Microdepression-focused recharge in a coastal wetland, La Plata, Argentina

Depression-focused recharge (DFR) has been well documented for the clay-till plains of western Canada, kettle lakes of southeastern Wisconsin, the Sandhills region of Nebraska, and elsewhere. In this study the importance of DFR in a fine-grained estuarine coastal plain environment with extremely low topographic relief is shown. The southwestern coastal plain of the Rio de La Plata near the city of La Plata, Argentina, extends 8-9 km inland from the river, and ranges in elevation from 0 to +3 m above sea level. The Holocene Postpampeano sediments, predominantly composed of silty clay in the study area, mantle the coastal plain and exert a major control on the hydrodynamics of groundwater of the region. The average hydraulic conductivity of the unit ranges from 1.0 X 10(-9) m s-1 when estimated on a small scale (single-well and oedometer tests) to approximately 1.0 X 10(-8) m s-1 when estimated on a larger scale (tritium profiles and seepage meters). A study of the groundwater flow system has established that recharge is occurring in the marshes of the central coastal plain, which are 0.5-1.0 m lower than the surrounding land. A broad, 50-cm-high groundwater mound (a significant feature in the context of a regional hydraulic gradient of 0.0005) was documented below the marshland using nests of monitoring wells. Recharge rates are further controlled by subtle, 10-40-cm elevation variations within the marsh. Four nests of six monitoring wells each were installed in the marsh and sampled for tritium. Recharge rates were then estimated through calibration simulations using a 1-D analytical solute transport model. In the two nests in the lower, wetter areas, recharge rates were estimated at 4-6 cm year-1, whereas estimated recharge rates in the higher, drier areas ranged from 0-3 cm year-1. These differences are consistent with: (1) near-neutral vertical hydraulic gradients in higher areas vs persistent downward gradients in lower areas, (2) chloride concentrations almost an order of magnitude lower in groundwater underlying lower areas than in that of higher areas due to flushing of older, more saline groundwater in the lower areas, and (3) isotopic evidence for evapotranspiration only in the higher areas. The present study suggests that ponding of surface water in local depressions can lead to significant recharge from coastal wetlands. This finding has implications for the study of water balance, soil salinity, and nutrient budgets in these environments.