PROTECTION OF BURIED DRIP IRRIGATION DEVICES

FROM ROOT INTRUSION THROUGH SLOW-RELEASE HERBICIDES

P. Van Voris¹, D.A. Cataldo ², and R. Ruskin³

This paper was published in the Proceedings of the Fourth International Micro-Irrigation Congress at Albury-Wadonga, Australia, October 23-28, 1988.

INTRODUCTION

Controlling plant root intrusion into belowground areas has been a subject of research at Battelle's Pacific Northwest Laboratories (BNW) since 1978. Original efforts were directed toward developing "chemical barriers" that would prevent plant roots from penetrating belowground repositories for buried nuclear and chemical wastes. Two basic requirements were imposed on development of such a system. First, the chemical herbicide inhibitor had to be environmentally safe and acceptable. Thus, it had to have a reasonably short environmental half-life, exhibit little soil mobility (which would prevent any contamination of surface and/or groundwaters), and limit plant root growth without adversely affecting the growth of aboveground biomass. Inherent in these criteria was the requirement for a method that would allow for controlled delivery from a protected reservoir. Second, since a buried system was required, the cost of placement dictated that it be bioactive for an extended period of time (10 to 100 years).

The joint venture between AGRIFIM Irrigation International (AGRIFIM) and BNW is best described as one of technology transfer, specifically the transfer of "Biobarrier Technology" developed to address nuclear waste management problems (Cline et al. 1982a; 1982b; Burton et al. 1983; 1986). The Biobarrier Technology is based on the principle of long-term controlled-release by means of a polymeric carrier delivery (PCD) system. The PCD system acts as a reservoir for the pure chemical, protecting it from chemical and biological degradation, while providing a method for controlled release. Thus, the bioactive chemical is released slowly in a controlled manner to the soil adjacent to the device. This PCD system provides a means of maintaining an effective dose for a substantial period of time in contrast to single application methods, which result in higher than necessary concentrations immediately after treatment followed by rapid degradation to a level below the minimum effective dose required for control (figure 1).

Ongoin g research at BNW has resulted in the development and production of a series of sustained release, polymeric carrier delivery Systems using trifluralin as the root-growth inhibiting chemical. These devices, which were developed for belowground burial, could be designed to protect a 1- to 2-inch soil horizon from root penetration for periods from 7 to 125 years (Cline et al. 1982b). This concept is currently being applied, under license from Battelle Memorial Institute, to produce and market a line of Biobarrier geotextile cloth products for commercial, industrial, and landscape use. In addition, a license has been negotiated for the production of Root-Shield, a polyisoprene sewer gaskets, incorporating this concept which will protect sewer lines from plant root intrusion for up to 50 years. Numerous other applications and products are currently being developed for commercialization using this technology. The lack of environmental concerns related to these applications has led to the rapid approval and issuance of an extended-use label by the U.S. Environmental Protection Agency.

Drip irrigation devices, such as those currently marketed by AGRIFIM and shown in Figure 2, are finding increased use in agricultural and landscape applications where water conservation is required, reduced energy use is mandated by use of lower pressures, or when long-term salinization of soils would result from extended sprinkler irrigation. However, there is one drawback if the drip emitters are to be used in a buried application The disadvantage is that the emitter uses a method for flow and/or pressure control, which is usually based on a restriction or tortuous path, and it is this point of constriction that is often subject to intrusion by fine plant roots.

The problem of root intrusion into drip emitter orifices can readily be resolved by adapting already developed sustained-release technology. Under license from Battelle Memorial Institute, AGRIFIM is producing an in-line drip emitter incorporating Biobarrier Technology. The goal of this joint venture was to develop an emitter that is protected from root penetration for a minimum of 5 to 15 years in buried applications.

The purposes of these research efforts were to test and evaluate new product lines incorporating Biobarrier Technology. Several aspects of "In-Line Drip Emitter" configuration and performance were addressed, these included:

1. total content of trifluralin contained in the emitter
2. rates of release based upon temperature, and
3. calculations of the functional life expectancy of the device with respect to inhibit root intrusion.

RESULTS AND DISCUSSION

Herbicide Incorporated Into the Emitter

The family of chemicals known as dinitroanilines, of which trifluralin (i.e., Treflan®) is one of the prime members, inhibits the division of cells at the end of the root tip in such a way that the root is unable to grow. It is effective, through direct contact and in vapor phase within soils, on both grasses and broadleaf plants. It is not known to bioaccumulate in plants (i.e., it is not systemic); thus it will not be transported through the food chain to wildlife, domestic animals, or humans. In the application of trifluralin for protection of buried drip irrigation, the growth of the aboveground portion of trees, shrubs, or grasses will not be adversely affected except through the indirect mechanism of limiting root mass. Trifluralin exhibits strong soil binding characteristics, and with an environmental half-life of approximately 60-days, will not be mobile and/or accumulate in the environment. These attributes make trifluralin uniquely suited for long-term sustained release applications.

Reservoir Size and Release Rates of the In-line Drip Emitters

The functional performance, or useful root-growth inhibiting life of "In-Line Emitters" incorporated with several levels of trifluralin, were evaluated based on the total content or reservoir of trifluralin within the emitter and the temperature dependent release rate of trifluralin from the devices.

Flowing diffusion cells were employed to obtain the rates of release from individual devices at three temperatures, namely 12⁰C +1⁰C, 25⁰C+1⁰C, and 39⁰C +1⁰C. Rates of release, as presented in Table 1, were measured at each temperature until a steady-state release is achieve Near-steady-state release rates for the lowest trifluralin concentration device decreased from 11.4 to 4.8 m g trifluralin/day/device, at 39⁰C and 26⁰C, respectively. No Treflan® was detected in eluents at 13⁰C, and it is assumed that trifluralin was below our detection limits of 5 ppb. The medium percent trifluralin concentration devices exhibited release rates of 49.8, 7.4, and 0.5 m g trifluralin/day/device at the three temperatures, respectively. Release rates for the highest percent concentration devices decreased from 110 to 1.3 m g trifluralin/day/device at 38⁰C and 13⁰C, respectively.

Table 1. Temperature-dependent Release Rates and Equilibration Time for In-Line Drip Emitters Containing Low, Medium, and High Percent (w/w) Trifluralin (Avg±sd, n=3)

Figure 3 represents a plot of data for the highest trifluralin concentration devices tabulated in Table 1. The response of release rate to temperature is marked; rates at 38⁰C are 100 m g trifluralin/device/day and decrease to approximately 1 m g trifluralin/ device/day at 12⁰C. Results for the medium and low concentration devices exhibited proportional trends in release rate. A comparison of emitter trifluralin content to release rate for the three temperatures was also performed. These data clearly show that while both are important, the influence of temperature on release is much greater than the emitter trifluralin content For instance, release rates at 38⁰C are directly related to the concentration of trifluralin in all devices. However, at 25⁰C the lowest concentration device exhibits a noticeable deviation from linearity; it shows no effective release at 12⁰C. The importance of these rate data will be discussed shortly in relation to performance.

Performance and Life Expectancy of Root-Growth-Inhibiting Drip Emitter

Two approaches are used to assess the useful life of trifluralin-incorporated drip irrigation devices. The most conservative is simply to estimate the number of days a release rate can be sustained, based on the total trifluralin content of a device. This minimum estimation is referred to as the shortest effective life (SEL). For the devices containing low, medium, and high percent concentrations of trifluralin, longevity (SEL) ranges from 39 to 49 years at 13⁰C. At 26 ⁰C, longevity is estimated at 4.8 to 9.1 years, depending on the concentration in the devices. This decrease in longevity with increasing temperature would indicate that at elevated temperatures the vapor pressure of the trifluralin is more important than diffusion through the matrix of the polymer, and may possibly be due to the comparatively short length of the diffusion path in these devices.

In practice the SEL calculation is conservative since it neither allows for the build-up of trifluralin in the soil adjacent to the emitter which reduces the rate of release, nor adjusts for the seasonal temperature fluctuations. Based on other trifluralin- incorporated devices which have been field tested over a seven-year period, the minimum effective life (SEL) of the devices can be increased by a factor of 2.4 to obtain the maximum effective life (MEL). Thus, at 15⁰C, the typical soil temperature at 6 to 9 inches below the surface in many areas of the world, the MEL for the medium and high concentration devices could be as long as 60 to 70 years, respectively. However, since these devices are employed in an irrigation application where they will be in contact with flowing water, which will reduce the gradient at the surface of the device, the MEL is expected to be roughly 60 to 70% of that calculated; however well wihin the stated objective of 5 to 15 years of root penetration protection.

While the longevity results for the trifluralin incorporated devices indicates that a sustained release can be maintained for periods of time well beyond that required, a second factor must be considered. This factor is the maintenance of an effective root-growth-inhibiting soil concentration of trifluralin immediately adjacent to the devices. Currently, based on soil sorption data and the soil half-life of trifluralin (approximately 60 days), the minimum release rate to soil should be approximately 0.4 to 0.7 ug/day, which the medium and high concentration devices meet or exceed This rate permits a localized build-up of trifluralin in the soil immediately adjacent to the emitter orifice, and thus affords protection against root intrusion, based on the conservative need for a soil concentration of 5 ppm for root inhibition.

CONCLUSIONS

Recommended Concentration of Trifluralin in Emitters

The in vitro performance evaluations of the three trifluralin-incorporated ^"In-Line Drip Emitter"configurations exhibit the expected temperature release rate responses characteristic of these sustained release delivery systems. The SELs of the medium and high percent concentration devices were determined to be 39 and 44 years at 13⁰C, and 4.8 and 6.6 years at 25⁰C. These are considered conservative estimates for these temperatures. The MELs based on soil/emitter diffusion gradients and lower seasonal temperature excursions will be longer, but we recommend not using the 2.4 correction factor pending further studies. The lowest percent trifluralin containing device does not perform in a reliable fashion and should not be considered

At soil temperatures of 15⁰C, both the medium and high percent trifluralin devices will meet the "Protective Threshold" values to protect the emitters from root intrusion

1 Senior Program Manager, Earth & Environmental Sciences Center, Battelle, Pacific Northwest Laboratories, Richland, Washington, USA.

2 Staff Scientist, Environmental Sciences Department, Battelle, Pacific Northwest Laboratories, Richland, Washington, USA.

3 Chairman, Agrifim Irrigation International, N.V., San Francisco, California, USA.

REFERENCES

Burton, F. G., D. A. Cataldo, J. F. Cline, and W. E. Skiens. 1983. "The Use of Controlled Release Herbicides in Waste Burial Sites." In Controlled Release Delivery Systems, eds. T.J. Roseman and S. Z. Mansdoff, pp.291-300. Marcel Dekker, Inc., New York.

Burton, F. D., W. E. Skiens, 3. F. Cline, D. A. Cataldo, and P. Van Voris. 1986. "A Controlled-Release Herbicide Device for Multiple-Year Control of Roots at Waste Burial Sites." J. Controlled Release 3:47-54.

Cline, J. F., D.A. Cataldo, W.E. Skiens and F.D. Burton. 1982a. "Biobarriers Used in Shallow Burial Ground Stabilization." Nucl. Technol. 58:150-153.

Cline, J. F., F. D. Burton, D. A. Cataldo, W. E. Skiens, and K. A. Gano. 1982b. Long-Term Biobarriers to Plant and Animal Intrusions of Uranium Tailings. PNL-4340, Pacific Northwest Laboratory, Richland, Washington. 70 pp.