Delineation of Preferred‐Flow Paths by Response to Transient Lagoon Exfiltration

Abstract

Observations of hydraulic head response to lagoon leakage into an extremely heterogeneous aquifer allow qualitative identification of spatial patterns in heterogeneity. In a 150–hour leakage experiment over such an aquifer, the fluid mass balance was estimated using a stage‐fluctuation model while transient response was observed in downgradient wells and springs. Based on timing with respect to pond leakage, water‐level response at different locations within the aquifer may be identified 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% above baseline flows, showing initial increase within ca. 40 hours. Synoptic water‐level rise patterns observed during the test describe enlargement of a recharge mound from beneath the pond and suggest the spatial pattern of PFPs to > 150 m distant. An analytical model assuming homogeneity (Hantush, 1967), applied to water‐level response for wells > 30 m from the pond, overestimated K by > 1 order of magnitude in comparison to slug‐test estimates. This discrepancy is attributed to the inferred presence of linearly continuous heterogeneities causing preferential flow. Large‐scale hydraulic tests such as this sample the spatial structure for conductive pathways and may be of utility in parameterization of numerical flow models. In contrast, slug‐test results give local parameter estimates, but more limited information on PFP distribution.