Influence of the geologic and geomorphologic characteristics and of crab burrows on the interrelation between surface water and groundwater in an estuarine coastal wetland

Abstract

► Tides modify the interrelation surface water/groundwater in coastal wetlands. ► The tidal wave flows into the aquifer according to its hydrologic properties. ► At the erosion scarps the tide propagates into the aquifer as a sub-horizontal flow. ► At the intertidal flats the tide enters the aquifer mainly as a sub-vertical flow. The interrelation between surface water and groundwater in intertidal flats is often studied through mathematical models. In many cases these models need to be supported by an integral analysis of the geologic, geomorphologic, hydrologic and biological characteristics of the environment that are to be obtained from field surveys. The marshy environment of the River Ajo in the Samborombon Bay wetland, Argentina, is a typical example of an estuarine coastal zone. Geologic and geomorphologic surveys were carried out, as well as measurements of surface water and groundwater level changes as a response of the aquifer to tidal forcing. The banks of the River Ajo are either scarped with storm flats, or mildly sloped with intertidal flats and numerous crab burrows. Sediments are mainly silty-clayey with low permeability, and lie over silty-sandy layers. At the erosion scarps the tidal wave enters the aquifer as a sub-horizontal flow through the pore space of the sediments. The tidal range in the aquifer depends on the lithological characteristics of the sediments and on the side changes of their hydraulic conductivity. The rise of the water table at high water and its subsequent fall are nearly sinusoidal, with a period similar to that of the tide at the river. At the intertidal flats, instead, the tidal wave enters the aquifer mainly as a sub-vertical flow through the crab burrows. As the crab burrows are not interconnected, they are not distinct pathways for preferential flow. Therefore, the groundwater flux into the river is very slow during low water, and the recovery of the water table takes a long time. The tidal influence upon the water table on both kinds of banks affects only a narrow strip of the aquifer. Not only are the characteristics of the marshy environment of the River Ajo representative of most of the Samborombon Bay wetland; they can also be extended to other similar coastal wetlands to help preserve these invaluable environments.