Question

Bioremediation was selected to clean up 1000 m³ soil contaminated with 1000 mg/kg n-nonane (C₉H₂₀), 1000 mg/kg toluene (C₇H₈), and 2000 mg/kg benzene 1,2-diol (C₆H₆O₂). The soil bulk density is 1,800 kg/m³.

(A) Estimate the average chemical structure/molecular formula of the contaminants (20%)

(B) Determine the most approriate electron acceotpr and metabolism mode for destruction of the above hydrocarbon mixture (20%)

(C) Estimate the amount (kg) of electron acceptor (20%) and nutrient (nitrogen (20%) and phosphorous (20%)) needs

Assumptions:

• Ammonia is used as the nitrogen source;
• Phosphorus need for microbial growth is approximately one-sixth of that for nitrogen source;
• The composition of cellular structure is C₅H₇O₂N.
• The energy factors for the conversion of contaminants to biomass are listed in the following table:

Electron acceptor	fsvalue
O2	0.5
NO3	0.3
SO4	0.1
CO2	0.2

Answer 1

Soil test results are most often presented either as a percentage of soil (e.g. % organic carbon) or as a weight per unit of soil (e.g. nitrogen, mg/kg). As bulk density is a measure of soil weight in a given volume, it provides a useful conversion from these units to an area basis unit (e.g. t/ha). The resulting number gives an easily understandable idea of the carbon storage or nutritional status of the soil on an area basis.
Soil analysis values can be converted to a meaningful volume for a paddock. For example:

i.e. 10,000 m2 in one hectare x 0.1 m soil depth

x 1.5 g/cm3 bulk density = 1,500 t/ha of soil.

20 × 1,500,000 kg = 30,000,000 mg-N/ha

= 30 kg-N/ha

Does gravel affect interpretation of soil test results?

Yes. Gravel tends to complicate soil characterisation and analysis, and needs to be considered on a case by case basis.
For soil analysis purposes, gravel is defined as any particle that is larger than 2 mm in diameter. As these particles are recognised as having little to no effect on the physical, chemical and biological functioning of the soil, they are removed prior to analysis (i.e. % organic carbon and nutrients are typically assessed on a less than 2 mm component). Therefore, if your soil contains a significant amount of gravel, this must be accounted for before multiplying up using bulk density, otherwise you will overestimate quantities on an area basis. Adding the gravel fraction back in after soil analysis will “dilute” certain results per unit area (e.g. total organic carbon, nitrogen and phosphorus).
Tables 1 and 2 give an indication of the multiplication factors when converting soil quality indicators from units in percentage (table 1) and milligrams per kilogram (mg/kg, table 2). After determining the bulk density and approximate gravel content of your soil, you can convert your raw analysis data by multiplying with the appropriate factor in table 1 or 2.

Table 1: Multiplication factors for converting percentage (e.g. total organic carbon) to tonnes per hectare (t/ha) for a 10 cm soil layer.

Gravel	Bulk Density (g/cm3)
(%)	1.2	1.4	1.6
0	12.0	14.0	16.0
10	10.8	12.6	14.0
20	9.6	11.2	12.8
30	8.4	9.8	11.2

Table 2: Multiplication factors for converting milligrams per kilogram (mg/kg, e.g. mineral nitrogen) of soil to kilograms per hectare (kg/ha) for a 10 cm soil layer.

Gravel	Bulk Density (g/cm3)
(%)	1.2	1.4	1.6
0	1.20	1.40	1.60
10	1.08	1.26	1.40
20	0.96	1.12	1.28
30	0.84	0.98	1.12