The Plains of San Au­gus­tine

Catron Courier - - Opinions & Editorial - By Dennis In­man

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.

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