Purpose: To characterize the physicochemical and microbiological properties of Salinas Valley tail water.
Methods: Six tail-water reservoirs, representative of those used in Salinas Valley vegetable production, were sampled monthly for 1 year. Physicochemical water characteristics, including turbidity, temperature, pH, dissolved oxygen, conductivity, and oxidation reduction potential, were measured each time. Samples were plated onto plate count agar and incubated at 37°C for 24 h to determine the aerobic plate count (APC). Coliform and generic Escherichia coli concentrations were determined with both the Colilert Quanti-Tray 2000 system and by filtration and plating on CHROMagar ECC.
Results: APC for the tail-water samples ranged from 2.43 to 5.74 log CFU/ml (mean: 3.83 log CFU/ml). Coliforms were detected at all time points and sites, with concentrations from 0.78 to 5.30 log CFU/100 ml or 0.30 to 5.19 log MPN/100 ml (mean: 3.91 log CFU/100 ml or 3.18 log MPN/100 ml). Forty eight of 60 (80 %) samples were positive (≥ 1 CFU/100 ml) for generic E. coli. E. coli and coliform levels were significantly lower with the MPN method than with the filtration method. Concentration of E. coli, coliforms, or aerobic plate counts did not correlate with any of the measured physicochemical parameters (Pearson’s correlation coefficients were not significant). Reservoir location was the main factor that significantly influenced E. coli concentration; one site consistently yielded higher E. coli concentrations. The E. coli geometric mean for this site was 486 CFU/100 ml or 248 MPN/100 ml compared to <38 CFU/100 ml or 20 MPN/100 ml for the other five sites.
Significance: Characterization of the microbial quality of tail water is a critical step to make informed decisions on its reuse.