PROTECTION OF SUBSURFACE DRIP IRRIGATION SYSTEMS FROM ROOT INTRUSION

R. Ruskin, CEO, Geoflow, Inc. San Francisco, CA 94127, USA
Karen R. Ferguson, President, Geoflow, Inc., Corte Madera, CA 94925, USA

SUMMARY

In the marketplace four different techniques are claimed to protect subsurface drip irrigation (SDI) systems from root intrusion. The literature, the claims of the manufacturers and the environmental regulations in the USA and are discussed. A direct comparison of the environmental risks of the three methods which use trifluralin (TFN) is presented.

KEYWORDS: Trifluralin, Treflan, subsurface drip irrigation, root intrusion

INTRODUCTION

SDI has considerable advantages in agriculture, landscape irrigation and dispersal of sewage. These advantages have given rise to considerable interest in SDI, however the risk of root intrusion has been a disincentive. In the market place four different techniques are claimed to protect subsurface drip irrigation (SDI) systems from root intrusion. I Mechanical barriers
II Addition of TFN directly to the irrigation water
III Incorporation of TFN into the emitter itself – "ROOTGUARD or Battelle Process"
IV Addition of TFN to the irrigation water from a slow release filter – "Techfilter"

The literature, the claims of the manufacturers and the environmental regulations in the USA are discussed. Commercial trademarks are used to make this paper more useful.

I MECHANICAL BARRIERS

It can be claimed that the smaller the passage of the dripper the less likely root intrusion is to take place; on the other hand, it can be claimed that the smaller the passageway of the dripper the more prone it is to plugging from impurities in the irrigation water. Studies at the Center for Irrigation Technology (CIT) (Jorgenson), CIT (Center for Irrigation Technology, 1994) and by Win Bui (Bui) have demonstrated this principle.

II ADDITION OF TFN DIRECTLY TO THE IRRIGATION WATER

This section is freely adapted and shortened from Technical analysis of the movement of TFN in the soil when applied through a subsurface drip irrigation system (Ruskin, 1997).

In 1993 the Gowan TFN 5 treatment system was introduced for application directly into the irrigation water. Concerns were raised with respect to hot spots and environmental safety concerns. As a result in February 1995 the Gowan label was substantially revised as was required by the EPA.

As is now required on the Gowan TFN 5 label dated Feb. 7, 1995 the amount of TFN which may be injected into the water is extremely small, being of the order of one teaspoon per hectare per annum. If correctly applied this is an effective dose. Furthermore, the technique of application requires the application cycle to be separated from the irrigation cycles. May addition of TFN in the irrigation water result in accumulation within the wetted area only, or is their a tendency for soil saturation and migration of TFN to surface and ground waters; and, is there a likelihood of excessive TFN accumulation in the wetted zone (hot spots)?

We demonstrate and quantify the risks of not exactly following the revised February 1995 label.

Materials and Methods

The approach taken was to apply TFN to a soil column via the water in the subsurface irrigation system as directed in the original 1993 Gowan TFN 5 label. Irrigation times were 12, 24 and 48 hrs. to evaluate the mobility of TFN under unsaturated and water saturating conditions. Two typical agricultural soils were used, a silt clay (Hawaiian/Kauai) and a silt loam (WA Palouse); these were contained in a standard perfusion column, and allowed for a determination of both accumulation at emitters, and vertical migration. Simulated application of TFN in irrigation water was to soils at 30% of field capacity (f.c.) and 90% of f.c.

Figure 1. Mobility and soil concentrations for TFN in Kauai soil with a moisture content of 30% f.c. Soil concentration profiles are for treatment durations of 12, 24 and 48 hrs.

Figure 2. Mobility and soil concentrations for TFN in Kauai soil with a moisture content of 90% f.c. Soil concentration profiles are for treatment durations of 12, 24 and 48 hrs.

Figure 3. Mobility and soil concentrations for TFN in Palouse soil with a moisture content of 30% f.c. Soil concentration profiles are for treatment durations of 12, 24 and 48 hrs.

Figure 4. Mobility and soil concentrations for TFN in Palouse soil with a moisture content of 90% f.c. Soil concentration profiles are for treatment durations of 12, 24 and 48 hrs.

Application Rates and Study Design.

Two hundred and forty 240 mL of water containing 0.0272 mL of TFN (Gowan-5) was applied to each soil column (700-800 g dry wt) per 12 hr period (240, 480, 960 mL added to each column irrigated for 12, 24, and 48 hr, respectively). The flow of diluted Gowan-5 solution to each of the columns was 20 ml/hr, and flow was initiated at 4 cm below soil surface. Replicate columns containing soils at 30% or 90% of field capacity were irrigated for periods of 12, 24 or 48 hrs. At the completion of the irrigation cycle, soil cores were taken vertically at 1 cm intervals. In addition, column effluents were analyzed for TFN that was not sorbed to the soil, and was carried with the water front. Results and Discussion

Results of the column studies are provided in Figures 1 through 4. Experimental studies have shown that the soil concentration of TFN required for weed control range from 0.5 to 2.0 ppm. This prevents the growth and elongation of roots into the emitter or prevents weed seedlings from establishing within the treated zone. The relative zones of efficacy and regulatory limits are crop dependent, and range from 0.5 to 2.0 lbs/acre

Figure 5. Mobility and soil concentrations for TFN in Kauai soil with a moisture content of 30% and 90% f.c. Soil concentration profiles are for a treatment duration of 12 hrs.

Fig. 5 demonstrates and Table 1 summarizes the comparative efficacy of applying TFN via buried drip irrigation water for 12 hrs. to either moist or saturated soils. It is clear that the zone of efficacy in moist, but not water saturated soils, is limited with TFN concentrations of 2 ppm being attained over radii of 1.5 to 6 cm from the emitter. For previously water saturated soils, the protective radii increase to from 13.5 to 18 cm from individual emitters.

 

12 hour test
Radius from the emitter Kauai Soil
(cm)
Radius from the emitter Palouse Soil
(cm)
30% Field Capacity
1.5
6.0
90% Field Capacity
13.5
18.0

Table 1. Effective soil radius within which soil TFN concentrations are greater than 2 ppm.

Breakthrough volumes and the concentration of TFN in column effluents were measured, and are tabulated in Table 2. The volume of column leachate increases with both initial soil moisture levels, and with duration of irrigation, as would be expected. The effect of soil moisture content on TFN break through from columns is evident in those soils at near water saturation prior to the irrigation cycle. This break through results from reduced sorption to soils at high moisture levels, and represents the primary mode for increasing soil coverage in the Gowan–5 treatment system. Conclusions
The Gowan–5 approach to broad area weed control using buried drip emitters has many complicating facets. From the data set generated, it is clear that application under saturated soil moisture conditions yields greater soil mobility of TFN (Figures 1–4). The soil profiles for 12 hr application scenario clearly show that protective TFN coverage/emitter is less than 9 cm for dry soils and less than 22 cm for water saturated soils. In each case however, soil TFN concentrations in the emitter profile are well beyond those levels supported by crop residue studies.

 

Soil Type
Soil Moisture
(% Field Capacity)
Irrigation Time
(hrs.)
Leachate Volume
(mL)
TFN Concentration
(ppm)
Kauai
30
12
0
0
   
24
245
0
   
48
1440
0
 
90
12
220
0
   
24
540
0
   
48
1880
1.14
Palouse
30
12
40
0
   
24
340
0
   
48
1200
0
 
90
12
210
0.5
   
24
550
1.53
   
48
1380
8.42

Table 2. Column leachate volumes, and TFN concentrations in leachates based on soil type and irrigation regime.

The increased mobility resulting from application into saturated soil conditions results in break through of TFN from columns (Table 2), which indicates that the normal high degree of soil sorption for TFN, which makes it an effective and environmentally safe pre-emergence herbicide, is circumvented by application to soils which are at or near water saturation. Mobility becomes troublesome from the standpoint of potential surface water contamination under higher soil water conditions, particularly in light, non-clay soils. Soil characteristics will also have a significant influence on TFN mobility and soil accumulation levels, and thus application methods will be quite difficult to regiment.

In all treatments studied, the actual and potential for accumulation of TFN in soil exceeds the application rates for which U.S. EPA has residue data. Concentration of TFN in those zones adjacent to the emitters range from 50 to 1400 ppm. For those crops where tested application rates are available, residue data are near the acceptable limits and increase soil loading rates (50–1400 ppm) can not be tolerated. Thus, situations where the Gowan–5 system results in high soil accumulation levels (› 10 ppm), food crops should not be grown due to the unavailability of supporting residue data.

The following general conclusion are indicated by the data generated:
1. The channeling seen in column data sets under water saturating conditions (notice unexpected dips in TFN conc.) would likely also occur in water saturated soils and result in hot and cold spots.
2. Column break through results indicate that a significant risk of water contamination may occur when applying this treatment system to water saturated soils.
3. The soil accumulations noted under both soil and both water treatments leads to TFN soil concentration (50-1400 ppm) that are so elevated that crops should not be grown under current U.S. EPA guidelines.
4. Therefor the addition of TFN to irrigation water as part of the irrigation cycle is not an acceptable practice and it is essential that both the maximum quantity of TFN and the application procedures be followed exactly as per the label.

Gowan Trifluralin 5 is the only grade of TFN which is registered with the EPA for this purpose, and may be used for most agricultural crops.

III SLOW RELEASE OF TFN FROM THE EMITTER ITSELF – BATTELLE PROCESS / ROOTGUARD®
At the Fourth International Micro-Irrigation Congress at Albury–Wodonga in October 1988 a paper titled Protection of buried drip irrigation devices from root intrusion through slow-release herbicides, (Van Voris) was presented. Field experience in the ten years since this presentation, has shown the results disclosed there–in to be valid.

Unlike the other two methods of applying the TFN to the water, the significant part of the ROOTGUARD process consists of a continuous slow release of the TFN from the plastic polymer into the soil whether the irrigation water is running or not. The TFN is released extremely slowly in the vapor phase and fixes in the soil immediately adjacent to the emitter. This zone of TFN treated soil creates a vapor back–pressure preventing the further release from the emitter, thereby reducing the amount of TFN in the soil, restricting the volume of treated soil, and increasing the life of the ROOTGUARD protection. Empirical evidence from the field indicates that this factor could account for as much as a five fold increase in life expectancy of the protection. Based upon this evidence the licensees of the ROOTGUARD technology now use a conservative factor of 2.4 times.

ROOTGUARD products are exempt from EPA registration but may not be used on root-crops.

IV ADDITION OF TFN TO THE IRRIGATION WATER FROM A FILTER
In 1996 Netafim introduced their ¾" and 1" disk filters (Techfilter) with TFN impregnated into the disks thereby releasing TFN into the irrigation water every time the system is operated. The instructions for use of the filter state that the herbicide bearing disks must be replaced every 2,400 hours of operation but not longer than two years of service.

The Techfilter is exempt from EPA registration however"Š.. the exemption is not applicable to that product when it is sold separately from the irrigation system it is designed to protect" (USEPA letter).

V A COMPARATIVE STUDY BETWEEN

  • the addition of Gowan TFN 5 directly to the irrigation water, and
  • the addition of TFN to the irrigation water from a slow release filter – "Techfilter", and
  • the incorporation of TFN into the emitter itself – "ROOTGUARD".

    • These three methods all use TFN as a barrier. A comparative analysis of the expected residual TFN concentrations in the soil of the three methods as applied for 200 emitters is presented. Gowan Procedure for addition of TFN to the irrigation water:
    Gowan TFN 5 label
    NOTE: This section is not in metric to be consistent with the US EPA registered label.

    EPA Reg. No.10163-99 EPA Est. No. 67545-AZ–l – See bottom of page 12 column 1 of the label:
    Subsurface drip applications – (Timing) – charge the irrigation system, begin application of Gowan TFN 5 immediately after all emitter points are functional. Shut off the irrigation system immediately after following completion of the Gowan TFN 5 application, allowing the TFN to bond to the treated soil. Resume irrigation 4 to 8 hours after the TFN application. (Rate Calculation) – apply 1.8 to 3.2 pints of Gowan TFN 5 per treated acre. The treated acreage is defined as the square footage area wetted by the irrigation system during the application period. ŠŠŠŠ In this type of application it is usually desirable to treat only 2 to 4 square inches around each irrigation emitter to prevent breakthrough weeds.

    1. Treated area per emitter = A, A =2 to 4 square inches Example: Take 4 square inches
    2. The area in square feet wet =B B = (A x (# emitters)) / 144 ,br> Example: If there are 200 emitters, then B = (4 x 200 )/ 144 = 5.556 square feet wetted
    3. The total area (in acres) wet by the system = C
    Example: C =5.556 / 43,560 = total acreage wetted by system = 0.0001275 acres
    4. C x (desired acre rate of TFN 5) = amount to be injected into the system.
    Example: If desired rate is 3.2 pints per acre, then 3.2 pints/acre x 0.0001275 acres = 0.000408 pints TFN 5 are to be injected into the 200 dripper irrigation system.
    5. TFN 5 contains 5 lbs. of TFN per gallon (from the label) i.e. 10 oz. per pint.
    Example: 0.000408 pints contains 0.00408 oz. of TFN
    Therefore the maximum amount of TFN applied per annum using the Gowan standard is 0.00408 oz. for the 200 dripper system. Correctly applied this is an effective dose to prevent root intrusion.

    ROOTGUARD/ Battelle Process:
    The spread sheet below shows some the various products manufactured using the Battelle process and the stable residual amount of TFN in the soil. The residual varies from 0.00423 to 0.006703 oz. in a 200 emitter system.

     

     
    Inline
    InPipe
    P/C
    Wt. of TFN per emitter (gms.)
    0.037
    0.115
    0.033
    Presume 95% used –amount used (gms.)
    0.03515
    0.10925
    0.03135
    Expected life at 20 deg. C (years)
    20
    42
    20
    Release rate presuming uniform rate over life (gms./day)
    4.82E-06
    7.13E-06
    4.29E-06
    Conc. in soil at each emitter at the end of 3.5 years (gms.)
    0.000642
    0.000950
    0.000573
    Amount in soil in a 200 emitter system (gms.)
    0.128402
    0.190041
    0.11452
    Amount in soil in a 200 emitter system (ozs.)
    0.004529
    0.006703
    0.00423

    This demonstrates that the ROOTGUARD/Battelle Process results are very similar to the February 1995 Gowan label application rates.

    From tests by Battelle:
    1) The elements which hold the TFN weigh 283.29 gms.
    2) The concentration is 20.9%
    Then the amount of TFN in the filter is 59.23 gms. = 2.09 oz.
    Released over two years = 1.04 oz.. per annum
    This is 255 times the amount permitted under the Gowan label.

    In the graph below Dr. Gerstl demonstrates the excess amount of TFN applied by Techfilter at all flow rates as compared to the Rootguard/Battelle Process as used by Geoflow. After adjustment for the method of test used by Dr. Zev Gerstl (Gerstl) to allow for the fact that the Battelle Process releases continuously into the soil in the vapor phase and thereby sets up a negative vapor pressure which slows down the process, the data from his study is consistent with the data from the Battelle studies used throughout this paper.

    From: Study to Compare the Release of TFN into Irrigation Systems for the Purpose of Root Intrusion Prevention. by Dr. Zev Gerstl, Volcani Center, Israel, under contract with Netafim. REFERENCES
    Jorgenson Greg, Solomon K., Evaluating Subsurface Drip Irrigation for Turfgrass: An Interim Report. 1990 Annual Conf. Of the Am. Soc. Of Irr. Cons., Oct. 26-29, Scottsdale, AZ.
    Ruskin R., P. Van Voris and D. A. Cataldo, Technical analysis of the movement of TFN in the soil when applied through a subsurface drip irrigation system, 1997 ASAE Annual International Meeting, Minneapolis Convention Center, Minneapolis, Minnesota, Aug. 10-14, 1997
    Center for Irrigation Technology, California State University, Fresno, Subsurface Drip Irrigation for Turfgrass: Emitter Observations, CIT Release 6/94
    Bui W., Performance of "Turbo Model " Drip Irrigation Tubes, Published as Paper # 704 in the Journal Series of the Experimental Station, Hawaiian Sugar Planters' Association.
    Gerstl Z. Institute of Soils and Water, Volcani Center, Israel. Study to Compare the Release of TFN into Irrigation Systems for the Purpose of Root Intrusion Prevention.
    United States Environmental Protection Agency certified letter dated June 2, 1997