HERMOSILLO BASINS GROUND-WATER-RESOURCE EVALUATION USING THERMONIC METHODS
William M. Turner, Ph.D.
In the early 1970's, the Mexican Government became alarmed at the rate of salt-water intrusion into the Kino Bay-Hermosillo-El Sahuaral coastal aquifer west of Hermosillo and El Sahuaral, Sonora, Mexico. Figure 1 shows the location of the Hermosillo Basin. Salt-water intrusion from the Sea of Cortez was caused by overpumping of the shallow aquifer by agricultural water users that began in the 1940's. By the early 1970's, large areas of irrigated agriculture were going out of production. By 1998, the over-exploitation of four decades had caused several drawdown cones to lower the water levels by 150 feet (50 m) below sea level.
The Kino Bay-Hermosillo-El Sahuaral coastal aquifer comprises an area of about 1,000 square miles (3,000 km2 ). It is a flat, wide valley. It rises in elevation from a few feet above sea level at the coast to about 600 feet (200 m) above mean sea level at Hermosillo about 70 miles (110 km) inland to the east.
To alleviate this problem, the Secretaria Recursos Hidraulicos (predecessor of CONAGUA) contracted with American Groundwater Consultants to evaluate the sustainable water resources of the "Hermosillo Basins." The Hermosillo Basins are defined as the alluvium-filled basins south and east of Hermosillo and east of the Kino Bay-Hermosillo-El Sahuaral aquifer. The Hermosillo Basins comprise an area of about 1,800 square miles (5,000 km2).
The objective of our study was to identify zones of maximum aquifer transmissivity leading to the coastal plain aquifer. Wells located in these zone would be pumped into a surface water conveyance channel for agricultural use.
HYDROGEOLOGICAL SETTING
The Hermosillo Basins area is comprised of thick sequence of interbedded silt, sands and gravels of Miocene to Recent age overlying a structurally controlled basement. Basement rocks are Late Mesozoic fine-grained clastics and interbedded volcanics cut by Early Tertiary granitic to granodioritic intrusives and overlain by rhyolite flows, pyroclastics, and ignimbrites. The Miocene to Recent alluvium was deposited as coalescing alluvial fans along numerous northwest-southeast-trending normal faults.
Rainfall in the area averages 7.9 inches (20 cm) per year. Recharge to the alluvium in this arid area takes place as mountain-front recharge and as recharge along line sinks along ephemeral stream channel. There are no perennial streams in the area.
In making its way to the sea to the west, the ground water is constrained to flow around bedrock inselbergs and through narrow alluvium-filled channels between the bedrock inselbergs.
PROCEDURE
AGW scientists carried out geological reconnaissance studies within the area. We measured ground-water levels and prepared a ground-water-level contour map to show the general direction of ground-water flow.
We measured temperature profiles in all accessible wells throughout the area to quantify the volumetric rate of flow and to evaluate relative transmissivity of the aquifer.
RESULTS
Ground-water temperature increases from the east to the west in the general direction of ground-water flow. Ground water in the recharge zones occurs at about 26oC (78oF). At the western edge of the study area, ground-water temperatures were more than 35oC (95oF)
The Thermonic analysis of all data defined zones of high aquifer transmissivity about 5 km (3 mi.) wide along which well fields were recommended.
Our analysis of the Thermonic data indicated a sustainable annual ground-water availability of 13 MCM (10,541 af/yr) without mining of the aquifers.
FURTHER READING
Andrews, R.W., 1981, Salt-water intrusion in the Costa de Hermosillo, Mexico: A
numerical analysis of water management practices, Groundwater, v. 19, n. 16, pp.
635-647.
Steinich, B., Escolero, O., and Marin, L.E., 1998, Salt-water intrusion and nitrate
contamination in the Valley of Hermosillo and El Sahuaral coastal aquifers,
Sonora, Mexico, Hydrogeology Journal,
v. 6, n. 4, pp. 518-526.
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