Lake responses following lanthanum-modified bentonite clay (Phoslock®) application: An analysis of water column lanthanum data from 16 case study lakes
Graphical abstract
Introduction
When assessing the use of management options for the restoration of impacted ecosystems it is essential that any potential unintentional impacts also be considered (Cullen and Boyd, 2008, Matthews and Turner, 2009, May and Spears, 2012). In lakes, geo-engineering using phosphorus (P) capping materials has been used as a management tool with which legacy P stores in bed sediments can be controlled (Hickey and Gibbs, 2009, Cooke et al., 2005, Spears et al., 2013a, Spears et al., 2013b). These legacy P stores can delay ecological recovery following reductions in catchment P loads for decades (Welch and Cooke, 2005, Søndergaard et al., 2003, Spears et al., 2012). Given that current water quality legislation more commonly provides guidance on deadlines by which water quality improvements must be made (e.g. 2015–2027 for the Water Framework Directive, WFD; EC2000/60/EC), research has focussed on identifying methods (e.g. Phoslock® and other P capping agents; biomanipulation; dredging etc.) for ‘speeding up’ the recovery of lakes following catchment management (Hickey and Gibbs, 2009, Jeppesen et al., 2007, Zhang et al., 2010). Recent evidence suggests that when internal P load and catchment P load reduction measures are applied simultaneously, rapid recovery can be achieved (Van Wichelen et al., 2007, Mehner et al., 2008). To meet this demand, novel products continue to be developed and proposed for use in lakes (Zamparas et al., 2012, Spears et al., 2013a, Spears et al., 2013b). Of increasing concern is the lack of understanding of the potential negative impacts on lake ecology and biogeochemical cycling associated with indirect effects of amendment products in lakes (Welch and Cooke, 2005, Vopel et al., 2008, Hickey and Gibbs, 2009, Egemose et al., 2010).
Phoslock® is a lanthanum (La) modified bentonite clay designed by the Commonwealth Scientific and Industrial Research Organisation (CSIRO) in the 1990s for the control of oxyanions (including dissolved P (DP)) in waste waters and sediments (US Patent 6350383; Douglas, 2002, Douglas, 2010). The incorporation of La into a bentonite carrier was deemed necessary to reduce the potential for negative ecological effects associated with the liberation of dissolved La3+ as described by Haghseresht et al. (2009). In recent years, Phoslock® has been increasingly used as a geo-engineering tool to control the release of legacy P stores from lake bed sediments to overlying waters (e.g. Robb et al., 2003, Lürling and Faassen, 2012, Lürling and Van Oosterhout, 2012, Meis et al., 2012, Van Oosterhout and Lürling, 2011).
Phoslock® is commonly applied from a barge, as slurry, where it acts to strip dissolved P en route through the water column. Once settled onto the bed, the product can enhance the capacity of lake bed sediments to retain P in an inorganic particulate form (Meis et al., 2012) that is not available to phytoplankton, and is stable under reducing conditions and within the pH range 5–9, commonly reported in eutrophic lakes (Douglas et al., 2000, Haghseresht, 2006, Robb et al., 2003, Ross et al., 2008). One common operational assumption is that La is not liberated from the bentonite carrier under natural conditions in lakes and that P incorporation into the bentonite matrix is the dominant mechanism of dissolved P removal from solution. However, little empirical evidence exists within the peer reviewed literature (with the exception of Haghseresht et al., 2009) with which the mechanisms of P removal from solution by Phoslock® may be quantitatively identified, although these details are available within confidential reports (Douglas, 2010). This is not the case for the formation of La–P complexes from dissolved species in solution, the mechanisms of which are relatively well documented in the literature (Firsching and Brune, 1991, Firsching and Kell, 1993, Diatloff et al., 1993).
Although the number of publications in which the control of P and/or algal abundance by Phoslock® has been demonstrated has increased in recent years (i.e. 18 publications since 2002 listed in Web of Science), no comprehensive meta-analysis of case study lakes has been conducted, to date. In addition, concern has been raised recently regarding the potential for release of filterable La (FLa) following Phoslock® application and the potential unintended ecological implications of this release (Stauber and Binet, 2000, Hickey and Gibbs, 2009, Lürling and Tolman, 2010). The speciation of FLa ions is also important when considering ecotoxicological impact and of all FLa species (i.e. La3+, La(OH)2+, and La(OH)2+) the La3+ ion carries the greatest risk of biological effects (Das et al., 1988). The application of large aerial loads of inorganic materials (e.g. Phoslock®) may also have a short term impact on aquatic ecology through a sudden increase in suspended matter concentration (Bilotta and Brazier, 2008, Wagenhoff et al., 2012).
A range of laboratory studies have quantified ecotoxicological thresholds related to both total La (TLa) and FLa on components of the aquatic environment (Table 1). However, variation in methodology makes it difficult to draw inferences from these laboratory based trials to the likely impact on populations of organisms in specific lakes under natural environmental conditions (Lürling and Tolman, 2010). However, this body of work can be used to provide an indicative range of threshold concentrations with which concentrations of FLa and TLa, observed in lakes following Phoslock® applications, can be assessed.
Here we use data from 16 case study lakes to which Phoslock® has been applied to address the following specific research questions: (1) to what type of lakes has Phoslock® been applied and at what range of doses?; (2) what are the ranges of TLa and FLa in treated lakes following application and are there common recovery trajectories across all lakes?; (3) what were the predicted La3+ concentrations in the treated lakes according to CHEAQS PRO modelling following Phoslock® application, (4) do reported FLa and TLa concentrations indicate potential issues when compared to laboratory controlled ecotoxicological test results?; and (5) what are the implications of these results for the use of Phoslock® as a eutrophication management tool in lakes?
Section snippets
Data availability and study site descriptions
The following analyses are founded on the results of a survey of the co-authors designed to gather case study information on lakes to which Phoslock® has been applied. Information on location, maximum fetch, mean depth, maximum depth, surface area, annual mean alkalinity, conductivity and pH in the year following product application and Phoslock® dose procedure was requested for each of the 16 lakes for which TLa, FLa or both TLa and FLa data were available for surface and/or bottom waters (
Responses in total and filterable lanthanum concentrations following Phoslock® application
The results of the co-author survey indicated that FLa and TLa data were available for 16 lakes to which Phoslock® had been applied (Fig. 1, Table 2).
Both surface and bottom water TLa and FLa concentrations were <0.001 mg L−1 in all lakes prior to the application of Phoslock® (Figs. 2 and 3). Surface and bottom water peak TLa concentrations during and in the month following the application of Phoslock® ranged from 0.026 mg L−1–2.300 mg L−1 and from 0.004 mg L−1 to 0.892 mg L−1, respectively.
Characterising the lanthanum recovery trajectory
Results of GAM analyses indicated that recovery trajectories for TLa and FLa in surface and bottom waters in lakes following an application of Phoslock® were well represented by a 2nd order decay relationship, with time, and that recovery reached an end-point between 3 and 12 months post-application. However, inspection of the raw data (Fig. 3) also indicates the occurrence of sporadic increases in TLa concentrations in later months (e.g. month 18; Fig. 3a and c), and that these increases were
Conclusions
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It was confirmed that release of FLa to the water column following Phoslock® application does occur, with peak FLa concentrations during application reported up to 0.414 mg L−1.
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Results of the GAM analyses indicated that recovery was achieved within 3 months in surface waters and 12 months in bottom waters, although the FLa GAM models were based on a relatively low number of case study lakes.
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Maximum reported estimates of Phoslock® in receiving waters did not exceed the EC50 values for Phoslock®
Acknowledgements
BMS was funded by the Natural Environment Research Council (NERC). Sebastian Meis was funded jointly by (NERC), Phoslock® Europe GmbH, and by the Deutscher Akademischer Austausch Dienst (DAAD Scholarship agreement number D/08/42393). We are grateful to the Loch Flemington Catchment Management Group, Scottish Natural Heritage, the Scottish Environment Protection Agency, Dundee City Council, the Regional Water Authorities Brabantse Delta and De Dommel (The Netherlands), the Province Noord-Brabant
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