Arsenic in groundwater in the Socorro Basin, New Mexico

Lynn Brandvold, New Mexico Bureau of Mines and Mineral Resources, Socorro, NM 87801


This study investigated the levels and distribution of arsenic in groundwater in the Socorro Basin and the relationship of arsenic to other groundwater parameters. It has been known for some time that high levels of arsenic occur in the groundwater, but the distribution of arsenic in the basin was not known. A total of 74 groundwater samples were collected and analyzed. The location of the wells extended from the Sevilleta Wildlife Refuge headquarters just north of the Walking Sands rest stop on I-25 to the Bosque del Apache Wildlife Refuge on the south. Arsenic distribution ranged from less than 2 ppb to 43 ppb. The wells north of Socorro contained low arsenic (<2-10 ppb ) with one exception. High arsenic levels (30 ppb or greater) were found in three separate areas; (1) the thermal springs located at the southwest corner of the Socorro city limits, (2) three wells located 2-3 miles below San Antonio and (3) the thermal well at the Bosque del Apache Wildlife Refuge. Eight wells have an intermediate arsenic range of 15-30 ppb. One is a relatively deep well (500 ft) located near the thermal springs and another is a more shallow well (200 ft) located in the center of town. Both of these wells are drinking water sources for the City of Socorro. In a previous study in the Albuquerque basin, arsenic concentration correlated with many geochemical parameters. In this study there were no correlations of arsenic with any geochemical parameters. The water also varied in general composition within the basin, from the springs with low sodium, low hardness, and low TDS; to wells with high sodium, low hardness, and high TDS; to wells with low sodium and high hardness, and high TDS.

A full report of this study has been published in the February 2001 issue of New Mexico Geology