|
publications > wri > 02-4050 > quantifying recharge/discharge > measured vertical head gradients
Interactions Between Surface Water and Ground Water and Effects on Mercury Transport in the North-central EvergladesBy Judson W. Harvey, Steven L. Krupa, Cynthia Gefvert, Robert M. Mooney, Jungyill Choi, Susan A. King, and Jefferson B. Giddings
Measured Vertical Head Gradients Compared with Hydrogeologic Model SimulationsFactors affecting recharge and discharge were examined using a simple hydrogeologic model of ground-water flow beneath levees. An analytical solution from Barlow and Moench (1998) was used to compute the hydraulic head distribution in a "leaky aquifer" with a hydraulic restricting layer at the top (representing peat). One-dimensional (horizontal) flow through an aquifer with uniform hydrogeologic properties is assumed in the model, and allows for vertical leakage across the peat. The stress applied to the model was a sudden 1-m change in head at the left boundary (representing an increase in the water level of a canal that is separated from the wetland by a levee). Because of the one-dimensional flow assumption, the head at the left boundary of the aquifer (in contact with the canal) is equal to the canal water level and is constant with depth. That boundary condition is common to many models of interactions between surface water and ground water, and, in the present case, represents the hypothetical situation where the canal fully penetrates the aquifer (fig. 15a). A constant surface-water level in the wetland was simulated using the "source bed" option, which holds the hydraulic head constant at the top of the restricting layer. This option is the best way to simulate how surface water that discharged from the aquifer would quickly flow away from the levee vicinity toward the wetland interior, leaving surface-water levels in the vicinity of the levee approximately constant for the period of the simulation. The hydrogeologic parameters used in model simulations were as follows: aquifer depth -60 m, peat depth -1 m, K in aquifer -30 m/d, K in peat -0.3 m/d, and specific storage in both aquifer and peat -0.001 m.
The primary result of the hydrogeologic simulation was determining that the effect of ground-water flow beneath the levee extended only 0.5 km into the wetland (fig. 15b). Even in a simulation with a K value for peat that was two orders of magnitude smaller than measured values (approximately 0.3 m/d), the distance of interaction increased only by approximately a factor of 2. Beyond 0.5 km, the simulated vertical head gradient across the peat was 3 x 10-6 or less, which is much smaller than typical vertical hydraulic gradients actually measured in interior areas of WCA-2A (typically 0.003 or higher, fig. 13). Even an improved two-dimensional model of ground-water flow beneath the levee probably would not change the observed mismatch between model and observations in the interior part of WCA-2A. Therefore, it was concluded that ground-water flow beneath levees is not an adequate explanation for a substantial part of the recharge and discharge that occurs in interior areas of WCA-2A. If water-level differences across levees and ground-water flow beneath the levee do not control recharge and discharge in interior areas of the wetlands, then what factors do? Other factors possibly controlling recharge and discharge may include fluctuations in surface-water levels in the wetlands. A constant surface-water level with no spatial or temporal changes in water-surface slope was assumed in the hydrogeologic model. Field observations discussed earlier indicated that surface-water levels and slopes change rapidly at times, because of water releases between WCAs. Surface-water slope in the wetland varies from essentially flat at certain times to a factor of 2 greater than the land-surface slope at other times. The effect of surface-water dynamics on interactions between surface water and ground water is discussed later as part of the interpretation of measured recharge and discharge fluxes. |
U.S. Department of the Interior, U.S. Geological Survey
This page is: http://sofia.usgs.gov/publications/wri/02-4050/measuredvert.html
Comments and suggestions? Contact: Heather Henkel - Webmaster
Last updated: 13 January, 2005 @ 12:09 PM (KP)
