The San Joaquin Marsh (SJM) is a constructed wetland located in Irvine, California within the Newport Bay watershed. A prominent feature of the habitat is the natural wetland treatment system, where urban runoff and groundwater travels through a series of ponds with vegetation, and the effluent (clean water) makes its way towards upper Newport Bay and the ocean. One major pollutant of emerging concern present in the marsh is selenium (Se), a naturally occurring element in the surrounding foothills around the area. At high levels, Se can be toxic to native aquatic plants and animals present in these ecosystems. This project quantifies the amount of Se removal via volatilization. Acrylic chambers were designed, assembled, and deployed clear at Pond 3 in the SJM to capture volatilized Se from bulrush and cattail utilizing carbon filters via a vacuum pump. The constructed chambers enclosed water and plants, allowing data collection over a period of multiple hours. Testing occurred at two sites with a total of 10 samples per site including: a minimum of 1 sample each for water, sediment, and plant for bulk selenium analysis per site; a minimum of 2 samples to quantify recovery; a minimum of 1 sample per site to test volatilization from open water; and a minimum of 4 samples (2 daylight, 2 overcast) at the end of each testing period. The rationale behind conducting this project is that there has not been work done to quantify the amount of Se removal in the SJM due to volatilization. Previous Se mass balance studies in the SJM presume that Se removal mechanisms include sediment sequestration, aquatic vegetation uptake, and volatilization as methylated Se, but there is no data on the quantity that is volatilized. It is theorized that Se in water gets trapped in sediment, which is then taken up by plants and formed to methylated Se that is volatile. The experiment focuses on measuring the amount of Se that is volatilized. This project fills the gap in the current mass balance of Se and contributes to research that develops a nature-based and environmentally friendly management option for Se in the Newport Bay watershed. The results estimated that 46.28 lbs/year of selenium may be removed through volatilization in the San Joaquin Marsh. Overall, this project showed that wetlands can serve as an effective and natural way to help remove selenium from contaminated water systems.
Selenium Volatilization in San Joaquin Marsh Plants
Summary
Technical Approach/Methodology
Most volatile selenium field measurements follow the method introduced in a 1999 technical report (Lin, Z et al, 1999). This describes a 0.71x0.71x0.76m plexiglass chamber with activated charcoal filters and a maximum capacity of 10 mg of selenium capture over 24 hours using a flow rate of 0.43 m3/hr. The chamber is inserted into the ground 5-10 cm deep to create an airtight seal. The literature determined that selenium recovery rates are close to 100% at flow rates below 0.42 m3/hr ( 7 L/min), above which there is a steady and sharp decline in percent recovery. The chamber design, filter design, experimental setup, and flow rate used in this project are modeled after this existing literature.
To conduct this research, two acrylic vacuum chambers were constructed to quantify volatilized selenium removal by bulrush and cattail in Pond 3 of the SJM. Each chamber is open at the bottom and partially submerged in the water, allowing the plants to be enclosed. Two openings in the chamber lid provide access to the air inside of the chamber through connected tubing systems.
One opening is connected to a vacuum pump that is powered by a battery located on land through tubing fitted with a charcoal filter. Using a low-flow vacuum pump, air containing volatilized selenium from inside the chamber is drawn through the filter for collection and analysis. The second port contains tubing with a charcoal filter that is open to the atmosphere, supplying oxygen to the enclosed plants while minimizing the entrance of ambient air that may contain selenium into the chamber.
Using ArcGIS systems, the total area of SJM was established by taking into account the surface area of ponds A-E and 1-5 and the surrounding vegetation. Surface areas of ponds and trails were subtracted to estimate total plant surface area. Advanced image processing methods may be utilized to enhance these estimates. The selenium concentration results were normalized for each plant in units of ug/hr/m². This was accomplished by first subtracting the control and recovery concentrations, dividing by its respective run time, multiplying by the weight of the charcoal in the filter, then dividing by the surface area of the enclosed plant.
Outcomes
The lab conducted Inductively Coupled Plasma Mass Spectrometry (ICP-MS) to measure selenium in the samples submitted. The concentrations found were normalized for each plant and then estimated for the entire marsh area to determine an annual selenium volatilization rate of 46.28 lbs/yr.