TCEQ says that ASARCO contaminated groundwater beneath El Paso Texas (this is within the Rio Grand alluvium)

"In accordance with the 1996 Agreed Order, ASARCO submitted the first Remedial Investigation (RI) Report on October 9, 1998. Subsequent RI Phase II, III & IV reports were submitted in July 2000, November 2001 and September 2003, respectively. The RI Report indicates the groundwater was impacted by plant operations. Since the process pond water and groundwater are interconnected, the process pond water is most likely the main source for metals in the groundwater"

surface soil ARSENIC AT 17,000 MG/KG at the ASARCO EL PASO SITE

"The elements which comprise the Chemicals of Concern (COC) at the [ASARCO] El Paso facility include arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), iron (Fe), lead (Pb), selenium (Se) and zinc (Zn). The most prevalent COCs are arsenic, lead, and cadmium. The highest on-site surface soil concentrations are arsenic at 17,000 mg/kg; lead at 49,000 mg/kg; and cadmium at 3,500 mg/kg."

http://www.tceq.state.tx.us/remediation/sites/asarco/media