WELL-SITE LOCATION IN THE NORTH-BOUNDARY CHANNEL OF THE CA?ADA DE LOS ALAMOS LAND GRANT - SANTA FE COUNTY, NEW MEXICO, U.S.A.
William M. Turner, Ph.D.
The Ca?ada de los Alamos Land Grant is situated about 12 miles (19 km) southeast of Santa Fe, New Mexico in the arid American Southwest. It is located in Figure 1. The Grant is more than 20 square miles (52 km2) in area and lies mostly on the Seton Village 7-1/2 MinuteYoycjj Rtj{D,Te9=#mfME|| Zopographical quadrangle map. The Grant traces its origin back to a grant of land from the King of Spain during the Spanish Colonial Period.
The Grant is bounded on the east by the foothills of the Sangre de Cristo Mountains. On the north it is bounded by the North Boundary Channel, an ephemeral stream channel. The southern boundary is the eastward flowing Gallisteo River. The western boundary has no geographic significance.
The property was purchased for residential development purposes and the developer encountered great difficulty in finding sufficient ground-water supplies.
AGW Consultants was contracted to conduct a hydrogeological evaluation and a Thermonic well-site location survey.
HYDROGEOLOGIC SETTING
The Ca?ada de los Alamos Grant has a gentle westward sloping surface that is eroded by a westward- flowing, ephemeral, drainage system into a series of parallel, broad hills separated by small valleys. The Grant is bordered to the east by the foothills of the Sangre de Cristo Mountains.
The Grant is comprised mostly of Ancha Formation of Pliocene age that unconformably overlies Precambrian basement metamorphic and granitic rock to the east. The Ancha Formation is poorly consolidated sand and gravel. It increases in thickness from a feather-edge on the eastern margin of the Grant to about 150 feet (46 m) at the west boundary of the Grant about 4 miles (6.5 km) to the west.
Driller's logs and the geologic map of the area indicate the Ancha Formation blankets an erosional paleo-surface of Precambrian crystalline rocks, Paleozoic and Mesozoic sandstone, limestone and shale and the Tertiary Tesuque Formation. The rocks in the subcrop produce little water to wells.
The area is block faulted and the Ancha Formation can change in thickness within a short distance. The water-bearing zone in the Grant is usually beneath the base of the Ancha Formation. However, where the Ancha Formation infills deeply incised paleo-channels in the subcrop, it is water-bearing. Because the channel deposits are well-sorted, coarse-grained material it can be a prolific aquifer.
Block faults occur primarily as north-south oriented, en echelon, step faults down-dropped to the west away from the mountain front. A second set of faults trends from northeast to southwest from the Precambrian basement. They can be traced on areal photography through the younger Ancha Formation as fracture traces. A major fracture trace coincides with the North Boundary Channel.
Our geologic analysis suggested that faults along the North Boundary Channel may have localized erosion of the subcrop and that water-bearing Ancha Formation may infill a subcrop paleo-channel.
Ground-water recharge comes from infiltration of surface-water runoff along the North Boundary Channel. Ground water may also occur within the fault zone underlying the North Boundary Channel. If the faults extend into the bedrock of the Sangre de Cristo Mountains to the east, they may drain the shallow ground-water system of the mountains into the Ancha Formation along the North Boundary Channel.
THERMONIC SURVEY
The objective of the Thermonic Survey in the area of the North Boundary Channel was to locate the axis the paleo-stream channel and the zone of highest ground-water-flow rate.
We drilled 50 Thermonic observation holes 20 feet (6 m) deep in two north to south lines across the North Boundary Channel. We initially spaced the holes 50 feet (15 m) apart. We equipped them with specially designed heat-flow measuring tubes in which we measured thermal data.
Mathematical methods of data analysis assume that we are dealing with a semi-infinite solid of uniform surface elevation, surface heating and thermal properties. In the real world this is seldom the case. AGW scientists developed proprietary methods to correct for effects of surface elevation, solar heat input and variable thermal characteristics of the soils.
RESULTS
AGW scientists made thermal measurements in the Thermonic observation holes. Based on the data, we constructed several additional holes to zero-in on the location of highest ground-water-flow rate. That is, based on our interpretation of the initial data from widely spaced Thermonic observation holes, we knew we were on zones of high ground-water flow rate. However, we were uncertain of the precise location of the zones. We closed in on our target zones using a Newtonian, interval-bisection method and drilled additional holes such that the hole spacing in the vicinity of the target zones was 25 feet (7.6 m).
We located two well sites on each north to south profile line. That is, instead of finding only one major buried paleo-channel, we found two. On the assumption that the paleo-channels were localized by basement faults, we projected them eastward where we found the faults in the exposes granite.
AGW scientists conducted an aquifer performance test on a water well, drilled into these narrow, fault-localized, ground-water conduits. The well produced 202 gallons per minute (12.7 l/s). The transmissivity of the Ancha Formation aquifer at the well site is 101,000 gpd/ft (1,252 m2/).
We conducted a thermal injection test on the well to determine the production zones. The test indicated that a thick, well-sorted, sequence of Ancha Formation had infilled a paleo-channel in the subcrop surface. Below the subcrop surface, there was no water production.
Today, this well is the major source of supply for the residents of the area.
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