Occurrence and Mobility of Arsenic in Groundwater Used for Public Supply in Southern Carson Valley, Douglas County, Nevada

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Over the past 15 years, Douglas County, Nev., has removed production wells in northern Carson Valley from use due to relatively high arsenic concentrations.  To maintain the supply of water to the public, the town of Minden has been providing water to Douglas County and Carson City.  Due to the projected increases in municipal demand, water resource managers are concerned that increasing pumping rates from wells in Minden may change groundwater chemistry and degrade the resource by potentially drawing in arsenic enriched water. The USGS Nevada Water Science Center, in collaboration with the Carson Water Subconservancy District, compiled available arsenic and related hydrogeochemical data from the Carson Valley aquifer system for the purpose of evaluating the sufficiency of available data for characterizing the vulnerability of public supply wells to arsenic contamination under increasing pumping stress.

Location map of arsenic study in Carson Valley, Douglas County, Nev.
Location of area of focused study and subareas (outlined in light green) in Carson Valley, Nevada.

BACKGROUND

Naturally occurring arsenic is one of the most common contaminants in groundwater in the western United States. The primary aquifers beneath Carson Valley are comprised largely of quaternary aged basin-fill deposits of weathered granitic and volcanic material, a factor often associated with relatively high groundwater arsenic concentration.

Conditions favorable to increasing arsenic concentrations in groundwater include, but are not limited to,

  • proximity to arsenic bearing rocks,
  • long groundwater flow paths,
  • the application of phosphate containing fertilizers, and
  • irrigation of arsenic enriched soils.

The vulnerability of groundwater resources to arsenic contamination is influenced by

  • the physical properties of the aquifer,
  • pumping rates,
  • well location and screened intervals relative to the groundwater flow system, and
  • geochemical environment.

Arsenic mobility and transport through the subsurface is largely controlled by the interaction of groundwater with aquifer sediments. Arsenite (As(III)), the reduced form of inorganic arsenic, usually exhibits greater mobility in groundwater than the oxidized form, arsenate (As(V)) largely due to the greater attraction of As(V) to aquifer sediments relative to that of As(III) at pH values exceeding 8.5. Arsenic speciation (form) is influenced by the relative redox condition of the aquifer environment. For example, in the vicinity of the Douglas County Airport, where arsenic speciation has been characterized, arsenic in groundwater collected at depths greater than 250 feet below land surface was found to be primarily As(III); however, in the upper 150 feet of the aquifer As(V) predominated (Paul and others, 2010).

The USGS Nevada Water Science Center assessed available chemical and physical data necessary to characterize the distribution of arsenic in Carson Valley to

  1. provide an evaluation of existing data on arsenic concentrations in groundwater in Carson Valley as available from local water purveyors, county, state, and federal databases;
  2. provide a graphical display of the spatial extent of known arsenic concentrations; and,
  3. identify data gaps and areas requiring additional monitoring of arsenic and redox related parameters within Carson Valley.

DATA

Data were compiled from the USGS NWIS database and as provided by local water purveyors.

Arsenic concentration data were requested from

  • the Carson Water Subconservancy District,
  • Douglas County Public Works,
  • Gardnerville Water Company,
  • Indian Hills General Improvement District,
  • Nevada Department of Environmental Protection,
  • South Lake Tahoe Public Utility District, and
  • the Town of Minden and Gardnerville Ranchos General Improvement District.

The data described in the linked presentation on this page are associated with untreated (unfinished, unblended) groundwater samples.

RESULTS

Arsenic in Carson Valley Data Summary (PDF) and associated data files

  • Limited data are available within the past 10-20 years from areas surrounding the focus area of concern, upgradient from production wells in the Minden-Gardnerville area.
  • Current data on arsenic, such as speciation and relative reduction/oxidation (redox) parameters are needed to understand the spatial distribution and redox conditions within the aquifer underlying the eastern and southern parts of Carson Valley. Understanding these conditions is essential in evaluating arsenic mobility.
  • Redox conditions vary from well to well. Existing data show many wells have redox conditions that may support the oxidized form of arsenic (As-V) whereas some conditions may support the reduced form (As-III).  Arsenic speciation data are needed to verify.
  • The limited data analyzed as part of this study suggest that higher concentrations of arsenic are found in samples collected from wells screened within the deeper regions of the aquifer.
  • Additional water level measurements are needed to evaluate how deep within the actual aquifer samples are being collected.
  • pH values are generally below what is critical for the desorption of As(V) from aquifer sediments; however, elevated As concentrations have been observed.
  • In some wells, arsenic concentrations responded to varying pumping rates (e.g. well 385412119401401; at rates above 200 gallons per minute, arsenic concentrations exceeded 10 µg/L; below 200 gallons per minute, arsenic concentrations remain below 10 µg/L).