Effects of Dredging and Lanthanum-Modified Clay on Water Quality Variables in an Enclosure Study in a Hypertrophic Pond

Abstract

An enclosure experiment was conducted between July and September 2009 to compare the effectiveness of a phosphate fixative, the lanthanum-modified bentonite clay Phoslock ® (LMB), dredging, and their combination in controlling eutrophication in a hypertrophic urban pond in Heesch, The Netherlands. In total, 25 water quality variables were monitored. Multivariate analysis revealed that the combination LMB-treated and dredged enclosures deviated most from the pond (reference) and the controls, and showed the strongest eutrophication reduction. Overall, dredging significantly increased transparency, lowered turbidity, and improved the oxygen conditions in the enclosures compared to non-dredged ones. Nonetheless, one dredged enclosure deviated dramatically from the others, which might reflect methodological issues with dredging. The LMB treatment appeared to be less effective at mitigating eutrophication than dredging, and phosphate concentrations even increased during the experiment in the LMB-treated enclosures. Chemical equilibrium modeling suggested that humic substances could have formed complexes with lanthanum (La) from the LMB, rendering it unavailable for intercepting P over the course of the enclosure experiment. Residual lanthanum concentrations in combination dredging and LMB treatments exceeded the Dutch standard 10-fold. Total zooplankton abundance, and particularly Cladocera, increased in all enclosures over the course of the experiment. The limited effect of LMB in the enclosure experiment and the violation of the Dutch La standard when combined with dredging disqualify LMB as an intervention agent in the restoration of the pond.

Combination of Measures to Restore Eutrophic Urban Ponds in The Netherlands

“…In each pond, a fish stock assessment was performed that revealed a very high fish biomass over 1000 kg ha −1 (Table 1, [31][32][33]) with a majority (77-85%) being sedimentresuspending fish, such as carp and bream ) from external and internal sources for ponds Dongen, Eindhoven (based on [28]), and Heesch. The critical P loading thresholds for clear to turbid (red lines) and turbid to clear (green lines) were determined with the PCLake Metamodel [30].…”

“…In each pond, a fish stock assessment was performed that revealed a very high fish biomass over 1000 kg ha −1 (Table 1, [31][32][33]) with a majority (77-85%) being sedimentresuspending fish, such as carp and bream…”

Response of sediment phosphorus partitioning to lanthanum-modified clay amendment and porewater chemistry in a small eutrophic lake

“…To our knowledge, few eld-based studies have accessed the geochemical conditions leading to the suppression of P uxes from the sediments in the presence of DOC. 40 These studies point to a key interplay between La 3+ , DOC, and Fe, [40][41][42] which we set out to better understand using eld data and thermodynamic modelling.…”

Response of sediment phosphorus partitioning to lanthanum-modified clay amendment and porewater chemistry in a small eutrophic lake.

“…To our knowledge, few field-based studies have accessed the geochemical conditions leading to the suppression of P fluxes from the sediments in the presence of DOC. 40 These studies point to a key interplay between La 3+ , DOC, and Fe [40][41][42] , which we set out to better understand using field data and thermodynamic modelling.…”