Environ. Sci. Technol. 2010, 44, 985–991
Chemical Lake Restoration Products: Sediment Stability and Phosphorus Dynamics SARA EGEMOSE,* KASPER REITZEL, FREDE Ø. ANDERSEN, AND MOGENS R. FLINDT Institute of Biology, University of Southern Denmark, Campusvej 55, DK-5230 Odense M, Denmark
Received October 26, 2009. Revised manuscript received December 18, 2009. Accepted December 21, 2009.
Laboratory experiments with sediments from three shallow Danish lakes were conducted to evaluate the effects of chemical lake restoration products during resuspension. Phosphorus (P) removal, sediment stability, sediment consolidation and color reduction were studied over time. The investigated products were aluminum (Al), Phoslock (a commercial bentonite product coated with lanthanum) and a combination of Al covered with bentonite (Al/Ben). All treatments effectively reduced the P concentration in the water. However, the treatments containing Al reduced the P concentration immediately after resuspension, whereas Phoslock required several days after resuspension to reduce the P concentration. Especially Phoslock, but also Al/Ben, increased the sediment stability threshold by 265% and 101%, respectively, whereas Al had no stabilizing effect. The fresh Al floc was resuspended 5× easier than untreated sediment. The largest consolidation of the sediment occurred with addition of Phoslock, followed by Al/ Ben, while Al alone had no effect. Enhanced consolidation may be of importance for macrophyte colonisation of organic sediment. Phoslock improved the light climate moderately by removing color, whereas Al was very effective in removing color. Ben/Al showed intermediate effects on color reduction. These findings are important when decisions are made on restoration method for a specific lake, which may be more or less wind exposed.
Introduction Although the external nutrient loadings in general have been reduced during recent decades (1, 2), many shallow eutrophic lakes will have difficulties meeting the EU Water Framework Directive (WFD) criteria in 2015 without restoration. Therefore, regional water action plans have been adopted to find possibilities for the lakes to meet the criteria of “moderate”, “high”, or “good” ecological condition (3, 4). The problem is often the release of accumulated phosphorus (P) from the sediment during summer (internal P loading) (2). This P supports phytoplankton blooms resulting in an impoverished light climate which affects the reestablishment of submersed rooted vegetation. Because P frequently limits the primary production in lakes (5), a chemical treatment is an obvious tool to achieve the WFD-criteria. Aluminum (Al) and the commercial clay * Corresponding author phone: +4565502774; fax: +4565930457; e-mail:
[email protected]. 10.1021/es903260y
2010 American Chemical Society
Published on Web 01/07/2010
product Phoslock are both used to prevent P release from sediments (6, 7). However, only limited information exists on the effects of chemical treatments on sediment stabilization and resuspension (8). The threshold for resuspension after chemical treatments may vary from the untreated lake sediment because the specific mass and surface of the added compounds are different. Thus, frequency and magnitude of resuspension events may change, directly affecting the light climate in the lake. Eutrophic shallow lakes are often exposed to resuspension events (9), because soft organic sediments prevent sufficient sediment consolidation or development of biological sediment stability created by diatoms and/or bacterial mats. Such highly unstable sediment also inhibits the reestablishment of submersed rooted macrophytes due to the low anchoring capacity of the sediment (10). The aim of this study was to investigate the initial effects of three products used for chemical restoration in shallow lakes subjected to wind driven resuspension events. The effects on P dynamics were studied along with the effects on erosion thresholds, erosion rates, color reduction, and sediment consolidation. These laboratory studies were compared to in situ measurements of free stream velocities in the lakes. The products studied were Phoslock (a commercial product consisting of bentonite coated with Lanthanum (La)) (11), aluminum chloride (Al), and a combination (Al/Ben) where Al was applied to bind P and afterward covered with Bentonite (not coated with La) to imitate the expected sediment stabilizing properties of Phoslock.
Experimental Section Surface sediment (0-5 cm), were collected by a grab corer, from the deepest area of three shallow Danish lakes: Lake Hostrup, Lake Vedsted and Lake Søbygård, during spring and summer of 2008. All three lakes have low mean depths (1.1-5.0 m), and general characteristics are listed in Table 1. The experiments were conducted with sediment from one lake at a time, due to the limited number of flumes available. In addition to the laboratory flume experiments, in situ free stream velocity measurements were performed in the three lakes (Table 1). Representative transects with 5-6 stations were defined including both wind exposed and protected areas. A Nortec Acoustic Doppler Velocimeters (ADV) was mounted on a long metal stick at a fixed position. During measurements the end of the metal stick was fixed into the sediment and the ADV was positioned about 20 cm above the sediment surface. Each measurement was performed at moderate wind speed (5-8 m s-1) and where the wind direction created the longest fetch (northwest to southwest or northeast to southeast). In addition, four years of meteorological data from the Danish Meteorological Institute (DMI) covering the areas were compiled, so they could be compared with the in situ measurements. The temporal resolution of this analysis was one measurement per hour. The data treatment implied sorting wind data to select periods with wind directions as above-mentioned and with wind speeds equivalent to or higher than 7 m s-1. Furthermore, the given wind speed should last longer than 2 h ensuring that the wind energy reached the sediment surface. Resuspension Experiment. The experiments were carried out in flumes made of two acrylic plastic tubes, one inside the other (diameter 50 cm, height 36 cm, flume channel 4.5 cm, volume 21.6 L, surface area 0.06 m2), fixed onto a base (Figure S1 of the Supporting Information). A rotating lid with six paddles induced the flow. The paddle speed was controlled VOL. 44, NO. 3, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Main Characteristics of the Three Danish Lakes Studied in the Experimenta
area max. depth mean depth dry weight loss of ignition in situ density mean grain size grain size
