Assimilable organic carbon concentrations in Southern California surface and groundwater

Abstract

Bacterial regrowth potential in Southern California surface and groundwaters was evaluated for assimilable organic carbon (AOC) concentrations using two bioassays. Untreated and ozonated groundwaters were tested to determine variation in AOC with well site, season and static water depth. The mean total AOC concentration (Pseudomonas P17 + Spirillum NOX) for untreated groundwater was 54 μg carbon (C) l−1. Ozone treatment resulted in a 2 to 10-fold increase in AOC with an average concentration of 254 μg cl−1. Highly colored groundwaters (containing up to 250 platinum color units, pcu) were ozonated with 7 mg l−1 which was sufficient to remove up to 75 pcu of color but often resulted in AOC concentrations exceeding 500 μg Cl−1. In comparison, AOC levels in untreated surface waters were generally greater than 100 μg Cl−1 and showed less of an increase after ozonation than did groundwaters. Total organic carbon (TOC) concentrations ranged from 0.2 to 5.7 mg l−1 and most groundwater samples contained less than 1 mg l−1 TOC. There was no correlation between AOC and chemical parameters such as DOC, color or u.v. absorbance (254 nm). Ratios of AOC to DOC ranged from 0.5 to 31% which suggests that the majority of the organic carbon in groundwater and surface waters is biologically stable. In summary, AOC levels appeared to vary with well location and possibly with season. Therefore, routine monitoring of source and treated water must be conducted on a site specific basis to determine bacterial regrowth potential or biofilm formation. Appropriate treatment strategies can then be developed, on an area by area basis, to remove or reduce AOC levels.