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Arsenic in New Mexico Groundwater

Arsenic (As) is an element that is toxic in small amounts and occurs in many minerals. The amount of naturally occuring arsenic in drinking water in New Mexico is among the highest in the United States. In 2001, the US EPA lowered the maximum contaminant level (MCL) for arsenic from 50 parts-per-billion (ppb) to 10 ppb (matching the standard for the World Health Organization). The 10 ppb standard is equivalent to 0.010 mg/l (milligrams per liter) as portrayed on the map below. This change affected many water systems in New Mexico that consequently became out of compliance with the new stricter standard.

Arsenic (mg/L)
> 0.051
0.021 - 0.050
0.011 - 0.020
0.0051 - 0.010
< 0.005
(Pan and zoom the map for more detail and click on sample points for more information.)

The abundance of arsenic is typically highest in organic-rich shale and volcanic rocks, particularly in light-colored, silica-rich volcanic rocks like those in the Jemez Mountains, which can contain as much as several hundred parts per million (ppm) of arsenic. Hydrothermal alteration can further concentrate arsenic around geothermal systems and ore bodies. Groundwater can then leach arsenic from these rocks with high arsenic concentrations.

Arsenic is tasteless, odorless, and colorless in water (at least in natural concentrations). The only way to know if water is contaminated with arsenic is to test it. There are several options to test for arsenic (and other contaminants) in groundwater. Simple test kits are available as are professional services and commercial analytical laboratories.

Municipal and larger community water systems have implemented treatment strategies to comply with the 10 ppb arsenic standard. Some smaller systems and many private wells are not in compliance. For domestic well owners, there are several treatment options. Reverse osmosis is probably the most cost-effective method, but there are other methods like anionic exchange (water softeners) and iron-oxide filters.

(Adapted from Dunbar, Chapin and Brandvold, 2002.)

Below are some resources regarding the geology of arsenic as well as risk and remediation.


  1. Bitner, Kelly; Thomson, Bruce; Chwirka, Joe, 2001, The cost of compliance with a new drinking water standard for arsenic in New Mexico, New Mexico Geology, v. 23, no. 1, pp. 10-12.
  2. Brandvold, Lynn A., 2001, Arsenic in ground water in the Socorro Basin, New Mexico, New Mexico Geology, v. 23, no. 1, pp. 1-8.
  3. Dunbar, N. W.; Chapin, C. E., 1994, The water future of Albuquerque and Middle Rio Grande basin, Proceedings of the 39th Annual New Mexico Water Conference, WRRI Report No. 290, p. 257-276.
  4. Dunbar, N. W.; Chapin, C. E.; Brandvold, L. A., 2002, Arsenic in New Mexico's water, New Mexico Earth Matters, v. 2, no. 2, pp. 1-4.
  5. Frost, Floyd J.; Craun, Gunther F.; Tollestrup, Kristine; Raucher, Robert; Chwirka, Joe; Stomp, John, 2001, Evaluation of costs and benefits of lower arsenic MCL-unprecedented high costs for uncertain benefits, New Mexico Geology, v. 23, no. 1, pp. 13-15.
  6. Reid, Kevin D.; Goff, Fraser; Counce, Dale A., 2003, Arsenic concentration and mass flow rate in natural waters of the Valles caldera and Jemez Mountains region, New Mexico, New Mexico Geology, v. 25, no. 3, pp. 75-82.
  7. Wust, Stephen L., 2001, Regulatory summary of the new arsenic standard, New Mexico Geology, v. 23, no. 1, pp. 9.