish the feasibility of using recycled water seeded with high concentrations of bacteriophage and bromide as tracers. The second experiment was completed in the summer of 1998, when similar conditions existed at the research basin, to measure more precisely the attenuation rates for MS2 and PRD1. The third infiltration experiment was conducted during the summer of 2000 to confirm the attenuation rates obtained from the second experiment. Extensive data were obtained for samples collected from the research basin itself and from depths of 0.3, 0.6, 1.0, 1.5, 3.0, and 7.6 m below the bottom of the research basin. Regression analysis of the ratios of bacteriophage to bromide yielded an attenuation rate for MS2 of approximately 1.1 and 2.5 log10 units over 3 m for the second and third field-scale experiment, respectively, and an attenuation rate for PRD1 of approximately 1.7 and 5.9 log10 units over 3 m for the second and third field-scale tracer experiments, respectively. Currently, bench-scale virus tracer experiments are being conducted to explain the different attenuation rates between the two field-scale infiltration experiments, especially for PRD1.