A preliminary assessment of land-surface subsidence in the El Paso area, Texas

The northeast and southeast parts of the El Paso area are underlain by Hueco bolson deposits as much as 9,000 feet thick. The deposits consist of lenses of gravel, sand, silt, and clay. In the Rio Grande Valley, about 400 to 450 feet of these deposits have been eroded and replaced with as much as 200 feet of alluvium. Ground water in the shallow alluvial aquifer in the Rio Grande Valley and in the Hueco bolson aquifer outside the valley is under water-table conditions, whereas ground water in the bolson aquifer in the valley is under leaky artesian conditions. Maximum water-level declines in the Hueco bolson aquifer are 110 feet east of the Franklin Mountains and 150 feet in the downtown El Paso area. For the shallow aquifer, the maximum declines have been 125 feet in the downtown area. Compressable materials in the freshwater zone of the aquifer range from 50 to 450 feet.

Recharge from the Rio Grande to the shallow alluvial aquifer has increased from an estimated 15,000 acre-feet during 1968 to 30,000 acre-feet during 1983, an increase of about 1,000 acre-feet per year. Leakage from the Rio Grande is expected to continually increase in the near future because of a continued decline in ground-water levels. The amount of leakages from the canals is much less than from the river.

Releveling of bench marks along lines to the northeast and the southeast of the Rio Grande, and along its channel commonly show land subsidence of about 0.2 foot. The maximum measured subsidence is 0.41 foot along the river in the Chamizal zone. No subsidence was detected at the Riverside Diversion Dam. A comparison of subsidence, water-level declines, and clay thickness along the three survey lines shows the expected correlation of greater subsidence with thicker accumulated clay material for a given decline in water levels. The preconsolidation stress was expected to range from 85 to 115 feet of water-level decline on the basis of subsidence studies in Arizona and California. A study of specific-unit compaction along the three survey lines shows that the values usually range between 1.0 to 2.5 x 10-5 feet per feet squared. These values are comparable to the ones computed in the Tulare-Wasco, California, and Houston-Galveston, Texas, areas following the exceedance of the local preconsolidation stress. Because of this comparability, the specific-unit compaction for future periods in the El Paso area probably will not increase dramatically when the preconsolidation stress is exceeded, if it has not already been exceeded.

In addition to regional subsidence, local subsidence is indicated by observable surface fractures but has not been verified by precise leveling. These local areas coincide with areas that historically were swamps along the Rio Grande.