Situation: There has been a spill of 25,000 L of diesel (= 21,250 kg as TPH) to the soil. The groundwater table is 18 m deep and the soil consists of fine sand, coarse sand, and gravel. The volume of the spill encompasses about 1,500 m3 (H x W x L = 15 x 10 x 10 m3) in vadose zone, beginning 3 m below the ground surface. The groundwater is 24 m deep, and the aquitard (confining layer) below the groundwater consists of bedrock. The groundwater hydraulic gradient is 0.0042 ft/ft and hydraulic conductivity is 50.0 m/day. The volume of the spill in saturated zone is found to extend over a diameter of approximately 20 m and a depth of 20 m, beginning 4 m below the water table. You are required to conceptually design a system to remediate the contaminated site as follows: a) for vadose zone, using SVE or bioventing processes; b) for saturated zone, using air sparging or bio-sparging methods; and c) for DNAPL, proposing a method and then designing the system. Given:
(a) Vadose zone soil properties: foc = 2.5%, φ = 40% (porosity), Θ = 15%, ρb = 1800 kg/m3,
(b) Groundwater soil properties: foc = 1.5%, φ = 40% (porosity), soil wet bulk density ρwb = 1960 kg/m3 (hint: you need find ρb in saturated soil)
(c) Characteristics of TPH: Vapor pressure = 0.014 atm at 200C; H = 5.9 x10-5 atm-m2/mol at 200C; MW = ~182, solubility = 5 mg/L; ρTPH = 850 kg/m3; Kow = 716; and molecular formula = C11H22O2 (assumed).
1. Given: the soil can retain 15 L THP/m3 of soil (i.e., solid + liquid, and gas phase) in vadose zone and 18 L THP/m3 of soil (i.e., solid + water phases) in saturated zone; contaminant partitioning in the vadose zone and saturated zone can be described by the following equation:
Mt = ρb soil*S*V + Θ*Cw*V + (φ – Θ)*H*Cw*V (for VZ)
Mt = ρb soil*S*V + Θ*Cw*V (for SZ)
where Mt = total quantity of contaminant in the vadose or saturated zone, (M); S = adsorbed chemical concentration, (MM-1); Cw = dissolved chemical concentration in pore water, (ML-3); Cg = vapor concentration; ρb = soil bulk density; Θ = volumetric water content; φ = volumetric air (gas) content; H = Henry’s law constant, (dimensionless); KSD = soil (solid) water partition coefficient (adsorption coefficient), (L3M-1 of soil); foc = fraction of organic carbon, (-); and Koc = the organic carbon partition coefficient (L3M-1 of soil). Estimate the phase distribution of the contaminants in (a) unsaturated, (b) saturated zone, and the dimension of the free product (DNAPL). Present your results in a table. Hint: the given 15 L/m3 of THP in vadose zone and 18 L/m3 in saturated zone are the total amount of THP that can be retained by the soil (including solid, water and air in VZ and solid and water in SZ). In other words, these two values are very similar to the Mtotal (= 30 g of toluene/kg soil) given on p. ¾ of my note 4. Other THP will be accumulated as LNAPL of free products. You need to use mass balance to calculate the mass of LNAPL, which will be located in groundwater between 18 and 22 m below the surface.
2. Assuming that you first evaluate the feasibility of bioremediation of the vadose zone. There are two methods to approach this: 1) using the method shown in Appendix B of my note 6 to estimate the electron acceptor (O2) and nutrient (N, P) needs for a) bioremediation, b) their delivery rates, and c) clean-up time needed for bioremediation; 2) using my note of “In-situ (Bio)remediation: Design Info” (p. 7 & 8) to estimate the electron acceptor (O2) and nutrient (N, P) needs for a) bioremediation and b) clean-up time needed for bioremediation. Given: the first-order degradation of TPH has a rate constant of 0.01/d, and the in-situ respiration test results are given:
Time (h): 5 10 20 30 40
O2 (%): 20 18 13.8 9.1 4.9
The background oxygen consumption is negligible. Hint: you can use the equations given in lecture notes to find out Ko2 and KB, and then find the cleanup time.
3. Design a SVE (or bioventing) system to bioremediate the contamination in the vadose zone. You need to identify the following: (a) system components and equipment, such as numbers of extraction and injection wells. Well locations and well construction (depth, screened interval, etc.); and (b) system operating parameters, such as radius of influence, extraction airflow rate, assuming an applied extraction vacuum of 10 psi and the well efficient of 80%. Well arrangement may be very different, depending on your selection of the processes. You may even not need any extraction well. Given: the pilot test monitoring results show that at extraction rate = 150 cfm,
Distance away from the extraction well (feet): 3 10 20
vacuum (inch H2O): 6.5 4 2.7
4. Design an air sparging (or biosparging) system for saturated zone remediation and a pump-and-treat system for NAPL treatment. You can make any reasonable assumptions if necessary. Again, the system component and equipment as well as system operating parameters need to be specified.