Andrew Robertson1, Kenneth Carroll2, Chris Kubicki2, Roland Purtschert3

1USGS New Mexico Water Science Center, Albuquerque, New Mexico, USA2New Mexico State University, Las Cruces, New Mexico, USA 3Climate and Environmental Physics, Physics Institute, University of Bern, Switzerland

Mesilla Basin/Conejos-MédanosStudy Area

• Study area lies within the Rio Grande Rift Basin.

• General groundwater flow direction is towards the river and from north to south.

• The Mesilla Basin is about 1,100 square miles (sq mi) and includes as much as 200 sq mi in Chihuahua (Hawley and Kennedy, 2004)

• Santa Fe aquifer is up to 3,000 feet thick and consists of late Cenozoic alluvial deposits. The Sante Fe is overlain by recent river deposits.

• The conjunctive use of water in the Mesilla Basin takes place under a myriad of legal constraints including the Rio Grande Compact, an international treaty, and a federal water project.

Generalized groundwater

flow direction

Motivation for work in Mesilla Basin/Conejos-Médanos

• Declining water levels, deteriorating water quality, and increasing use of water resources by municipal, industrial, and agricultural water users on both sides of the international border have raised concerns about the long-term availability of the resource in the region (Alley, 2013).

• In 2014, initial NAWQA results suggest that geothermal waters and brines are sources of natural contaminants to public supply wells in the Rio Grande aquifer system.

Dissolved-solids concentration at selected sites in the Rio Grande study

area from 1934 to 1999 (Wilcox, 1968; Williams, 2001); from Moyer and

others, 2013

Mesilla

Basin

Reach

Geochemical and isotopic investigation of deep groundwater and salinity in the Mesilla Basin, New Mexico

Conventional groundwater sampling

• Sampled 17 nested wells in the Mesilla Valley (and 7 single-completion wells in adjacent basins)

• Constituents:• major ions, • trace elements,• stable isotopes of water (δ18O, δD), • strontium (87Sr/86Sr),• boron (δ11B) isotopes,• uranium isotopes (234U/238U), • carbon isotopes (δ13C and 14C) of dissolved inorganic

carbon, • noble gas concentrations, and• helium isotope (3He/4He) ratios.

Area of

upward

vertical

gradients

Geochemical and isotopic investigation of deep groundwater and salinity in the Mesilla Basin, New Mexico

Conventional groundwater sampling – Residence Time

*pmC – Percent modern carbon relative to the NBS I oxalic acid standard, corrected for decay since 1950, but not normalized for 13C fractionation.

Helium accumulation

ModernMixed

Geochemical and isotopic investigation of deep groundwater and salinity in the Mesilla Basin, New Mexico

Conventional groundwater sampling – Evaporite Dissolution

Saline, brackish & geothermal water

(Witcher and others, 2004)

0

200

400

600

800

1,000

1,200

1,400

1,600

1,800

2,000

- 500 1,000 1,500 2,000 2,500 3,000

Wel

l dep

th (

feet

bel

ow

lan

d s

urf

ace)

Cl/Br

Fresh

Brackish

Saline

Geochemical and isotopic investigation of deep groundwater and salinity in the Mesilla Basin, New Mexico

Conventional groundwater sampling - Geothermal

Geochemical and isotopic investigation of deep groundwater in the Mesilla Basin, New Mexico

Ultra-trace groundwater sampling

• Sampled 8 wells within the Mesilla Valley for• Krypton -81 (81Kr); half-life = 229,000 y• Krypton -85 (85Kr); half-life = 10.756 y• Argon -39 (39Ar); half-life = 269 y

• Conservative behavior, advantageous decay rates

Geochemical and isotopic investigation of deep groundwater in the Mesilla Basin, New Mexico

Ultra-trace groundwater sampling by degassing

• Current extraction efficiency is approximately 50%.

• ~1600 gallons to extract 40L of dissolved gases.

• Head loss of ~1.5 gpm primarily due to 250 mm filters

• General procedure1. Leak test (~20 psi) with nitrogen,2. Evacuate system with vacuum,3. Purge system with dissolved groundwater gas, and4. Collect sample.

• 85Kr and 39Ar is being analyzed by Low Level Counting (LLC) at University of Bern.

• 81Kr and 85Kr is being analyzed at Argonne National Laboratory with Atom Trap Trace Analysis (ATTA).

Geochemical and isotopic investigation of deep groundwater in the Mesilla Basin, New Mexico

Preliminary results

• Tritium results agree with 85Kr results indicating no atmospheric contamination.

• The 81Kr analysis show no decay in 6 samples analyzed.

• The 39Ar piston flow ages were far younger than the radiocarbon ages in the samples collected from wells in the southern basin.

• High Helium/Neon (He/Ne) indicate mantle and radiogenic sourcing of He and possibly terragenic production of 39Ar.

*R/Ra is the ratio of 3He/4He in the sample to the ratio in air

Geochemical and isotopic investigation of deep groundwater in the Mesilla Basin, New Mexico

Summary

• Need for continued analysis of groundwater flow patterns, and salinity sources and contributions

• Complex groundwater flow system without clear depth-dependent relations and multiple sources of salinity

• Benefits of multi-tracer approach• Distribution and source of dissolved solids• Determine groundwater source and mixing

• Benefits of ultra-trace sampling approach• Constrain age estimates• Additional conservative tracer for confirming mixing models

Geochemical & isotopic investigation of deep groundwater and salinity in the Mesilla Basin/Conejos-Médanos, NM, TX, Chih.

Questions?