Year: 1963
Type: report
Abstract: A large caldera in the Jemez Mountains of north-central New Mexico was studied for its potential as a source of water supply for the town of Los Alamos, N. Mex.This caldera is roughly 12 miles in diameter.The floor of the depression is divided into a network of valleys by numerous domes of rhyolite.Two valleys in the eastern half of the depression, the Valle Grande and the Valle Toledo, compose the area investigated for this report.The eastern margin of the study area is about 5 miles west of Los Alamos.Altitudes in the area range from slightly less than 8,500 feet on the valley floor to about 11,250 feet at the top of the highest rhyolite dome.The crest of the rim ranges from 9,500 to 10,500 feet.The Jemez Mountains consist of a volcanic pile whose interior has collapsed.Rhyolite domes were extruded in the caldera area and the lowland area remaining between rhyolite domes and a caldera rim was, for a time, a lake.The lake gradually filled with debris derived largely from the rhyolite domes and, in part, from rocks of a caldera rim.The rocks that crop out in the area are volcanic and sedimentary.An older sequence of flow rocks (Chicoma volcanic formation of Smith, 1938) is of probable Pliocene and Pleistocene ages, a younger series of extrusive domes and tuff (Bandelier rhyolite tuff of Smith, 1938) is of probable Pleistocene age.The sedimentary rocks are lacustrine deposits of Pleistocene age and surface alluvium consisting of terrace, fan, and channel deposits, all younger than the volcanic rocks.The Chicoma volcanic formation of Smith (1938) and the Bandelier rhyolite tuff of Smith (1938) do not contain important aquifers in the area.The same is true of the rhyolite domes, but the blocky crust and the porous rhyolite of the domes make them important as areas of recharge for the sedimentary rocks of the caldera fill.Probably little if any water flows off the domes; it infilters downward through the porous rhyolite and into the porous strata in the lakebeds and perhaps the alluvial fan material on the edges of the valleys.Several clayey members interfinger with the pumiceous sand and gravel of the caldera fill.Test drilling indicated that individual clay members are as much as 20 feet thick in the Valle Toledo; however, at places in that valley, the clayey members unite to form a clayey zone as much as 80 feet thick.In Yl Y2 CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATESthe Valle Grande, a thick zone of clay overlies a thicker zone of pumiceous sand and gravel.The maximum known thickness of the upper clay and underlying sand and gravel are 295 feet and 880 feet, respectively.The pumiceous sand and gravel of the lake sediments form the principal aquifers in the Valley Toledo and the Valley Grande, and ground water in these aquifers is confined under pressure by clay zones.A pumping test in the Valle Toledo indicated that the coefficient of transmissibility of the aquifer in that valley is about 50,000 gpd (gallons per day) per ft, and its coefficient of storage is about 0.005 The specific capacity of the pumped well used in the test was about 50 gpm (gallons per minute) per ft of decline in pressure.A pumping test in the Valle Grande indicated that the coefficient of transmissibility for the aquifer in that valley is at least 25,000 gpd per ft; the coefficient of storage was not determined.The specific capacity of the discharging well was about 10 gpm per ft of drawdown.Properly constructed wells developing water from the lake deposits might be expected to yield more than 1,000 gpm each.Large-discharge wells should be spaced far enough apart to avoid undue interference among the wells.Considerable water could be pumped from storage but the amount of water perennially available to wells in each valley would be the amount of water issuing from springs in their respective valley about 2,200 acre-feet in the Valle Grande and about 1,600 acre-feet in the Valle Toledo.Pumping from wells would deplete the flow of the springs and thereby interfere with surfacewater rights downstream.Water from the lake deposits is high in silica content but low in total dissolved solids.The water is soft.Water in the Valle Toledo is relatively high in fluoride content and would have to be mixed with low fluoride water before using in a municipal system.
DOI: 10.3133/wsp1619y
Language: English
Subfield: Water Science and Technology
Field: Environmental Science
Domain: Physical Sciences
Open Access status: gold
OpenAlex ID: W48334296