Field evaluation of passive capillary samplers in monitoring the leaching of agrochemicals

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AN ABSTRACT OF THE THESIS OF
Florian M. Brandi -Dohrn for the degree of Master of Science in Bioresource
Engineering presented on November 17, 1993.
Title: Field Evaluation of Passive Capillary Samplers in Monitoring the Leaching of
Agrochemicals
Redacted for Privacy
Abstract approved:
John S. Selker
Soil solution samplers have certain inadequacies that limit their range of
possible applications. Passive Capillary Samplers (PCAPS), which apply suction to the
soil pore-water via a fiber glass wick, have shown promising results in preliminary
experiments in regard to collection efficiency of water and of bromide tracers. The
objectives of this study were to evaluate PCAPS under non-steady state field conditions
with respect to (1) effect of installation procedure and operational characteristics, (2)
ability to estimate the soil-water flux, and (3) ability to estimate the mean concentration
of agrochemicals. At the same time, samplers were used to (4) evaluate the effect of a
cereal rye (Secale cereale (L.)) cover crop on NO3- leaching.
Thirty-two PCAPS and 32 suction cup samplers were installed below the root
zone at a depth of 120 cm in a Willamette Variant loam wet soil (fine loamy mixed
mesic Pachic Ultic Argixeroll). Samplers were installed in an ongoing cover crop/crop
rotation study. Regarding overall performance, flux measurements were within 20 %
of the native values as determined by a water balance. The air release from the sample
bottles was a point of concern and might have slowed down the sampling rate. The
installation procedure introduced bias into volume and concentration measurements of
the part of the PCAPS closest to the refilled trench. The leachate concentration as
calculated using the arithmetic mean of suction cup sampler measurements holds a
significant bias, deviating by up to 97 % for bromide concentrations. Phosphate was
not detected by the suction cup samplers indicating that ceramic cups should not be
used for phosphate sampling. Matrix and preferential flow could clearly be
distinguished using the PCAPS, showing that PCAPS are a valuable tool to assess the
hydrology and solute transport mechanisms of a field site.
The cover crop reduced NO3--N leaching significantly at the recommended
N rate as evaluated by PCAPS. The cover crop reduced the seasonal mean NO3--N
concentration at the recommended N rate from 13.5 mg LI to 8.1 mg LI, which is
under the E.P.A. drinldng water quality standard of 10 mg 1.-1. The total NO3--N mass
lost under the fallow treatment at the recommended N rate was 48 kg N ha-' which
compares to 32 kg N ha-' under the cover crop treatment. Given the increasing
problems with nitrate contamination of ground water, programs to support the
cultivation of catch crops in conjunction with nitrogen soil testing should be considered
as a relatively easy, effective, and biologically sound means to reduce nitrate
concentrations in the recharge to the ground water in agricultural settings.
Field Evaluation of Passive Capillary Samplers in Monitoring the
Leaching of Agrochemicals
by
Florian M. Brandi -Dohrn
A THESIS
submitted to
Oregon State University
in partial fulfillment of
the requirements for the
degree of
Master of Science
Completed November 17, 1993
Commencement June 1994
APPROVED:
i
1
Redacted for Privacy
Assistant Professor for Bioresource Engineering in charge of major
Redacted for Privacy
Head of department of Bioresource Engineering
Redacted for Privacy
Q
Dean of Gradu hool
Date thesis is presented November 17. 1993
Typed by Florian Brandi-Dohrn.
Acknowledgments
I would like to thank in the first place Dr. John S. Selker, my advisor. His
friendly, uncomplicated, and enthusiastic ways made it a lot of fun working with himon
this project. I really appreciate his friendship, his patience in regard to my never-ending
questions, his creative help in many matters, and, last not least, his financial support. I
also would like to thank the Water Resources Research Institute for its financial
support of this project.
Special thanks to my committee members Dr. Marshall J. English, Dr. Herb
Huddleston, Dr. Richard Dick, and Dr. William B. Shepard for their advice.
Many sincere thanks to everybody, who helped in one way or another to make
this project possible. I especially thank Bob Christ for his invaluable help with the
sample collection; Joan Sandeno for the help with the ion chromatograph; Ming-Che
Wang for his assistance on the project; Bob Mittelstadt, Jeff Lawrence, Seong-Baeg
Lee, Winnie Zhang, Alex McIntosh, Gunther Gassier, Nick Rinard, and Mario Hess for
their unforgettable work in the field; James Hartmann and Bob Schneckenburger for
their great way of teaching and explaining to me the magic of a machine work shop;
Alex McIntosh, Winnie Zhang, Ken Jones, Scott Gibbson, Megan Schneckenburger,
and Joel Hashimoto for their help in putting the samplers together; Bob
Schneckenburger for his very educational help in keeping all the computers running;
Doug Thomas and Troy McCoy for the help with the sample analysis and data entry;
Catherine Gallagher and Abdellatif Boussaid for proof-reading.
For all the discussions and brainstorming I thank my fellow students,
especially Shaun Kelly, with whom I had the most valuable discussionson bike rides,
John Knutson, with whom I had the best discussions not only in 'Squirrels', Bob
Mittelstadt, and Weidong Cao.
My great respect and thanks go to the entire staff and faculty of the
department of Bioresource Engineering, who always made me feel welcome and helped
me in many ways.
Especially, I would like to express my greatest appreciation, respect, and
warm thanks to my parents for their unquestioning support. The same applies to my
friends and room mates, whom I not only admire for their patience in listening to my
laments about how long it takes to write a thesis.
The very fact that we are seeking a thing usually stands in the way ofour finding it.
(African Proverb)
Table of Contents
Chapter I. Introduction
1
Chapter II. Materials and Methods 2
Characterization of Site 2
Soil Description 2
Climate 4
Management
5
PCAPS
8
Construction 8
Installation 10
Suction Cup Samplers
12
Construction 12
Installation 12
Tracer Application
13
Samples
14
Sample Collection 14
Analytical Procedures
14
Statistical Methods 16
Chapter III. Field Evaluation of Passive Capillary Samplers (PCAPS) 19
Introduction
19
Results
25
Operational Characteristics 25
Soil-Water Retention and K. 25
Wick Suction and Soil Matric Potential 28
Correlation Between Measurements of Single Bottles 30
Bias in Measurements of Bottle no.1 31
Breakthrough of the Sorbing Tracers 34
Estimation of Chemical Composition of Soil Solution 36
Difference between Flow-Weighted and Arithmetic Means of
Concentration 36
Comparison of Concentration Measurements as Obtained by
Suction Cup Samplers and PCAPS 43
Collection Efficiency of the PCAPS as Estimated with a Water Balance 49
Collection Efficiency of the PCAPS as Estimated with Bromide Mass
Recovery
51
Discussion
54
Trench Effect 54
PCAPS Collection Efficiency
55
Number of Samplers Required 55
Implications for the Selection of the Appropriate Sampler 56
Conclusion 58
Chapter IV. Effect of a Cereal Rye Cover Crop on NO3--N Leaching 60
Introduction 60
Results and Discussion
63
Statistical Independence of Measurements 63
Amount of Percolate
63
Effect of Cover Crop and N Application Rateon the NO3--N
Concentration of the Recharge to the Ground Water 69
Trend in Cover Crop Effect 70
NO3- -N Mass Loss Due to Leaching
71
Long-term use of Cover Crops
73
Phosphate Leaching
73
Conclusions 76
References
77
Appendix A 84
Table of Figures
Figure 1. Climatic data at the NWREC 1961 - 1991 (courtesy of the Oregon
Climate Service).
5
Figure 2. Layout of plot and placement of PCAPS (drawnto scale). 7
Figure 3. Crossectional view of PCAPS (drawn to scale). 9
Figure 4. Crossectional view of installed PCAPS (drawn to scale).
11
Figure 5. Standard curve for Rhodamine WT (x30 aperture) in the presence of
0 - 10 mg L-1 blue dye.
15
Figure 6. Standard curve for blue dye FD&C #1 (wavelength 630 nm).
16
Figure 7. Ring infiltrometer measurements and fitted Brutsaert model 27
Figure 8. Matric potential and flux at a depth of 114 cm in Willamette Variant
loam, wet.
28
Figure 9. Flux- matric potential relationship for braided 2.93-cm medium
density Amatex fiber glass wick (Knutson and Selker, 1993b) and for
the Willamette Variant loam, wet 30
Figure 10. Percent deviation of flux and bromide concentration in bottle no.1
from sampler mean.
33
Figure 11. Percent deviation of flux and bromide concentration in bottle no.3
from sampler mean.
34
Figure 12. Rhodamine WT and blue dye concentrations as observed in
32 PCAPS.
36
Figure 13. Correlation coefficient r between flux and normalized NO3--N
concentrations, and mean flux. 38
Figure 14. Correlation coefficient r between flux and bromideconcentrations,
and mean flux. 40
Figure 15. Arithmetic and flow-weightedmeans for bromide concentrations. 41
Figure 16. Arithmetic and flow-weightedmeans for Rhodamine WT and blue
dye. 43
Figure 17. Deviation of mean NO3--N concentration insuction cup samplers
from flow-weighted mean NO3--N concentration in PCAPS. 45
Figure 18. Histogram of bromide concentrations collected in suctioncup
samplers on day 221. 47
Figure 19. Deviation of mean bromide concentration in suction cup samplers
from flow-weighted mean bromide concentration in PCAPS. 47
Figure 20. Bromide breakthroughas observed by suction cup samplers and
PCAPS, not shifted and shifted according to approximatesample
collection time.
48
Figure 21. Mean bromide breakthrough and fitted convection-dispersion
equation (CDE) using CXTFIT
52
Figure 22. Average daily and cumulative infiltration for the plotsreceiving zero
N rate.
64