2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 19
Presentation Time: 8:00 AM-6:00 PM

The Variable Conductivity Interface – Expanded Usefulness for the Image Well Technique

CONGDON, Roger D., USDA Forest Service, 333 Broadway Blvd SE, Albuquerque, NM 87102, rcongdon@fs.fed.us

The image well (IW) technique has long been used to simulate either no-flow or constant head boundaries near a pumping well (PW) in a confined, saturated aquifer; and is described in virtually every introductory hydrogeology text book. However, because the technique is a result of the properties of superposition, it is possible to use it to simulate pumping near an interface with a lower hydraulic conductivity (K) if the storage coefficient (S) is constant. MODFLOW was utilized to simulate K ratios in multiples of 1000, 300, 100, 30, 10, 5 and 2:1 in a scenario where the pumping well (1 l/s) is located 100m from the interface in a 10-meter-thick aquifer with K of 0.0001 m/s. K across the interface was adjusted downward per the multiples given above. S was constant at 0.001. Comparison of drawdown in simulated observation wells (OW) in MODFLOW and with variable image-well pumping rates using the analytical Theis solution made it possible to develop a regression for generating a pumping-ratio coefficient. As the ratio of pumping rate to drawdown for a given K ratio is a linear function in each OW, generation of an IW rate matching drawdown in each MODFLOW scenario is trivial. A series of at least three pumping rates at the IW was employed which results in a first order regression with an r2 of 1.00. A curve fit of this IW/PW ratio versus the K ratio from MODFLOW yields the fifth order equation y = -2.44x5 + 8.45x4 - 11.59x3 + 8.74x2 - 4.32x + 1.17. This equation enables the correct IW pumping ratio to be calculated for any combination of K ratios, PW distance from the interface, pumping rates, or aquifer properties; as long as S is constant.