Dissolved Contaminants

Rainwater can dissolve some contaminants from their sources and convey them to surface water bodies including stream, river, lakes, and ocean. Examples of dissolved contaminants include heavy metal cations, nutrient anions (e.g., nitrate, phosphate), and organic chemicals (e.g., benzene, pesticides, hormones, pharmaceuticals, fire retardants). Some of these organic chemicals are known as contaminants of emerging concerns (CEC). The fate and transport of these contaminants in soil and water depend on soil properties (e.g., mineralogy, organic carbon content, surface charge, physical and chemical heterogeneity), contaminant properties (e.g., water solubility, pKa, surface functional groups or charges, Kow) and water chemistry (pH, ionic strength, and other chemical compositions). My research in this category answers the following questions:

  • How do the geochemical triggers (e.g., changes in pH, ionic strength, dissolved organic carbon) affect the mobility wide range of dissolved contaminants in soils?
  • How do the physical (e.g., the presence of preferential flow, mobile-immobile water interaction) and chemical non-equilibrium (e.g., concentration gradient) affect the transport of dissolved contaminants in subsurface soils?

Collaborators:

Selected publications on this category are provided below.


Subsurface Release of Cesium and Strontium: Relative Contribution of Colloids, Natural Organic Matter and Major Cations

The field setup to examine colloid transport is shown below.

During a rainfall, heavy metal and radioactive cations can be released from the contaminated subsurface soil via cation exchange, colloid- and natural organic matter (NOM)- facilitated transport, but the relative contribution of each of the processes for the contaminant release is unclear. We contaminated an instrumented soil pedon within Oak Ridge Reservation with cesium and strontium and applied synthetic rainwater at low and high ionic strengths and NOM concentrations to examine the release of sorbed cesium and strontium.

Competing cations are more effective in releasing metals compared to colloids or dissolved organic carbon.
Competing cations are more effective in releasing metals compared to colloids or dissolved organic carbon.

During simulated rainfalls, cesium and strontium were released from the contaminated soil primarily via cation exchange and colloid-facilitated transport, not NOM-facilitated transport, and transported through preferential flow paths. The relative contribution of both processes depended on ionic strength of the infiltrating water: at low ionic strength (< 1 mM), colloid-facilitated transport was relevant, whereas cation exchange dominated the release of cesium and strontium at high ionic strength (> 1 mM). These results are useful to assess the release potential of heavy metal and radioactive cations in field conditions.  This research is submitted to  Environmental Science and Technology.


Fate and transport of selected estrogen compounds in Hawaii soils: Effect of soil type and macropores

Transport of an estrogen is enhanced in the presence of treated wastewater (right) compared to control (1 mM CaCl2) in four soils from Hawaii.
Transport of an estrogen is enhanced in the presence of treated wastewater (right) compared to control (1 mM CaCl2, left) in four soils from Hawaii.

The fate and transport of estrogen compounds in the environment is of increasing concern due to their potential impact on freshwater organisms, ecosystems and human health. The behavior of these compounds in batch experiments suggests low mobility, while field studies indicate the persistence of estrogen compounds in the soil with the possibility of migration to surface water as well as groundwater. To better understand the movement of these chemicals through soils, we examined their transport in three different Hawaiian soils and two aqueous matrices. The transport of these compounds was enhanced in the presence of macropore in soil and dissolved organic carbon in water. Organic carbon content and mineralogical composition of the soils had a profound effect on the transport of the estrogens. This work is published at Journal of Contaminant HydrologyPDF.


Release of Major and Trace Elements from Smelter Slag Separated from the Upper Columbia River

(Design of the fluidized bed reactors and batch reactors to examine the relase of major and trace elements from the slag fractions)

The release of major ions and trace elements from two slag fractions isolated from sediments collected in the upper Columbia River were examined in batch reactors under oxic (air‐equilibrated) and near‐anoxic (nitrogen‐purged) conditions and in fluidized bed reactors.

Release of heavy metals from slags elevated their concentration in the river water.
Release of heavy metals from slags elevated their concentration in the river water.

The results of the batch reactor experiments established that many of the major ions and trace elements present in the slag were released to Columbia River water at concentrations above the concentrations of those elements in the Columbia River water. Relatively high concentrations were observed for the release of iron, aluminum, manganese, copper, lead, zinc, antimony, arsenic, and selenium from both slags. Over time, the concentrations of most of the elements that occur as anions increased and the concentrations of most of the cations decreased. Oxic conditions resulted in the release of more of the major cations, silicon, barium, lithium, manganese, and strontium, whereas near‐anoxic conditions resulted in the release of more aluminum, arsenic, phosphorus, and selenium. The results of the fluidized bed reactor experiments agree with the results of the batch experiment. Relatively high concentrations were observed for the release of iron, aluminum, manganese, titanium, cadmium, copper, lead, cobalt, nickel, zinc, antimony, arsenic, and selenium from both slags. Significant colloidal fractions were present for iron, aluminum, cadmium, cobalt, copper, lead, titanium, and zinc. These results suggest that slag in the river sediment could serve as a source of major or trace elements and may pose threats to aquatic life.

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