CAN CONTAMINANT TRANSPORT BE SIMULATED ASSUMING STEADY-STATE FLOW?

Groundwater flow models are commonly developed as a first step in simulating contaminant transport processes, or for the determination of well capture zones. For most field sites, snapshots of piezometric head or concentrations are obtained at fixed times. The temporal sparseness of the field data typically leads to the development of steady-state groundwater flow models based on average recharge and surface water levels. Transient conditions are often simulated using several steady-state models to represent the various snapshots that have been obtained. The impact of seasonal weather variations and changing anthropogenic operating practices such as occur at pumping wells are ignored.

A transient groundwater flow model was developed for Toms River, New Jersey. Spatially and temporally variable recharge was developed in a GIS framework for a 30+ year time span using actual daily temperature and precipitation data and the HELP3 hydrologic model. Monthly and daily well pumping volumes for the 8 wells in the domain were used for the same period. Sufficient data on the response of the aquifer to the spatially and temporally varying recharge and the temporally varying pumping were available in the form of historical water level measurements and continuous water level recorders. The comprehensive data set significantly aided in the development and calibration of a transient groundwater flow model.

Groundwater flow simulations clearly show that the system cannot be represented assuming steady state flow. Beyond the impact on groundwater flow expected with varying pump rates, temporally varying recharge is essential for model calibration. Contaminant transport simulations using averaged recharge and pump rates also resulted in an inability to describe the chemical data available for the aquifer. Consistent with the development of spatial-temporal averaging of the advection diffusion equation over a representative elementary volume, the impact of temporal averaging is to increase the dispersivities required for transport modeling. The research very clearly demonstrates the inadequacy of the steady-state assumption and temporal averaging for the simulation of contaminant transport in an aquifer with pumping wells.