Transient seepage during Class-V Underground Injection Control Well Borehole Infiltration Tests in Vashon Advance Outwash, Puget Lowland, Washington
Porter, Matthew J
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The radius of influence and height of groundwater mounding of Class-V Underground Injection Control (UIC) well infiltration within Quaternary Vashon Advance Outwash (Qva) is investigated and implications for regional regulation and design are considered. Class-V UIC wells for stormwater management infiltrate treated stormwater at depth beneath the land surface. The model method and sensitivity analysis outlined in this study can be used as part of well interference and down gradient impact assessment evaluating the adequacy of proposed infiltration facility design and the potential impacts to slope stability located within the zone of influence of the infiltration system. Transient seepage flow above the water table for an axisymmetric layered model with a far field constant head boundary using site-specific conditions such as the stratigraphy, location of water table, and grain size is modeled with finite-element (FE) SEEP/W. The saturated hydraulic conductivity (Ksat) in the model (Kmodel) is calibrated to quasi-steady state stepped-flow rate infiltration tests. FE results are then used as ground truth to judge the adequacy of borehole permeameter (BP) solutions, six of which are reviewed for analytical estimation of Ksat (Kanalytical). Finally, a sensitivity analysis exposes which parameters control the radius of influence and Kmodel. The radius of influence herein is defined as the radius of the bulb of saturation surrounding the well, and the length of mound measured where the mound decays 90% from the maximum amplitude in the x-direction from the well. BP solutions make numerous assumption to translate infiltration rate into saturated hydraulic conductivity of the vadose zone. A combination of the Zhang (1998) and Reynolds (2010) solutions best match isotropic Kmodel. A major assumption in BP solutions is isotropic soil, yet, the Reynolds (1983) solution accurately predicts Kmodel under a vertical anisotropy ratio (Kh: Kv) of 10:1. Overall, these solutions match model results with approximately ±10% mean error. Based on a sensitivity analysis, the most dominant material property affecting the radius of influence is the vertical anisotropy of hydraulic conductivity. This vertical anisotropy present in the Qva is controlled by the degree of stratification. Increasing the vertical anisotropy decreases mound height and increases the radius of influence. This result agrees with analysis by Sumner et al. (1999) and disagrees with that by Carleton (2010) on groundwater mounding beneath infiltration basins. Increasing the vertical anisotropy from isotopic to 10:1 is accompanied by an average increase of 30% in Kmodel. Changing the volumetric water content curve to represent finer textures gives a small increase in the amplitude of the mound and radius of influence. Decreasing the effective porosity significantly reduced the time required for the mound to reach a quasi-steady state condition. These parameters dictate the shape and extent of the saturation bulb surrounding the UIC well and the mounding of the water table beneath. The results have implications for current impact assessment and guidance on UIC design from Washington State Department of Ecology. First, certain BP methods can be used for in-situ determination of Ksat from borehole infiltration tests, streamlining the modeling process for impact assessments. The sensitivity of the groundwater mound amplitude and radius of influence to model variables suggest UIC wells under certain site conditions are less likely to meet requirements for treatment capacity and separation from the groundwater table. Finally, both unsaturated flow and a vertical anisotropy of 10:1 should be considered in future assessments of slope stability within the zone of influence of infiltration. This project was supported by Associated Earth Sciences Inc. (AESI) in Kirkland, Washington.