Groundwater Residue Sampling Design - American Chemical Society

Chapter 21

Techniques for Collecting Soil Samples in Field Research Studies

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Frank A. Norris, Russell L. Jones, S. Dwight Kirkland, and Terry E. Marquardt Rhone-Poulenc Ag Company, P.O. Box 12014, Research Triangle Park, NC 27709

A variety of techniques have been developed for collecting soil samples in unsaturated zone studies on the movement and degradation of agricultural chemicals. The appropriateness of a sampling technique will depend on the desired depth of the soil core, soil properties, and the location of the agricultural chemical residues in the soil profile. Determining the amount of soil residues immediately after a chemical has been sprayed on the soil surface is often difficult. Data from a study comparing several techniques for measuring the amount of an agricultural chemical sprayed onto the soil surface indicate that using filter paper disks or collecting soil samples using 77 mm diameter tubes provided the best measurement of the amount applied. Soil sampling is used in environmental fate studies to measure the amount of an agricultural chemical in the soil as a function of depth at a specific time. During the past decade, the nature of unsaturated zone environmental fate studies has expanded from sampling surface soils in simple dissipation studies to complete sampling of the soil profile down to the water table in groundwater research studies. The numbers of such studies, as well as the number of soil samples collected during a study, has been increasing rapidly. Therefore, much attention has been devoted to comparing existing soil sampling methods and making modifications to improve the performance of these methods. The choice of sampling techniques is dependent on the location of residues in the soil profile, the nature of the soil, and the properties of the agricultural chemical being studied. The objective of this paper is to discuss the advantages and problems of these sampling techniques and is an update of the information in a previous summary (l). The information in this paper is intended to apply only to relatively non-volatile compounds which are stable during the time required for sample

0097-6156/91/0465-0349$06.00/0 © 1991 American Chemical Society

In Groundwater Residue Sampling Design; Nash, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

350

GROUNDWATER RESIDUE SAMPLING DESIGN

collection. This paper is not a discussion of the design of studies which use soil sampling; this topic is discussed in a companion paper (Jones and Norris in this work).


Sampling Procedures A variety of sampling techniques have been used to collect soil cores in unsaturated zone studies of agricultural chemical residues. Basically the techniques can be divided into two types. One approach is to collect soil in intact cores or core segments. The other is to remove soil from a hole as it is deepened. Collection of soils in intact cores or core segments is usually performed by pushing sampling tubes or split spoon samplers into the soil. Depending on the diameter of the sampling tube and the sampling depth, these tubes are inserted manually, using hydraulic equipment, or sometimes hydraulically inside a hollow stem auger. The most common of the sampling devices which extract soil as the hole is deepened is the bucket auger. Another procedure has been to place a tube in the soil and then remove the soil in this tube by use of an implement such as a spoon. Another technique (2) uses a golf cup cutter. Sampling Tubes. The collection of soil samples in intact segments is probably the most widely used sampling technique. In general, sample tubes used to collect soil samples in environmental fate studies should be greater than about 50 mm to insure a representative sample. This will provide more than the amount of soil usually required for an analysis. Sample tubes of this diameter may be inserted manually to a depth of about 0.5 m, or with rather simple equipment down to a depth of 1 to 2 m. Below this depth drilling equipment (hollow stem auger and split spoon sampler) is usually required. The cost and elapsed time required to collect samples using drilling equipment usually makes collection of more than a few cores impractical. Regardless of the equipment used to insert the sampling tube, each tube should be carefully washed prior to each use. Sampling tubes are usually used to collect an entire soil core which is then subdivided into segments of the desired depth increments. Two problems can arise as a result. First, soil near the top of the core is often spread along the sides of the tube as the tube is pressed downward. This has been confirmed by the observation of pesticide granules along the side of the tube, and also in recent dye studies (V. Clay, Mobay, personal communication, 1989). The movement of surface soil along the sides of the sampling tube has been shown to result in contamination of deeper increments, especially when residues at the soil surface are relatively high (RhonePoulenc, unpublished data). One approach to eliminating this contamination when different depth increments are being collected in an single intact sample core is to remove the outside of the entire core, perhaps after freezing. Another procedure is to collect the first depth increment, enlarge the hole diameter, and insert a lining to prevent surface soil from falling into the hole prior to the collection of the remaining



21. NORRISETAL.

Collecting Soil Samples in Field Research Studies 351

depth increments. The other problem associated with sampling tubes is that the sample core length in the tube may be either longer or shorter than the actual depth of the hole as a result of compaction or expansion, making it difficult to divide the soil core properly into the desired depth increments. This problem is most likely to arise when soil texture changes or rock is encountered in the soil profile. Because of possible of cross-contamination between depth increments from a single intact core and compaction or expansion of the core, the authors' preference is to re-insert the tube into the hole for each depth increment with the upper 50 mm of soil from each tube (except for the uppermost sample from each hole) discarded to reduce any residues introduced into the hole during the raising and lowering of the auger. Bucket Augers. In field research studies involving the collection of soil samples at depths greater than about 1.2 m, bucket augers have been the most widely used collection technique. This technique has been used by the authors to manually collect samples as deep as 7.8 m in a variety of soils. Normally the authors use a conventional bucket auger, but in some coarse soils a sand bucket auger is needed to keep the sample from falling out of the bucket as it is raised out of the hole. A bucket auger technique should not be used to collect samples just after an agricultural chemical has been sprayed onto the surface of the soil, since the shape of the auger tends to push the upper 10 mm of soil towards the outside of the hole. The major concern with the use of a bucket auger has been the introduction of soil into the bottom of the hole during the raising and the lowering of the auger. This problem can be minimized by using only trained personnel to collect samples, discarding the upper 50 mm of each bucket (except the uppermost bucket from each core), and by using a clean auger for each sample. The authors clean augers using a detergent with ammonia and a toilet brush, followed by a clean water rinse. An advantage of the bucket auger is the ability to collect a larger diameter core (often an 83 mm diameter auger is used) than obtained with most sample tube procedures, perhaps resulting in a more representative sample. Other Excavation Techniques. These procedures, such as removal of soil inside a tube placed in the soil with an implement such as a spoon or the golf cutter approach (2), are usually restricted to no deeper than the upper 0.5 m. They may be followed by other sampling techniques such as tube samplers or bucket augers to obtain deeper samples. Sampling Immediately After Application Collection of soil cores immediately after application of an agricultural chemical requires special consideration. The purpose of the initial post-treatment sampling is to demonstrate that an application has been made and confirm the application rate. However, because of the variability of soil samples, an application rate based on the amount of material applied is usually more accurate than a rate determined by soil analyses. If the agricultural chemical is incorporated into the soil, then the same technique used to collect soil cores during the rest of the study is usually most


352



appropriate. However, if the chemical is sprayed or broadcast onto the soil surface, then a different sampling technique is usually required. In the authors experiences, the collection of soil samples just after a spray or broadcast application has often resulted in considerably less residues than would be predicted from the amount of chemical actually applied. To determine which sampling techniques are most appropriate, a study was performed in which 10 different techniques (Table I) were used to assess the amount of chemical applied. These techniques included one conventional soil core sampler, four bucket auger procedures, two sample tube techniques, two buried dish procedures, and one filter paper measurement procedure. Sand was used in some of these procedures to cover the soil surface (either before or after inserting the sampling device) because a previous study indicated that it would improve recovery. Twisting the auger while placing it onto the soil was tested also to see if this would minimize the outward movement of the upper 10 mm of soil due to the shape of the auger. The test was carried out on a Norfolk loamy sand soil at the Rhone-Poulenc research facility near Clayton, North Carolina. An application of aldoxycarb [2methyl-2-(methylsulfonyl)propionaldehyde 0-(methylcarbamoyl)oxime], chosen because aldoxycarb is easily analyzed, was made at a nominal rate of 3.36 kg ha"^ to a narrow strip of land approximately 50 m in length using a single pass of a tractormounted sprayer with four spray nozzles mounted 0.5 m apart. The treatment area was divided into four subplots each 10 m long, with a 5 m buffer on each end. In each subplot there were 40 sampling locations: 10 transects one meter apart, each with four spray positions. Two spray positions were located directly underneath the middle two spray nozzles and the other two spray positions were located halfway between spray nozzles. For each of the sampling techniques, one sample was collected from each of the four spray positions in each of the four subplots for a total of 16 samples per sampling technique. For each spray position, the order of the sampling techniques within a subplot was selected randomly. A subsequent analysis of the data indicated no significant variations in residue concentrations as a result of plot location. The individual analyses and averages are shown in Table II. Statistical analyses by a variety of tests show that the data is divided into two distinct groups. The first group with lower residues consisted of the standard core sampler and the four bucket auger methods. The second group consisted of the two buried dishes, the two sampling tubes, and the filter paper. Twisting the auger as it was placed in the soil may have slightly increased the recovery but the recovery was still not acceptable. Pouring sand over the soil did not seem to significantly increase recovery in any of the techniques. Therefore, the use of sampling tubes, buried dishes, or filter paper were the most acceptable procedures for determining the amount applied to soil. Although this study indicates that filter paper was a good method, other more recent experiments conducted by the authors with other agricultural chemicals have been less satisfactory, perhaps due to the chemical not being totally adsorbed, inability to extract the chemical from the paper, or degradation of the chemical on the paper.


21. NORRIS ET AL.



Table I. Techniques Used in Immediate Post-Application Sampling Study

1.

Conventional soil core sampler pressed to a depth of 80 mm.

2.

Bucket auger (83 mm diameter) placed on the ground before twisting to a depth of 80 mm.

3.

Bucket auger, same as 2, except auger was twisted as it was placed on the ground.

4.

Bucket auger, same as 3, except approximately 25 mm of soil was placed on top of soil in auger before withdrawing the auger from the hole.

5.

Bucket auger, same as 2, except approximately 25 mm of sand was placed on the sample site before augering.

6.

Plastic dish (80 mm in diameter, 80 mm high) buried flush with soil surface filled with soil removed from hole.

7.

Plastic dish, same as 6, except dish covered with a layer of sand prior to removal from ground.

8.

Copper tubing (77 mm inside diameter, 80 mm long) pressed into soil immediately after application.

9.

Copper tubing, same as 8, except approximately 25 mm of sand was placed on sample site before pressing the tube into the soil.

10.

Filter paper (Whatman Grade No. 1; 90 mm diameter) placed on ground surface prior to application.



Aldoxycarb Residues: Individual Analyses (mg) core 1 core 2 core 3 core 4 core 5 core 6 core 7 core 8 core 9 core 10 core 11 core 12 core 13 core 14 core 15 core 16 coefficent of variation (%) Average of Analyses (mg) (kg/ha)

Sample Diameter (mm)

Description:

Method No.:

0.99 0.94 1.07 1.14 0.75 0.80 0.83 0.78 1.19 0.86 1.25 0.98 1.17 0.78 0.85 1.13 18

0.97 1.56

0.10 2.05

89

Bucket Auger, no twist

2

0.140 0.099 0.095 0.100 0.081 0.160 0.080 0.130 0.110 0.100 0.084 0.130 0.098 0.086 0.093 0.073 24

25.4

Soil Core Sampler

1

1.10 1.77

1.24 1.59 1.13 1.19 0.81 0.80 0.90 0.91 0.91 1.30 1.22 1.64 0.97 1.37 0.74 0.90 25

89

Bucket Auger, twist

3

1.25 2.01

1.60 1.05 1.14 1.28 1.01 1.21 1.32 1.00 1.32 1.40 1.08 1.23 1.53 1.41 1.44 0.95 16

89

Bucket Auger, sand added

4

1.16 1.87

1.30 1.52 1.42 1.14 1.15 0.96 0.85 0.84 1.13 1.42 0.93 1.25 1.22 1.20 1.40 0.84 19

89

Bucket Auger, into sand

5

1.48 2.94

2.04 1.46 1.88 1.50 1.37 1.81 1.80 0.50 1.80 1.76 1.28 1.47 0.83 0.85 2.10 1.22 31

89

Buried Dish

6

1.41 2.80

1.75 1.23 1.36 1.45 1.31 1.62 1.48 1.47 1.22 1.31 1.53 1.56 1.73 1.48 0.95 1.03 16

80

Buried Dish, sand added

7

Table II. Results of Immediate Post-Application Sampling Study

1.58 3.39

1.31 1.68 1.49 1.71 1.65 1.92 1.41 1.80 1.70 1.31 2.18 1.67 1.09 1.79 1.19 1.38 18

77

Sample Tube

8

1.42 3.04

1.32 1.60 1.62 1.14 1.14 1.04 1.53 1.67 1.31 1.02 1.81 1.38 1.39 2.20 1.28 1.22 22

77

Sample Tube, into sand

9


1.90 2.98

2.17 1.99 1.98 2.17 1.97 1.78 1.66 1.51 2.03 2.08 1.77 1.69 2.14 1.44 1.87 2.07 12

90

Filter Paper

10

21. NORRIS ET AL


Therefore, suitability studies should be performed prior to application with the specific chemical under study.


Recommendations The authors' assessment of the various sampling procedures is summarized in Table III. A variety of sampling techniques will produce acceptable results; therefore the choice of technique may depend on the sampler's past experience, available equipment, and the expected depth of the chemical in the soil. The authors' preferences are: For measuring the amount of an agricultural chemical sprayed on the soil surface, the use of filter paper in addition to 77 mm diameter sampling tubes is recommended. At other times, when an agricultural chemical is concentrated in the upper 10 mm of soil, the use of the 77 mm diameter sampling tubes has produced satisfactory results. When deeper cores are required, but there are still residues concentrated at the soil surface (for example, just after an application but residues from previous applications are present throughout the soil profile), the sampling tubes can be used to collect a sample from the upper 0.15 m of soil followed by a bucket auger procedure to collect samples from deeper depths. After a soil incorporated application or at any sampling interval where residues are not concentrated in the upper 10 mm of soil, the authors prefer the bucket auger technique described elsewhere (3). One exception is in stoney soil where some type of split spoon sampler (4) is appropriate. For soil cores deeper than about 7.8 m, the use of hollow stem augering combined with split spoon sampling (5) is recommended.

Several important guidelines should always be followed to reduce potential contamination during the collection of soil samples regardless of which sampling technique is used. Samples should always be collected by conscientious, trained personnel. Careful attention must be given to proper labeling of samples. Because of the large number of soil samples usually collected at a sampling interval, it is quite easy to confuse the identity of some samples. Therefore, care must be taken to make sure that each sample is placed into the proper prelabeled container. Also concern about cleanliness must be paramount in the minds of everyone. A l l sampling equipment should be thoroughly washed between samples (the wash water should not be discarded in the test plot). All sampling equipment and sample containers should be kept off the ground and (as much as possible) kept away from dust which might contain agricultural chemical residues. Hands should not be placed inside the sample bags during collection or processing. Sample containers (before and after addition of samples) should not be transported in vehicles used to transport agricultural chemicals. Nor should containers or collected samples be stored in the same areas used to store agricultural chemicals, analytical standards, or application equipment.


356


Table ΙΠ. Summary of Sampling Techniques


Technique

Suitable for Sampling Surface Residues?

Maximum Sampling Depth (m)

Comments

Filter Paper

yes

can be used only to measure the amount sprayed on the soil surface

Buried Dishes

yes

can be used only to measure the amount applied to the soil surface.

Core Sampler

no

0.5

Golf Cup Cutter

yes

0.1

Excavation of Soil Inside a Ring

yes

0.5

Bucket Auger

no

7.8

Sampling Tubes Manual Placement Hydraulic Placement Hollow Stem Auger

yes yes no

0.5 2 >10

the small diameter appears to result in poor recovery of surface residues

Literature Cited 1. 2. 3. 4. 5.

Jones, R. L. InEnvironmentalFateofPesticides;Hutson, D. H.; Roberts, T. R, Eds.; Progress in Pesticides in Biochemistry and Toxicology, vol. 7; John Wiley and Sons: Chichester, U.K., 1990; pp. 27-46. Day, E. W.; Decker, O. D.; Griggs, R. D.; West, S. D. Proc. Sixth International Congress of Pesticide Chemistry (IUPAC), 1986, abstract 6B-16. Jones, R. L.; Hornsby, A. G.; Rao, P. S. C. Pestic. Sci. 1988, 23, 307-325. Jones, R. L.; Rourke, R. V.; Hansen, J. L. Environ. Toxicol. Chem. 1986, 5, 167173. Pacenka, S.; Porter, K. S.; Jones, R. L., Zecharias, Y. B.; Hughes, H. B. F. J. Contam.Hydrol.1987, 2, 73-91.

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September 5, 1990


Groundwater Residue Sampling Design - American Chemical Society

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