Prepared as part of the National Water-Quality Assessment Program, Source Water-Quality Assessment
By James A. Kingsbury, Gregory C. Delzer, and Jessica A. Hopple
Source water, herein defined as stream water collected at a water-system intake prior to water treatment, was sampled at nine community water systems, ranging in size from a system serving about 3,000 people to one that serves about 2 million people. As many as 17 source-water samples were collected at each site over about a 12-month period between 2002 and 2004 for analysis of 258 anthropogenic organic compounds. Most of these compounds are unregulated in drinking water, and the compounds analyzed include pesticides and selected pesticide degradates, gasoline hydrocarbons, personal-care and domestic-use compounds, and solvents. The laboratory analytical methods used in this study have relatively low detection levels‚ commonly 100 to 1,000 times lower than State and Federal standards and guidelines for protecting water quality. Detections, therefore, do not necessarily indicate a concern to human health but rather help to identify emerging issues and to track changes in occurrence and concentrations over time.
About one-half (134) of the compounds were detected at least once in source-water samples. Forty-seven compounds were detected commonly (in 10 percent or more of the samples), and six compounds (chloroform, atrazine, simazine, metolachlor, deethylatrazine, and hexahydrohexamethylcyclopentabenzopyran (HHCB) were detected in more than one-half of the samples. Chloroform was the most commonly detected compound‚Äîin every sample (year round) at five sites. Findings for chloroform and the fragrances HHCB and acetyl hexamethyl tetrahydronaphthalene (AHTN) indicate an association between occurrence and the presence of large upstream wastewater discharges in the watersheds. The herbicides atrazine, simazine, and metolachlor also were among the most commonly detected compounds. Degradates of these herbicides, as well as those of a few other commonly occurring herbicides, generally were detected at concentrations similar to or greater than concentrations of the parent compound. Samples typically contained mixtures of two or more compounds. The total number of compounds and their total concentration in samples generally increased with the amount of urban and agricultural land use in a watershed.
Annual mean concentrations of all compounds were less than human-health benchmarks. Single-sample concentrations of anthropogenic organic compounds in source water generally were less than 0.1 microgram per liter and less than established human-health benchmarks. Human-health benchmarks used for comparison were U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Levels (MCLs) for regulated compounds and U.S. Geological Survey Health-Based Screening Levels for unregulated compounds. About one-half of all detected compounds do not have human-health benchmarks or adequate toxicity information for evaluating results in a human-health context.
During a second sampling phase (2004‚Äì05), source water and finished water (water that has passed through all the treatment processes but prior to distribution) were sampled at eight of the nine community water systems. Water-treatment processes differ among the systems. Specifically, treatment at five of the systems is conventional, typically including steps of coagulation, flocculation, sedimentation, filtration, and disinfection. One water system uses slow sand filtration and disinfection, a second system uses ozone as a preliminary treatment step to conventional treatment, and a third system is a direct filtration treatment plant that uses many of the steps employed in conventional treatment. Most of these treatment steps are not designed specifically to remove the compounds monitored in this study.
About two-thirds of the compounds detected commonly in source water were detected at similar frequencies in finished water. Although the water-treatment steps differ somewhat among the eight water systems, the amount of change in concentration of the compounds from source- to finished-water samples generally did not differ systematically at one or among the water systems for compounds with similar detection frequencies in source and finished water. Additionally, changes in concentration over time in source water of some compounds, for example seasonal changes in atrazine concentrations, usually were reflected in the associated finished water.
Some compounds detected in source water were removed or transformed during treatment and, therefore, were not detected in finished water. These included aromatic hydrocarbons with one or more methyl groups, 3,4-dichloroaniline (diuron degradate), the organophosphate insecticides (diazinon and malathion), and fipronil. On the basis of results for matrix spikes, decreases in concentration of these compounds or nondetections in finished water likely were due to degradation or transformation as a result of chlorine disinfection.
The annual mean concentration of all compounds detected in finished water were less than established human-health benchmarks, and concentrations of most compounds were several orders of magnitude less than human-health benchmarks. With the exception of one detection of atrazine at one site, maximum measured concentrations of all commonly detected compounds in finished water were less than established human-health benchmarks. The annual mean concentration of atrazine at this site was 0.72 microgram per liter, which is less than the MCL of 3 micrograms per liter. Most source- and finished-water samples contained mixtures of 2 or more compounds, and more than one-half of both source- and finished-water samples contained mixtures of 14 or more compounds. Degradates of commonly detected herbicides often were present at a concentration similar to or greater than of that the parent compound in both source and finished water and contributed to the number of compounds detected in mixtures.
For additional information contact:
Director, USGS South Dakota Water Science Center
U.S. Geological Survey
1608 Mt. View Rd.
Rapid City, SD 57702
World Wide Web: http://sd.water.usgs.gov