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Semiconsolidated sand aquifers

	The sediments that compose semiconsolidated sand aquifers (yellow) are in the general shape of a wedge that thickens seaward.

The principal water-yielding aquifers of North America can be grouped into six types: aquifers in unconsolidated sand and gravel, semiconsolidated sand aquifers, sandstone aquifers, carbonate-rock aquifers, aquifers in interbedded sandstone and carbonate rocks, and aquifers in basalt and other types of volcanic rocks.

These aquifers consist of semiconsolidated sand interbedded with silt, clay, and minor carbonate rocks. Porosity is intergranular, and the hydraulic conductivity of the aquifers is moderate to high. The aquifers underlie the Coastal Plains of the eastern and southern United States, and they are of fluvial, deltaic, and shallow marine origin. The aquifers are in a thick wedge of sediments that dips and thickens coastward; in places, the sands of the aquifers are more than 650 meters thick. The varied depositional environments of these sediments have caused complex interbedding of fine- and coarse-grained materials. Accordingly, some aquifers are local whereas others extend over hundreds of square kilometers. The numerous local aquifers can be grouped into several regional aquifer systems that contain groundwater-flow systems of local, intermediate, and regional scale. Water in topographically high recharge areas is unconfined, but, it becomes confined as it moves coastward. Discharge is by upward leakage to shallower aquifers or to saltwater bodies in coastal areas. Because flow is sluggish near the ends of regional flow paths, the aquifers commonly contain unflushed saline water in their deeply buried, downdip parts. Where shallow aquifers have been heavily pumped near the coasts, saltwater intrusion has locally contaminated the groundwater. During 1985, more than 30 million cubic meters per day was withdrawn from these aquifers.

This map of semiconsolidated sand aquifers in the United States shows the shallowest principal aquifer. In some places, other, sometimes more productive, aquifers underlie those mapped. Only small areas of some aquifers may be shown on the map because they are covered in many places by other aquifers closer to the land surface. In other places, local aquifers, such as those along stream valleys, might overlie the aquifers mapped. Local aquifers are not shown because of the scale of the map. Some aquifers in sedimentary rocks are overlain by confining units, and the aquifers extend into the subsurface beyond the areas shown on the map.

PRINCIPAL SEMICONSOLIDATED SAND AQUIFERS
		Semiconsolidated sand aquifers at or near the land surface.		Limit of continental glaciation. North of this line, glacial sand and gravel aquifers overlie bedrock aquifers in many places.

Semiconsolidated sand aquifers include:

• Coastal lowlands aquifer system (Gulf Coast)
• Texas coastal uplands aquifer system
• Mississippi embayment aquifer system
• Southeastern Coastal Plain aquifer system
• Northern Atlantic Coastal Plain aquifer system

See also: Unconsolidated sand and gravel aquifers

Publication source

• Introduction and National Summary (chapter A)