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ARS Home » Midwest Area » West Lafayette, Indiana » National Soil Erosion Research Laboratory » Research » Publications at this Location » Publication #219685

Title: Predicting Atrazine Levels in Water Utility Intake Water for MCL Compliance

item Pappas, Elizabeth
item Huang, Chi Hua

Submitted to: Journal of Environmental Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/2/2008
Publication Date: 8/22/2008
Citation: Pappas, E.A., Huang, C. 2008. Predicting Atrazine Levels in Water Utility Intake Water for MCL Compliance. Journal of Environmental Science and Technology. 42(19):7064-7068.

Interpretive Summary: United States municipal water quality standards require that levels of the weed killer atrazine, which is commonly used in agriculture, not exceed 3 parts per billion (ppb). To establish whether drinking water meets this standard, water utilities must sample their water on a quarterly basis (every 3 months). Since most of the atrazine that runs off farm land and into potential drinking water sources does so during the first few runoff – producing rainfall events following field atrazine application (usually in mid - May), the potential for atrazine levels to be high in municipal water is greatest during May and June, especially on days when rainfall is sufficient to produce field runoff. Since the second quarter of the year includes the months of April, May, and June, and runoff does not occur every day, the quarterly sampling requirement allows for the possibility to be unaware of an atrazine pollution problem if the quarter 2 sample is taken in April and / or on a day when a runoff – producing rainfall event does not occur. Additionally, atrazine levels change so fast during quarter 2 that there is a significant element of chance associated with whether any one sample will exceed 3 ppb. The results of this research indicate that basing the second quarter atrazine level on the average of weekly samples, rather than on just one sample, would greatly reduce the uncertainties associated with the timing of sampling. The impact of these findings are that regulatory agencies may use this information to validate or update sampling requirements, and water utilities may better understand how to time the employment of extensive treatment for atrazine in their water treatment process.

Technical Abstract: To protect human health, atrazine concentrations in drinking water must not exceed its maximum contaminant level (MCL) of 3 ug/L. The United States Environmental Protection Agency (USEPA) mandates that municipal water providers sample quarterly to determine MCL compliance. Atrazine levels were monitored along tile-fed drainage ditches draining to a major drinking water source and used to predict MCL exceedance of intake and finished drinking water. Water samples were collected daily at 8 monitoring sites located at the outlets of sub basins draining 298 – 19,341 ha (736 to 47,794 ac). Flow-weighted average (FWA) atrazine concentrations ranged from 0.9 – 9.8 ug/L, and were above MCL for the majority of sites. However, relatively low MCL exceedance frequencies (10 – 20%) make this problem difficult to detect using quarterly sampling methods. Random chance was found to heavily influence the current method of MCL exceedance determination, in that altering the required quarterly sampling schedule by as little as one day could reverse MCL compliance status. In order to reduce the impact of random chance and have a 95% probability of detecting any sample exceeding atrazine MCL in a drainage system exceeding atrazine MCL on a FWA basis, sampling frequency would need to be every seven days or more often during the second quarter, when the potentials for field atrazine losses and temporal variability of atrazine concentrations are highest.