MODEL POST AUDIT FOR THE SUNDRE AQUIFER, NORTH DAKOTA AND IMPLICATIONS FOR GROUND WATER MODELING

A post audit of a ground water model constructed in 1987 for the Sundre aquifer was conducted. The model was constructed to predict the effects of a planned well field. Ground water monitoring data for the period 1968-1998 was evaluated and used to construct a water budget for the aquifer system before and after the well field was constructed. This allowed examination of the aquifer as it responded to development and of the performance of the Sundre aquifer ground water model.

Observation wells in the Sundre aquifer demonstrated average drawdown greater than 44 feet during the period 1976 to 1998. The ground water model of the Sundre aquifer did not accurately represent the Sundre aquifer. This is due to an inadequate conceptual model of the aquifer system. The model predicted that leakage from the overlying Minot/Lower Souris aquifer would account for 98 to 99% of pumpage from the Sundre aquifer. The findings of this study indicate that leakage accounts for 33% while lateral flow in the aquifer provides 35 to 53% of pumpage. The remaining 14 to 32% came from storage in the Sundre and Minot/Lower Souris aquifers, or from bedrock sources.

Given the data available at the time the model was constructed, it would be considered a good model. A model post audit was not conducted as a routine part of model construction as is suggested by most modeling textbooks. If this step is omitted, planning and management can be adversely affected as calibration errors accumulate over time. Additionally, by not modeling the entire hydrologic system, the system boundaries had a larger affect than anticipated which would only become apparent in a model post audit.