Environmental Fate of Aquatic Herbicides

Transcription

1 Environmental Fate of Aquatic Herbicides UF-IFAS Aquatic Weed Control Short Course Michael Netherland, Ph.D US Army Engineer Research & Development Center US Army Corps of Engineers

2 What Happens After You Spray? What happens to the plant Injury - Senescence/Decay - Degradation What happens to the herbicide Molecule broken into non-herbicide constituents - Degradation Microbial processes responsible for plant decay are responsible for herbicide decay Organic decomposition 2

3 Submersed biomass does not correlate with a significant removal (uptake) of herbicide 1-4% uptake 3

4 Emergent Treatments Bulk of product is initially absorbed by emergent plant tissue 4

5 Amount that enters the water depends on the density of the plant stand 5

6 Is Herbicide Released from Treated Vegetation? -Can be metabolized by tolerant plants -Non-herbicidal compounds -Can be sequestered in vacuoles -Can be bound to organic constituents -Can be slowly released from decaying plant matter

7 Herbicide Dispersion Aquatic herbicides disperse in water movement from the treatment site Flow, winds, scale of treatment - drive dispersion Concentrations can be rapidly diluted Dilution is not the same as degradation Dilution can result in concentrations being too low for herbicidal activity 7

8 Rate of Dispersion Major influence on Efficacy e.g. Fluridone vs. Diquat Product Dispersion TARGET Control Area Thermal Gradient US Army Corps of Engineers Engineer Research and BUILDING Development STRONG Center

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11 13 Herbicides Registered for Aquatic Use Copper (1900 s) 2,4-D (1950 s) Endothall (1960) Diquat (1962) Glyphosate (1977) Fluridone (1986) Triclopyr (2002) Imazapyr (2003) Carfentrazone (2004) Penoxsulam (2007) Imazamox (2008) Flumioxazin (2011) Bispyribac (2011) 11

12 Primary Mode of Degradation Photolysis (5/13) Inactivation followed by microbial (2/13) Microbial Degradation (3/13) Hydrolysis (2/13) Copper (1/13) elemental Biologically inactive but never broken down 12

13 Photolysis (5 of 13 Products) Energy from sun breaks chemical bonds Ultraviolet (uv) light - THINK SUNBURN!! Different wavelengths of uv for different molecules Fluridone and Penoxsulam half-lives 20 to 50 days Triclopyr, Imazapyr, and Imazamox half-lives average 5 to 14 days 13

14 Photolysis Photolysis occurs in the top inches of lake water UV light penetration Does water depth impact the rate of degradation? Can photolysis occur below a thermocline? Can photolysis occur in sediments (fluridone?) Can photolysis occur below a dense plant mat? Molecules split by photolysis into smaller parts Photolysis, microbial, hydrolysis,

15 Photolytic Degradation of Fluridone

16 Photolysis and Efficacy Fluridone and Penoxsulam - submersed Efficacy is dependent on product longevity Imazapyr, Imazamox, Triclopyr emergent Herbicide in plant tissue not subject to photolysis 16

17 Inactivation (3/13) Diquat, Glyphosate and Copperinactivated in water Half-lives can be hours to days (water quality) Impacts efficacy on submersed plants Molecules exist as ions in water Diquat ++ Glyphosate Copper++ Ionic bonds = inactivation No further herbicidal activity (water or soil) 17

18 Clay and Organic Matter = High surface area and full of negative charges Diquat = ++ charges Once diquat is bound to sediment microbial degradation proceeds very slowly - half-life > 1000 days 18

19 Copper (micronutrient) Copper is inactivated but does not degrade Rapidly removed from water column Forms numerous complexes and ultimately resides in sediment Biologically active? Florida heavy use - private sector (algae) very limited use in public waters 19

20 Microbial Degradation (3/12) Endothall and 2,4-D microbial degradation is the primary pathway Source of carbon (energy/food) for microbes Some microbes specialize on herbicides Bispyribac complex molecule broken down by microbes All aquatic herbicides are subject to microbial degradation (one key exception)

21 Factors that Influence Microbial Degradation Temperature We use cold temperature to extend exposure Prior application Efficient microbe populations can be selected Water (aerobic) vs. Sediment (anaerobic) Oxygen generally enhances rate of breakdown Complexity and size of the molecule

22 Enhanced Degradation Enhanced microbial degradation is welldescribed in terrestrial settings Soil half-life of 2,4-D can decrease with increasing use pattern As long as enhanced degradation does not impact efficacy, it is viewed as positive

23 Enhanced Fluridone Degradation In several FL lakes with significant use history, we observed: 1) Rapid degradation of fluridone in dark!! Half-lives of fluridone < 2 days Narrowed issue to enhanced microbial activity Product was rendered ineffective

24 What is the Opposite of Enhanced Degradation? 24

25 Lake Tomahawk, WI acres of 2,4-D at 0.5 ppm 1 st Time Lake had been treated 2,4 D, ppb ½ Life in Water = > 50 days 3 days of exposure will kill milfoil May 14May 28 Jun 10 Jul 8 Jul 26 Sample Date Severe reductions or loss of 21 species including tolerant monocots - bulrush, lily, vallisneria

26 Hydrolysis (2/13) Both Flumioxazin and Carfentrazone are degraded by hydrolysis Process can result in rapid degradation ph-dependent (flumioxazin) ph 5-4 days ph 7-24 hours ph 8 -??????? - between 22 min. and 24 hr ph 9 22 minutes 26

27 Miscellaneous Questions Are aquatic herbicides subject to volatilization? What is adsorption and desorption? Think fluridone and organic matter How does degradation of herbicides from granular formulations compare to liquids? Breakdown in hydrosoils tends to be slower 27

28 SUMMARY Herbicides subject to degradation Primary & Secondary Modes of Degradation 2,4-D (2, 1) Triclopyr (1,2) Diquat (3, 2) Endothall (2) Flumioxazin (4,2) Fluridone (1,2) Glyphosate (3, 2) Imazamox (1, 2) 1) Photolysis 2) Microbial Degradation 3) Inactivation 4) Hydrolysis