An important gap in the understanding of the hydrology of the Middle Rio Grande Basin, central New Mexico, is the rate at which water from the Rio Grande recharges the Santa Fe Group aquifer system. Several methodologies-including use of the Glover-Balmer equation, flood pulses, and channel permeameters- have been applied to this problem in the Middle Rio Grande Basin. In the work presented here, ground-water temperature profiles and ground-water levels beneath the Rio Grande were measured and numerically simulated at four sites. The direction and rate of vertical ground-water flux between the river and underlying aquifer was simulated and the effective vertical hydraulic conductivity of the sediments underlying the river was estimated through model calibration.

Seven sets of nested piezometers were installed during July and August 1996 at four sites along the Rio Grande in the Albuquerque area, though only four of the piezometer nests were simulated. In downstream order, these four sites are (1) the Bernalillo site, upstream from the New Mexico State Highway 44 bridge in Bernalillo (piezometer nest BRN02); (2) the Corrales site, upstream from the Rio Rancho sewage treatment plant in Rio Rancho (COR01); (3) the Paseo del Norte site, upstream from the Paseo del Norte bridge in Albuquerque (PDN01); and (4) the Rio Bravo site, upstream from the Rio Bravo bridge in Albuquerque (RBR01). All piezometers were completed in the inner-valley alluvium of the Santa Fe Group aquifer system. Ground-water levels and temperatures were measured in the four piezometer nests a total of seven times in the 24-month period from September 1996 through August 1998.

The flux between the surface- and ground-water systems at each of the field sites was quantified by one-dimensional numerical simulation of the water and heat exchange in the subsurface using the heat and water transport model VS2DH. Model calibration was aided by the use of PEST, a model-independent computer program that uses nonlinear parameter estimation. Mean vertical hydraulic conductivities were estimated by model calibration and range from 1.5x10^-5 to 5.8x10^-6 meters per second (m/s). Mean simulated vertical ground-water flux for the BRN02 piezometer nest is 3.30x10^-7 m/s; for the COR01 piezometer nest is 3.58x10^-7 m/s; for the PDN01 piezometer nest is 4.22x10^-7 m/s; and for the RBR01 piezometer nest is 2.05x10^-7 m/s. Comparison of the simulated vertical fluxes and vertical hydraulic conductivities derived from this study with values from other studies in the Middle Rio Grande Basin indicate agreement between 1 and 3.5 orders of magnitude for hydraulic conductivity and within 1 order of magnitude for vertical flux.