ARS | Publication request: Implications of sampling frequency to herbicide conservation effects assessment

Submitted to: Journal of Soil and Water Conservation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 4, 2008
Publication Date: October 31, 2008
Citation: Pappas, E.A., Huang, C., Bucholtz, D.L. 2008. Implications of sampling frequency to herbicide conservation effects assessment. Journal of Soil and Water Conservation. 63(6):410-419.

Interpretive Summary: Farmers use herbicides to control weeds and help ensure a good crop yield. Although herbicides are beneficial to crop yield, many of them are toxic to humans in sufficient quantities. When it rains and herbicides are washed from farm fields into drainage ways, they may make their way into a public drinking water supply, posing a human health hazard. For this reason, the USEPA has set maximum contaminant levels (MCLs) for some herbicides in public drinking water. Atrazine, which is a herbicide commonly used in corn fields, has an MCL of 3 micrograms per liter ('g/L). In order to meet this standard, water utilities must measure the atrazine in their water quarterly (four times per year), and the annual average of four must be below MCL, or they will have to spend extra money to clean these chemicals out of the water or find an alternate water source. In this study, herbicide levels were measured daily for two growing seasons in drainage from an area where corn fields are common. This drainage feeds a large drinking water treatment plant downstream. Most of the herbicides measured were not found to be above their MCL very often, if at all. However, atrazine was detected in the farm drainage above its MCL 12.7% of the time, and the average atrazine level in the drainage water was higher than MCL. We found that municipal drinking water plants could pass or fail the MCL test, depending on which four days during the year were sampled. Sampling in May or June, or on stormy days usually produced higher atrazine readings, and atrazine levels varied a lot from day to day. This research provides information which can impact regulatory decisions among regulatory agencies, water treatment decisions among water utilities, experimental design among researchers, and consumer awareness.

Technical Abstract: Herbicide losses from row crop agriculture represent potential human health hazards. In particular, atrazine concentrations in drinking water must not exceed its maximum contaminant level (MCL) of 3 'g/L. Atrazine, simazine, alachlor, acetochlor, metolachlor, and glyphosate were monitored along tile-fed drainage ditches draining to a major drinking water source. Water samples were collected daily at 7 monitoring sites located at the outlets of sub basins 298 - 4303 ha (736 to 10,634 ac) during the 2004 and 2005 cropping seasons (April ¿ November). Flow-weighted average (FWA) atrazine concentrations ranged from 0.9 ¿ 12.2 'g/L. Of the three ditches having FWA atrazine concentrations above MCL, the MCL was exceeded in an average 16 % of daily samples. Simazine and metolachlor were also frequently detected. Sampling timing and frequency were found to impact calculated MCL exceedance, and altering the required quarterly sampling schedule by as little as one day could reverse MCL compliance status.