CAJA RURAL DEL SUR:
4. INFORMACIÓN POST EMISIÓN
4.1.5 Información a las Agencias de Calificación
Particle Size Analysis (PSA) is based on the important relationship between particle size distribution and the hydraulic conductivity of a hydrogeological unit, which generally increases with coarser particles (e.g. Shepherd, 1989). PSA equations are usually based on a threshold grain size, of which a certain percentage of the sample is finer (Hazen, 1892). For instance, the d10, the particle size at which 10% of the sample is finer, serves
as the key input parameter in many PSA empirical relationships (e.g. Hazen, 1892; Krumbein and Monk, 1943; Carrier, 2003). Other equations which use a single parameter are based on the percentage of clay (Puckett et al., 1985) or sand and clay particles within a sample (e.g. Rawls and Brakensiek, 1989). However, the latter method is only
applicable to soils with maximum 70% of sand. Hence, it is possibly not applicable to coarse-grained glaciofluvial sediments.
The main advantages of PSA are its relatively easy and inexpensive sample collection and analysis. Additionally, the environmental impact of PSA is lower than that of methods which require piezometers (i.e. pumping tests, slug tests) (Table 5.1). However, PSA also has various limitations such as high variability in results, small scale representation of aquifer parameters, and strong dependence on the formula that is used (e.g. Brassington, 2007; Ronayne et al., 2012). However, despite these limitations, PSA has been widely used in a variety of hydrogeological settings including fluvial environments (e.g. Song et
al., 2009), desert alluvium (e.g. Alyamani and Sen, 1993) and proglacial outwash plains
118 PSA at the Skaftafellsjökull foreland was determined from sediment samples collected as close as possible to the water table (~0.60 m below ground). The samples were collected from lakeshores, relict glaciofluvial channels, the transect area, and from near the
piezometers within the monitored area (Figure 3.13). PSA was obtained by a Coulter® LS230 laser Grain Size Analyser (GSA), which provides the volume percentage finer than the following grain sizes: 2, 1, 0.5, 0.25, 0.125, 0.063, 0.003, 0.001, 0.0005, 0.00025, 0.000125, and 0.0000625 mm. However, some of the coarser samples were too angular to be analysed on the GSA. Therefore, the PSA for these samples were obtained by wet sieving, using mesh sizes of 2, 1, 0.5, 0.25, 0.125 and 0.063 mm. These samples
contained a very small amount of material below 0.063 mm. Hence, analysis of finer particle sizes was not taken on these samples.
This study compared between three PSA equations, which focus on different particle sizes: Hazen (1892), Puckett (1985), and Alyamani and San (1993). The Hazen (1892) method has been extensively used to estimate the hydraulic conductivity of clean sands, including those from glacial environments (e.g. Hazen, 1892; Robinson et al., 2008). The Hazen method estimates hydraulic conductivity based on Equation 5.2:
𝑲 = 𝑪(𝒅𝟏𝟎)𝟐
Equation 5.2 The Hazen equation (1892)
Where K is hydraulic conductivity (m/day), C is a coefficient based on both grain size and sorting, and d10 is the particle size diameter (mm) of which 10% of the sample is finer.
When comparing various PSA equations, Bradbury and Muldoon (1990) observed that the Hazen method consistently underestimated field-obtained results by one-two orders of magnitude. However, it also provided the closest results to laboratory measured values, despite a consistent overestimation by one order of magnitude. Additionally, Goodman
119 (1999) has also reported that the Hazen method predicted lower values and was more consistent than other methods which consider sorting, such as Krumbein and Monk (1943). In addition to its reported relative consistency, the Hazen method was also used in this study because of its previous usage in proglacial environments (e.g. Robinson et al., 2008).
In addition to the d10, the Hazen equation also includes an empirical coefficient (C), which
is related to sediment grain size and sorting (Brassington, 2007). It has also been
suggested that the C parameter is influenced by sediment compaction, with higher Hazen coefficient assigned to looser material (Uma et al., 1989). Although it is usually assumed that the coefficient is equal to 100, a review has shown that cited Hazen coefficients can range between 1 and 1000 (Carrier, 2003). However, values as high as 1300 have also been suggested (Brassington, 2007). The C coefficients which were used in the current study were 350 (lowest value suggested) for fine-grained sediments and 1000 for coarse- grained sediments (Brassington, 2007).
The Alyamani and Sen (A&S) (1993) equation was developed in order to overcome the bias problems associated with PSA equations that only use a single parameter. A&S have suggested that finer particles carry a higher physical impact on hydraulic conductivity, hence, the central tendency chosen in a one parameter equation is usually biased toward fine grain sizes. They also suggested that one parameter fails to fully represent the whole grain-size distribution curve. Therefore, a single parameter does not yield consistent results with respect to actual values of hydraulic conductivity (Alyamani and Sen, 1993). In order to overcome these limitations, they developed an alternative approach, which is based on a portion of the curve, using its slope, intercept and the difference between the
120
𝐾 = 1300 [𝐼0
+ 0.025(𝑑50
− 𝑑10)]2
Equation 5.3. The Alyamani and Sen (A&S) equation (1993).
K denotes hydraulic conductivity (m/day), d50 and d10 are the grain sizes which are 10%
and 50% coarser than the remaining of the sample, respectively. I0 (mm) is the grain size
diameter where the d10 and d50 values intersect the horizontal axis (Figure 5.3).
Figure 5.3. Determination of the I0 (the particle diameter which corresponds to the
intersection of d10 and d50) from PSA data.
The I0 is located where the dotted line crosses the x axis.
The third equation for measuring hydraulic conductivity which was tested in this study has been developed by Puckett et al. (1985) (Equation 5.4).
𝑲 = (𝟒. 𝟑𝟔 × 𝟏𝟎−𝟓) × 𝒆(−𝟎.𝟏𝟗𝟕𝟓×%𝒄𝒍)
121 K is hydraulic conductivity (m/sec) and % cl is the percentage of the total sample that is finer than 0.002 mm. The obtained values from this equation were later converted to m/day. This method is specifically designed for sediment with high clay contents. It was chosen due to the high proportion of fine-grained sediment at the Northern Oasis and the eastern lakeshore of the Instrumented Lake.
Sorting was calculated according to Equation 5.5 (Folk, 1986).
𝜎1
= 𝛷84
− 𝛷16
4
+
𝛷95
+ 𝛷5
6.6
Equation 5.5. Calculation of sediment sorting coefficient (after Folk, 1986).
1
is the sorting coefficient and
84,
16,95,5 are the phi values at the respective 84,16, 95, and 5 percentiles. These coefficients are accompanied by a verbal description of the sorting (Table 5.3, Appendix 3).