Initial sampling

The facilities available at the CSIRO laboratories enabled only two samples per week to be analysed. To reduce the processing time, soil from the following sample points was processed for analysis, see Figure 4.1. Five samples from snig track at (0,0), (0,3), (0,-2), (2,0), (4,0), four from the sediment tongue (2m, 5m, 8m, 14m) and 2 from the undisturbed areas. Thus only 11 of the 26 soil samples were processed.

Drying

Samples were oven dried at 30° C for 24 hours. Sieving

Burch et al (1988) had found that the isotope activity of particle size classes assisted in identifying erosion sources and investigations of activity levels with respect to particle size were therefore incorporated into this study.

To separate samples into selected particle size distributions the samples were sieved through five sieves, 1.0 mm, 500 urn, 250 urn, 125 urn and 63 urn. The weight of soil retained on each sieve and the retainer was obtained.

Grinding

Grinding of the samples is necessary to obtain a fine homogeneous powder for casting. That part of the sample retained on the 1.0 mm was set aside. This is the standard procedure adopted in the CSIRO laboratories (Murray, personal communication). Three classes were separated for grinding, according to the particle size. First the part of the sample retained on the 500 urn sieve, called ’> 500 urn’, second the parts of the sample retained on the 250 urn, the 125 urn and the 63 urn sieves, called '63-500 urn', third the part of the sample retained on

the retainer, called '< 63 um'. The reason for grouping the soil into the three different classes was to enable the activity levels of radio isotopes, especially Cs-137 and Be-7, in these fractions to be examined. Using a ring grinding machine, classes one and two can be ground to a fine powder. The third class of samples does not need to be ground because in it the particles are fine. After grinding, the classes of powder were weighed and put into different plastic bags. Table 4.1 shows the weight of powder in each of the classes at the respective sampling points. Particle size WEIGHT OF SAMPLE(gm) (0,0) (0,3) (0,-2) (2,0) POINT1 (4,0) (2m) (5m) (8m) (14m) Sitel Site2 >500um (class 1) 167.1 148.7 254.9 538.2 273.7 426.5 373.2 434.8 155.3 280.1 156.5 63-500um(class 2) 100.7 118.3 224.7 273.7 159.2 278.2 156.1 274.8 169.2 238.7 168.9 <63um(class 3) 19.5 42.2 70.7 56.5 47.1 32.7 29.8 18.8 68.0 59.2 19.9 1) See figure 4.1.

Table 4.1. Total sample weight of three classes from each point after grinding.

Casting

The maximum weight of each sample is used to choose the appropriate mould. The maximum proportion of ground powder in a mould is about 70%, the remainder being resin. Sample weights in the mould are 40 gm and 250 gm for the disc and the cup moulds respectively. If the sample is insufficient then solid resin powder, called filler ( ground to less than 150 urn), is used to fill the mould. This minimizes settling of the sample during curing of the resin and avoids the uncertainties in homogeneity that may otherwise occur. Approximate quantities of sample, filler and resin for 40 gm of sample in the disc and 250 gm of sample in the cup are shown in Tables 4.2 and 4.3.

The total sample weights for classes one and two were always more than 40 gm and the cup mould was used for these. Sample weights for class three were less than 40 gm for some samples and the disc mould was used for these. There were 22 samples in cup moulds and 11 in disc moulds.

Before casting all the parts of mould are washed clean. When dried, a release agent is put on the inner and outer surfaces o f moulds. Wax is applied when the release agent has dried. The release agent and the wax facilitate extraction of the sample from the mould.

Sample weight (gm) Filler (gm) Resin (gm) Hardener (ml)

Total max. weight (gm) 250 - 150 3 400 200 - 125 2 350 150 - 150 2 350 100 50 125 1-2 300 <100 75 100 1 300

Table 4.2 Approximate quantities of sample, resin, filler and hardener for making a cup mould.

Sample weight (gm) Filler (gm) Resin (gm) Hardener (ml)

Total maxi: weight (gm) 45 - - 1 67-70 35 - 25 1 60-62 30 - 30 1 60 20 7 32 1 60 10 12 28 1 50-53

Table 4.3. Approximate quantities of sample, resin, filler and hardener for making disc mould.

RADIOISOTOPE SAMPLE PREPARATION

Cast date: 16.2.89

Cast code: ED029 Sample code:EN89006/l

Project /Sample: EDEN, Sediment tongue 2 meter, >500 urn Wt. container & Rod : ...38.357 gm

Total sample weight :...248.05 gm (A) Hardener added :... 2 ml

Total wt. mixture: ...403.752 gm (B) Final cast weight: ... 357.672 gm (C)

Net sample in cast = (A) x (C) - 248.05 x 357.672

(B) 403.752

= 219.71 gm

Figure 4.2. Proforma for sample preparation

An example of the proforma used for the preparation and casting of the samples is shown in Figure 4.2.

A plastic container and an iron stirring rod are put on a balance and the balance

tared. The sample is poured into the container and the total sample weight recorded which is (A)

in Figure 4.2. Resin, Filler and hardener (catalyst) are added as shown in Tables 4.2 and 4.3 to

provide the appropriate mixture weight. The resin and soil are mixed thoroughly by stirring

with the iron rod. The total mixture weight is recorded on the form, (B) in the proforma, Figure

4.2, and the sample is then cast. The mixture is poured into the assembled mould , the lid put

on and left overnight to harden.

The next day the mould is pulled apart and the samples taken out. As shown in

a a

Figure 4.3 the disc has a volume of 33 cm and the cup 201cm . The moulds of disc, cup and

samples casted are also shown in Plate 4.2. The moulds are washed with water and weighed

when dry. The weight is recorded on the proforma (C). The total sample (A) cannot be placed

in the disc or cup because a portion of the mixture of soil and resin is left in the container while

pouring and some comes out the holes in the lid when it is pressed on. The calculation for the

<---& 6 5--- > Disc 3 Volume = 33 cm <- -& 72 & 90 o vO \ / o r - Cup 3 Volume = 201 cm

Figure 4.3. Cross section of disc and cup after casting ( Source : Murray, et al, 1987)

Plate 4.2. Moulds of cup and disc and top and bottom view of casting. 1 = Cup mould, 2 = Disc mould, A = Top view of cup, B = Bottom view of cup, C = Top view of disc, D = Bottom view of disc.

4.5. Detection of cups and discs with the spectrometer