Event based analysis of chlorothalonil concentrations following application to managed turf



Chlorothalonil concentrations exceeding acute toxicity levels for certain organisms have been measured in surface water discharge events from managed turf watersheds. The duration of exceedence and the timing of these events related to precipitation/runoff and time since application, however, have not been explored. Chlorothalonil concentrations were measured from discharge waters draining a managed turf watershed in Duluth, Minnesota, USA, between 2003 and 2009. The median chlorothalonil concentration was 0.58 µg/L. Approximately 2% of all measured concentrations exceeded the 7.6 µg/L median lethal concentration (LC50) acute toxicity level for rainbow trout. One-twentieth the LC50 concentration, equivalent to the level of concern (0.38 µg/L) for endangered species, was exceeded 31% of the time during the present study. The concentrations that exceeded the LC50 threshold were associated with eight rainfall/runoff events. Low dose exposures are a more important biological concern than acute occurrences. Exceedence concentrations associated with acute effects were significantly (p < 0.05) correlated to time since application and were measured only in the fall following extensive application. A conflict exists between the transportability of chlorothalonil as suggested by its chemical properties and the data collected in the present study. With respect to course-wide golf course application, avoiding application until after the major autumn rainfall period but before the first snow coverage is recommended to reduce occurrence of chlorothalonil concentrations that exceed toxic levels associated with acute and chronic levels of concern. Environ. Toxicol. Chem. 2013;32:684–691. © 2012 SETAC


Fungicides are a critical management option for treating fungal diseases in agricultural crops, urban lawns, and recreational turf. The most widely used fungicide in the United States is chlorothalonil 1, a polychlorinated aromatic fungicide (Table 1). The U.S. Environmental Protection Agency (U.S. EPA) 2 estimated that approximately 6.8 million kilograms per year were used in both agricultural and urban settings in 1999 to 2000. Upwards of five million kilograms of chlorothalonil active ingredient are applied annually to agricultural crops 3. In comparison, 1.8 million kilograms of active ingredient are used in the urban landscape. Approximately 34% of all chlorothalonil is applied to peanuts, while application to potatoes accounts for nearly 12% 1. Recently, chlorothalonil has emerged as one of the few fungicides available in the United States to control Asian soybean rust 4. Recreational turf or golf course use comprises approximately 10% of the annual applied active ingredient 1. In cold, humid climates, golf course superintendents may apply chlorothalonil extensively as a snow mold retardant 5, 6. Chlorothalonil is also used to a much lesser extent to treat dollar spot and other turfgrass diseases.

Table 1. Selected physical and chemical properties of chlorothalonil and guidelines for human and aquatic protection
  1. LC50 = median lethal concentration; EC50 = median effective concentration.

Physical/chemical property 23

 Chemical nametetrachloroisophthalonitrile
 CAS number1897-45-6
 Molecular weight (g/mol)265.92
 Water solubility (mg/L) at 25°C0.6–1.2
 Melting point (°C)250–251
 Log (KOW) octenal water coefficient2.88
 Henry's law constant (Pa m3/mol)0.022
 Adsorption coefficient (KOC) (ml/g)1,300–14,000
 Field dissipation half-life (days)48

Drinking water standards and guidelines 24

 MCL (United States) (µg/L)
 One-day exposure health advisory level (HAL) (µg/L)200
 Canadian fresh-water guidelines (µg/L)0.18
 Canadian salt-water guidelines (µg/L)0.36

Aquatic toxicology 13

 LC50 (µg/L) rainbow trout (acute)7.6–250 for 4 d duration
 LC50 (µg/L) frog or toad (acute – tadpole or larvae) 245 for 2 d duration
 LC50 (µg/L) water flea (acute)129–202 for 2 d duration
 EC50 (µg/L) water flea (chronic)97 for 2 d duration

Due to its low water solubility and high soil sorption, chlorothalonil has not previously been perceived as an environmental concern when used on recreational turfgrass (Table 1) 7. Chlorothalonil is widely applied on golf course turfgrass in late fall and is moderately persistent in turfgrass thatch and soil 6, 8. These conditions result in chlorothalonil persistence in the environment with availability for long-term transport 8, 9. In a review of pesticide fate in turfgrass, Magri and Haith 8 speculated that pesticides strongly absorbed to turfgrass foliage, thatch, and soil may be sequestered from exposure to runoff and leaching. However, highly sorbed pesticides in the turfgrass environment with longer degradation half-lives, such as chlorothalonil, may have greater transport losses due to long-term exposure to runoff and leaching.

Chlorothalonil has been observed in surface waters associated with golf courses at low background concentrations with occasional large concentration spikes 6, 10. King and Balogh 6 discussed median annual and monthly inflow and outflow chlorothalonil concentrations and mass loadings in surface discharge from Northland Country Club golf course in Duluth, Minnesota, USA. Detailed storm events with peak concentrations and duration of potential exposure above toxicological limits were not specifically addressed. The median outflow concentration of chlorothalonil (0.58 µg L−1) from six years of monitoring (2003–2008) was significantly greater than the median inflow concentration (below the detection limit). However, the authors did find occasional peak concentrations of chlorothalonil as great as 48.1 µg/L in discharge from the golf course that occurred during late-October through mid-November. King and Balogh 6 did not report other peak concentrations.

Chlorothalonil has both acute and chronic effects on fish and other aquatic organisms 11–13. The analysis by King and Balogh 6 showed that most discharge concentrations from the golf course were well below the human health limits, as well as the median lethal concentration (LC50) and median effective concentration (EC50) for aquatic species. However, fish and other aquatic species can be adversely impacted with prolonged exposure to lower concentrations of chlorothalonil. Chlorothalonil losses in runoff from golf courses can represent a biologically meaningful pathway for exposure of aquatic organisms such as salmonids 11.

It has been documented that chlorothalonil concentrations measured in surface water discharge from golf courses may exceed published LC50 and EC50 concentrations 6, 11 that have biological relevance for aquatic species. However, the duration and frequency of these exceedences has not been explored. The analysis by King and Balogh 6 did not assess the timing, duration, and concentration of other event peak concentrations or the duration of chlorothalonil concentrations above toxicological thresholds.

The primary objectives of the present study were to relate event mean concentrations of chlorothalonil to discharge, rainfall, and time since application, and to interpret and compare those findings with respect to a conservative toxicological endpoint for chlorothalonil, specifically the LC50 for rainbow trout (Oncorhynchus mykiss). Additionally, because biological impacts are deemed relevant on specific needs and management goals and depend on dose, duration, and frequency of exposure, we present and discuss concentration exceedences for one-half the LC50, which corresponds to the U.S. EPA's restricted use level of concern, and one-twentieth of the LC50, which is equivalent to the U.S. EPA's endangered species level of concern 14.


The methods used for the present analysis were described fully in the monitoring results manuscript by King and Balogh 6. The following section describes those methods plus the methods that are unique to the present study.

Site description

The experimental site was located on Northland Country Club (NCC) golf course located in Duluth, Minnesota, USA. Specifically, the study area was a 21.8 hectare subarea of the golf course that contained seven greens (0.3 hectares), eight tees (0.5 hectares), 10.5 fairways (3.95 hectares), grass roughs (8.1 hectares), and 8.95 hectares of unmanaged mixed northern hardwoods (Fig. 1). A small stream enters the study area at the inlet and empties into a small detention pond, which was once used for irrigation. After the water leaves the pond, it meanders approximately 700 meters through the study area until it exits at the outflow collection site and eventually into Lake Superior. There is a 37 m elevation change across the study area, with slopes ranging from 3 to 25%. Approximately 80 hectares of low-density housing and forested area feed the inflow site. A small area of typical urban housing is located on the east side of the inflow portion of this upper watershed.

Figure 1.

Plan view of Northland Country Club golf course and study area. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com]

Northland Country Club soils are characteristic of re-worked lacustrine clay deposits, moderately deep (3–6 m) over bedrock. The dominant soil on NCC is the Sanborg (fine, mixed, active, frigid, Oxyaquic Glossudalfs)−Badriver (fine, mixed, active, frigid, Aeric Glossaqualfs) complex (92%). The parent material is noncalcareous clayey lacustrine deposit over calcareous clays. Perched water table conditions on the site are common and are caused by the dense subsurface horizons and fine-textured soils.

Northland Country Club is located in a temperate-continental climatic region. The area is characterized by warm, moist summers and cold, dry winters. The average monthly maximum summer temperature (May–August) ranges from 16 to 25°C (62 to 77°F), and the average monthly maximum winter temperature (December–March) ranges from –9 to 0°C (16 to 32°F). Average annual (1949–2008) precipitation measured at the Duluth International Airport during the period of April through November was 648 mm (standard deviation = 123 mm). The stream bed at the outlet is typically frozen from the end of November through the end of March.

Northland Country Club is managed at a moderate to intense level. Greens and tees were seeded with creeping bentgrass (Agrostis palustris Huds. A. stolonifera L.). Fairways were primarily creeping bentgrass with some Kentucky bluegrass (Poa pratensis L.). The roughs were a mixture of annual bluegrass (Poa annua L.) and Kentucky bluegrass. Pesticide application at NCC is used primarily for weed control and to eradicate turfgrass disease. Aerial weighted chlorothalonil application averaged 3.2 kg/hectare of active ingredient (Table 2). Chlorothalonil is used primarily to retard snow mold and generally is applied in late fall to the primary playing areas (Table 2). Chlorothalonil is generally applied as part of a tank mixture of fungicides. It is also used in small amounts at targeted locations at other times during the growing season to treat dollar spot and other turfgrass diseases.

Table 2. Chlorothalonil application timing and amount of active ingredient applied to study area greens, tees, and fairways during the study period (2003–2009)
DateBrandActive ingredient (%)Application amount (kg)Primary application locationa
  • a

    Some application times and areas include collars (areas around the green) and approaches (areas in front of green).

10/27/03Concorde5458.7Greens and fairways
10/14/04Concorde5413.8Tees and fairways
10/24/05Pegasus L544.2Greens
10/25/05Pegasus L547.4Tees
10/26/05Pegasus L5452.4Fairways
10/19/06Manicure 6FL5434.0Fairways
10/20/06Manicure 6FL5430.6Fairways
10/24/06Manicure 6FL547.4Tees
10/25/06Manicure 6FL544.2Greens
6/20/07Daconil Weather Stik541.4Greens
7/16/07Daconil Weather Stik541.4Greens
10/30/07Manicure 6FL547.4Tees
10/31/07Manicure 6FL545.8Fairways
11/1/07Manicure 6FL5443.0Fairways
8/5/08Daconil Zn38.52.0Greens
10/27/09Daconil Weather Stik547.87Tees
10/28/09Daconil Weather Stik5445.6Fairways

Data collection

Discharge and water quality samples were collected by a combination of grab samples and automated sample collection. In the summer of 2002, two, 3-foot H-flumes with stilling wells and approach sections were installed in the stream that bisects the study area (Fig. 1). One flume was positioned at the inflow, and another was placed at the outflow. The H-flumes were instrumented with Isco 4230 bubblers programmed to record stage on 10-minute intervals. Stage was converted to discharge using the standard 3-foot H-flume stage-discharge rating curve. Precipitation was collected on site using Isco 674 tipping bucket rain gauges. Rain gauges were located at both the inlet and outlet data collection sites for backup purposes. Rainfall was assumed to be uniform over the study area. Isco 6700 automated samplers were used and programmed to collect discrete flow proportional samples every 132 m3 (35,000 gallons). Grab samples were collected approximately weekly to supplement automated water collection. Water samples were collected from April 1 to November 30 each year. Samples were collected in 350-ml glass bottles and transported to the laboratory following collection, at a minimum of once per week but typically twice per week. Samples were stored at 4°C and analyzed within 14 d after collection, consistent with the recommended holding times for chlorothalonil 15, 16.

Chemical analysis

Analysis for chlorothalonil was conducted using enzyme linked immunosorbent assay (ELISA) and methods outlined by Strategic Diagnostics 17. Reactivity or assay interference resulting from the presence of similar chemicals was assumed negligible because these other chemicals were not applied to the golf course. Once collected in the field, the samples were stored at or below 4°C until analysis, which occurred within 2 weeks 15, 16. The samples were first syringe–filtered through a 0.45-µm pore diameter membrane filter. Once filtered, a prescribed sample volume was added to the bottom of prelabeled test tubes followed by 250 µl of the enzyme conjugate and then 500 µl of paramagnetic linked antibody particles specific to the pesticide. Each sample tube was then mixed and allowed to incubate for 30 min. After incubation, a magnet was applied to the base of the tube and the liquid was poured off. One milliliter of wash agent was then added and decanted, and the process was repeated. A total of 500 µl of color reagent were then added to each tube, and the liquid was vortexed and allowed to incubate for 20 min. Following the incubation period, 500 µl of an acidic solution were added to stabilize the sample. The samples were then analyzed using a spectrophotometer. Concentration was determined colorimetrically by interpreting the reading to an established standard three-point curve (Fig. 2). Blanks were analyzed with each batch run and produced concentrations below the detection limit. The method detection limit was 0.07 µg/L, and the method range spanned from the detection limit to 5 µg/L. When sample concentrations were determined to be greater than the method range, samples were diluted and reanalyzed.

Figure 2.

Mean absorbance ratio (B/Bo, where B is the absorbance at 450 nm observed for a standard divided by the absorbance at the zero standard, Bo) and concentration points for Enzyme Linked ImmunoSorbent Assay (ELISA) calibration curves used for chlorothalonil (n = 126) during study period (2003–2009). Error bars represent one standard deviation.

The ELISA procedure has been shown to be an accurate and cost-effective approach for analyzing chlorothalonil in water samples 18. Lawruk et al. 18 demonstrated a significant correlation (r = 0.984 and slope of 1.14) between chlorothalonil concentrations determined by immunoassay and gas chromatography (GC) methods. The greater chlorothalonil concentrations with the immunoassay approach compared to GC analysis techniques were attributed to losses incurred during the GC extraction process. This theory was corroborated with data indicating only 91% recovery of spiked water samples using the GC method. The authors concluded that the immunoassay methodology was a cost-effective, time-efficient alternative to using gas chromatography–mass spectrometry analysis techniques.

Two alternative quality assurance approaches were used in the present study. The first was a measure of standards within the ELISA procedure. Figure 2 provides this information and can be summarized as showing that equal variance was observed in the samples regardless of concentration. Second, because chlorothalonil is generally not applied to the watershed that drains into the golf course, the concentrations measured at that site ultimately provided a measure of quality assurance. A review of the 1,111 data points collected at the inlet indicated that the median concentration at the inlet was below the detection limit, as suspected. This provides further evidence that the methods used were valid and that the concentrations measured at the outlet can be reported with confidence.

Data analysis

Dates of major chlorothalonil application (fall application for snow mold control) were spread, in general, over a 3- or 4-d period. To determine the time since application, a single time-weighted date of application for major applications was calculated for each year. The time-weighted date was calculated by multiplying the date and time of application by the amount applied, then summing over the application period and dividing by the sum of the application amount. Because time of application was not available, for calculation purposes we assumed a time of application of 12 noon. For example, in 2003, the time weighted date/time of the fall application was calculated as ([23-Oct-2003 12:00 × 9.9 kg active ingredient] + [27-Oct-2003 12:00 × 58.7 kg active ingredient]) ÷ (9.9 + 58.7 kg active ingredient). This resulted in a date/time of 26-Oct-2003 22:08 with an amount of 68.6 kg active ingredient. The time-weighted date of application was used to calculate the time since application by subtracting that date from the rainfall/runoff event date.

An event was defined as any precipitation occurrence in which at least 6.35 mm of precipitation fell and in which there were no more than 6 h without recorded precipitation. If the duration was greater than 6 h between recorded precipitation points, a new event was identified. The discharge associated with each event was determined as the cumulative flow from the time the precipitation event began to the point at which the discharge rate was within 5% of the discharge rate when the event began. Flow-weighted event concentrations were determined by multiplying each concentration collected during an event by the volume of water associated with the sample, and then summing those values and dividing by the total event discharge.

As a point of reference, we compared our findings to the LC50 for rainbow trout (7.6 µg/L) as well as fractions of the LC50 (one-half and one-twentieth) that correspond to important toxicological thresholds often used by regulatory agencies 14. The LC50 for rainbow trout was the most conservative value that we found published for chlorothalonil. In addition, duration of exposure to chlorothalonil must be greater than 4 d to manifest the acute effect for 50% of the population. Notably, concentrations and durations less than those required to meet the LC50 will still have dire consequences for a percentage of the exposed population. Reported metrics in the present study include exceedence probability, duration above the defined threshold, and duration of exposure assuming the organism of interest was present throughout the sampling period.

To determine the duration of exceedence, we needed to determine the time on the rising limb of the chemograph and the time on the receding limb of the chemograph when the concentrations were equal to the LC50 or other thresholds. These times were determined through linear interpolation using the concentration and time from the data point immediately preceding the time, as well as the concentration of the point above the threshold on the rising limb and the time and the first point on the receding limb when the concentration was below the threshold. The duration above the threshold was determined as the difference in these two times.

Graphical techniques and a multiple linear regression were used to identify the most important factors in the transport of chlorothalonil from all rainfall/runoff events during the study period. The dependent variable was flow-weighted event concentration, and the independent variables were precipitation amount, runoff volume, and time since major fall application. The statistical analysis was conducted using SigmaStat 3.5 statistical software and a significance level of 0.05 19.


Precipitation, discharge, and chlorothalonil concentrations were measured at NCC from April 1 to November 30, 2003 through 2009. A total of 1,400 water samples were collected at the outlet of the watershed. Based on the 1,400 flow proportional samples, the median concentration for all 1,400 outflow samples was 0.58 µg/L, the 25th percentile concentration was 0.17 µg/L, and the 75th percentile concentration was 1.45 µg/L. The 95th percentile concentration was 4.12 µg/L. The probability of any concentration exceeding the LC50 threshold for rainbow trout (7.6 µg/L) was 1.87%. The probability of exceeding half of this concentration (3.8µg/L) was 6.14%, and the probability of exceeding one-twentieth of the LC50 (0.38 µg/L) was 57.95% (Fig. 3). Only 3.5% of the 1,400 collected water samples had concentrations below the 0.07 µg/L detection limit. Thus, from a toxicological point of view, the implications to rainbow trout or any similarly sensitive or affected species could be significant. Although the LC50 was exceeded only for a small percentage of samples, the low-dose, lengthy exposures might be more detrimental to sensitive aquatic species.

Figure 3.

Probability of exceedence plot for all flow proportional chlorothalonil concentrations (n = 1,400) measured at Northland Country Club during the study period (2003–2009). The 7.6 µg/L concentration is representative of the median lethal concentration (LC50) for rainbow trout 13. The 3.8 µg/L concentration represents one-half of the LC50 and the 0.38 µg/L concentration represents one-twentieth of the LC50.

Duration refers to the time an organism is exposed to a given condition. With respect to rainbow trout and the 7.6 µg/L threshold used in the present study, for a full manifestation of the LC50—that is, for lethality for 50% of the population—the duration of exposure should be for 4 d. Because the data collected in the present study was on a flow proportional basis, the time associated with each sample was variable, which prevented calculation of standard uniform time-weighed concentrations. As a proxy for time-weighted concentrations, we took each sample and determined its representative time to provide an estimate of exposure time. Assuming constant exposure over the life of the study, exposure to concentrations greater than the 7.6 µg/L LC50 threshold could be expected for 16.2 d, or 1% of the study period. Likewise, exposure to one-half the LC50 (3.8 mg/L) could be expected for 37.6 d, or 2.2% of the time. Exposure to concentrations greater than one-twentieth of the LC50 (0.38 mg/L) could be expected for 530.7 d, or 31.1% of the study period time. These findings suggest that lengthy exposure to low doses could be expected and might result in lethality of sensitive organisms.

We recorded 145 rainfall/runoff events during the study period. Eight events with peak concentrations exceeding the LC50 (7.6 µg/L) for the rainbow trout threshold were identified in this 7-year study (Table 3). In the event of October 29, 2004, two different peaks were measured that exceeded the threshold. The exposure duration of 4 d for the LC50 for rainbow trout was approached on two of the eight events, suggesting that this exposure would be lethal for a significant portion of the sensitive organisms. While the 4-d duration is published as the exposure time for the full manifestation of the acute effects of the LC50 for rainbow trout, it is understood that lethal effects could and would be expected with concentrations and durations less than the published LC50 values. The rainfall/runoff event that began on November 12, 2003 had a duration of 3.81 d when the concentration was above the LC50 threshold of 7.6 µg/L. Similarly, the event beginning on November 9, 2005 had an exceedence duration of 3.97 d. Among the other events with concentrations exceeding the 7.6 µg/L threshold, only one other event had an exceedence duration greater than 1 d. The remaining five events had durations of much less than 1 d. Among the 145 observed discharge events, only 2% of the events had concentrations that exceeded the 7.6 µg/L threshold with durations that approached the 96 h acute toxicity level. Given the greater concentrations and lengthy durations above the threshold concentration, it would be expected that many of the organisms sensitive to chlorothalonil would suffer significant losses. Furthermore, exposure to concentrations exceeding the more sensitive one-half LC50 was much greater (Table 3), indicating that the biological impacts on sensitive organisms associated with these exposures could and would be significant. For all nine hydrographs with peak concentrations exceeding the LC50 threshold, the one-twentieth threshold was surpassed for the entirety of each storm discharge event. These findings call for the development and implementation of management practices to address events leading to the exceedence of the LC50s and the prolonged exposure to low doses.

Table 3. Characteristics of runoff events exceeding 7.6 µg/L chlorothalonil in stream outflow at Northland Country Club
Date of peak eventPeak concentration in streamflow (µg L−1)Flow-weighted event concentration (µg/L)Time-weighted event concentration (µg/L)Volume of precipitation (mm)Duration of chlorothalonil concentration above 7.6 µg L−1 threshold in stream flow (days)Duration of chlorothalonil concentration above 3.8 µg L−1 threshold in stream flow (days)

Greater chlorothalonil concentrations were generally measured with greater flow rates (Fig. 4). These greater flow rates were associated with storm event runoff throughout the year but were clearly evident in the spring and fall (Fig. 5). Chlorothalonil losses were closely related to application timing. Primary peak concentrations occurred in fall after application, and secondary peaks occurred in the spring when residual chlorothalonil was still present (Fig. 5). The secondary peaks measured in the spring were a result of chlorothalonil's presence in the turfgrass environment, its sorption to the thatch and soil 20, and its persistence in the environment indicated by its greater degradation half-life 8.

Figure 4.

Relationship between flow rate (L/s) and chlorothalonil concentrations (µg/L) collected at Northland Country Club during the sampling period of 2003 through 2009 (n = 1,400). Dashed lines represent the median lethal concentration (LC50) (7.6 µg/L), one-half the LC50 (3.8 µg/L) and one-twentieth the LC50 (0.38 µg/L) for rainbow trout 13.

Figure 5.

Time series plot of chlorothalonil concentrations (n = 1,400) and application timing of chlorothalonil active ingredient (a.i.). Dashed lines represent the median lethal concentration (LC50) (7.6 µg/L), one-half the LC50 (3.8 µg/L) and one-twentieth the LC50 (0.38 µg/L) for rainbow trout 13.

Application timing prior to a storm event was more critical in generating concentrations that exceeded the threshold than the amount of precipitation or runoff volume (Fig. 6 and Table 4). A marked difference was observed in the concentrations generated shortly after application in the fall compared to concentrations generated with spring runoff. Precipitation amount was not as critical as just having a precipitation/runoff event (Fig. 6). Five of the eight events occurring in the fall had approximately 12.7 mm of rainfall or less (Table 3). Similar findings with respect to precipitation events following application have been reported for other turfgrass pesticides 6, 21, 22. Ma et al. 21 highlighted the critical importance of time to precipitation following application in both minimizing losses and predicting concentrations. In the regression analysis of the present study, flow-weighted event concentrations were significantly correlated (p < 0.001) to time since application (Table 4). Precipitation amount and discharge volume were not identified as significant factors in the model. From a directional standpoint, however, the positive signs of the coefficients in the model indicate that as precipitation increased, the concentrations increased. Likewise, the negative signs associated with discharge volume and time since application indicated that as these variables increased in magnitude, the concentrations decreased. In 2006 and 2007, by delaying application until after the major fall rainfall events occurred, no peak concentrations exceeded the rainbow trout LC50 threshold (7.6 µg/L), and only three measurements across both years met the one-half LC50 (3.8 µg/L) criteria. These findings suggest that while a short window exists for applying chlorothalonil after the major rainfalls in the fall, the results are significant with respect to reducing peak concentrations to levels below important toxicological thresholds.

Figure 6.

Bubble plot of flow-weighted event concentrations (bubbles) and their relationship to event discharge and time since application (A) and their relationship to precipitation and time since application (B); the larger the bubbles, the greater the event concentration. The red bubbles correspond to those events with peak concentrations exceeding the reference value of 7.6 µg/L, which is equivalent to the median lethal concentration threshold for rainbow trout. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com]

Table 4. Linear model and statistics for flow-weighted event concentrations using 145 rainfall runoff events from Northland Country Club during the study period (2003–2009)a
VariableCoefficientStandard errort valueProbability
  • a

    Flow-weighted event concentration = constant + precipitation + runoff volume + time since application r2 = 0.346, F = 24.893 (p < 0.001).

Constant4.040.40210.04< 0.001
Runoff volume−0.00001860.000029−0.6400.523
Time since application−0.01190.00141−8.461< 0.001

Appropriate timing of pesticide application in relation to precipitation and runoff is well recognized as a management practice to reduce the risk of pesticide losses in discharge water from turf 23. This is more difficult when extensive application of fungicide is applied to turfgrass for snow mold control on golf courses in the cool, humid portions of the United States and Canada. For snow mold control, the final, end-of-season application of fungicides typically occurs in the last week of October through the first week of November. In the northern United States and southern Canada, at least one or two rain events often will occur after the final fungicide application and prior to ground freezing and first snowfall. Comparing the timing of application to precipitation and subsequent discharge concentrations of chlorothalonil in the years 2003, 2004, 2005, 2008, and 2009 to 2006 and 2007 suggests a possible solution (Fig. 5 and Table 3). Delaying the application of chlorothalonil until the second or third week of November would significantly reduce the probability of a rain event occurring after the extensive application of snow mold control fungicides. A one- to two-week delay in application will reduce the likelihood of discharge concentrations exceeding both acute toxicity thresholds and as well those levels of concern used for endangered and threatened species effects determinations.


Event-based chlorothalonil concentrations were analyzed for a 7-year period (2003–2009) in discharge waters from Northland Country Club golf course in Duluth, Minnesota, USA. Event mean concentrations were correlated to precipitation amount, discharge, and time since application and compared to a series of conservative biological thresholds for rainbow trout. Eight rainfall runoff events produced concentrations that exceeded the acute toxicity LC50 for rainbow trout (7.6 µg/L). Events with concentrations exceeding the acute toxicity threshold (7.6 µg/L) occurred in late fall in five of the seven study years, including two occasions when measured concentrations were significantly greater than the LC50 (47.2 and 48.1 µg/L). Two of the events had duration times that approached the 4-d exposure time (3.81 and 3.97 d) required for the full manifestation of the acute effects. More important, though, might be the duration for which sensitive organisms were exposed to low doses of chlorothalonil. Chlorothalonil concentrations greater than the U.S. EPA's endangered species level of concern 14, or one-twentieth of the LC50 (0.38 mg/L), occurred during 31.1% of the study period duration.

Exceedence concentration and duration were correlated primarily to time to rainfall since the last major application. The amount of precipitation was not an integral component. Major application amounts in late fall for snow mold control followed by precipitation shortly after application generated greater peak concentrations. Similarly, smaller application amounts applied in the spring and summer for dollar spot also generated peak event mean concentrations, but the biological relevance of these event mean concentrations was much less when compared to the measured concentrations following the major fall applications.

Based on the findings in the present study, a conflict exists between the transportability of chlorothalonil in water as suggested by its chemical properties, particularly water solubility and adsorption coefficient, and what the present study measured. The present study's findings indicate that chlorothalonil transport associated with rainfall/runoff events can be significant. Once applied, chlorothalonil persists in the environment and concentrations in discharge waters may be substantial and have significant biological relevance and impact, especially as they relate to low-dose, lengthy exposures.

As regulations in the United States and abroad become more stringent, attention will need to be focused on all pesticides, regardless of their chemical properties. However, data collected in the natural environment for many pesticides is lacking, and more data collection efforts will be required. Simply relying on established chemical properties and expectations observed and derived in controlled laboratory experiments might be misrepresentative and misleading with respect to the true environmental characterization of certain chemicals and pesticides.

With respect to chlorothalonil and its course-wide use for retarding snow mold, the following recommendation should be considered: Avoid application for course-wide snow mold protection until after the last major rainfalls and before the first snow cover events. For the course and climate in the present study, this is roughly the third week of November. The label recommendations do allow for multiple applications if snow cover is intermittent. As shown in the present study, if fall application of chlorothalonil is delayed until the third week of November, the likelihood of greater peak concentrations in the fall is reduced significantly. Secondary peak concentrations in the spring might still occur but at concentrations below biologically relevant concentrations.


The authors acknowledge the assistance of S.B. Scadlock, I. Leland, E. Burgess, H. McMains, and S. Hess in the data collection, processing, and analysis necessary to complete this study. Without their dedication, perseverance, and attention to detail, the present study would not have been possible. The authors are also indebted to D. Kolbry (past superintendent), C. Tritabaugh (current superintendent), and J. Ryan (current assistant superintendent) for granting us access to the course and sharing their management strategies and records. We also thank the members of NCC for their insight in recognizing the importance of this type of research and their willingness to permit us to conduct this study on their course. We also acknowledge and thank the U.S. Golf Association Green Section for its support and financial contributions.