Arsenic in Drinking Water Supplies in Socorro, New Mexico
Some observations

Lynne Brandvold

Under the 1996 amendments to the Safe Drinking Water Act, Congress required that the Environmental Protection Agency (EPA) propose a new standard for arsenic by January 1, 2000 and finalize the rule by January 1, 2001. The then current standard of 50 parts per billion (ppb) had been adopted by the U. S. Public Health Service in 1942. EPA did not meet this schedule and proposed the new maximum contaminate level (MCL) on June 22, 2000. At that time interested parties were asked to submit comments on MCL levels of 3, 5, 10, and 20 ppb by September 20, 2000. The EPA then evaluated over 6,500 pages of comments from 1,100 commenters. In the fall of 2000, Congress extended the deadline for the final rule until June 2001. However, on January 16, 2001 the EPA Administrator signed the final rule to establish the MCL as 10 ppb. On January 20, 2001, President Bush's chief of staff issued a directive to put any rules published in the Federal Register but not yet in effect on hold for 60 days. This applied to the arsenic rule which appeared in the Federal Register on January 22, 2001. In April of 2001, the EPA Administrator Christine Whitman asked the National Research Council (NRC) to perform an "expedited review" of a possible standard between 3 ppb and 20 ppb. The EPA requested that NRC review new studies regarding health effects received after September 20, 2000, and to review EPA's risk analysis of arsenic. The review report was issued in the fall of 2000 and recommended that the MCL should be lowered from 50 ppb and that the EPA risk estimates were calculated appropriately. On February 22, 2002 the arsenic in drinking water rule became effective; the date by which systems must comply with the new 10 ppb standard is January 23, 2006.

How does this affect Socorro? Socorro, with a population of about 9,000, has six sources of supply for drinking water. Two of the sources are thermal springs and four are wells ranging in depth from 97-500 ft. The water serves as drinking water for those in the immediate areas surrounding the wells or springs. The water is not blended into one source thus removal treatment will be required at each source that contains 10 ppb arsenic or greater. The source locations are shown in Figure 1 with color-coded arsenic ranges. The six sources and the levels of arsenic in each are listed in Table 1 along with the Interim EPA standard and the new MCL.

Table 1. Levels of arsenic in drinking water supplies and comparison with EPA values.

Drinking water source Arsenic (ppb)
Eagle Pitcher well
Olson well
School of Mines well
Industrial well
Socorro Springs
Sedillo Springs
Old EPA Standard
New EPA Standard

Four of Socorro's six sources contain greater than 10 ppb. The two springs contain the highest levels at 40 ppb. The Industrial Well and the School of Mines Well contain an intermediate level of 24 ppb. The remaining two sources are below the 10 ppb level--but just barely. Thus some type of treatment to remove the arsenic will need to be installed at four of the sources.

The water varies not only in differing amounts of arsenic but in other water quality parameters as well. Some of these other parameters are listed in Table 2. Sodium and calcium are not listed but the springs and the Industrial Well are predomimately sodium carbonate type water and the other three wells are calcium carbonate type water.

Table 2. Some Selected Parameters in Socorro Drinking Water Sources
Source TDS (ppm) Hardness (ppm CaCO3) Fe (ppb) Mn (ppb)
Socorro Springs 230 68 3 14
Sedillo Springs 230 69 3 12
Industrial Well 710 222 125 136
School of Mines Well 666 317 110 740
Olsen Well 421 217 5 50
Eagle Picher Well 366 187 105 75

It is interesting to note that the springs, which contain the highest levels of arsenic, also have the best water quality! Three of the wells contain high amounts of the nuisance parameters iron and manganese. Iron and manganese are not a concern for public health, but water sources containing high iron and/or manganese will result in brown to black deposits in toliets, dishwashers, etc. and in the clothes that you wash, particularily white clothes.

Removing arsenic from drinking water supplies will be expensive. In New Mexico 20% of the systems exceed 10 ppb As. The estimated annual costs of compliance range from $49 to $60 million depending on the treatment technology. For small systems, like Socorro, the estimated monthly increase in the cost of water per household is between $50-$90. This would increase the monthly bill for residents up to 5 times--no small amount for a town with a below average income!