Chapter 26
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Triazines in Waters of the Midwest: Exposure Patterns
R. Peter Richards and David B. Baker Water Quality Laboratory, Heidelberg College, 310 E. Market Street, Tiffin, OH 44883
Concentrations of triazine herbicides in waters receiving runoff from agricultural lands are seasonal in nature, with highest concentrations in the six weeks to two months following application, and lower to non-detectable concentrations during the rest of the year. Inriversand streams, concentrations are highest during runoff from storm events in this post-application period, and lower during base flow periods. This pulsed exposure pattern has important implications for possible impacts on aquatic ecosystems, since algae may have time to recover from the highest concentrations during intervening low flow periods. Typically, atrazine dominates the triazines: atrazine concentrations usually exceed the sum of cyanazine, simazine, and the atrazine breakdown products DIA and DEA, particularly during times of the year characterized by elevated concentrations. With improvements in analytical technology, agricultural pesticides are being found more and more frequently in surface water throughout the midwestern United States. Concentrations are highly variable in time and space. While many different herbicides and insecticides are occasionally detected in natural waters, only four or five of the pesticides quantified by standard GC/MS techniques are found above detection limits consistently enough to provide useful information on patterns of occurrence intimeand space. All five are herbicides; they are atrazine, alachlor, metolachlor, cyanazine, and metribuzin. Two of these are S-triazines: atrazine and cyanazine. Atrazine is by far the most widely studied of these herbicides. Consequendy it is the compound most frequendy used as an example of pesticide behavior in the environment. A third S-triazine, simazine, is sometimes detected, usually in low concentrations. In addition, certain breakdown products are often detected, particularly the atrazine breakdown products des-ethyl atrazine (DEA) and de-isopropyl atrazine (DIA). In specific situations these breakdown products 336
©1998 American Chemical Society In Triazine Herbicides: Risk Assessment; Ballantine, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.
337 can have higher concentrations than the parent compounds. Triazine herbicide use is generally most intense in the midwestern states of Ohio, Indiana, Illinois, and Iowa. This paper illustrates patterns of triazine concentrations in midwestern surface waters, drawing on detailed datasets (1-3), and creates a context in which the possible impacts of triazines on aquatic ecosystems may be better understood and evaluated.
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Spatial Patterns of Triazine Use County-level herbicide use estimates (4) for the period 1978-1989 have been widely used for depicting spatial patterns of herbicide use; relatively complete sets of maps for herbicides and crops are available in several recently published documents (5,6). Similar datasets of county-level herbicide use have been prepared by the Agricultural Research and Statistical Service for the years 1991, 1993, and 1994. Atrazine is used almost exclusively on corn, with minor applications to sorghum. Its area of most intense use is the corn belt of the upper Mississippi River watershed, particularly the states of Iowa, Illinois, Indiana, and Ohio. The use distribution, properties, and environmental occurrence of atrazine are thoroughly discussed by Solomon et ai (7). Cyanazine is used on corn and cotton. The use patterns for cyanazine in the late 1980s were very similar to those of atrazine, but the quantities used were much smaller (4-6) and have declined since, because cyanazine is being phased out of use. Simazine is used primarily on corn and alfalfa and in orchards. It is used in much smaller quantities than atrazine in the midwest, but is also used extensively along the southern east coast and along the west coast(4-6) Temporal Patterns of Herbicide Concentration in Surface Water Patterns in Streams and Rivers. Various aspects of temporal patterns of triazine concentrations in rivers and streams have been described in a number of papers in the literature. Early works (e.g. 8-11) focused on atrazine runoff from fields into rivers and streams. Recent works (e.g. 7, 2, 7, 72, 14) have broadened the focus to include other triazine and non-triazine herbicides, and possible ecological and human health effects from exposure to these compounds. Triazine herbicides move to streams and rivers primarily in runoff from rainfall events. As a consequence, concentrations typically increase and decrease more or less in parallel with stream discharge (7). In some systems, triazines may also move to streams and rivers through shallow groundwater pathways (75); this pathway is more important during low flow conditions than during storm runoff. The relative importance of direct runoff and groundwater baseflow is a function of surficial and bedrock geology and a number of other factors. Triazines are transported in rivers and streams primarily in the dissolved state. On an annual basis, triazine herbicides tend to occur in highest concentrations during the two or three months following application, declining to low to nondetectable concentrations by mid to late fall, and remaining at low concentrations
In Triazine Herbicides: Risk Assessment; Ballantine, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.
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338 until the following application season (7, 16). In most of the midwest, the period during which elevated concentrations occur typically begins in May and extends into July. Cyanazine concentrations are usually below detection limit by late summer or early fall, whereas it is not uncommon to find small but detectable quantités of atrazine in storm runoff throughout the year, in basins where agricultural land use is dominant. Simazine concentrations in midwestern rivers are generally too small to reveal a very clear annual pattern, except that detections are almost entirely limited to the two months following application. The typical patterns described above and in the following sections are the result of non-point processes. Occasionally, high concentrations which are not associated with storm runoff have been observed in streams and rivers monitored by the Water Quality Lab. Sometimes these occur simultaneously for two compounds known to be applied together. These concentrations rarely persist long enough to appear in two successive samples (which are usually three days or less apart). These are most likely the result of point-source inputs, either accidental spills or emptying excess herbicide from a spray tank. While apparently infrequent, of short duration, and of unpredictable timing, these point-source events often have higher concentrations than most non-point storm-runoff events. Storm Runoff and Pulsed Exposures. Because of the importance of storm runoff in the transport of triazine herbicides, rivers and streams are characterized by pulses of elevated concentration which result from the passage downstream of the runoff from different storms (Figure 1). Smaller streams tend to have wider extremes of concentration - higher maximum concentrations during storm runoff and longer periods of low concentration between storms - than larger rivers, which carry water resulting from contributions from different tributaries whose maximum concentrations enter at different times. A l lriversand streams, however, show the pulsed exposure pattern. The pulsed nature of triazine runoff has important implications for possible ecosystem effects of herbicide exposures, because these herbicides act by inhibiting photosynthesis rather than by inflicting direct damage on cells, and plants such as algae may recover to a large extent between pulses of high concentration. Pulsed concentrations also hinder assessments of long-term human exposures through drinking water, because it is difficult and expensive to carry out a sampling program sufficiently detailed to obtain a reliable estimate of the average over time of the rapidly fluctuating concentrations. The quarterly sampling program required by the Safe Drinking Water Act (13) is clearly inadequate to deal with such extreme fluctuations. Major differences in average and extreme concentrations (Table I) occur among different compounds and in watersheds of different sizes and geographic locations, due to differences in crop distributions and pesticide use rates, differences in hydrologie residence times, and compound-specific differences in rates of breakdown.
In Triazine Herbicides: Risk Assessment; Ballantine, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.
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10.0 Triazines by Immunoassay
Sangamon River Monticello, IL 1990
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