A Numerical Simulation for Movement of Contaminated Water through a Wellbore

Thursday, 18 December 2014
Tracy Nishikawa, Zachary P Stanko, Nicholas F Teague and John A Izbicki, USGS California Water Science Center San Diego, San Diego, CA, United States
A 3D groundwater-flow model was developed using MODFLOW and MODPATH particle tracking to simulate movement of water through a 293-m public-supply well under unpumped and pumped conditions. The well, located in the Rialto-Colton groundwater basin in Southern Calif., penetrates two water-bearing units, separated by a low-permeability layer, and has been inactive for 8 years as a result of perchlorate contamination; field data show 13 kg of perchlorate is redistributed annually through the well from the upper to the lower unit. The local-scale (13.7 km2) model included one pumping well and was calibrated to wellbore flow and head data. The parameter-estimation software, PEST, was used to estimate hydraulic conductivities (K) for each of 18 homogeneous and isotropic model layers. The PEST problem was ill-posed, which was partially mitigated by applying singular value decomposition and Tikhonov regularization. Prior information included lithology from drillers’ logs and approximate regional water-level gradients. The model simulated: 1) unpumped wellbore flow and contaminant redistribution from the upper, contaminated unit to the lower, uncontaminated unit; and 2) pumped wellbore flow and contaminant recapture. Model results indicated that a vertical head gradient of about 1% could vertically redistribute the perchlorate through the unpumped well. The longitudinal extent of the redistributed contaminant plume in the direction of the regional horizontal gradient was strongly dependent on the calibrated K values. In addition, the simulated horizontal distribution of contaminants in the transverse direction matched observed concentrations collected from a nearby well at various depths. Under pumped conditions, 67% of the tracked particles were recaptured within 10 years of resuming pumping at 13,250 L/min. Sensitivity analyses revealed that perchlorate recapture estimates are unaffected by small uncertainties in K or the regional gradient imposed by the boundary conditions.