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Floating Vegetative Islands

March 2012 (volume 7 - issue 2)

Contributed by Pete Weiss, Valparaiso University and John Gulliver, St. Anthony Falls Laboratory, University of Minnesota

Funded by the Minnesota Local Road Research Board

Metal pollution from urban runoff is widespread and increasing metal concentrations in receiving waters are causing public concern. Thus, the removal or reduction of the toxic metals in stormwater is a focus of many watershed managers. For example, the United States Environmental Protection Agency (USEPA) mandates the use of dissolved metals to set and measure compliance with water quality standards.

An effective method for reducing dissolved contaminants in stormwater and wastewater involves the use of vegetation. Plants have the ability to remove dissolved nutrients, metals, and other pollutants from water (Kamal et al. 2004, Mazen 2004). In fact, popular commercially available products, such as the in-ground StormTreat system (2010), take advantage of the ability of vegetation to remove dissolved contaminants from water.

An alternative approach to in-ground systems is to use floating mats or islands that house hydroponic vegetation. In these systems specially selected vegetation is planted in a floating island that can be anchored to the bottom. The roots of the vegetation dangle in the water below. As the water moves past the roots, contaminants are removed from the water by surface adsorption to the roots and, in some cases, uptake by the plants into root and stem matter. These systems mimic nature as it is not uncommon for mats of wetland plants to break away from shoreline populations and float out into the middle of a pond (Figure 1).

Figure 1: Naturally occurring floating mats of wetland vegetation in a Twin Cities detention pond.

Floating islands of vegetation have been used for water remediation in Germany, Japan, China, England, Taiwan, and Korea (Nakamura and Mueller 2008) to reduce shoreline erosion, provide habitat for wildlife, improve landscaping, and improve water quality. Their most common application, however, is to improve water quality. For example, Billore et al. (2009) report that floating vegetative islands have been used to remove significantly more nutrients and biochemical oxygen demand in River Kshipra (India) than a typical vegetated lagoon. Another example is RiverFIRST project, the winner of the Next Generation of Parks Design Competition for the Minneapolis Riverfront, which proposes to use 8 acres of floating islands in the Mississippi River to help restore water quality. In the United States, companies such as Beemats (2011) and Floating Island International (2011) now provide this technology for water quality improvement.

Although floating vegetative islands are becoming more popular and contaminant uptake by plants is well documented, little is known about the effect that the water velocity past the roots has on the plant uptake of contaminants such as dissolved metals. A research project was thus undertaken to determine the short-term and long-term effect of increased water velocity past the roots of hydroponically grown Scirpus validus Vahl (soft stem bulrush). This species was chosen because it is a common wetland plant in the northern temperate climates and is well suited to grow under hydroponic conditions.

In the experiment, synthetic stormwater with typical dissolved heavy metal concentrations of cadmium, copper, lead, and zinc (10, 50, 300, 250 ppb, respectively) was recycled past the roots of hydroponically grown soft stem bulrush for a total of three weeks. Groups of plants were subject to different but constant water velocities of 6 cm/s, 3-4 cm/s, and 1 cm/s over the three week period. After three weeks of continuous operation, the plants were removed, dried, and the stems separated from the roots. Results revealed that water velocity had no impact on the cadmium and lead concentration in the stems while there was a slight upward trend in stem copper and zinc concentrations with velocity. As an example, the results for stem copper concentration as a function of water velocity past the roots is shown in Figure 2.

Figure 2:Stem copper concentration as a function of water velocity.

Note that the bioconcentration factor (BCF) is defined as the metal concentration in the plant material (mg/kg) at harvest divided by the initial concentration in the water (mg/kg) (Zayed et al. 1998). The dashed horizontal line shows the metal background concentration in the plant material when it was obtained from the nursery. However, for all four dissolved metals, the concentration in or on the roots was a function of water velocity for low water velocities (up to 4 cm/s) but was not impacted by higher water velocities (4-6 cm/s). As an example, the root copper concentration as a function of water velocity is shown in Figure 3.

Figure 3: Root copper concentration as a function of water velocity.

In a second experiment, all plant roots were exposed to a constant water velocity of about 2 cm/s and the variation in metal concentration in the plant roots and stems were investigated over a period of 76 days. The results showed that for roots, metal accumulation continues at a relatively rapid pace for 20 to 30 days with a significant decline in the rate of accumulation after that period.

The second experiment also showed that exposing the roots to a water velocity of 2.0 cm/s caused the BCFs of the roots at day 76 to increase by a factor of 7.0, 2.0, 13.8, and 1.4 for Cd, Cu, Pb, and Zn, respectively as compared to control plants exposed to stationary water. Zinc stem concentrations were increased by a factor of 1.6 while the stem concentrations of the other metals were not noticeably affected by water velocity.

In conclusion, it was determined that increasing the water velocity across the roots can significantly increase cadmium, copper, lead, and zinc concentrations in the roots of hydroponically grown Scirpus validus. The data also suggests that stem zinc and copper concentrations can also be increased in this manner. The increase in metal accumulation in roots was found to occur as early as three weeks after exposure and extend to 76 days. These results support the notion that metal concentration in or on the roots and accumulation of some metals in the stems can be increased over an entire growing season by exposing the roots to moving water.

References

• Beemats web page, www.beemats.com, accessed on July 6, 2011.
• Billore, S. K., Prashant; and J.K. Sharma. 2009. Treatment performance of artificial floating reed beds in an experimental mesocosm to improve the water quality of river Kshipra. Water Sci. &Tech., 60 (11), 2851-2859.
• Floating Island International webpage, www.floatingislandinternational.com, accessed on July 6, 2011.
• Kamal, M., A. E. Ghaly, N. Mahmoud, and R. Cote. 2004. Phytoaccumulation of heavy metals by aquatic plants. Environ. International 29, 1029-1039.
• Mazen, A.M.A. 2004.  Accumulation of four metals in tissues of Corchorus alitorius and possible mechanisms of their tolerance. Biologia Plantarum 48(2), 267-272.
• Nakamura, K. and G. Mueller. 2008. Review of the performance of the artificial floating island as a restoration tool for aquatic environments. World Environmental and Water Resources Congress, May 12-16, Honolulu, Hawaii, USA.
• StormTreat Systems web page, www.stormtreat.com, accessed on July 6, 2011.
• Zayed, A., Gowthaman, S., and N. Terry. 1998. Phytoaccumulation of trace elements by wetland plants: I. duckweed. Jour. Environ. Qual., 27, 715-721.

We want to hear from you!!!

Let us know your thoughts, experiences, and questions by posting a comment. To get you thinking, here are a few questions:

• What other applications could floating vegetative mats prove beneficial?
• Contaminant uptake is limited due to the rather large volume of water compared to the plants surface area and ability to uptake contaminants. How could the system be adapted to further increase uptake?