A GEOPHYSICAL FIELD INVESTIGATION OF INFILTRATION DURING RAIN EVENTS IN THE RIPARIAN ZONE

Geophysics was used to monitor function of the riparian zone during storm events. Numerous chemical processes make the riparian zone an effective buffer for nonpoint source pollution. To fully understand where these chemical processes occur, one must also understand the physical processes or the pathways that water takes. An important focus is the unsaturated zone because of its proximity to the pollution sources that are being buffered by the riparian zone such as agricultural runoff. During storms the unsaturated zone can be wetting from either infiltration or backflow from a rise in the stream water level where a gaining stream temporarily becomes a losing stream. To track the pathway of the water that wets the unsaturated zone during a rain event, a transect perpendicular to the stream was monitored along the riparian zone of the east branch of White Clay Creek at the Christina River Basin CZO in Avondale, PA. This transect is ideal because of its natural, undisturbed state and topographic contrast on either side of the stream (a broad floodplain to the west and a narrow floodplain with steep hillslope to the east). Electrical resistivity tomography (ERT) was employed with repeated runs every 15 minutes throughout the storm to monitor changes in resistivity. Time lapse inversion increases sensitivity to changes in soil moisture in the resistivity profiles. A SuperSting® was used with a dipole-dipole array and 28 electrodes with a 0.7 m spacing (18.9 m transect) for greater penetration and 0.5m spacing (13.5 m transect) for greater resolution. Soil moisture probes installed at two locations along the line confirmed trends observed in the ERT. Infiltration at this site occurred from the top down, not from backflow. Wetting of the unsaturated zone occurred abruptly within a half hour of an 11 mm/hour rain event. Differing intensities of infiltration were observed along the ERT profiles. For example, the 11 mm/hour event had three patches of 1-2 m at saturation surrounded by less saturated ground. Geophysics provided more information about heterogeneity and suggested locations for additional soil moisture probes.