2007 Ground Water Summit

Monday, April 30, 2007 : 9:50 a.m.

Utilization of Wellbore Flow and Depth-Dependent Water-Quality Data for Modifying Well Design to Reduce Arsenic Concentrations

Loren F. Metzger, Christina L. Stamos and John A. Izbicki, U.S. Geological Survey

Arsenic concentrations in water from many wells screened in the alluvial aquifer underlying the San Joaquin Valley near Stockton, California, approach or exceed the maximum contaminant level (MCL) of 10 µg/L. Wellbore flow and depth-dependent water-quality data were collected under pumped conditions from a 410-ft-deep well yielding water with high arsenic concentrations to determine the vertical distribution of flow and arsenic concentrations in the well bore. Analysis of the data indicates that below a depth of 250 feet the arsenic concentration of the wellbore flow was as high as 15 µg/L. On the basis of the velocity-log and depth-dependent water-quality data, the well below a depth of 250 ft was backfilled, reducing the length of the screened interval by about 60 percent. The well modification resulted in a decrease in well yield from 2,400 to 1,300 gal/min and a reduction in arsenic concentration of the surface discharge from almost 10 µg/L to 8 µg/L.

 A numerical flow model at the wellbore scale was developed and calibrated to the wellbore-flow and water-level data under unpumped conditions.  The model was verified using the wellbore flow data collected under pumping conditions.  Calibrated hydraulic conductivity values ranged from 30 ft/day in the shallower part of the aquifer to 1 ft/day in deeper parts of the aquifer, which roughly corresponds to the range of values expected from the lithologies at the wellbore.  Given the availability of lithologic and wellbore-flow data collected under both pumped and unpumped conditions, wellbore-flow models of this type could be developed and calibrated for other wells in the area.  These models, in conjunction with water-level and depth-dependent arsenic data, could be used to predict arsenic concentrations in the surface discharge at different pumping rates under changing water levels to help modify the construction of other wells that have high arsenic concentrations.


The 2007 Ground Water Summit