Lien T.H. Nguyen

Field validation of sediment zinc toxicity

A field study was conducted to validate concentrations of zinc in freshwater sediments that are tolerated by benthic macroinvertebrate communities and to determine whether a relationship exists with the acid volatile sulfide (AVS)-simultaneously extracted metal (SEM) model. In both the lake and riverine systems, one sediment type was high in AVS and one low in AVS, which resulted in zinc-spiked sediments that ranged from low to high SEM to AVS ratios. The colonization trays were sampled seasonally, ranging from 6 to 37 weeks of exposure, and were evaluated using several appropriate benthic indices. Results of the field evaluations at the four test sites confirmed the validity of the AVS-SEM model, predicting benthic macroinvertebrate effects correctly 92% of the time. In sediments where the SEM to AVS ratio or the AVS and organic (OC)-normalized fractions exceeded 8 and 583 micromol/g of OC, toxicity was observed from the zinc-spiked sediments. Conversely, when the SEM to AVS ratio or OC-normalized AVS fractions were less than 2 or 100 micromol/g of OC, no toxicity was observed. In the range of 148 to 154 micromol/g of OC, toxicity varied in two treatments. Total zinc concentrations in sediments showed no relationship to benthic effects. The most impaired benthic community occurred in the high-gradient stream sediments, which had low OC and AVS concentrations and SEM to AVS ratios of 33 and 44 in the spiked sediments. Five to six benthic metrics were depressed at SEM to AVS ratios of 8.32 and 9.73. The no-observed-effect level appeared to be near a SEM to AVS ratio of 2, with slight to no effects between ratios of 2.34 and 2.94. No sites with ratios of less than 2 showed any adverse effects.

Characterizing sediment acid volatile sulfide concentrations in European streams

Sediment acid volatile sulfide (AVS) concentrations were measured in wadeable streams of a wide variety of ecoregions of western Europe (84 sites in 10 countries and nine ecoregions) to better understand spatial distribution and ecoregion relationships. Acid volatile sulfide has been shown to be a major factor controlling the bioavailability and toxicity of many common trace metals, such as Cd, Cu, Ni, Pb, and Zn. Sediment characteristics varied widely. The ratio of the sum of the simultaneously extracted metals (SEM) to AVS ranged from 0.03 to 486.59. The sigmaSEM-AVS ranged from -40.02 to 17.71 micromol/g. On a regional scale, sediment characteristics such as dominant parent soil material showed significant trends in AVS distribution and variation by ecoregion. Total Fe and Mn were correlated weakly with SEM concentrations. Three AVS model approaches (i.e., the SEM:AVS ratio, SEM-AVS difference, and carbon normalization) were compared at threshold exceedance levels of SEM/AVS > 9, SEM-AVS > 2, and SEM-AVS/foc > 150 micromol/g organic carbon (OC). Only 4.76% of the sediments exceeded all three AVS thresholds; 22.6% of the sediments exceeded two models; and 13% of the sediments exceeded one model only. Using the SEM:AVS, SEM-AVS, and fraction of organic carbon models, and including site-specific data and regional soil characteristics, ecoregions 1 (Portugal), 3 (Italy), 4 (Switzerland), and 9 (Belgium/Germany) had the highest potential metals toxicity; ecoregions 13 and 8 (Belgium/France) showed the lowest potential toxicity. However, because AVS can vary widely spatially and temporally, these data should not be considered as representative of the sampled ecoregions. The general relationship between AVS levels and sediment characteristics provides some predictive capability for wadeable streams in the European ecoregions.

Field measurement of nickel sediment toxicity: Role of acid volatile sulfide

A field experiment was performed in four freshwater systems to assess the effects of Ni on the benthic macroinvertebrate communities. Sediments were collected from the sites (in Belgium, Germany, and Italy), spiked with Ni, and returned to the respective field sites. The colonization process of the benthic communities was monitored during a nine-month period. Nickel effect on the benthos was also assessed in the context of equilibrium partitioning model based on acid volatile sulfides (AVS) and simultaneously extracted metals (SEM). Benthic communities were not affected at (SEM - AVS) ≤ 0.4 µmol/g, (SEM - AVS)/fraction of organic carbon (f(OC)) < 21 µmol/g organic carbon (OC). Sediments with (SEM - AVS) > 2 µmol/g, (SEM - AVS)/f(OC) > 700 µmol/g OC resulted in clear adverse effects. Uncertainty about the presence and absence of Ni toxicity occurred at (SEM - AVS) and (SEM - AVS)/f(OC) between 0.4 to 2 µmol/g and 21 to 700 µmol/g OC, respectively. The results of our study also indicate that when applying the SEM:AVS concept for predicting metal toxicity in the field study, stressors other than sediment characteristics (e.g., sorption capacity), such as environmental disturbances, should be considered, and the results should be carefully interpreted.

WHOLE SEDIMENT TOXICITY TESTS FOR METAL RISK ASSESSMENTS: ON THE IMPORTANCE OF EQUILIBRATION AND TEST DESIGN TO INCREASE ECOLOGICAL RELEVANCE

Current laboratory-based approaches for predicting metal toxicity in sediments exhibit a number of limitations. The most important are (1) a lack of sufficient equilibration resulting in unrealistically low pH values or unnaturally high porewater metal concentrations and (2) an inadequate test design regarding the metal concentrations selected for spiking. The present study illustrates that by explicitly accounting for these limitations, one obtains reliable and environmentally realistic toxicity data, thus advancing the metal risk assessments of sediments. To this end, a toxicity test design with natural sediments was developed in which the administered metal concentrations were selected to comprise a range of the difference between the molar concentration of simultaneously extracted metals and acid volatile sulfides (SEM-AVS) closely surrounding zero. In addition, the test design presented includes a 35- or 40-d equilibration period with overlying water renewal during which conductivity, pH, and metal concentrations in the overlying water are monitored. This allows toxicity testing to start after equilibrium for these parameters has been reached. This test design was applied to Ephoron virgo (Olivier, 1791), Gammarus pulex (Linnaeus, 1758), and Lumbriculus variegatus (Mueller, 1774) exposed to Zn and Pb. These tests indicated that the general concept of absence of toxicity when SEM-AVS<0 could not be rejected. However, the onset of Zn toxicity occurred at lower concentrations than generally assumed.

Single versus combined exposure of Hyalella azteca to zinc contaminated sediment and food.

The amphipod Hyalella azteca was exposed for 28 d to different combinations of Zn contaminated sediment and food. Sediment exposure (+clean food) resulted in increased Zn body burdens, increased mortality and decreased body mass when the molar concentrations of simultaneously extracted Zn were greater than the molar concentration of Acid Volatile Sulfide (SEM(Zn)-AVS>0), suggesting that dissolved Zn was a dominant route of exposure. No adverse effect was noted in the food exposure (+clean sediment), suggesting selective feeding or regulation. Combined exposure (sediment+food) significantly increased adverse effects in comparison with sediment exposure, indicating contribution of dietary Zn to toxicity and bioaccumulation. The observed enhanced toxicity also supports the assumption on the presence of an avoidance/selective feeding reaction of the amphipods in the single sediment or food exposures. During 14 d post-exposure in clean medium, the organisms from the same combined exposure history received two feeding regimes, i.e. clean food and Zn spiked food. Elevated Zn bioaccumulation and reduced reproduction were noted in amphipods that were offered Zn spiked food compared to the respective organisms that were fed clean food. This was explained by the failure of avoidance/selective feeding behavior in the absence of an alternative food source (sediment), forcing the amphipods to take up Zn while feeding. Increasing Zn body burdens rejected the assumption that Zn uptake from food was regulated by H. azteca. Our results show that the selective feeding behavior should be accounted for when assessing ecological effects of Zn or other contaminants, especially when contaminated food is a potential exposure route.