# Estimation of Saprolite Thickness Needed to Remove _E. coli_ from Wastewater
## 3 Results and Discussion
### Effective Thickness of Saprolite  
The thicknesses of saprolite below septic drainlines that would be needed to filter _E. coli_ from wastewater were estimated using the mean values of the pore volume measurements. Thicknesses ranged from 36 to 113 cm and had a mean value of 58 cm across all sites (Table 2). The largest thickness (113 cm) was found for a saprolite consisting of coarse sand with only 4% silt and 5% clay. Coarse sands have a majority of the sand grains in the 0.5 to 1.0 mm diameter class. These particles form relatively large packing pores that _E. coli_ would move through without filtration. While pore size distribution (from water retention analyses) was used to estimate the saprolite thicknesses needed to filter _E. coli_ from wastewater, such measurements are time-consuming and expensive to conduct. Using other measures related to pore size to estimate the saprolite thicknesses would be more convenient for field personnel to use. As shown in Table 2, saprolite thicknesses for OSWMSs that were < 60 cm had textural classes of loamy sand, sandy loam, and clay loam. Saprolite thicknesses between 68 and 74 cm had textural classes of sand and loamy sand. Despite the apparent relationship between texture and thickness of saprolite for removing _E. coli_, using textural class alone to determine saprolite thickness in the field may not produce a reasonable value because loamy sand textures showed a broad range in saprolite thicknesses. Correlation coefficients were used to identify other properties that could be used to estimate saprolite thickness (Table 2). Percentages of silt and sand, along with K\({}_{\text{sat}}\), had significant r values as shown. Regression equations relating silt and sand percentages along with K\({}_{\text{sat}}\) to saprolite thickness are shown in Figure 2. A quadratic relationship between the thickness of saprolite and silt percentage had an R\({}^{2}\) of 0.74 (Figure 2A). Sills are defined as particles having diameters between 2 to 50 \(\upmu\)m. Fine sinks (2 to 10 \(\upmu\)m diam.) are close to the size of _E. coli_ and should form packing pores that can filter the organism out of wastewater. Saprolite materials with silt percentages <16% were generally estimated to require >60 cm thickness to filter _E. coli_ (Figure 2A). A linear relationship was found between sand percentage and saprolite thickness (Figure 2B). The sand relationship contained two samples (site 5\({}^{\prime}\)s C4 horizon and site 7, Table 2) with identical values for sand percentage and saprolite thickness, so only 13 data points appear in Figure 2B. A linear relationship was used because quadratic terms were found to be insignificant (i.e., \(p\) values >0.05). The R\({}^{2}\) value for the sand relationship was lower than the R\({}^{2}\) value found for the silt relationship (Figure 2A). Sand may not be as effective a filter for _E. coli_ as silt. The size of sand particles range between 0.050 mm (very fine sand) to 2.00 mm for very coarse sand. Pores between the larger sand particles may be too large to filter _E. coli_. Finer sands would form smaller pores than the larger fractions and aid in the filtration of _E. coli_. The relationship between saprolite thickness and Ksat is shown in Figure 2C. Thirteen values are reported because Site 5\({}^{\prime}\)s C4 horizon did not have an in situ Ksat measured. The quadratic (nonlinear) relationship shown in Figure 2C was significant with a relatively high R2 of 0.73. The high R2 was due in large part to one high Ksat value (233 cm/d) as shown for site 10. Without considering site 10, there was no significant relationship between in situ measured Ksat and thickness of saprolite needed for removing _E. coli_. Based on the data presented in Figure 2C, in situ Ksat measurements do not provide any reasonable estimates of saprolite thickness needed to filter _E. coli_. We should note that in general, we expect that the _E. coli_ is not a good indicator of the _E. coli_.