Herbicide Accumulation in Recycled Irrigation Water

Introduction

Some growers I’ve spoken with have concerns over the potential for herbicide contamination of their irrigation water, especially if they recycle water captured from their container beds.  This article will discuss pathways for herbicides to contaminate ponds, and best management practices (BMPs) that minimize the potential for herbicide accumulation in retention ponds.  This information is targeted towards container growers.  

Herbicides are one of the primary methods of weed control used in container crops.  Most herbicides are applied in granular formulations, some nurseries apply spray herbicides.  Herbicides are typically broadcast applied over the entire container bed.  Due to labor costs, herbicides are not applied individually to each container.  Broadcast-applied herbicides either fall in the containers or between them.  We will examine individually what happens to herbicides that either fall in or between containers.  

Fate of herbicides in container media

In a nutshell, herbicides commonly used by nursery growers move very little in containers.  When applied at the recommended rate and incorporated with irrigation, (as instructed by the label) most herbicides remain in the top 1 inch of the media, with some moving as far as 1.5 inches below the container surface.  Herbicides rarely (if ever) leach through the entire container and out the drain holes.  While this may be a logical conclusion based on the chemical properties of herbicides, low solubility and high adsorption to organic matter, it has also been verified by several scientific experiments.  

In a peat-based media, 96% of applied isoxaben remained in the top 2 inches, with no detectable isoxaben below 4 inches (Rouchaud et al., 1999).  Over 99% of oxadiazon applied to a 3:1 pinebark:peat or 7:1 pinebark:sand media remained in the top 1 inch of the container, with no oxadiazon detectable below 1.25 inches (Wehtje and Gilliam, 1993).  Oryzalin is only slightly more mobile, with 99% remaining in the top 1.5 inches of the pinebark:sand media and 99% in the top 0.75 inch of the pinebark:peat media.  Decreased movement in the pinebark:peat media is likely due the higher organic matter content and increased cation exchange capacity (CEC) in that mix (due to incorporation of peat instead of sand).  Oxyfluorfen also has a low probability of leaching from containers, and similar to oryzalin, it’s been shown that peat is more absorptive of oxyfluorfen than redwood bark alone (Horowitz and Elmore, 1991).

Herbicides have been detected in container leachate at very low levels (Gilliam et al., 1993).  At their highest levels, oxadiazon was found of 100 ppb (parts per billion), pendimethalin at 3 ppb, and oxyfluorfen (from Rout 3G) at 5 ppb.  Just to put this in perspective, oxyfluorfen at 5 ppb is 0.0000008 times the concentration normally applied to soil for effective weed control (assuming Goal is applied at 2 lb ai/Acre in 40 gpa).  In other words, imagine diluting your normal oxyfluorfen spray 1.2 million times; that’s the approximate concentration coming out of the bottom of the container.  

With herbicides currently used in container production, leaching of the herbicide through the  container and out of the drainage holes is not a risk for contaminating retention ponds.

Fate of herbicide that falls between containers

Container spacing
Because herbicides move little in bark-based media, and virtually no herbicide has been detected leaching from the bottom of containers, herbicide accumulation in retention ponds is a function of the amount of herbicide that falls between containers.  Herbicide that falls between containers is called non-target herbicide loss.  The further containers are spaced, the more herbicide falls between them instead of in them.  When containers are jammed pot-to-pot, roughly 79% of applied herbicide falls into the containers while 21% falls between.  If pots are spaced just 3 inches apart, only 35% of the herbicide falls in the containers while 65% falls between.   

Apply herbicides while containers are still jammed prior to and after overwintering, or before they are otherwise spaced out.  This will drastically reduce the amount of herbicide falling between containers with potential for movement into retention ponds.

Sequence of application timing
Most of the herbicide that accumulates in ponds is released during the first irrigation event following application.  Isoxaben (from Snapshot) concentration in a pond peaked during the day of application (90 ppb), and steadily declined each day thereafter until it was nearly undetectable (10 ppb) 21 days after application (Wilson et al., 1993).   In another experiment, a majority of the detectable isoxaben and trifluralin (from Snapshot) was detected within the first 9 days.

Staggering herbicide applications, instead of treating the entire nursery at one time, will reduce the total amount of herbicide entering a pond at any given point.  

Groundcover
Material used under containers will affect herbicide movement.  A study measuring movement of trifluralin and isoxaben (from Snapshot) demonstrated that gravel reduced herbicide movement compared to plastic or woven polypropylene (plastic resulted in greatest movement) (Wilson et al., 1994).  The authors noted that when herbicides were applied to areas covered with gravel only, herbicide granules likely fell down between stones and were thus removed from the irrigation flow.

Vegetative filter strips

Vegetative filter strips are areas with dense vegetation (usually grass) through which runoff must flow before entering a retention pond.  Vegetative filter strips can remove pesticides, nutrients, and sediment from flowing water.  Because herbicides used in containers have low solubility and high coefficients of adsorption (Koc, the affinity for a chemical to bind to soil), it is likely that herbicides move while attached to some type of sediment.  
    
Vegetative filter strips remove herbicides in several ways.  Water moving through a vegetative filter strip is slowed to a point where sediment is dropped out of the flow.  As mentioned previously, much of the herbicide moving in runoff water is attached to sediment or soil particles, and as those particles drop out of the runoff flow, so too will the herbicide.  

Research at Mississippi State University has demonstrated that soil organic matter increased by more than two-fold when covered with vegetation, compared to bare soil.  Increased organic matter increases the adsorptive properties of soil, thus attracting and tying up herbicides and preventing their movement into ponds.  Higher organic matter also increases microbial populations and activity (thus increasing herbicide breakdown).  In their work, they also found that herbicide half life on ground covered with vegetation was 12 days, compared to 100 days on bare soil.

Vegetative filter strips can be a strip that surrounds the retention pond through which irrigation runoff must flow before entering.  Or, the entire ditch can be lined with vegetation.  In a mock container nursery, a grassed ditch removed roughly 20% of isoxaben and 50% oryzalin in nursery runoff (Briggs et al., 1994).

So what’s the threat?

To what extent do herbicides accumulate in retention ponds?  It will, of course, depend on many factors, although there are two studies that have evaluated herbicide accumulation from container nurseries.  One study found the highest concentrations for oryzalin and oxyfluorfen (from Rout) peaked at 0.15 ppm (parts per million) through the first day after application, and declined thereafter (Keese et al., 1994).  By two weeks after application, the herbicides were nearly undetectable, and by 4 weeks herbicides were not detectable (below 1 ppb).  

A separate study monitored herbicide levels in a retention pond on a 50 acre container nursery (Camper et al., 1994).  Samples were collected over two years, and were analyzed for pendimethalin, oryzalin, and oxyfluorfen (from Rout and OH2).  Pendimethalin levels peaked at 4 ppb, oryzalin at approximately 0.15 ppm (similar to the above mentioned study) and oxyfluorfen at 9 ppb.  To put this into perspective, oxyfluorfen (when applied as a spray herbicide) is typically applied at about 6000 ppm, which is roughly 600,000 times more concentrated than what was found in the above experiment.

In both studies, herbicide concentrations declined shortly after peaking, indicating that degradative processes (microbial, chemical, and photo-degradation) prevent accumulation from repeated herbicide applications.

I have heard anecdotal reports of herbicide injury to crops resulting from contaminated ponds.  It is not my intent to argue with growers who have reported this, however, I find herbicide contamination unlikely when products are used according to label instructions, and under normal nursery conditions.

Summary

Herbicide accumulation in ponds, to the point that it will cause injury to crops when reapplied through irrigation, is uncommon.  However, when it occurs, it likely results from a combination of:  excessively high herbicide rates, large areas treated with herbicides that drain into a small pond, and applying herbicides broadcast to containers that are spaced far apart.  To minimize herbicide accumulation in ponds, consider the following BMPs:
  • Apply herbicides to jammed containers.
  • Include peat as a component in your container media.
  • Do not apply herbicides to the entire container yard at one time.  If possible, spread out the timing of applications.
  • Develop grassed waterways, or vegetative filter strips, to remove sediment and herbicides from irrigation runoff before they enter retention ponds.
  • If granular herbicides are used, consider using gravel under containers, without using weed fabrics or plastic.

Literature Cited

  • Briggs, J.A., M.B. Riley, and T. Whitwell.  1994.  SNA Proceedings 39:57-59.
  • Camper, N.D., T. Whitwell, R.J. Keese, and M.B. Riley.  1994.  J. Environ. Hort 12:8-12.
  • Gilliam, C., D. Fare, and D. LeCompte.  1993.  SNA Proceedings 38:312-314.
  • Horowitz, M. and C.L. Elmore.  1991.  Weed Technology 5:175-180.
  • Keese, R.J., N.D. Camper, T. Whitwell, M.B. Riley, and P.C. Wilson.  1994.  J. Environ. Qual. 23:320-324.
  • Rouchaud, J., O. Neus, M.C. Van Labeke, K. Cools, and R. Bulcke.  1999.  Weed Science 47:602-607.
  • Wehtje, G.R., C.H. Gilliam, and B.F. Hajek.  1993.  HortScience 28:126-128.
  • Wehtje, G.R., C.H. Gilliam, and B.F. Hajek.  1994.  HortScience 29:824.
  • Wilson, C., T. Whitwell, and M. Riley.  1993.  SNA Proceedings 38:46-49.
  • Wilson, C., T. Whitwell, and M. Riley.  1994.  SNA Proceedings 39:53-57.

Disclaimer:  This article is for educational purposes only.  Mention of a specific product should not be interpreted as an endorsement, nor should failure to mention a product be considered a criticism.

 

 

 

 

 

Herbicides form a chemical barrier over the container surface, and under normal conditions do not move below approximately 1 inch.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Grass surrounding this pond filters sediment from the flow of water, reducing fertilizer and pesticide levels before it reaches the pond.

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