Practical Design Calculations for Groundwater and Soil Remediation - Chapter 6

Kuo, Jeff "Groundwater remediation" Practical Design Calculations for Groundwater and Soil Remediation Boca Raton: CRC Press LLC,1999 chapter six Groundwater remediation This chapter starts with design calculations for capture zone and optimal well spacing. The rest of the chapter focuses on design calculations for commonly used in situ and ex situ groundwater remediation techniques, including bioremediation, air sparging, air stripping, advanced oxidation processes, and activated carbon adsorption. VI.1 Hydraulic control (groundwater extraction) When a groundwater aquifer is contaminated, groundwater extraction is often needed. Groundwater extraction through pumping mainly serves two purposes: (1) to minimize the plume migration or spreading and (2) to reduce the contaminant concentrations in the impacted aquifer. The extracted water often needs to be treated before being injected back into the aquifer or released to surface water bodies. Pump and treat is a general term used for groundwater remediation that removes contaminated groundwater and treats it above ground. Groundwater extraction is typically accomplished through one or more pumping or extraction wells. Pumping of groundwater stresses the aquifer and creates a cone of depression or a capture zone. Choosing appropriate locations for the pumping wells and spacing among the wells is an important component in design. Pumping wells should be strategically located to accomplish rapid mass removal from areas of the groundwater plume where contaminants are heavily concentrated. On the other hand, they should be located to allow full capture of the plume to prevent further migration. In addition, if containment is the only objective for the groundwater pumping, the extraction rate should be established at a minimum rate sufficient to prevent the plume migration. (The more the groundwater is extracted, the higher the treatment cost.) On the other hand, if groundwater cleanup is required, the extraction rate may need to be enhanced to shorten the reme-diation time. For both cases, major questions to be answered for design of a groundwater pump-and-treat program are ©1999 CRC Press LLC 1. What is the optimum number of pumping wells required? 2. Where would be the optimal locations of the extraction wells? 3. What would be the size (diameter) of the wells? 4. What would be the depth, interval, and size of the perforations? 5. What would be the construction materials of the wells? 6. What would be the optimum pumping rate for each well? 7. What would be the optimal treatment method for the extracted groundwater? 8. What would be the disposal method for the treated groundwater? This section will illustrate common design calculations to determine the influence of a pumping well. The results from these calculations can provide answers to some of the above questions. VI.1.1 Cone of depression When a groundwater extraction well is pumped, the water level in its vicinity will decline to provide a gradient to drive water toward the well. The gradient is steeper as the well is approached, and this results in a cone of depression. In dealing with groundwater contamination problems, evalua-tion of the cone of depression of a pumping well is critical because it repre-sents the limit that the well can reach. The equations describing the steady-state flow of an aquifer from a fully penetrating well have been discussed earlier in Section III.2. The equations were used in that section to estimate the drawdown in the wells as well as the hydraulic conductivity of the aquifer. These equations can also be used to estimate the radius of influence of a groundwater extraction well or to estimate the groundwater pumping rate. This section will illustrate these applications. Steady-state flow in a confined aquifer The equation describing steady-state flow of a confined aquifer (an artesian aquifer) from a fully penetrating well is shown below. A fully penetrating well means that the groundwater can enter at any level from the top to the bottom of the aquifer. Kb(h2 − h1) 528log(/r )r 2.73Kb(h2 − h1 log(/ )1 for American Practical Units [Eq. VI.1.1] for SI where Q = pumping rate or well yield (in gpm or m3/d), h , h = static head measured from the aquifer bottom (in ft or m), r1, r2 = radial distance from ©1999 CRC Press LLC the pumping well (in ft or m), b = thickness of the aquifer (in ft or m), and K = hydraulic conductivity of the aquifer (in gpd/ft2 or m/d). Example VI.1.1A Radius of influence from pumping a confined aquifer A confined aquifer 30 ft (9.1 m) thick has a piezometric surface 80 ft (24.4 m) above the bottom confining layer. Groundwater is being extracted from a 4-in (0.1 m) diameter fully penetrating well. The pumping rate is 40 gpm (0.15 m3/min). The aquifer is relatively sandy with a hydraulic conductivity of 200 gpd/ft2. Steady-state drawdown of 5 ft (1.5 m) is observed in a monitoring well 10 ft (3.0 m) from the pumping well. Determine a. The drawdown in the pumping well b. The radius of influence of the pumping well Solutions: a. First let us determine h1 (at r1 = 10 ft): h1 = 80 – 5 = 75 ft (or = 24.4 – 1.5 = 22.9 m) To determine the drawdown at the pumping well, set r at the well = well radius = (2/12) ft = 0.051 m and use Eq. VI.1.1: ()2)((00 )30 h2 −75 528log[(/2 1)2/ 1]0 2 or [(.0 1)5( 14)4]0 2.73[.()2(00 .)0]()(0410. ) 9 1 h2 − 22 9 log(.0 051/ .3 )0 2 So, the drawdown in the pumping well = 80 – 68.7 = 11.3 ft (or = 24.4 – 21.0 = 3.4 m). b. To determine the radius of influence of the pumping well, set r at the radius of influence (r ) to be the location where the drawdown is equal to zero. We can use the drawdown information of the pumping well as ()2).((00 30) 68 7 − 80 528log[(2 12 r] or ® rRI = 270 ft ©1999 CRC Press LLC [(0.15)(1440)]= 2.73[(200)(0.0410)](9.1)(21.0- 24.4) ® rRI = 82 m RI Similar results can also be derived from using the drawdown infor-mation of the observation well as ()2)((00 3)0 75− 80 528log[10/rRI ] RI or [(0.15)(1440)]= 2.73[.(200 .)0.0410 9 1 )22 9− 24.4 ® h2 = 78 m Discussion 1. In (a), 0.041 is the conversion factor to convert the hydraulic conduc-tivity from gpd/ft2 to m/day. The factor was taken from Table III.1.A. 2. Calculations in (a) have demonstrated that the results would be the same by using two different systems of units. 3. The “h – h ” term can be replaced by “s – s ,” where s and s are the drawdown values at r and r , respectively. 4. The differences in the calculated r values in (b) come mainly from the unit conversions and data truncations. Example VI.1.1B Estimate the groundwater extraction rate of a confined aquifer from steady-state drawdown data Use the following information to estimate the groundwater extraction rate of a pumping well in a confined aquifer: Aquifer thickness = 30.0 ft (9.1 m) thick Well diameter = 4-in (0.1 m) diameter Well perforation depth = full penetrating Hydraulic conductivity of the aquifer = 400 gpd/ft2 Steady-state drawdown = 2.0 ft observed in a monitoring well 5 ft from the pumping well = 1.2 ft observed in a monitoring well 20 ft from the pumping well Solutions: Inserting the data into Eq. VI.1.1, we obtain Kb(h2 − h1) 528log(/ )1 ©1999 CRC Press LLC (.400 .30 2 0 −1 2 528log(20) 5 ... - tailieumienphi.vn