Calibration and Reliability in Groundwater Modelling: Credibility of Modelling

(Proceedings of ModelCARE 2007 Conference, held in Denmark, September 2007). IAHS Publ. 320, 2008, 250-256.

The use of groundwater age as a calibration target

LEONARD F. KONIKOW¹, GEORGE Z. HORNBERGER¹, LARRY D. PUTNAM², ALLEN M. SHAPIRO¹ & BRENDAN A. ZINN¹

1 US Geological Survey, MS 431, Reston, Virginia 20192, USA

lkonikow@usgs.gov

2 US Geological Survey, Rapid City, South Dakota 57702, USA

Abstract Groundwater age (or residence time), as estimated on the basis of concentrations of one or more environmental tracers, can provide a useful and independent calibration target for groundwater models. However, concentrations of environmental tracers are affected by the complexities and mixing inherent in groundwater flow through heterogeneous media, especially in the presence of pumping wells. An analysis of flow and age distribution in the Madison aquifer in South Dakota, USA, illustrates the additional benefits and difficulties of using age as a calibration target. Alternative numerical approaches to estimating travel time and age with backward particle tracking are assessed, and the resulting estimates are used to refine estimates of effective porosity and to help assess the adequacy and credibility of the flow model.

Key words groundwater age; numerical simulation; solute transport; model calibration; porosity