NGWA Event and Education Recordings Archive

The ability to identify clogging and mounding at recharge basins is critical for maximizing infiltration performance. Clogging has long been considered the primary cause of reduced infiltration rates at recharge basins. However, an increasing number of basins are accepting treated water, reducing the influence of clogging. In such basins, water table mounding and/or perching may have a greater impact on performance. We hypothesize that these mechanisms can be identified based on unique relationships expected between infiltration rate declines and changes in vertical head gradients. To test this hypothesis, we instrumented the Orange County Water District’s newest recharge basin, La Palma Basin. This basin receives solely purified water. The basin was instrumented with paired distributed temperature sensing (DTS) cables and shallow piezometers. Infiltration rates were calculated throughout the basin floor, using heat as a tracer from the paired DTS cables installed on the basin floor and 0.5 m below ground surface. Head gradients were calculated using stage and shallow piezometric water level measurements. Clogging was identified in the deepest part of the basin where infiltration rates declined to asymptotically low values and head gradients increased to unity over a period of two months. These measurements indicate unsaturated conditions (i.e., a hydraulically disconnected basin and water table). Adjacent to the clogging zone, both infiltration rates and head gradients declined, indicating mounding or perching beneath the basin. These findings suggest that both clogging and mounding can affect performance in a single basin. The spatial and temporal density of DTS measurements proved to be an effective tool for identifying heterogeneous clogging and mounding conditions.