New Mexico Geological Society Annual Spring Meeting — Abstracts

To replace the headworks that divert water from the Rio Grande to the low-flow diversion channel at San Acacia, the water table has been lowered from its pre-construction altitude of 4661 feet to 4647 feet. This paper discusses the geologic and hydrologic factors and construction constraints that controlled the design and locatidn of dewatering wells.

Strata at the site include the Sierra Ladrones Formation, colluvium (with a large basalt slump block?) interfingering with alluvium, fill and riprap: and post-dam silt and sand. Slug tests and a pumping test indicate that the hydraulic conductivities of the strata range from about 1 to more than 500 gpd/ft².

Near the headworks the pre-construction water table had a complex shape with slopes as large as .06 ft/ft. It intersected the river surface and the Socorro Main Canal near its bottom. The free-water surfaces of the river and canal were within 100 feet of three sides of the proposed excavation. The low-point on the water table was the low-flow channel.

During construction, a temporary earth-fill dam Quilt on the river bed, extending from the San Acacia Dam to the north bank, allows the river level to rise to an altitude of 4667 feet and discharge 250 cfs through a 108-inch conduit to the canal. Thus, during construction the water table was both higher and lower than before construction.

Seven wells from 30 to 60 feet deep pump from 30 to 95 gpm. Together with a central sump, they dewater the site and keep the water table below the temporary dam's surface. Four observation wells and four piezometers monitor the water-table altitude around the site.

All dewatering wells should have been drilled into the Sierra Ladrones Formation, but boulders in the colluvium and fill prevented the construction of ideal wells. Only two wells or test holes reached the Sierra Ladrones Formation.