STREAM-AQUIFER INTERACTIONS: PUMPING TESTS NEAR A STREAM WITH AND WITHOUT STREAM DISCHARGE

Over the last years, the problem of pumping induced stream depletion was subject to a considerable amount of theoretical work. However, little field work has been done to evaluate commonly applied physical concepts and mathematical models.

In order to investigate the applicability of existing models of stream depletion, two pumping tests were performed at a test site at the perennial Prairie Creek, Nebraska. The test site is instrumented with a pumping well at a distance of about 55 m from the stream and a monitoring network of 67 piezometers screened at different aquifer depths. The two tests were performed over 6 and 3 days respectively with a constant pumping rate of 6480 m³/day. The unconfined aquifer has a saturated thickness of 18 m and is in direct hydraulic connection with the 10 to 20 m wide stream.

Due to the extremely variable weather conditions during the year 2000, a unique data set was collected. In May, a first pumping test was performed under seasonally typical stream discharge conditions (0.3 m³/s). In August, a second test was performed under drought conditions, when no discharge was observed several hundred meters upstream and downstream from the test site.

The time-drawdown data from the first pumping test were analyzed using 2D analytical models to obtain estimates on the hydraulic properties of the aquifer and the stream-aquifer interface. The second test was interpreted independently in terms of aquifer parameters only by applying a validated analytical 3D model for pumping tests in unconfined aquifers without stream interaction.

Comparison of aquifer and interface parameters from the two data sets and different models allows for the direct verification of internal and inter-model consistency. The analyses with the 3D model yield consistent aquifer parameters over the entire domain. Aquifer parameters obtained from the 2D and 3D models compare well in certain spatial zones. The 2D models provide additional parameters on the stream-aquifer interface, which are similar at each stream bank, but vary between the banks.