MODELING AND ISOTOPIC ANALYSIS TO ASSES ARTIFICIAL RECHARGE IN THE RIALTO-COLTON BASIN, SAN BERNARDINO COUNTY, CALIFORNIA

The goal of water banking is to supplement ground-water resources and offset overdraft during dry periods. Ground-water age can be used to assess the movement of water through an aquifer and the feasibility of using an aquifer for water banking. Three different geochemical and isotopic models are being developed to assess the impact of a water-banking program in the Rialto-Colton Basin, San Bernardino County, California.

A one-dimensional mass-balance model is being used to determine the mass transfer of constituents between the dissolved phase and aquifer materials, and to determine the carbon-14 (C¹⁴) ground-water age at specific wells along four flow paths through the Rialto-Colton Basin. The C¹⁴ isotope can be used to determine the age of ground water on timescales ranging from recent to more than 20,000 years before present. To determine age at a given location in an aquifer, the C¹⁴ activity is measured, and the age calculated as the time required to reduce the activity of the influent to that measured by radioactive decay, with corrections for geochemical reactions. The dominant reactions that affect C¹⁴ activities in the Rialto-Colton Basin are silicate weathering, clay precipitation, cation exchange, calcite precipitation or dissolution, and oxidation of organic matter. Ground-water age ranges from 200 to 16,000 years before present.

A three-dimensional finite-element reactive-transport model (Hydrogeobiochem) is being developed to simulate C¹⁴ activities, which are calibrated with measured data, over the entire aquifer. This model includes the geochemical reactions from the one-dimensional modeling and steps for computing C¹⁴ activities and age. The goal of this model is to assess the movement of water through the aquifer and study the mixing of different recharge sources, including artificially recharged imported water.

A two-dimensional solute-transport model is being developed to compute the ground-water age distribution via the transport of water as a solute through porous media. Results from this model are compared to the results from a cross-section of the three-dimensional reactive-transport model to evaluate the use of the ground-water age equations in the two-dimensional solute-transport model as a tool for simulating ground-water age when carbon-isotope data are not available.