Chemical Quality Series 12: A survey of organic carbon and trihalomethane formation potential
Drinking-water standards promulgated by the U.S. Environmental Protection Agency (EPA) underscore the importance of detailed water-quality data for sources used in conjunction with public water supplies. It has been recognized that reaction of the chlorine (used as a disinfectant) with organic matter present in surface waters can lead to formation of trihalomethanes (THMs) in concentrations which may exceed the 100 µg/L maximum contaminant level (MCL) established by the EPA. However, relatively little consideration has been given to ground-water sources with regard to the organic carbon content of the waters, the potential for THM formation, or the presence of ammonium ion (NH4+), which both consumes chlorine and serves to reduce THM formation. The successful operation of water-treatment plants for public supplies is dependent upon many factors, such as operator skill and the condition of the equipment, but of similar importance is a detailed knowledge of water quality for the sources being employed.
A survey of Kansas ground waters to determine their concentrations of total organic carbon (TOC) and trihalomethane-formation potential (TFP) was conducted in the spring of 1986. Wells were carefully selected, based on well logs made during construction, to represent particular geologic intervals. Thirty-four samples were collected from public water supply wells and 16 samples were collected from private water wells. Samples from 11 alluvial aquifers, four unconsolidated aquifers in Quaternary and Tertiary formations, and four consolidated aquifers in Cretaceous, Permian, Pennsylvanian, and Cambrian-Ordovician rocks were taken.
The mean and median TOC concentrations were 1.03 ± 0.76 and 0.84 mg/L, while the mean and median TFP concentrations were 46.7 ± 39.5 and 30.6 µg/L, respectively. The mean TFP yield was 0.242 ± 0.07 µmol per mg of TOC, and the TFP concentration in micromoles per liter was very strongly correlated (r = 0.953) with TOC. Only 8% of the samples had a TFP concentration exceeding the present MCL for THMs of 100 µg/L, but 56% exceeded 25 µg/L and 90% exceeded 10 µg/L, suggesting that many Kansas water-supply systems using ground water might have difficulty meeting a substantially lower THM standard.
The average instantaneous THM (ITHM) concentration was only 6.95 µg/L, while the average terminal (TTHM) concentration was 35.6 µg/L. Hence, only a small fraction of the THM concentration to which consumers might be exposed is formed prior to distribution. For the 21 TTHM samples having a free chlorine residual at the end of the incubation period, TTHM was strongly correlated with both TOC (r = 0.819) and TFP (r = 0.926), suggesting that either of these might be a good surrogate measure for TTHM.
TOC (and TFP) appeared to be unrelated to aquifer (well screen) depth, but both were clearly much higher in the alluvial aquifers, all of which were located at relatively shallow depths. TOC also appeared to be unrelated to the inorganic constituents present in the samples, with the exception of a subset of samples from alluvial aquifers having high concentrations of NH4+ (>0.1 mg/L) and Fe + Mn (>1.0 mg/L). For these samples, TOC was strongly correlated with both NH4+ (r = 0.676) and Fe + Mn (r = 0.991). These relationships merit further investigation, since all of the constituents involved pose problems for water-treatment plants.
In Kansas, efforts to control THMs in public water supplies from groundwater sources should focus primarily on alluvial aquifers, especially those having high concentrations of TOC, NH4+, Fe, and Mn. TOC and TFP may be useful surrogates for TTHM and could be used as a basis for exemptions from monitoring requirements. Use of combined chlorine appears to be the simplest and most effective means of limiting THM formation, but the necessary precautions must be taken to ensure that the microbial quality of the drinking-water supply is not compromised.
