DELINEATION OF PREFERRED-FLOW PATHS BY RESPONSE TO TRANSIENT LAGOON EXFILTRATION

Observations of hydraulic head response to lagoon leakage into an extr emely heterogeneous aquifer allow qualitative identification of spatia l patterns in heterogeneity. In a 150-hour leakage experiment over suc h an aquifer, the fluid mass balance was estimated using a stage-fluct uation model while transient response was observed in downgradient wel ls and springs. Based on timing with respect to pond leakage, water-le vel response at different locations within the aquifer may be identifi ed as either within or outside the influence of preferred-flow paths ( PFPs). Flows at springs 150-200 m from the pond increased by 20-60% ab ove baseline flows, showing initial increase within ca. 40 hours, Syno ptic water-level rise patterns observed during the test describe enlar gement of a recharge mound from beneath the pond and suggest the spati al pattern of PFPs to > 150 m distant. An analytical model assuming ho mogeneity (Hantush, 1967), applied to water-level response for wells > 30 m from the pond, overestimated K by > 1 order of magnitude in comp arison to slug-test estimates. This discrepancy is attributed to the i nferred presence of linearly continuous heterogeneities causing prefer ential flow. Large-scale hydraulic tests such as this sample the spati al structure for conductive pathways and may be of utility in paramete rization of numerical flow models. In contrast, slug-test results give local parameter estimates, but more limited information on PFP distri bution.