USGS Fact Sheet 134-98
|Herbicide or metabolite|| Report-
| Mean of
¹Concentration at or above reporting limits.
²Based on 608 reservoir outflow samples.
³Based on 511 reservoir outflow samples.
Most of the reservoirs sampled during this study had detectable concentrations of at least one or more herbicides and (or) metabolites (table 1). Atrazine, its two metabolites, deethylatrazine and deisopropylatrazine, and the alachlor metabolite alachlor ESA were the most frequently detected compounds, all being detected in more than 60 percent of the samples. Cyanazine and metolachlor were also detected in about 50 percent of the samples. The highest herbicide concentration detected was for atrazine [12.4 micrograms per liter (mg/L)], and the highest metabolite concentration was for alachlor ESA (19.7 mg/L). Of the parent herbicides, atrazine, the most extensively applied herbicide in the Corn Belt, occurred at the highest frequency (82 percent) and in the largest concentration (mean of 1.36 mg/L for samples with detectable concentrations). Alachlor, the second most extensively applied herbicide after atrazine in 1992, was detected less frequently (29.8 percent of samples) and at smaller concentrations (mean of detectable concentrations of 0.27 mg/L) than either metolachlor or cyanazine. However, alachlor ESA was detected in a majority of the samples (70.2 percent) and had the highest mean concentration (1.74 mg/L) of all of the parent herbicides or metabolites analyzed. Alachlor ESA is more mobile and thought to be less toxic than alachlor, and its higher frequency of detection and larger concentrations in streams as compared to alachlor have been documented in other studies (Thurman and others, 1996; Kalkhoff and others, 1998).
During the 18-month study, the mean sum of the individual concentrations of alachlor and alachlor ESA was 1.9 mg/L when at least one of the two compounds was detected. The mean sum of atrazine and its metabolites was also 1.9 mg/L, implying that alachlor and atrazine were used in nearly equal amounts in the study area, a finding consistent with use data (U.S. Department of Agriculture, 1993). Similarly, the mean sum of the individual concentrations of cyanazine and its metabolites was 1.0 mg/L, about one-half the sum for alachlor and atrazine plus their metabolites. This result is also consistent with the fact that cyanazine usage in the study area was about one-half that of atrazine or alachlor.
The temporal variability in the concentrations of two herbicides and two metabolites in reservoir outflows are shown in the graphs. Concentrations of the parent herbicides, in this case atrazine and alachlor, are generally smallest in the winter and early spring prior to planting. Concentrations of atrazine peaked in the summer gradually decreasing through the fall and winter. Concentrations of alachlor also peaked in the summer and were small or undetectable for the rest of the year. Concentrations of the metabolites deethylatrazine and alachlor ESA also varied seasonally but tended to peak later in the summer and appeared to be less variable as compared to the associated parent herbicides.
The variability in the concentrations of herbicides and metabolites in reservoir outflow is bly affected by the reservoir residence time. Lake Vermillion in Illinois is a small reservoir with a volume of about 11.4 million cubic meters (9,200 acre-feet) and a relatively short residence time (about 9 days). As a result, water is flushed through the reservoir rapidly, and concentrations of herbicides and associated metabolites exhibit a temporal distribution similar to that of unregulated streams (see graphs). During both 1992 and 1993, concentrations of both herbicides and metabolites in Lake Vermillion outflow were low in late April, peaked rapidly in early July, and then decreased rapidly by early October. In contrast to Lake Vermillion, Hillsdale Lake in Kansas (see photograph) has a much larger volume of about 89 to 160 million cubic meters (72,000-130,000 acre-feet) and a residence time of nearly 8 months. As a result, herbicides and metabolites delivered to Hillsdale Lake in the spring and early summer are stored and subsequently delivered downstream in smaller concentrations and for longer periods of time.
Statistical models were used to estimate mean herbicide concentrations in each reservoir outflow as a function of the physical characteristics of the reservoir, the land and chemical use in the drainage basin, soil characteristics, topographic conditions, and climatic variables. Results from the models indicate that the amount of herbicide used in a drainage basin is the primary process affecting the concentration of the herbicide in reservoirs. The models also indicate that when drainage basins have steep slopes and poorly drained clay-rich soils, the receiving reservoirs tend to have higher herbicide concentrations. These findings suggest that best-management practices targeted at reducing the use of herbicides and reducing the loss of herbicides to surface- and ground-water systems will be the most successful in lowering herbicide concentrations in reservoirs.
The presence of herbicides and associated degradation products in reservoirs has implications that could affect the management of public-water supplies in the Midwest. Water from Midwestern reservoirs and streams that receive reservoir outflow are frequently used for public supply. A common characteristic of the 15 herbicides and associated degradation products detected in reservoir outflows during this study is that they are relatively water soluble; thus, they are readily transported in water. Studies have shown that conventional water treatment (coagulation, sand filtration, and chlorination) is ineffective at removing herbicides such as alachlor and atrazine from drinking water and, because of the similar chemical nature, would probably also be ineffective at removing their metabolites. As a result, additional treatment processes such as ozone (Adams and Randtke, 1992) or activated carbon (Najum and others, 1991) need be used to adequately treat herbicide-laden water. Because reservoirs tend to lengthen the period of time that elevated herbicide concentrations are present in the water column, additional treatment may be required during most of the year when the water source is located either in or downstream from a reservoir.
The U.S. Environmental Protection Agency (1996) has established either a Maximum Contaminant Level (MCL) or a Health Advisory Level (HAL) for 9 of the 11 parent herbicides analyzed in this study (table 1). MCLs are legally enforceable drinking-water regulations, whereas HALs are drinking-water criteria and nonenforceable. Because MCLs and HALs are based on annual average concentrations, one or more exceedances of the specified value does not necessarily indicate noncompliance. MCLs or HALs have not been established for any of the six herbicide metabolites analyzed during this study or for the combination of the parent herbicides and associated metabolites. However, if the concentrations of the associated metabolites are added to the concentrations of the parent herbicides, the number of reservoirs with mean annual outflow concentrations that exceeded the parent compound MCL or HAL increases. For example, in 1992, 17 reservoirs would have exceeded the MCL for atrazine on the basis of atrazine and its associated metabolites compared to 6 reservoirs on the basis of atrazine only. Similarly, 14 reservoirs would have exceeded the HAL for cyanazine in 1992 on the basis of cyanazine and its associated metabolites compared to 6 reservoirs on the basis of cyanazine only. If alachlor ESA were added to the concentration of alachlor, 11 reservoirs would have exceeded the MCL in 1992, and 16 reservoirs would have exceeded the MCL in 1993. However, unlike the atrazine and cyanazine metabolites, alachlor ESA is a chlorine-free compound upon degradation; thus, it is considered to be less toxic than the parent compound (Baker and others, 1993). Therefore, a comparison of the sum concentration of alachlor plus alachlor ESA to established MCLs for alachlor probably is not valid.
--John K. Stamer, William A. Battaglin, and Donald A. Goolsby
This Fact Sheet is based largely on information contained in the following publications and references cited therein:
For additional information and selected readings about the Midcontinent Herbicide Project,
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