Source relationships between streambank soils and streambed sediments in a mercury-contaminated stream

Purpose

In contaminated streams, understanding the role of streambank and streambed source contributions is essential to developing robust remedial solutions. However, identifying relationships can be difficult because of the lack of identifying signatures in source and receptor pools. East Fork Poplar Creek (EFPC) in Oak Ridge, TN, USA received historical industrial releases of mercury that contaminated streambank soils and sediments. Here, we determined relationships between the contaminated streambank soils and sand-sized streambed sediments.

Materials and methods

Field surveys revealed the spatial trends of the concentrations of inorganic total mercury (Hg) and methyl mercury (MeHg), Hg lability as inferred by sequential extraction, particle size distribution, and total organic carbon. Statistical tests were applied to determine relationships between streambank soil and streambed sediment properties.

Results and discussion

Concentrations of Hg in streambank soils in the upper reaches averaged 206 mg kg^?1 (all as dry weight) (n?=?457), and 13 mg kg^?1 in lower reaches (n?=?321), while sand-sized streambed sediments were approximately 16 mg kg^?1 (n?=?57). Two areas of much higher Hg and MeHg concentrations in streambank soils were identified and related to localized higher Hg concentrations in the streambed sediments; however, most of the streambank soils have similar Hg concentrations to the streambed sediments. The molar ratio of Hg to organic carbon, correlation between MeHg and Hg, and particle size distributions suggested similarity between the streambank soils and the fine sand-sized fraction (125–250 μm) collected from the streambed sediments. Mercury in the fine sand-sized streambed sediments, however, was more labile than Hg in the streambank soils, suggesting an in-stream environment that altered the geochemistry of sediment-bound Hg.

Conclusions

This study revealed major source areas of Hg in streambank soils, identified possible depositional locations in streambed sediments, and highlighted potential differences in the stability of Hg bound to streambank soils and sediments. This work will guide future remedial decision making in EFPC and will aid other researchers in identifying source–sink linkages in contaminated fluvial systems.

     

Exploring the factors influencing the hydrological response of soil after low and high-severity fires with post-fire mulching in Mediterranean forests

Despite ample literature, the influence of the individual soil properties and covers on the hydrological response of burned soils of forests has not clearly identified. A clear understanding of the surface runoff and erosion rates altered by wildfires and prescribed fires is beneficial to identify the most suitable post-fire treatment. This study has carried out a combined analysis of the hydrological response of soil and its driving factors in burned forests of Central-Eastern Spain. The pine stands of these forests were subjected to both prescribed fire and wildfire, and, in the latter case, to post-fire treatment with mulching. Moreover, simple multi-regression models are proposed to predict runoff and erosion in the experimental conditions. In the case of the prescribed burning, the fire had a limited impact on runoff and erosion compared to the unburned areas, due to the limited changes in soil parameters. In contrast, the wildfire increased many-fold the runoff and erosion rates, but the mulching reduced the hydrological response of the burned soils, particularly for the first two-three rainfalls after the fire. The increase in runoff and erosion after the wildfire was associated to the removal of the vegetation cover, soil water repellency, and ash left by fire; the changes in water infiltration played a minor role on runoff and erosion. The multi-regression models developed for the prescribed fire were accurate to predict the post-fire runoff coefficients. However, these models were less reliable for predictions of the mean erosion rates. The predictions of erosion after wildfire and mulching were excellent, while those of runoff were not satisfactory (except for the mean values). These results are useful to better understand the relations among the hydrological effects of fire on one side and the main soil properties and covers on the other side. Moreover, the proposed prediction models are useful to support the planning activities of forest managers and hydrologists towards a more effective conservation of forest soils.     

Hydrogeochemistry of a stream draining sulfide-bearing postglacial sediments in Finland

The influence of hydrological conditions and overburden type on the stream-water quality in a catchment effected by fine-grained sulfide-bearing sediments was studied. Water samples for measurement of pH and electric conductivity were collected at 16 sites along the main course of the stream during autumn and spring high water flow and during low and intermediate water flow. On one occasion (spring high-water flow) samples for analysis of total organic carbon were collected at these sites. Seventeen water samples for analyses of chemical elements (ICP-MS and ICP-AES) were taken within the catchment basin during autumn high water flow. The pH and conductivity of snow, and the geochemistry of the sulfide sediments were also studied. The occurrence of fine-grained sediments within the catchment was estimated from topographic and geologic maps and by visit to areas were the overburden type could not be determined from the maps. The hydrological conditions had a strong influence on the water quality. During high water flow in spring and autumn the concentrations of ionic components (conductivity) and hydrogen ions in the stream water were high, while the concentrations were much lower during low and intermediate water flow and when the ground was frozen. Extensive leaching of the sulfide-bearing sediments, which had higher contents of S and metals than the fine fraction of till in the study area, resulted in elevated contents of Li, Ni, Zn, Co, B, Al, Mg, U, Sr, Ca, K and Na in the stream water during autumn high water flow. The contents of Cr, Ba, Pb, Si and Fe in stream water were not much influenced by the type of overburden, while V was depleted in drainage waters from the sulfide sediments. Humic substances flocculated in the water in the middle section of the stream, which may have been caused by di- and trivalent metal cations leached in large quantities from the oxidizing sulfide sediments.     

Sediment transport from continuous monitoring in a perennial Mediterranean stream

Sediment transport in the Ribera Salada stream was estimated from continuous suspended and bedload monitoring between 2005 and 2008. The Ribera Salada basin is a mountainous perennial river in the Southern Pyrenees and is representative of the extensive forestry land use in this upland region. Water and sediment fluxes have been analysed with the aim of determining the thresholds, duration, ranges and relative contributions of the two sediment transport modes (suspended and bedload), and their variability in relation to the basin's annual hydrology. The stream's hydrology in the first two sampling years was average in the context of a 10-yr series of the basin, while 2007–2008 can be classified as a wet year. The specific total sediment load during the study period amounted to 12 t km⁻²y⁻¹, a low value compared with similar Mediterranean counterparts. The maximum load (31.5 t km⁻²y⁻¹) was observed during the wettest year of the monitoring programme. During average years most of the total load (> 90%) was transported in suspension; consequently, suspended sediment transport is more frequent through time. In contrast, bedload occurs sporadically during floods that exceed certain hydraulic thresholds, corresponding to a flow equalled or exceeded 4% of the time, that yields an average shear stress of ∼ 35 N m⁻². Under such conditions bedload transport becomes relevant and, if the threshold is frequently exceeded as in relatively wet years such as 2007–2008, may constitute the majority of the total sediment load (74%). This paper provides new evidence of the distinct role of sediment transport modes in stable fluvial environments where only sporadic inputs of surplus energy (flow discharge) determine the dominant mode, magnitude and duration of their respective contribution.     

Source apportionment of lake bed sediments to watersheds in an Upper Mississippi basin using a chemical mass balance method

Sediment samples were collected from three known sources contributing to a depositional basin of the Upper Mississippi River, USA. The heavy silt fraction (2–63 μm, density >2.95) isolated from the sediments was analyzed for 42 elements by inductively coupled plasma-mass spectrophotometry (ICP-MS) to give signature profiles characteristic of each source catchment. Known proportions of heavy silt fractions from each source were mixed and the composite samples analyzed in the same way. The proportions of components in the composite samples were compared to their apportionments calculated statistically using the US Environmental Protection Agency (USEPA) chemical mass balance (CMB) air quality receptor model. This showed that the CMB technique can apportion sediments to their catchment sources reliably. The technique was then applied to dated sediments of Lake Pepin, in the Upper Mississippi basin, to apportion them to their contributing catchments. The results show that almost all the recent sediments deposited in the basin originated in the Minnesota River catchment, and that the sediment load increased over time as anthropogenic impacts increased. This technique could be used elsewhere to determine sediment provenance and identify changes in sediment sources over time.     

Deposition of trace metals in stream sediments

Sediment samples have been collected downstream from a Pb smelter and analyzed for Cd, Cu, Fe, Pb, and Zn. The concentrations of these trace metals decrease with increased distance from the smelter. The decrease in trace metal contant has been utilized to provide an indication of how rapidly artificially high levels of trace metals are reduced to the carrying capacity of a natural water. Equilibrium calculations are utilized to predict the water's intrinsic trace metal carrying capacity.     

Extracellular enzyme activity within hyporheic sediments of a small lowland stream

The activity of extracellular α-glucosidase, β-glucosidase and β-xylosidase were measured in stream water, interstitial water and hyporheic sediments of a small lowland stream. Percent contribution of all the activity in the solution phase (<0.2 μm) to the total enzyme activity was low (15-40%), indicating that the enzyme activities are predominantly associated with bacterial cells. Generally, hyporheic sediments dominated the enzyme activity. Stream water, interstitial water and sediments showed activites that decreased in the order β-glucosidase>α-glucosidase>β-xylosidase. The fine fraction of the sediment (<0.063 mm) had much higher enzymatic activities than the coarser fraction (0.063-1 mm). All the extracellular enzymes, had higher activities during summer, period was attributed to the maximum input of allochthonous organic carbon into the sediments being at this time. A low ratio of α-glucosidase to β-glucosidase activity in the sediments, and no obvious spatial and seasonal changes in the ratio of β-xylosidase to β-glucosidase activity, suggested that cellulose is a predominant carbon source for hyporheic heterotrophic bacteria.     

Ripening of clayey dredged sediments during temporary upland disposal a bioremediation technique

Background and Goal  In the Netherlands about 40 million m³ of sediment has to be dredged annually for both maintenance and environmental reasons. Temporary upland disposal is the most widely adopted alternative for dredged sediments worldwide. For good management of temporary disposal sites, knowledge is needed on the processes controlling the behavior of the sediments during disposal. Therefore, a review of the literature was made to get an integrated overview about processes that take place during temporary disposal. Ripening  After disposal of clayey sediments, the following spontaneous dewatering processes can be distinguished: sedimentation, consolidation, and ripening. Sedimentation and consolidation are relatively fast processes, whereas ripening can take up to several years. In a remediation perspective, the ripening of sediments is the most important dewatering process. Ripening, which may be subdivided into physical, chemical, and biological ripening, transforms sediment into soil. Physical ripening is the irteversible toss of water and results in the formation of soil prisms separated by shrinkage cracks. Continuing water loss causes a breaking up of the prisms into aggregates. The aggregates produced by this ongoing desiccation process usually remain quite large (>50 mm) and can only be further broken down by weathering processes like wetting and drying or by tillage. As a result of the aeration caused by physical ripening, also chemical and biological ripening take place. Chemical ripening can be defined as the changes in chemical composition due to oxidation reactions and leaching of soluble substances. Biological ripening is the result of the activity of all kinds of soil fauna and flora that develop as a result of aeration, including both the larger and the microscopic forms of life. Decomposition and mineralization of soil organic matter caused by micro-organisms can be seen as the most important aspect of biological ripening. Many interactions exist between the different ripening processes. Conclusions and Outlook  Oxygenation of the dredged sediment is improved as a result of the natural ripening processes: the air-filled porosity increases, the aggregate size decreases, and the initially high respiration rates caused by chemical and biological ripening decreases. Therefore, conditions in the disposal site become more favorable for aerobic biodegradation of organic pollutants like Polycyclic Aromatic Hydrocarbons (PAH) and mineral oil. It is concluded that the naturally occurring process of ripening can be used as a bioremediation technique. Ripening in an upland disposal site is an off-site technique, and therefore, the process could be enhanced by means of technological interference. However, it is concluded that the knowledge currently available about upland disposal is not sufficient to distinguish critical process steps during the ripening and bioremediation of PAH and mineral oil polluted sediments because of the complex relationships between the different ripening processes and bioremediation.     

Transformation and sorption of fipronil in urban stream sediments

Fipronil is an urban-use insecticide, and the increased use has led to its frequent detections in urban streams. Most studies on the environmental fate of fipronil so far have focused on soils, and little is known about its behavior in sediment-water systems. In this study, we investigated the transformation and sorption of fipronil in urban stream sediments from California, incubated under facultative and anaerobic conditions. Degradation of fipronil in sediments generally followed exponential decay kinetics, and the first-order half-lives of fipronil were only 4.6-18.5 days in anaerobic sediments. The persistence of fipronil under facultative conditions was considerably longer, with half-lives from 25 to 91 days. Sterilization generally decreased the dissipation of fipronil, indicating that microbial activity was an important factor in fipronil transformations in sediments. Under facultative conditions, fipronil sulfide and sulfone were observed, while only fipronil sulfide was detected in anaerobic samples. The sorption coefficient K d consistently increased with organic carbon contents of sediments. In the same sediment, K d usually also increased with contact time, suggesting decreased availability for aged residues. Results from this study showed that the stability of fipronil in sediments depends closely on the oxygen status and that due to the readily conversion of fipronil to the sulfone and sulfide metabolites, the overall risk assessment of fipronil in surface aquatic systems should take into consideration fipronil as well as its metabolites.     

合流制排水系统降雨径流污染物特征及初期冲刷效应

合流制排水系统降雨期间高污染溢流污水直接排入水体对水体质量有明显影响,其径流污染物输出特征和初期冲刷效应受排水区类型、面积、降雨特性等多种因素影响。为此,选取昆明市典型合流制排水系统明通河流域不同类型集水区进行降雨径流监测,研究合流制排水系统降雨径流污染物输出特征及初期冲刷效应。结果表明：合流制排水系统初期合流污水污染物质量浓度明显高于后期,不同污染物输出特征、形态差别较大,溶解态氮是N污染物的主要赋存形态,而P污染物主要以颗粒态形态存在。管道沉积物是合流污水重要的污染来源。年内次降雨污染物平均质量浓度以雨季初期降雨最高,中期降雨质量浓度最低,雨季后期质量浓度较中期略有上升。不同类型集水区N、P质量浓度排序为：居民区〉城市综合型〉综合型〉城郊结合型。降雨量是影响初期冲刷效应的关键因素,雨量大的降雨初期冲刷效应明显。     

Soil microbial recolonisation after a fire in a Mediterranean forest

The capacity of different microbial groups to recolonise soil after a fire event will be decisive in determining the microbial community after the fire. Microbial recovery after a wildfire that occurred in Sierra la Grana (Alicante province, southeast Spain) was tracked for 32 months after the fire. Colony forming units (CFUs) of different microbial groups, microbial biomass, soil respiration, bacterial growth (leucine incorporation) and changes in the microbial community structure (phospholipid fatty acid (PLFA) analysis) were determined directly after the fire and four times during the recovery period. Direct effects were reflected by low values of most microbiological variables measured immediately after the fire. Microbial biomass increased during the first year after the fire but was below the unburned reference site 32 months after the fire. Bacterial activity and soil respiration showed the highest values immediately after the fire, but decreased to values similar to that of the unburned reference site or even lower (respiration) 32 months after the fire. Colony forming units of bacterial groups estimated by the plate count method peaked 8 months after the fire, but then decreased, showing values similar to the unburned reference site at the end of the study, with the exception of spore formers, which were 20 times higher than the reference site 32 months after the fire. Fungal CFUs were more sensitive to the fire and recovered more slowly than bacteria. Fungi recovering less rapidly than bacteria were also indicated by the PLFA pattern, with PLFAs indicative of fungi being less common after the fire. The recovery of microbial biomass and activity was mirrored by the initially very high levels of dissolved organic carbon being consumed and decreasing within 8 months after the fire. The wildfire event had thus resulted in a decrease in microbial biomass, with a more bacteria-dominated microbial community.     

Indicator bacteria in water and stream sediments near the snowy range in Southern Wyoming

Water samples were taken at eight pools in streams in the Nash Fork watershed after snowmelt in the summer of I980. The watershed is below the Snowy Range escarpment, and principal land uses include skiing and other winter sports, sheep and cattle grazing, camping and fishing, and summer homes. Water samples taken before and after disturbing streambottom sediment were analyzed for fecal coliforms and fecal streptococci. The levels of these indicator bacteria were low, except where sheep and cattle had recently grazed. Recreation use had little effect on bacterial densities. Disruption of streambottom sediment increased the mean concentration of fecal coliforms 1.7 times and fecal streptococci 2.7 times. Stream sediment was not a significant secondary source of fecal coliforms but appeared to harbor residual fecal streptococci that had survived and multiplied.     

Modelling post-depositional transport of PAHs in aquatic bed sediments using CoReTranS

Purpose

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous and persistent contaminants in aquatic bed sediments. A better understanding of their in-bed fate and transport is therefore key in minimising the risk to the environment over time through various remediation and monitoring strategies. Since ecological effects and risks are related to contaminant concentrations, this study developed CoReTranS, a predictive model that simulates one-dimensional organic contaminant reaction and transport in bed sediments.

Materials and methods

CoReTranS was benchmarked against analytical solutions of simplified reactive transport models and validated using a published study of marsh sediments contaminated with petroleum-derived hydrocarbons from Wild Harbour, West Falmouth, MA, USA.

Results and discussion

The CoReTranS model effectively predicted the vertical distribution of PAHs in the Wild Harbour sediments as confirmed by the modelling results from the published study. The CoReTranS model was also used to interpret results from a published study of PAH-contaminated fjord sediments from Kitimat Arm in British Columbia, Canada. Specific insights into the post-depositional fate and transport of selected PAHs in the Kitimat fjord sediments were obtained by comparing the measured concentration-depth profiles with the numerical results from the CoReTranS model. Key parameters such as effective diffusivity of contaminants and burial velocities of sediment particles were shown to possibly account for the predicted concentrations-depth profiles in the Kitimat fjord sediments.

Conclusions

As demonstrated, CoReTranS can simulate reactive transport models in order to predict PAH concentration profiles in porewater under site-specific conditions. The information derived from the use of the CoReTranS model highlighted practical application of such information by engineers to site-specific risk assessment and remediation.     

Mineralogy and heavy metal contents of soils and stream sediments in a rural region of Western Germany

This paper presents the results of a study of clay mineralogy and Pb, Zn, Cu, Rb, Sr, Y and Zr analyses of soils and stream sediments of the Schwarzach watershed, a drainage system located in a secluded rural region of eastern Bavaria, far removed from major industrial installations.Clay fractions of soils and stream sediments differ significantly in their mineralogy. The soils average 50% secondary chlorite and 40% illite, whereas recent stream sediments average 20% chlorite and 70% illite. These assemblages are possibly interconvertible under the influence, or on withdrawal, of Schwarzach water. Proportions of clay minerals in fossil stream sediments average approximately 30% chlorite, 50% illite, and 20% kaolinite with up to 10% of 10–14 Å interstratified minerals.Surface soil layers have been enriched in Pb, as have recent stream sediments. The latter have also been enriched in Zn and Cu. The increased amounts of Pb plus its close association with organic C indicate atmospheric deposition of Pb and incipient eutrophication of the Schwarzach River.     

Lead mobilisation in the hyporheic zone and river bank sediments of a contaminated stream: contribution to diffuse pollution

Purpose

Past metal mining has left a legacy of highly contaminated sediments representing a significant diffuse source of contamination to water bodies in the UK and worldwide. This paper presents the results of an integrated approach used to define the role of sediments in contributing to the dissolved lead (Pb) loading to surface water in a mining-impacted catchment.

Materials and methods

The Rookhope Burn catchment, northern England, UK is affected by historical mining and processing of lead ore. Quantitative geochemical loading determinations, measurements of interstitial water chemistry from the stream hyporheic zone and inundation tests of bank sediments were carried out.

Results and discussion

High concentrations of Pb in the sediments from the catchment, identified from the British Geological Survey Geochemical Baseline Survey of the Environment (GBASE) data, demonstrate both the impact of mineralisation and widespread historical mining. The results from stream water show that the stream Pb load increased in the lower part of the catchment, without any apparent or significant contribution of point sources of Pb to the stream. Relative to surface water, the interstitial water of the hyporheic zone contained high concentrations of dissolved Pb in the lower reaches of the Rookhope Burn catchment, downstream of a former mine washing plant. Concentrations of 56???g?l^?1 of dissolved Pb in the interstitial water of the hyporheic zone may be a major cause of the deterioration of fish habitats in the stream and be regarded as a serious risk to the target of good ecological status as defined in the European Water Framework Directive. Inundation tests provide an indication that bank sediments have the potential to contribute dissolved Pb to surface water.

Conclusions

The determination of Pb in the interstitial water and in the inundation water, taken with water Pb mass balance and sediment Pb distribution maps at the catchment scale, implicate the contaminated sediments as a large Pb supply to surface water. Assessment of these diffuse contaminant sources is critical for the successful management of mining-impacted catchments.     

Experimental management of Mediterranean temporary pools for conservation of the rare quillwort Isoetes setacea

Mediterranean temporary pools represent unique species-rich habitats with many rare and endangered species. The Roque-Haute Natural Reserve is one of the few sites in France with Isoetes setacea Lam., a very rare Mediterranean quillwort, considered as a keystone species determining a priority habitat for the European Union. Land uses changes have favoured the invasion of the pools by several shrub and tree species (e.g., Ulmus minor, Fraxinus angustifolia subsp. oxycarpa). Clearing shrubs in an experimental area of one pool resulted in the increase of I. setacea as compared to control, combining a higher cover in previously occupied quadrats and more quadrats occupied. Litter accumulation had a negative effect on the population of Isoetes but this effect was detectable only in the cleared area. Light attenuation was shown experimentally to be detrimental to the vegetative and regenerative production of I. setacea. Clearing of the shrubs and removal of the litter led to an increase in the I. setacea populations and therefore appears to be an effective technique for restoring these plant communities invaded by shrubs. Long-term shrub control should be ensured by reintroducing sheep grazing in order to enhance the sustainable conservation of this oligotrophic habitat.     

The extent of drying influences the flush of respiration after rewetting in non-saline and saline soils

Drying and rewetting are common events in soils during summer, particularly in Mediterranean climate where soil microbes may be further challenged by salinity. Previous studies in non-saline soils have shown that rewetting induces a flush of soil respiration, but little is known about how the extent of drying affects the size of the respiration flush or how drying and rewetting affects soil respiration in saline soils. Five sandy loam soils, ranging in electrical conductivity of the saturated soil extract (ECe) from 2 to 48 dS m⁻¹ (EC2, EC9, EC19, EC33 and EC48), were kept at soil water content optimal for respiration or dried for 1, 2, 3, 4 or 5 days (referred to 1D, 2D, 3D, 4D and 5D) and maintained at the achieved water content for 4 days. Then the soils were rewet to optimal water content and incubated moist for 5 days. Water potential decreased with increasing drying time; in the 5D treatment, the water potential ranged between −15 and −30 MPa, with the lowest potentials in soil EC33. In moist and dry conditions, respiration rates per unit soil organic C (SOC) were highest in soil EC19. Respiration rates decreased with increasing time of drying; when expressed relative to constantly moist soil, the decline was similar in all soils. Rewetting of soils only induced a flush of respiration compared to constantly moist soil when the soils were dried for 3 or more days. The flush in respiration was greatest in 5D and smallest in 3D, and greater in EC2 than in the saline soils. Cumulative respiration per unit SOC was highest in soil EC19 and lowest in soil EC2 Cumulative respiration decreased with increasing time of drying, but in a given soil, the relationship between water potential during the dry phase and cumulative respiration at the end of the experiment was weaker than that between respiration rate during drying and water potential. In conclusion, rewetting induced a flush in respiration only if the water potential of the soils was previously decreased at least 3-fold compared to the constantly moist soil. Hence, only marked increases in water potential induce a flush in respiration upon rewetting. The smaller flush in respiration upon rewetting of saline soils suggests that these soils may be less prone to lose C when exposed to drying and rewetting compared to non-saline soils.     

Recovery of aquatic bacterial populations in a stream after cessation of chemical pollution

Tims Branch was studied for 1 yr to determine the effects of an annual loading of about 229 900 kg of various chemicals on the bacterial populations of a small fast flowing stream. The 10 yr discharge of chemical waste to this stream was terminated in 1973 and a 2 yr study of the stream's recovery was begun. Upper Three Runs, an adjacent nonpolluted stream, was used as a control. Total culturable bacterial counts, per cent chromagenic bacteria and diversity were used as indices of recovery. Plate count agar was used for culturing aquatic bacteria present in both systems. Diversity in both systems was more affected by seasonal temperatures than the presence of chemical pollution. Mean total counts dropped from 187 × 10³ in the polluted stream to approximate those of the control stream (170 × 10³) following cessation of pollution. In the chemically affected stream, per cent chromagens increased from 15% during pollution to 25% approximately 1 yr after pollution was stopped. The latter percentage was comparable to that of the control stream (28%) during the period of study.     

The importance of geological controls on the natural distribution of mercury in lake and stream sediments across Canada

The Geological Survey of Canada (GSC) has surveyed a significant portion of Canada using systematic stream and lake surveys under the National Geochemical Reconnaissance (NGR) program. Total mercury (Hg) data, available for most of the sites, reveal significant natural variation. Much of the observed variation in Hg concentration can be directly related to the composition of the bedrock, regolith and glacial deposits in the surrounding watershed. Some of the highest Hg values within the sediments of Ontario lakes occur southwest of Thunder Bay in an area underlain by shales known to be naturally enriched in Hg and other trace metals.