LOW-ENERGY ALTERNATIVES
dispersivity was 0.1 m, and the effective molecular diffusion coefficient was 0.00001 m2/d. The concentration boundary of contaminant plumes was 1.0 mg/L.
A no-action and three active remediation alternatives were modeled, including: (a) a 0.5-m thick permeable reactive bar- rier of minimum length to prevent the contaminant plume from traveling offsite; (b) linear transects of non-pumped wells with filter media spaced 0.5 m apart, of minimum number to prevent offsite contamination; and (c) an extraction-injection well pair, each well pumping a minimum (identical) rate to prevent offsite contamination. The permeable reactive barrier, non-pumped wells, and well pair all occupied linear transect(s) located 5 m downgradient of the initial contaminant plume and oriented perpendicular to the regional hydraulic gradi- ent. In the extraction-injection well scenario (c), one well injected clean water, considered treated above ground after extraction from the other well. Locations and pumping rates of wells were adjusted through an iterative process to identify a minimum number of wells in the non-pumping filter media wells scenario (b) and pumping rate in scenario (c) necessary to contain the plume onsite.
Vertically, wells and trenches traversed the entire aquifer. The hydraulic conductivity and effective porosity of reactive filter media was set at 100 m/d and 0.35, respectively. In the model, the permeable reactive barrier and non-pumped wells were contaminant sinks with a concentration of 0 mg/L. All groundwater flow and mass transport simulations used the preconditioned conjugate gradient and generalized conjugate gradient solvers, respectively. Mass balance errors were less than 0.01%.
Results And Discussion
Without intervening, the initial contaminant plume was simulated to migrate to the eastern boundary after 470 d (Figure 1). Therefore, natural attenuation alone could not contain the plume onsite. When it reached the boundary, the plume had grown in size but decreased in concentration, due to the effects of hydrodynamic dispersion and dilution by clean groundwater (Figure 1).
Figure 2. Map of remaining contaminant plume after 500 d with permeable reactive barrier (top) and non-pumped wells (bottom); contours in mg/L.
Alternative (b) required a minimum of 31 non-pumped wells along parallel cross-gradient transects, one containing 26 wells and the other containing 5 wells, to contain the contaminant plume (Figure 2). Wells along the second, shorter transect captured enough contamination moving past the first transect to contain the plume onsite (Figure 2). After 720 d, the plume reached its farthest point of advance, 17.5 m from the eastern boundary. Similar to scenario (a), scenario (b) removed the plume after 910 d; however, scenario (b) required more than 30 wells to contain the plume onsite.
An extraction-injection well pair separated by 7.5 m and discharging only 2.6 m3/d also contained the contaminant plume, removing it after 950 d (Figure 3). The pair’s midpoint was positioned slightly (approximately 1.1 m) south of the initial plume’s long axis. This slight asymmetry reflects the geometry of the groundwater flow field induced by the well pair, creating a small tendency for southerly movement in this example (Figure 3). As with other alternatives, scenario (c) allowed the contaminant plume to travel past the remedial structure, but not offsite. After 920 d, the plume reached its maximum distance of 9.5 m from the eastern boundary of the model domain.
Figure 1. Map of initial contaminant plume (top) and residual plume after 470 d with no remedial structure (bottom); contours in mg/L.
Thus, each active scenario contained and removed the contaminant plume within a similar time-frame; however, the extraction-injection well pair required the least infrastruc- ture. While the well pair requires above-ground treatment of contaminated water, it does not involve costly excavation and
www.aipg.org
For scenario (a), a 10.5 m-long permeable reactive barrier was simulated to contain the plume onsite (Figure 2). The bar- rier was centered on the long axis of the initial contaminant plume, reflecting the plume’s symmetry in a uniform initial groundwater flow field. While the simulated contaminant plume moved past the barrier, it did not reach the eastern boundary of the model domain (Figure 2). After 770 d, the plume reached its farthest point of advance, 17.75 m from the eastern boundary. After 910 d, the plume was gone; that is, concentrations at all model cells dropped below 1 mg/L.
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56