Electronic Soil Moisture Measurements in Federal Insecticide

Chapter 10

Downloaded by UNIV OF MISSOURI COLUMBIA on January 6, 2018 | http://pubs.acs.org Publication Date: December 15, 2002 | doi: 10.1021/bk-2002-0842.ch010

Electronic Soil Moisture Measurements in Federal Insecticide, Fungicide, and Rodenticide Act Field Dissipation and Prospective Groundwater Studies 1

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Nathan J. Snyder , Juliet M. Cartron , Ian van Wesenbeeck , 1

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Les S. Carver , and Amy M. Ritter 1

Waterborne Environmental, Inc., 897B Harrison Street, Leesburg, VA 20175 Greenhorne and O ' M a r a , Inc., 9001 Edmonston Road, Greenbelt, MD 20770 Dow AgroSciences, Building 306, A2, 9330 Zionsville Road, Indianapolis, IN 46268-1053 2

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The draft United States Environmental Protection Agency (USEPA) guidelines for Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) Terrestrial Field Dissipation and Prospective Ground-Water (PGW) studies have soil-moisture monitoring requirements. The measurement methodology, frequency, and locations are at the discretion of the registrant. Time Domain Reflectometry (TDR) was used to monitor soil moisture at two PGW study sites, conducted to fulfill part of the registration requirements for a soybean herbicide in the United States. Measurements of soil moisture were continuous from the soil surface to a 3.6-m depth, close to the water table, and recorded hourly. A bromide tracer was applied and transport monitored through the laboratory analysis of soil and soil pore-water samples. A calibrated model of the system hydrology was developed and used to predict vadose zone leaching, ground-water recharge, and tracer movement. Model predictions were compared with field observations. The water and tracer movement predictions, made possible with intensive monitoring, contribute to the understanding of test system dynamics for both dissipation and PGW studies and provide valuable insight to the field conditions between the discrete sampling events. © 2003 American Chemical Society

Arthur et al.; Terrestrial Field Dissipation Studies ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

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Introduction

The draft U S E P A guidelines for FIFRA Terrestrial Field Dissipation and Prospective Ground-Water (PGW) studies include soil-moisture monitoring requirements to better understand the study sites hydrology and potential test substance movement (1, 2). The draft Terrestrial Field Dissipation study guidelines state that "The soil water content can affect the mode of degradation, degree of microbial activity, potential for volatization, plant growth, and potential for movement (up or down in the soil profile). In order to interpret routes and patterns of dissipation of the test substance, the soil-water content needs to be measured on a regular basis to adequately determine thefluxof soil water. Various methods of measuring soil water include tensiometers, time domain reflectometry (TDR), neutron probes, gypsum blocks, and direct measurement of the moisture content of the soil samples" (/). The draft P G W guidelines state that "soil water content throughout the site should be measured at least monthly (2). The guidelines specifically suggest that soil-moisture measurements are important for determining the study site water balance, water movement, and solute transport. Measurements should be made near other instrumentation and at least when samples are collected from lysimeters. Instrumentation as listed with the dissipation guidelines are appropriate in P G W studies. The two study guidelines do not explicitly state that continuous monitoring is essential, although soil-moisture changes are a dynamic process in the agricultural environment and continuous monitoring provides a dramatically different view of the soil environment than could be obtained through discrete sampling methods. The rapid fluctuations in moisture content are evident in the continuous moisture monitoring data presented in the case studies. Two general methods are listed as options for monitoring soil moisture, those that measure potential and those that measure water content. Soil water potential methods include watermark sensors, gypsum block, or tensiometers. Soil water potential measurements require a water content characterization curve to obtain volumetric water content. The characterization curves are soil specific and change in drying and wetting conditions, making direct application of the collected potential data in models or other uses difficult. Time Domain Reflectometry (TDR) and several other technologies offer measurement of soil mosisture content directly. This technology enables continuous, remote monitoring at many depths with reasonable accuracy without calibration.


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The case studies presented in this paper present data obtained using T D R technology for soil-moisture measurement. The data presented will make clear that non-continuous monitoring would provide an incomplete picture of an agricultural site receiving rainfall and irrigation water inputs simultaneously with rapid leaching and significant évapotranspiration. The collected data are used to calibrate the hydrology component of the Pesticide Root Zone Model ( P R Z M , version 3.12) for each site. The calibrated model is used to predict potassium bromide tracer movement which is compared to field observations.

Objective The objective of this paper is to present the experiences of the authors in implementing the moisture monitoring requirements of the draft Terrestrial Field Dissipation and P G W study guidelines. Instrumentation considerations are discussed including the sensor selection, location, and installation in relation to the two case studies. The case studies illustrate the use of the data in model calibration and the overall understanding of water dynamics at a field study site. This paper is not intended to fully cover all available options for monitoring and modeling or to discuss the theory behind the instrumentation, modeling, or water movement in general.

Case Studies

Methods Two P G W studies were initiated in the spring of 1999 as part of the registration requirements for a soybean herbicide. Following detailed site characterization, the two sites were instrumented with suction lysimeters, monitoring wells, pipe lysimeters, meteorological monitoring equipment, and a soil-moisture and water-level monitoring system. Each site received applications of the test substance and a conservative potassium bromide tracer as pre-plant bare ground broadcast spray with light incorporation.

PGW Study Site Description The Southeastern U S site is located in North Carolina, on soils identified as Conetoe, sandy loam (loamy, mixed, thermic Arenic Hapludults). The


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Midwestern U S site is located in Indiana, on soils identified as Tyner, loamy sand (mixed, mesic Typic Udipsamments). Both sites receive irrigation from center pivot systems. Pore-water Br" analysis was performed on samples from suction lysimeters installed at depths 90, 180, 270 cm at both sites with an additional 365 cm depth at the Indiana site. Soil Br" analysis was performed on samples collected from a 0-60 cm depth in 15 cm increments. Characteristics of the two sites follow: NORTH CAROLINA Conetoe sandy loam Arenic Hapludults • Well drained • Moderately rapid permeability • Formed on Atlantic Coastal Plain stream and low marine terraces Annual Precipitation 130 cm Depth to water 2.6 to 3.8 m K B r application 126 kg/ha

INDIANA Tyner loamy sand Typic Udipsamments • Excessively drained • Rapid permeability • Formed on Wisconsinan age sandy outwash plains and terraces Annual Precipitation 102 cm Depth to water 4.4 to 5.0 m K B r application 153 kg/ha

Moisture Monitoring Instrumentation Identical on-site automated weather stations and soil-moisture monitoring systems (Environmental Sensors, Inc., Moisture Point™ TDR, Victoria, B C , Canada) were installed by Waterborne Environmental, Inc. (Leesburg, V A ) , at each study site along the edge and just outside of the treated area (Figure 1). The weather station/TDR system measures and records soil-moisture as well as meteorological parameters. A Campbell Scientific Inc. (Logan, U T ) CR10X data logger is utilized for multiplexer control and data storage. Meteorological sensors are wired directly to die CR10X data logger and measurements are taken every 15 seconds and recorded hourly. Meteorological measurements include precipitation, solar radiation, wind speed and direction, air and soil temperature, and relative humidity. A solar panel, sized appropriately for each location, was installed to ensure a continuously charged battery supply to the data logger and Moisture Point™ system. Data were downloaded remotely on a regular basis via modem and cellular phone. Soil-moisture measurements were made at three locations along the edge of the treated area (Figure 1). The T D R system measures and records volumetric water content hourly from a series of nine T D R profiling probes. The Moisture Point™ profiling probe system employs conventional T D R technology with a


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Site Layout Schematic

Single Location T D R Probe: Profile View


Wires To Data logger System TDR Probe Cluster Borehole

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Datalogger Enclosure ο

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Meteorological Instrument Post

Electronic Soil Moisture Measurements in Federal Insecticide

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