R. Douglas Evans

Empirical evidence of the importance of sediment resuspension in lakes

This paper reviews the empirical evidence for bottom sediment resuspension, which has been documented in many aquatic systems, and attempts to put those observations into a predictive framework. The evidence suggests that resuspension is episodic in all systems with the spatial extent of resuspension being determined by the wind speed, duration of disturbance, effective lake fetch and lake morphometry. The rates of deposition of resuspended material vary greatly with measurements recorded between 0.5 and 50 g m−2 d−1. In many systems and at many times of year resuspended material comprises the majority of the flux of particles to lake bottoms. An order-of-magnitude estimate of the long-term contribution of resuspended material to the total flux of particulate matter is 85%.

Measurement of mercury methylation in sediments by using enriched stable mercury isotopes combined with methylmercury determination by gas chromatography–inductively coupled plasma mass spectrometry

A novel technique for the calculation of mercury methylation rates in sediments by using enriched stable mercury isotopes is described. The method takes advantages of the ability of an inductively coupled plasma mass spectrometry (ICP-MS) instrument to measure individual isotopes. An ICP-MS instrument was used as a detector for the determination of methylmercury compounds after separation by gas chromatography (GC). CH3Hg+ was isolated from sediments by distillation, converted to methylethylmercury by sodium tetraethylborate and analysed after purge-and-trap pre-collection on a Tenax adsorber and thermodesorption onto the GC column. Detection limits were found to be ≈ 1 pg (as Hg) absolute or 0.02 ng g–1 dry sediment. The precision was ≈ 4% relative standard deviation when 250 pg of methylmercury were processed. The accuracy of the GC–ICP-MS technique was demonstrated by analysis of an International Atomic Energy Agency certified reference material (IAEA CRM 356) Harbor Sediment, giving a concentration of 5.40 ± 0.40 ng g–1, compared with the certified value of 5.46 ± 0.38 ng g–1. Mercury methylation was investigated by spiking sediments with stable enriched mercury isotopes at in situ mercury concentrations not perturbing the system. More than 3% of the mercury added to a lake sediment was methylated during a 21 d incubation period. Isotope ratios of total mercury differed significantly from isotope ratios of methylmercury at the end of the experiment, suggesting that the system was still not in equilibrium after 21 d.

Evaluation of210Pb dating in lake sediments using stable Pb,Ambrosia pollen, and137Cs

We used sediment chronology data from fourteen published studies of lake cores across much of North America and Scandinavia in order to make a large scale comparison of the different dating techniques. The uncertainty of210Pb derived dates was determined using common sediment event markers: the stable Pb rise, the137Cs rise, and theAmbrosia pollen rise. For all data combined, the 95% confidence intervals for the stable Pb rise and theAmbrosia rise, were approximately 30 years. These 95% confidence intervals are slightly higher than those derived by First-Order Error analysis performed by others on210Pb derived dates. When comparing the concordance of two210Pb models (CRS and CIC) against markers of known history, we found that the CRS model dates (constant rate of supply) had consistently better agreement than the CIC model dates (constant initial concentration). Major discrepancies between137Cs and210Pb were common, but were consistently more severe in sediments of soft water lakes pointing to an inability of sediments with low mineral content to ‘immobilize’ Cs.

Sources of mercury contamination in the sediments of small headwater lakes in south-central Ontario, Canada

To determine the significance of mercury contamination of remote lakes in three regions of south-central Ontario, Canada, sediment cores were collected at several stations in each of fourteen lakes. Profiles of Hg concentration with depth were similar to those found in many parts of the world and indicate a substantial increase in Hg loading to these lakes in recent decades. Surficial sediment concentrations (0–2 cm) ranged from 120 to 700 ng/g dry matter.Whole-lake burdens of Hg, used to determine the source of the contamination, were similar for all lakes in the study. The average anthropogenic component of the burden for all lakes was 0.79 mg/m2. This amount represents approximately one-half of the total burden in the sediments. There was a relationship between the size of a lakes catchment and the whole-lake burden of Hg in lakes in one region of the study area but not the remaining lakes. It is concluded that the major source of the Hg in the study lakes is atmospheric deposition of material originating from outside the catchments and that the contribution from catchment washout is variable.

Rapid determination of plutonium in urine using flow injection on-line preconcentration and inductively coupled plasma mass spectrometry

A rapid technique for the determination of Pu in urine samples using flow injection (FI) on-line preconcentration, desolvation nebulization using a highly sensitive APEX device and inductively coupled plasma mass spectrometry with a dynamic reaction cell (ICP-DRC-MS) was developed. Using the DRC avoided the necessity to separate Pu from U, and thus made the technique more rapid. Effective preconcentration and matrix separation for urine samples was achieved using TRU resin as the ion exchanger. A detection limit for 242Pu of 1.9 pg L−1 was achieved for raw, undigested urine samples using a 10 ml sample volume. The reproducibility of the method was demonstrated by recovery measurements performed on urine that had been collected from several non-exposed volunteers and spiked with Pu. 205Tl and 175Lu were used as an internal standard and yield tracer, respectively. The method provides a rapid (∼11 min) means of measuring Pu in urine with no off-line sample pre-treatment, at levels below the regulatory requirements for drinking water.

Method for the estimation of organic-bound and crystal-bound metal concentrations in bivalve shells

The object of this study was to provide a technique to separate and analyze the adsorbed heavy metals in the nacreous layer of the shells of Elliptio complanata from that of the isomorphically substituted metals, thereby determining the relative distribution of the total metal content between the two nacre components. In doing so, it will test the hypothesis that the shell is a legitimate and better index of bioavailability. By understanding the distribution of trace metals through the tissues and into the shell nacre components, it allows for a better understanding of the uptake processes that occur and ultimately the significance of the observed concentrations.

The History of Atmospheric Deposition of Cd, Hg, and Pb in North America: Evidence from Lake and Peat Bog Sediments

Lake and ombrotrophic peat bog sediments record increases in the concentrations and accumulation rates of Cd, Hg, and Pb for most of temperate North America for the last 100 years. These increases are largely related to the burning of coal, smelting of nonferrous metals, the transportation industry, and the industrial production of chlorine.

Paleolimnological evidence of recent acidification in two sudbury (Canada) lakes

Abstract As a result of an increase in SO 2 emissions from the metal mining and smelting activities in the vicinity of Sudbury, severe detrimental effects have been reported on aquatic ecosystems. However, documentation of the time and rates of lake acidification are not available. The purpose of the present study is to provide this information by utilizing the stratigraphic analysis of sedimentary diatoms from Hannah and Clearwater lakes. The study indicates that, in Hannah Lake, acidification occurred soon after the roasting of ore started at Copper Cliff in the 1880s. Between about 1880 and 1975, the inferred lake water pH declined from a high of 6.0 to a low of 4.6. After the lake was neutralized in 1975 its measured pH increased from 4.3 to ∼ 7.0. This increase was also indicated by an increase in diatom-inferred pH. In Clearwater Lake, acidification commenced after 1930. Between about 1930 and 1970 the pH declined from ∼ 6.0 to ∼ 4.2. Due to reduction in SO 2 emissions, no further pH decline has occurred since about 1970. The study indicates that, in addition to the existing buffering capacity of the lakes, the lakes distance from the point source and past changes in smelting practices may have greatly influenced the onset of acidification in Sudbury lakes.

Long distance transport of anthropogenic lead as measured by lake sediments

Long-term atmospheric deposition of Pb has been measured using lake sediment cores. The average ‘whole-lake’ Pb burden was measured for several lakes in each of four regions located throughout Ontario and Quebec, Canada. Since the regions were remote to any point source of Ph, large scale variation in atmospheric transport could be studied. The results indicate that Pb deposition is highest in South-Central Ontario, where lake burdens ranged from 312 to 432 mg m−2, and that it decreases in an eastward direction into Quebec. Lowest deposition was at the site farthest north, Schefferville, Quebec, where fallout was 31 to 42 mg m−2, less than one-tenth of that previously observed in South-Central Ontario. Lead-210 dating of sediment cores from the northern site showed that input to these remote lakes has occurred over the same time period as in the southern sites.

In vitro and whole animal evidence that methylmercury disrupts GABAergic systems in discrete brain regions in captive mink

The effects of mercury (Hg) on key components of the GABAergic system were evaluated in discrete brain regions of captive juvenile male American mink (Neovison vison) using in vitro and in vivo (whole animal) experimental approaches. In vitro studies on cortical brain tissues revealed that inorganic Hg (HgCl(2); IC50=0.5+/-0.2microM) and methyl Hg (MeHgCl; IC50=1.6+/-0.2microM) inhibited glutamic acid decarboxylase (GAD; EC 4.1.1.15) activity. There were no Hg-related effects on [(3)H]-muscimol binding to GABA(A) receptors (IC50s>100microM). HgCl(2) (IC50=0.8+/-0.3microM) but not MeHgCl (IC50>100microM) inhibited GABA-transaminase (GABA-T; EC 2.6.1.19) activity. In a whole animal study, neurochemical indicators of GABAergic function were measured in brain regions (occipital cortex, cerebellum, brain stem, and basal ganglia) of captive mink fed relevant levels of MeHgCl (0 to 2microg/g feed, ppm) daily for 89d. No effects on GAD activity were measured. Concentration-dependent decreases in [(3)H]-muscimol binding to GABA(A) receptors and GABA-T activity were found in several brain regions, with reductions as great as 94% (for GABA(A) receptor levels) and 71% (for GABA-T activity) measured in the brain stem and basal ganglia. These results show that chronic exposure to environmentally relevant levels of MeHg disrupts GABAergic signaling. Given that GABA is the main inhibitory neurotransmitter in the mammalian nervous system, prolonged disruptions of its function may underlie the sub-clinical impacts of MeHg at relevant levels to animal health.