All of these publications recommend growing in raised beds when yard soil contamination is above 400 ppm. However, overtime, risks to contamination of new, clean soil in a raised bed include resuspension of exposed surrounding surface soils, tilling too deeply into contaminated soil beneath the raised bed media or plants with deep roots, which may reach past the clean soil. While both UMass Amherst and the University of Connecticutrecommend using a liner under a raised bed to avoid the latter two mechanisms of resuspension, only the University of Connecticut specifies their recommendation as landscape fabric liner, though research based evidence was not cited. Many studies have looked at Pb uptake and partitioning in plants and contaminated soils, but few studies have looked at physical barriers to limit or prevent Pb movement from contaminated soil into raised bed environments where produce is grown. Therefore, this study supplements gaps in current literature pertaining to use of barriers in a raised-bed garden in Pb contaminated areas. The objective of this study is to evaluate barriers to prevent Pb mobility from contaminated ground entering into the uncontaminated raised bed environment. Ideally, a specific barrier for lead exclusion will be identified. Within each container, contaminated media was layered to simulate contaminated ground soil and uncontaminated raised bed media . Between the media layers in each container, barrier treatments were applied . Barrier material treatments included PremleneTM Neoprene , landscape fabric , and a no-barrier control. The media used in this study is similar to standard potting mixes used in raised beds. Media was mixed in large batches using 4 cubic feet of peat moss , 18 cubic feet of 5/8th inch screened pine bark , 4 cubic feet of washed large grain sand , 10.5 pounds of Osmocote 19-5-9 , 0.5 pounds of Micromax , and 8 pounds of dolomitic lime . Each treatment container was first filled with 5 gallons of contaminated media . To attain a final concentration of 500 ppm, media was spiked with a 10,000 ppm Pb standard in a 5% nitric acid solution diluted to concentration using municipal water.
Inductively coupled plasma-optical emission analytical spectrometry was used to analyze total Pb in media to confirm intended initial concentrations . The contaminated media layer was then covered and sealed to the edges of the containers using the experimental barriers . After applying the barrier treatments,ebb and flow trays each container was filled with 15 gallons of uncontaminated media into which the experimental plants were to be grown . All treatment containers were arranged in a complete randomized design with three replications per barrier treatment. Brassica rapa was selected to be grown out in the simulated raised bed conditions. Chinese Cabbage plants were replicated five times per treatment plot .Seeds of Brassica rapa ‘Mibuna’ were planted in 50 cell trays using SunGro Metro-Mix 830 on March 11, 2020 and April 11, 2020 and grown in a greenhouse. Temperatures through the first and second trials ranged between 53˚F and 78˚F. Transplants were watered daily with overhead irrigation twice a day for five minutes. Transplants were fertilized using Peter’s ProfessionalTM water soluble 20-20-20 fertilizer at 400 ppm every other week. The seedlings were thinned to one plant per cell. Two weeks after seeding, five plants were transferred into each raised bed treatment container at 12 inch spacings . These containers were located under a hoop house with open sides where temperatures ranged between 53˚F and 78˚F. The plants were hand-watered daily and Bifenthrin insecticide was applied at recommended rates to control aphids, , flea beetles , and cross-striped cabbage worms . After 30 days of growth in the treatment containers, all plants were harvested at the base of the stem and the above ground portion of the plant was weighed for fresh weight in grams. All plants were divided between two dryers at an average of 60˚C and dried to a constant weight before grinding through a 1 mm sieve in preparation for analysis. Inductively Coupled Plasma-Optical Emission Spectrometry procedure was used to analyze total Pb accumulated in the plant tissues . The neoprene rubber sheeting was also analyzed for Pb content using a similar procedure as above. A respirator and goggles were the personal protective equipment used each time Pb standards and loose contaminated media were handled. Upon conclusion of this study, contaminated materials were disposed of by Louisiana State University Hazardous Waste Disposal . Data were analyzed with the statistical program SAS Proc GLM with Tukey and Excel for Microsoft 365 .The effects of lead on final harvest weight of Brassica rapa did not vary significantly across barrier treatments which indicates that growth was not affected by the treatments applied .
The large error bar values reflect the wide range of weight variability within each treatment . Lead accumulation data suggests that neither neoprene rubber or landscape fabric significantly excluded lead uptake in Brassica rapa ‘Mibuna’ compared to the no barrier control treatment . There were no significant differences in Pb uptake between the neoprene rubber and landscape fabric barriers nor between landscape fabric and the no-barrier control treatment . Furthermore, the plants in neoprene and fabric treatments exceeded the reference value published by the Chinese National Food Safety Standard for maximum levels of contaminants in foods. Not only did the neoprene rubber sheet treatment exceed the threshold discussed above, the accumulation reported in this treatment was significantly higher than the no barrier treatment. Plausible explanations for the unexpected results include: pores in the landscape fabric may have allowed some contaminated soil movement; contamination could have occurred via the barrier materials themselves; or the plant roots did not grow deeply enough to reach the uncontaminated-contaminated media interface of the simulated raised bed to accumulate substantial lead in the plant tissues. ICP-OES extractions were subsequently carried out on the neoprene rubber revealing the material contained a concerning amount of lead , possibly contributing to the elevated lead accumulation in the Chinese cabbage plants. Studies of other systems which use neoprene corroborates this conclusion by demonstrating other occurrences of lead leaching from neoprenematerials. Lead analysis was not performed on the landscape fabric because the accumulation data of the plant tissue did not indicate increased Pb levels in this treatment as compared to the control treatment. Upon conclusion of this study, having no barrier is still not recommended as sufficient to prevent Pb uptake in vegetable crops. While the no barrier control treatment had similar Pb values as the landscape fabric treatment , further investigations of barrier materials is recommended. In this study,4×8 flood tray the plants were only allowed to grow for 30 days, whereas a homeowner may let them grow for 40 or 50 days. If plants were allowed to grow for a longer period of time, there is a chance that higher accumulation in the no barrier treatment may have been observed.
Future projects related to this research should evaluate other impermeable barriers such as other plastics like visqueen, and landscape fabrics of other densities as well as longer harvest intervals.Florida’s human population has increased dramatically over the last four decades; increasing from approximately 4.95 million in 1960 to ca. 19.7 million in 2010 which makes Florida the fourth most populous state behind only California, Texas and New York. Moreover, Florida’s population has typically grown considerably faster than the overall U.S. population; for instance, Florida’s population increased 14.2% for the eight-year period 2000-2008, while the overall U.S. population increased by only 7.8% . This rapidly increasing human population is placing a great deal of stress on Florida’s ecosystems, especially its watersheds, freshwater supply and coastal communities. As a result, riparian zones have become a major focus of watershed initiatives to improve degraded stream systems because alterations of these transitional ecotones can have significant effects on the aquatic ecosystems that they bor-der. Natural tributaries are common in suburban and urban settings, but they are often highly disturbed , eutrophic from excessive inorganic nutrient loading and have reduced plant diversity, especially of shrubs and trees. Substantial research has found that high levels of nutrients in lotic systems can significantly affect the ecology of recipient rivers, lakes and estuaries. Of special concern are levels of inorganic nitrogen and phosphorous which are the two nutrients most often associated with algal blooms, although the strength of the relationship is often affected by other environmental factors such as irradiance and water temperature. In addition, the relative strength of top-down and bottom-up factors in limiting phytoplankton abundance and/or diversity vary temporally. For instance, Vanni and Temte found that grazing by zooplankton was more important in limiting phytoplankton populations in a lake during the spring, but nutrient limitation was more important in the summer. Removal of native vegetation, especially trees and shrubs, while facilitating access and maintenance of urban streams, can greatly affect ecosystem functions including in-stream processing of nutrients and pollutants. Loss of riparian forest has been found to influence stream parameters such as light fluxes, temperature, and nutrient levels. For instance, Peterjohn and Correll calculated that surface water that had filtered across approximately 50 m of riparian forest removed about 45 kg/ha of nitrate nitrogen per year; additionally, riparian forest sequestered more nutrients than nearby cropland. Wahl et al. found outflow of inorganic nitrogen from urbanized deforested stream was more than twice that of a forested riparian watershed.
Sabater et al. found that open nutrient-rich tributaries with warmer temperatures had significantly higher levels of algal biomass than shaded nutrient-poor streams. These results are consistent with other studies that have found interactions of various degrees between light levels, temperature and nutrient levels. Transitional zones such as wetlands and riparian ecotones also provide important habitat for invertebrate communities and provide refugia for juvenile fish and amphibians. According to Richardson et al. , riparian management was initially developed to protect and improve fish habitat. Loss of tree and shrub communities, which is common along urban and suburban tributaries, will result in decreased leaf litter. Reduced leaf litter has been associated with decreased detritivore populations, which leads in turn to decreased abundance of aquatic and terrestrial insect prey available for fish. In addition, Renöfält and Nilsson found that when they experimentally disturbed riparian vegetation by cutting along a river in Sweden, plant species composition was unaffected, but species richness was reduced. Reduction of vegetation in riparian and marine habitats may have cascading effects on higher trophic levels. For instance, arthropod abundance was significantly higher in riparian sites that contained supralittorial vegetation compared to a site in which the vegetation had been removed. Riparian reforestation projects, incorporating native shrubs, forbs and other understory species, have the potential to ameliorate nutrient loading and reduce rates of species loss in urbanized areas. Riparian strips also serve as corridors for both plants and wildlife by facilitating the movement of colonizing individuals between suitable patches of habitat as well as gene flow among otherwise isolated populations. Restoration projects are becoming increasingly common and provide important opportunities to conduct research on the effects of scale on basic ecological principles such as nutrient cycling and species-area relationships. It has been suggested that riparian forest widths of 30-100 m may be necessary to provide sufficient habitat to maintain pre-logging densities of some wildlife.The current study suggests that partial restoration of native vegetation of much smaller fragmented patches along first-order tributaries of the St. Johns may provide local benefits to the watershed including reduction of nutrients associated with eutrophication and algal blooms and modest increases in biodiversity over a relatively short time. The St. Johns River is Florida’s largest river and it is an important natural and cultural resource for central and north Florida. Although it is a primary source of fresh water for Florida and it was designated as a National Heritage River in 2002, high levels of inorganic fertilizers enter the watershed from commercial and residential areas, significantly reducing water quality. Continued loading of inorganic nitrogen and phosphorous is likely facilitated by typical urban riparian management techniques such as tree removal and regular mowing, which are designed to facilitate maintenance and rapid removal of large volumes of water after storms, reduces the structure and complexity of the ecosystem. This increase in nutrient enrichment has resulted in frequent and extensive algal blooms in the river over the past ten years.