ORIGIN, ISOTOPIC COMPOSITION, AND RESIDENCE TIME OF WATER IN THE ROOT ZONE OF A BALD-CYPRESS DOMINATED WETLAND

The hydrology of the root zone in a wetland dominated by bald cypress trees is being studied as part of a larger investigation of the hydrologic controls on the growth rate of the wetland trees. Sky Lake is an oxbow lake located on the ancestral floodplain of the Mississippi River in northwestern Mississippi. The lake is surrounded by a vegetated fringe with a large population of century-old bald cypress. Lake levels vary seasonally by over 4 m, fully inundating the vegetated fringe during high water, and leaving the fringe fully aerated during low water.

The hydraulic gradient is being monitored using a series of nested piezometers along an elevation transect through the vegetated fringe. The piezometers are completed at depths of 0.6, 1.8 and 3.0 m. During the Summer and Fall of 2002, lake levels were low. The vertical hydraulic gradient was consistently downward in all nests. The horizontal gradient was more complex. The gradient in the shallow piezometers consistently sloped toward the lake, but the 3.0 m piezometer maintained a hydraulic high in the center of the vegetated fringe for both seasons. Pressure response in all the piezometers to changes in lake level was rapid, but flow rates appear to be extremely slow. Water removed from the piezometers during water sampling took several weeks to recover.

Preliminary oxygen isotope data demonstrate three processes. First, substantial evaporation from the lake is evident from elevated summer d¹⁸O values for lake water (+3‰ VSMOW) relative to stream inflow values (-1.3 to -4.9‰). Second, contributions from Hurricane Lili (Sept. 21, 2002) was apparent from a significant drop in the precipitation d¹⁸O value. Normal values during this time of year were in the range of -3 to -6‰. Hurricane Lili delivered water with a d¹⁸O of -11‰. Lastly, the d¹⁸O of groundwater in the root zone is more similar to inflowing stream water than to the enriched summer lake-water, which is consistent with the hydraulic gradient observed in the 1.8 m piezometers. Monitoring is continuing to see if gradients and source water reverses when lake levels are high during the winter months.