MODELING REGIONS OF COMPLEX FLOW: INTERPRETING ABNORMAL PUMPING TESTS AT LAWRENCE LIVERMORE NATIONAL LABORATORY

Unexpected drawdown patterns in hydraulics tests at Lawrence Livermore National Laboratory have prompted an interest in the extent meter-scale heterogeneity can have on subsurface flow patterns in alluvial aquifers. Hydraulic monitoring of six observation wells within a ~60m² region was conducted to assess response to short-term extraction. Results surprisingly revealed both highly variable responses between observation wells and anomalous drawdown patterns within individual wells. General comparisons with respect to spatial relationships and screened intervals suggest that communication between wells is extremely vertically limited. Further assessment of drawdown patterns revealed anomalous responses unique to individual wells, consisting of 1) delayed responses, 2) decreased drawdown during periods of extraction, and 3) continued drawdown after pumping ceased.

Often heterogeneities at the meter scale are overshadowed by higher-order heterogeneities and neglected to aid model simplification. This is especially apparent when the modeled region exceeds a few hundred meters, as is often the case for water resource interests. Sites concerned with contamination, however, can be greatly disrupted by these flow disturbances and therefore require a higher resolution understanding of the subsurface. Complexities apparent in hydraulic tests allude to the geological heterogeneities we are unable to view. Interpretation of these tests, along with a geologically-founded conceptual model of the region, were utilized to construct numerical flow models of the area. Fifty-one models were constructed with varying geological parameters (e.g. facies dimensions and orientations) in an attempt to capture the general character of these observed drawdown anomalies. By capturing these patterns we are able to learn more about the geologic character capable of producing the anomalous drawdowns observed.