The Plains of San Augustine
The San Augustin Plains or watershed is a closed basin 1,993 square miles in size. Closed basin means that there is no outlet surface stream. Therefore, the watershed gets no water from outside sources. There are no perennial streams within the basin. The water that is in the basin is considered ancestral water mainly from the past Pleistocene ice age. The recent water additions in the last couple of centuries has been less than evapotranspiration rate, as water has not accumulated to form a lake as it did in past Pleistocene Era. That large lake has long since evaporated.
Two counties share the watershed; Catron County’s portion is approximately 1,551 square miles in size, a little over three quarters of the watershed, while Socorro County is about 441 square miles in size. Land ownership within the basin is approximately as follows: BLM 222.4 square miles, FS 438.8 square miles, private land 830.7 square miles, and state land 500.1 square miles. The actual plains portion of the watershed is approximately 603 square miles in size or about 30% of the watershed. The Blodgett and Titus report of, 1973 at the time of the study concluded that the basin was receiving approximately 100,000 acre feet of water per year from rain and snow; however the basin seemed to be losing that same amount from evapotranspiration and leakage. They assumed that the main source of leakage was along the southern boundary. Since the time of the study New Mexico has been in an extended drought and probably not receiving anywhere near that amount of water annually. The NM state geologic map shows many faults that cross the watershed boundary and more recent geologic mapping have identified even more faults.
The San Augustin Plains is a topographically high area which means groundwater in the basin will seek to flow in all direction away from the basin. The gradient flows from the basin have been calculated by a report done by RG Myers, JT Everheart, and CA Wilson in 1994 titled Geohydrology of the San Augustin Basin, Alamosa Creek Basin upstream from Monticello Box, and upper Gila Basin in parts of Catron, Socorro, and Sierra Counties, New Mexico. This is an excellent publication and should be read if you are interested in the groundwater resources in this area.
The basin is made up of mainly volcanic rocks and sediments. The under lying geologic rocks are from the Cretaceous Period. The basin deposits are approximately 4,000 feet thick in the graben trough or approximately three quarters of a mile thick. Outside of the graben structure the sediments are considerably shallower. As the volcanic rocks were being erupted the basin was being pulled apart by basin and range tectonic activity. This formed the graben, a down dropped block of the earth’s crust; that resulted in the San Augustin Plains. The graben is a complex structure, with a northeast, southwest block and a north-south block; approximately 495 square miles in size which is about 25% of the overall watershed. The western part of the graben is approximately 268 square miles in size and the eastern block is about 216 square miles in size. The southwestern end of the western graben is mainly saline in nature and takes up about 45% of this structure.
Much of the sediment now filling the graben or basin is volcanic ash. The ash deposits over time have been turned into silt/clay layers; these layers do not hold a large quantity of extractable water. The material near the watershed margins are generally coarser in
nature and therefore have a higher porosity to yield more water but the extraction wells would have to be much deeper. Wells along the watershed boundary would not be consistent in water production, because of the massive volcanic bedrock. Wells in the interior of the basin are not consistent in water production either as the inter fingering of course materials with the finer materials creates perched aquifers. There are currently 1,025 wells within the basin and 9,327 acre feet have been allocated under the state permit system.
The Augustin Plains Ranch, LLC which is totally within Catron County but abuts Socorro County, has proposed thirtyseven wells along the northwest graben structure and would like to pump 54,000 acre feet of water annually. The ranch is approximately 18,199 acres is size and is about 1.4 % of the overall watershed.
They have drilled one well south of US 60 to depth 3,510 feet, the resulting well log is inconclusive as to whether or not they hit groundwater. Since they plugged the well I would guess that it was not a successful well. They drilled another well north of US 60 to a depth of 1,510 feet and hit water at 510 feet, a pump test yielded 2,000 gallons per minute, however it was not disclosed how long the test was done or what the recovery rate was.
The ranch even though it is a small part of the total private land within the basin, with this proposal would control the vast majority of the groundwater in the basin. If the LLC is granted this permit to extract 54,000 acre feet of water annually it opens the door for other request for water. It has been suggested that if the Office of the State Engineer does grant this permit it will have a profound adverse effect on the groundwater level within ten years of the start of pumping. The well field as proposed is definitely targeting the graben structure in hopes of the same results from the 1,500 foot well.
As of this time they are only hitting 50% with their drilling efforts.
The LLC is proposing to place infiltration structures across many of the ephemeral draws that cross through their land, in an effort to recharge their well field. These structures would require per-
mits from the state as they would be considered points of diversion. If these draws flow water during extreme runoff events they will only be able to capture a very small percentage of any rainfall that occurs within the basin. Most snow melt would never reach their recharge basins, unless there is a rare rain on snow weather event.