To meet the needs for increased use of reclaimed water and to provide data for regulatory decisions, a 1/2-acre research basin was constructed in the San Gabriel River Coastal spreading grounds to study chemical and microbial processes related to such recharge practices. Hydrologic, chemical, and microbiological data were collected during an artificial recharge experiment between April 4 and June 1 1994, when 12.5 acre-ft of reclaimed water was delivered to the research basin for 17 days. Water samples were collected to determine the changes in concentrations of the major ions, selected trace metals, nutrients, oxygen-18, deuterium, and nitrogen-I 5. The populations of nitrifying, denitrifying, nitrate-reducing, and coliform bacteria were determined from selected water samples collected during the recharge event In addition, core material was collected to determine the lithology, permeability, and cation exchange capacity of the top 10 feet of soil beneath the research basin. The sediment beneath the research basin consists of moderately-sorted, coarse sand with a fine-grained layer extending from about 31 to 37 feet below the bottom of the research basin. The fine-grained layer was confirmed by EM logs and appears to be continuous beneath the research basin. Vertical hydraulic conductivities from permeameter measurements ranged from 55 to 85 ft/day, and can decrease to less than l ft/day. Slug tests yielded a horizontal hydraulic conductivity of approximately 85 ft/day. The initial vertical flux of reclaimed water was 7.02 ft3/day and horizontal flux 236 of groundwater beneath the research basin was 0. 76 ft3/day, suggesting vertical flow dominated during the early stages of the recharge experiment. Total and fecal coliform bacteria populations varied during the recharge experiment from none to >300 cfu/100mL, suggesting the bacteria migrated with the reclaimed water. Nitrogen-transforming bacteria were present in high numbers before and during the recharge experiment. Hydrogen and oxygen isotopes confirm displacement of preexisting water beneath the research basin was rapid and nearly complete, and nitrogen-15 isotopes suggest denitrification was occurring during the recharge experiment. Suboxic conditions suggested by low dissolved oxygen concentrations were present at the start of the experiment. As the reclaimed water percolated beneath the research basin, the conditions became oxic and nitrification of ammonia occurred. After 4 days, suboxic conditions returned and nitrate was removed by denitrification. The study showed the presence of the thin clogging layer composed of fine-grained, organic-rich sediment at the floor of the research basin can exert a strong influence on the infiltration rate. The presence of coliform bacteria exceeded the maximum contaminant level (MCL) during the second day of recharge to a depth of l0 feet. The drinking water MCL was exceeded by nitrite within the research basin and by manganese within the aquifer.