Evaluation of Microorganism Survival Under Model Temperature and Total Dissolved Solids Conditions for the Floridan Aquifer – Preliminary Indications

Bench-scale microcosm studies were performed to determine the effect of total dissolved solids (TDS) as sodium chloride and temperature on survival of several organisms. Two model TDS levels and three temperatures were evaluated: 500 and 3,000 mg/l NaCl at 20 degrees and 30 degrees C each. Subsequent experiments involved temperatures of 7° C also. These conditions were established using NaCl and sterile distilled water. The organisms evaluated consisted of 2 individual organisms and 3 populations. The individual organisms were MS-2 bacteriophage and PRD-1 bacteriophage. Microbial populations evaluated were fecal coliforms, enterococci, and coliphage, all isolated from natural surface waters. Samples were taken from the survival experiments at varying intervals for a period of 0 to 30 days, until the endpoint of less than 0.5 viable organisms per ml was reached. For each organism, the inactivation over time was determined using established media culture (bacteria) or agar overlay plaque analysis (bacteriophage). Microcosms with the higher salinity conditions (3,000 mg/l TDS) supported survival better and inactivation proceeded more rapidly at higher temperatures. Preliminary experiments were compared to later trials which employed a refined protocol that involved better controls on the organisms and a near-neutral pH. Enterococci inactivation rates were reduced in trials with neutralized pH. However, to evaluate the survival of these microorganisms in actual environmental conditions, the naturally-occurring physical, biological, and chemical conditions found in the Floridan aquifer, natural waters must be investigated. On-going research will examine inactivation rates in lower TDS ranges, from 100 – 1000 mg/L, the survival of organisms in natural vs. artificial water, and field monitoring of ASR sites for microbial detection. These data will help determine the potential adequacy of natural aquifer conditions within a recharge zone around an ASR well for inactivating microorganisms introduced from surface water.