HR: 13:30h
AN: NB43D-01 INVITED     [Abstracts]
TI: Scale Dependence of Measurements of Surface-water and Groundwater Interactions in Everglades Wetlands
AU: * Harvey, J W
EM: jwharvey@usgs.gov
AF: U.S. Geological Survey, 430 National Center, Reston, VA 20192 United States
AB: The Everglades is an outstanding example of a subtropical peatland with a distinctive plant community adapted to the low-nutrient, calcium bicarbonate-type water. Interactions between surface water and groundwater were fundamental in developing the hydrological and chemical signatures that define the Everglades habitat. One of the changes resulting from decades of Everglades management for flood control is believed to be increases in recharge and discharge fluxes in the Everglades. Resulting effects that may need to be addressed by the Everglades restoration include increased rates of storage of contaminants (phosphorus, sulfur, mercury) in peat soils that could prolong the legacy of water-quality contamination, as well as increased discharge from deeper parts of the aquifer of dissolved salts emerging near canal and levee systems that move into the wetlands to influence plant community structure in profound ways. The present research compared results from several independent methods, including estimation of recharge and discharge fluxes from (1) hydraulic-head gradients and hydraulic conductivity in peat; (2) rates of vertical movement in the subsurface based on bromide tracer injections, and (3) rates of vertical movement in the subsurface based on modeling the subsurface distribution of naturally occurring isotopes (²²⁴Ra, ²²³Ra, ³H, and ³H/³He). The measurements revealed that surface-water and groundwater interactions are best described as bi-directional, with both recharge and discharge occurring within relatively close proximity. Most of the surface water that is recharged remains within peat porewater or the top 7 m of the underlying aquifer for variable periods or time (ranging from weeks to centuries) before being returned by discharge back to surface water. Inherent differences were found in the sensitivity of each method to different timescales of surface-subsurface water exchange. For example, tracer measurements in peat were sensitive to the relatively large component of recharge and discharge fluxes driven by high-frequency (weekly to monthly) reversals in the hydraulic gradient that result from heavy precipitation and/or sudden surface-water releases through water-control structures. In contrast, tritium modeling was not sensitive to those high-frequency signals, and was instead sensitive to the much smaller component of recharge and discharge fluxes that are driven deeper into the aquifer (limit of approximately 8 m) by lower frequency fluctuations in hydraulic gradient resulting from longer term (interannual to decadal) variations in the Everglades water balance. Use of complementary measurement approaches therefore revealed a "scale-dependence" of measurements of surface-water and groundwater interactions in the Everglades that must be considered for specific applications. For example, rates of recharge and discharge based on fast-timescale exchanges between surface water and peat porewater are appropriate for problems involving the possible remobilization of very high levels of nutrients stored in areas of the Everglades that were previously enriched by drainage from agricultural areas.
UR: http://water.usgs.gov/nrp/jharvey/site/index.html
DE: 1829 Groundwater hydrology
DE: 1871 Surface water quality
DE: 1890 Wetlands
SC: North American Benthological Society [NB]
MN: 2005 Joint Assembly