Water Resources Professional Project Reports

Karen Torres

Document Type

Technical Report

Publication Date

8-1-2013

Abstract

Arsenic is a naturally occurring chemical element closely linked to geologic units and structural elements within aquifers of the Southern Espanola Basin in Santa Fe County, New Mexico. Public water systems are required by the US Environmental Protection Agency (EPA) to reduce arsenic levels below the Maximum Contaminant Level of 10 μg/L but this mandate does not cover private domestic wells. The New Mexico Environment Department (NMED) is aware of this issue and regularly conducts water fairs which allow homeowners to have a chemical analysis performed of their well water at no cost. A water fair conducted in 2009 collected over 300 samples of domestic well water which were analyzed for major anions/ cations, heavy metals and trace elements. These data are used in this study to perform a geospatial analysis of 200 domestic wells to investigate the arsenic occurrence in the Santa Fe Embayment of the Southern Española Basin. The goal is to determine if conditions exist in which arsenic levels may exceed the EPA maximum contaminant level of 10 ppb. To benefit from recent geologic mapping and geochemical characterization, the Santa Fe Embayment of the Espanola Basin was chosen as the project area. This allows incorporation of existing surface and groundwater chemistry and isotopic data into this study. Wells are plotted on geologic and aquifer maps for a regional view of structure, stratigraphy and aquifer type. Four zones of elevated arsenic, named Northwest, Southwest, South and Mountain Front respectively with unique Stratigraphic and structural features were identified. Each zone is described by its geology, hydrology and geochemical attributes. Spatial and chemical correlations with respect to arsenic were evaluated for each area. Groundwater with elevated arsenic in the South and Mountain Front zones consistently occurs in the saturated volcaniclastic Espinaso Formation, especially if groundwater is saturated with chlorite minerals. In the Southwest zone, wells with elevated arsenic were not drawing water from the volcaniclastic unit but from Lithosome E of the Tesuque Formation, which contains volcanic clasts and reworked Espinaso Formation. The North Zone, which is composed of sand and gravels of Lithosome S of the Tesuque Formation, does not have the same relationship to volcanic rocks at depth. This area is highly faulted and exhibits an upward hydraulic gradient allowing for upwelling, which may be the source of arsenic and is consistent with previous findings. Desorption through ion exchange of arsenic with metal oxides may also contribute to elevated arsenic in groundwater. A combination of both up-welling and desorption is proposed which serves as controls for arsenic concentration in Lithosome S of the Tesuque Formation within the Santa Fe Embayment of the Espanola Basin.

Language (ISO)

English

Keywords

Groundwater--Pollution--New Mexico-Santa Fe County., Arsenic--Environmental aspects--New Mexico--Santa Fe County., Water--Pollution--New Mexico-Santa Fe County.

Comments

A Professional Project report submitted in partial fulfillment of the requirements for the degree of Master of Water Resources, Water Resources Program, University of New Mexico.

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