Regional Geologic Controls on Environmental and Human Health

Sediments in Cr and Se study.

The composition and structure of earth materials play a fundamental role in many environmental and human health issues. Processes governing the mobilization of bedrock-sourced constituents of environmental concern in the near-surface environment and their integration into ecologic systems are complex. Yet understanding these processes is required for effective environmental risk management. This project specifically addresses the geochemistry of toxic trace elements with particular focus on chromium (Cr) and selenium (Se). Project objectives are to:

  • Characterize mineralogy, weathering, and geochemistry of anthropogenic perturbations that increase the potential for human and environmental health issues associated with chromium and selenium.
  • Provide geologic, geochemical, and land use context for elevated Cr(VI) in groundwater for both technical and general audiences: generate maps identifying additional areas of potential concern.
  • Provide stakeholders with quantitative geologic and geochemical data to assess mitigation strategies for watersheds exceeding selenium water quality standards.


Characterization of Environmental Impacts of Chromium (Cr) and Associated Potentially Toxic Elements From Ultramafic-hosted Mineral Deposits

Contact: JoAnn Holloway, Email:

It is well-documented that gold and mercury mining have resulted in the mobilization of vast amounts of toxic trace metals (e.g. mercury, arsenic) that pose environmental and human health issues, but the extent to which chromite mining has resulted in similar issues including the mobilization of chromium and asbestos is relatively unstudied. Previous studies have shown that Sacramento Valley soils have high concentrations of chromium due to weathering of ultramafic rocks in the Coast Ranges and Sierra Nevada. The impact that chromite mining in the Coast Range, Sierra Nevada and Klamath Range on chromium concentrations in mine sites and downstream sediments and soils beyond natural weathering processes has not been assessed. The task objective is to evaluate the impact of historic chromium mining on trace metal mobilization in adjacent mined (Elder Creek) and unmine (Thomes Creek) watersheds with similar geology. Task workflow includes the 1) collection and analysis of rock, soil, sediment, and stream water samples in paired sites impacted and not impacted by historic chronium mining to assess potential mining impacts; 2) interpretation of geomorphic and hydrologic processes that govern transport of chromium-bearing sediments; and 3) evaluation of the interaction between soils, sediment, and surface water biogeochemistry.

Geologic sources of Chromium in California and Selenium in Utah

USGS scientist sampling soils.
USGS scientists sampling soils and sediments.

Contact: Jean Morrison, Email:

The weathering of regionally extensive geologic units can be a significant source of contaminants across the globe. During weathering, phases that are not in equilibrium with ambient conditions (e.g. pyrite, chromite, organic matter, etc.) are easily altered, releasing contaminants into the environment where they are transported from the site or fixed in secondary weathering products. Semi-arid and arid climates play a key role in accumulation of soluble weathering phases, providing large reservoirs of easily mobilized contaminants to watersheds and their rivers when hydrologic or climatic conditions change. This interplay of geology, hydrology, climate, and land use controls natural, nonpoint-source contamination and understanding their interactions is key to determining if contamination can be mitigated and designing mitigation strategies when appropriate. The task objective is to identify the key variables in the cycling of chromium in California soils and salt and Se in Utah soils. Results will help land managers assess contaminant supply on different landscapes and identify processes that release and transport the contaminants within and from the watershed. The workflow for this task involves the 1) collection and analysis of soil, sediment, and water samples attributable to natural background vs. historic mining/agricultural activity; 2) assessment of mineralogical and speciation controls on contaminant supply and mobility; and 3) construction of watershed cycles to help quantify contaminant reservoirs and fluxes.

Characterization of Geochemical Processes Controlling Release of Potentially Toxic Cr(VI)

Contact: Christopher Mills, Email:

Chromium is present in two oxidation states, Cr(III) and Cr(VI) under typical soil and groundwater conditions. Cr(III) is relatively non-toxic, while Cr(VI) is a known carcinogen and irritant through inhalation and potentially through ingestion. Chromium(VI) is one of the most common contaminants at U.S. EPA Superfund sites. Elevated concentrations of Cr(VI) in groundwater in many areas of the world have recently been attributed to weathering of ultramafic rocks and subsequent oxidation of Cr(III) on Mn oxides. Task objectives are to characterize the processes leading to the the formation of Cr(VI) through the use of advanced geochemical tools and laboratory experiments. Major planned activities are: 1) collect and characterize dust samples from California for chemistry, Cr(VI), and mineralogy; 2) develop capability to measure Cr stable isotopes by multicollector-inductively coupled plasma-mass spectrometry and apply measurements to solid and dissolved phase Cr samples; and 3) perform Cr(VI) extractions, selective extractions for iron and manganese oxides, and laboratory experiments on samples collected from areas where Cr(VI) in groundwater is unusually high with no likely industrial source to characterize Cr(VI) formation mechanisms.