Geology topics

D. P. Krabbenhoft

Publications and source records attributed to D. P. Krabbenhoft.

At least 37 records · Page 2Linked to original sources

USGS2007

Mercury and organic carbon dynamics during runoff episodes from a northeastern USA watershed

Mercury and organic carbon concentrations vary dynamically in streamwater at the Sleepers River Research Watershed in Vermont, USA. Total mercury (THg) concentrations ranged from 0.53 to 93.8 ng/L during a 3-year period of study. The highest mercury (Hg) concentrations occurred slightly before peak flows and were associated with the highest organic carbon (OC) concentrations. Dissolved Hg (DHg) was the dominant form in the upland catchments; particulate Hg (PHg) dominated in the lowland catchments. The concentration of hydrophobic acid (HPOA), the major component of dissolved organic carbon (DOC), explained 41–98% of the variability of DHg concentration while DOC flux explained 68–85% of the variability in DHg flux, indicating both quality and quantity of the DOC substantially influenced the transport and fate of DHg. Particulate organic carbon (POC) concentrations explained 50% of the PHg variability, indicating that POC is an important transport mechanism for PHg. Despite available sources of DHg and wetlands in the upland catchments, dissolved methylmercury (DmeHg) concentrations in streamwaters were below detection limit (0.04 ng/L). PHg and particulate methylmercury (PmeHg) had a strong positive correlation ( r 2 = 0.84, p < 0.0001), suggesting a common source; likely in-stream or near-stream POC eroded or re-suspended during spring snowmelt and summer storms. Ratios of PmeHg to THg were low and fairly constant despite an apparent higher methylmercury (meHg) production potential in the summer. Methylmercury production in soils and stream sediments was below detection during snowmelt in April and highest in stream sediments (compared to forest and wetland soils) sampled in July. Using the watershed approach, the correlation of the percent of wetland cover to TmeHg concentrations in streamwater indicates that poorly drained wetland soils are a source of meHg and the relatively high concentrations found in stream surface sediments in July indicate these zones are a meHg sink.

Water, Air, & Soil Pollution↗

USGS2007

Mercury and methylmercury in water and bottom sediments of wetlands at Lostwood National Wildlife Refuge, North Dakota, 2003-04

Certain ecosystem types, particularly wetlands, have environmental characteristics that can make them particularly sensitive to mercury inputs and that can result in large mercury concentrations in fish or other aquatic biota. To provide information needed to make effective management decisions to decrease human and wildlife exposure to methylmercury in northern prairie pothole wetlands, the U.S. Geological Survey, in cooperation with the North Dakota Department of Health, conducted a study to assess mercury and methylmercury concentrations in wetlands at the Lostwood National Wildlife Refuge (the Refuge) in northwest North Dakota. In April 2003 and 2004, water and bottom-sediment samples were collected from 44 individual wetlands that were classified as one of four wetland types. Many factors that may affect methylmercury production were considered in the study. The prairie pothole wetlands at the Refuge had large ranges in major environmental characteristics. Hydrologic differences, most notably semiannual wetting and drying cycles, that are intrinsic to prairie pothole wetlands affected methylmercury concentrations. This likely resulted from the stimulation of anaerobic microbial activity following reflooding of soils, particularly soils containing substantial organic carbon. Among the four wetland types considered for this study, seasonal and semipermanent wetlands generally had the largest methylmercury concentrations. Regardless of wetland type, however, methylmercury concentrations at the Refuge are large in relation to reported concentrations for natural aquatic systems.

North Dakota↗

USGS2006

Investigation of mercury exchange between forest canopy vegetation and the atmosphere using a new dynamic chamber

This paper presents the design of a dynamic chamber system that allows full transmission of PAR and UV radiation and permits enclosed intact foliage to maintain normal physiological function while Hg(0) flux rates are quantified in the field. Black spruce and jack pine foliage both emitted and absorbed Hg(0), exhibiting compensation points near atmospheric Hg(0) concentrations of ∼2−3 ng m - 3 . Using enriched stable Hg isotope spikes, patterns of spike Hg(II) retention on foliage were investigated. Hg(0) evasion rates from foliage were simultaneously measured using the chamber to determine if the decline of foliar spike Hg(II) concentrations over time could be explained by the photoreduction and re-emission of spike Hg to the atmosphere. This mass balance approach suggested that spike Hg(0) fluxes alone could not account for the measured decrease in spike Hg(II) on foliage following application, implying that either the chamber underestimates the true photoreduction of Hg(II) to Hg(0) on foliage, or other mechanisms of Hg(II) loss from foliage, such as cuticle weathering, are in effect. The radiation spectrum responsible for the photoreduction of newly deposited Hg(II) on foliage was also investigated. Our spike experiments suggest that some of the Hg(II) in wet deposition retained by the forest canopy may be rapidly photoreduced to Hg(0) and re-emitted back to the atmosphere, while another portion may be retained by foliage at the end of the growing season, with some being deposited in litterfall. This finding has implications for the estimation of Hg dry deposition based on throughfall and litterfall fluxes.

Environmental Science & Technology↗

USGS2006

Mercury in water and biomass of microbial communities in hot springs of Yellowstone National Park, USA

Ultra-clean sampling methods and approaches typically used in pristine environments were applied to quantify concentrations of Hg species in water and microbial biomass from hot springs of Yellowstone National Park, features that are geologically enriched with Hg. Microbial populations of chemically-diverse hot springs were also characterized using modern methods in molecular biology as the initial step toward ongoing work linking Hg speciation with microbial processes. Molecular methods (amplification of environmental DNA using 16S rDNA primers, cloning, denatured gradient gel electrophoresis (DGGE) screening of clone libraries, and sequencing of representative clones) were used to examine the dominant members of microbial communities in hot springs. Total Hg (THg), monomethylated Hg (MeHg), pH, temperature, and other parameters influential to Hg speciation and microbial ecology are reported for hot springs water and associated microbial mats.

Applied Geochemistry↗

USGS2006

Atmospheric mercury speciation in Yellowstone National Park

Atmospheric concentrations of elemental mercury (Hg 0 ), reactive gaseous Hg (RGM), and particulate Hg (pHg) concentrations were measured in Yellowstone National Park (YNP), U.S.A. using high resolution, real time atmospheric mercury analyzers (Tekran 2537A, 1130, and 1135). A survey of Hg 0 concentrations at various locations within YNP showed that concentrations generally reflect global background concentrations of 1.5–2.0 ng m − 3 , but a few specific locations associated with concentrated geothermal activity showed distinctly elevated Hg 0 concentrations (about 9.0 ng m − 3 ). At the site of intensive study located centrally in YNP (Canyon Village), Hg 0 concentrations did not exceed 2.5 ng m − 3 ; concentrations of RGM were generally below detection limits of 0.88 pg m − 3 and never exceeded 5 pg m − 3 . Concentrations of pHg ranged from below detection limits to close to 30 pg m −3 . RGM and pHg concentrations were not correlated with any criteria gases (SO 2 , NO x , O 3 ); however pHg was weakly correlated with the concentration of atmospheric particles. We investigated three likely sources of Hg at the intensive monitoring site: numerous geothermal features scattered throughout YNP, re-suspended soils, and wildfires near or in YNP. We examined relationships between the chemical properties of aerosols (as measured using real time, single particle mass spectrometry; aerosol time-of-flight mass spectrometer; ATOFMS) and concentrations of atmospheric pHg. Based on the presence of particles with distinct chemical signatures of the wildfires, and the absence of signatures associated with the other sources, we concluded that wildfires in the park were the main source of aerosols and associated pHg to our sampling site.

Science of the Total Environment↗

USGS2005

Using stable isotopes of water and strontium to investigate the hydrology of a natural and a constructed wetland

Wetlands cannot exist without water, but wetland hydrology is difficult to characterize. As a result, compensatory wetland mitigation often only assumes the proper hydrology has been created. In this study, water sources and mass transfer processes in a natural and constructed wetland complex were investigated using isotopes of water and strontium. Water isotope profiles in the saturated zone revealed that the natural wetland and one site in the constructed wetland were primarily fed by ground water; profiles in another constructed wetland site showed recent rain was the predominant source of water in the root zone. Water isotopes in the capillary fringe indicated that the residence time for rain is less in the natural wetland than in the constructed wetland, thus transpiration (an important water sink) was greater in the natural wetland. Strontium isotopes showed a systematic difference between the natural and constructed wetlands that we attribute to the presence or absence of peat. In the peat‐rich natural wetland, δ 87 Sr in the pore water increased along the flowline due to preferential weathering of minerals containing radiogenic Sr in response to elevated Fe concentrations in the water. In the constructed wetland, where peat thickness was thin and Fe concentrations in water were negligible, δ 87 Sr did not increase along the flowline. The source of the peat (on‐site or off‐site derived) applied in the constructed wetland controlled the δ 87 Sr at the top of the profile, but the effects were restricted by strong cation exchange in the underlying fluvial sediments. Based on the results of this study, neither constructed wetland site duplicated the water source and weathering environment of the adjoining natural wetland. Moreover, stable isotopes were shown to be effective tools for investigating wetlands and gaining insight not easily obtained using non‐isotopic techniques. These tools have potential widespread application to wetlands that have distinct isotopic endmember sources.

Ground Water↗

USGS2005

Trace metal concentrations in shallow ground water

Trace metal clean sampling and analysis techniques were used to examine the temporal patterns of Hg, Cu, and Zn concentrations in shallow ground water, and the relationships between metal concentrations in ground water and in a hydrologically connected river. Hg, Cu, and Zn concentrations in ground water ranged from 0.07 to 4.6 ng L −1 , 0.07 to 3.10 μg L −1 , and 0.17 to 2.18 μg L −1 , respectively. There was no apparent seasonal pattern in any of the metal concentrations. Filtrable Hg, Cu, and Zn concentrations in the North Branch of the Milwaukee River ranged from below the detection limit to 2.65 ng Hg L −1 ,0.51 to 4.30 μg Cu L −1 , and 0.34 to 2.33 μg Zn L −1 . Thus, metal concentrations in ground water were sufficiently high to account for a substantial fraction of the filtrable trace metal concentration in the river. Metal concentrations in the soil ranged from 8 to 86 ng Hg g −1 , 10 to 39 μg Cu g −1 , and 15 to 84 μg Zn g −1 . Distribution coefficients, K D , in the aquifer were 7900,22,000, and 23,000 L kg −1 for Hg, Cu, and Zn, respectively. These values were three to 40 times smaller than K D values observed in the Milwaukee River for suspended particulate matter.

Ground Water↗

USGS2005

Assessing the potential for re-emission of mercury deposited in precipitation from arid soils using a stable isotope

A solution containing 198 Hg in the form of HgCl 2 was added to a 4 m 2 area of desert soils in Nevada, and soil Hg fluxes were measured using three dynamic flux chambers. There was an immediate release of 198 Hg after it was applied, and then emissions decreased exponentially. Within the first 6 h after the isotope was added to the soil, ∼12 ng m - 2 of 198 Hg was emitted to the atmosphere, followed by a relatively steady flux of the isotope at 0.2 ± 0.2 ng m - 2 h - 1 for the remainder of the experiment (62 days). Over this time, ∼200 ng m - 2 or 2% of the 198 Hg isotope was emitted from the soil, and we estimate that ∼6% of the isotope would be re-emitted in a year's time. During the experiment, dry deposition of elemental Hg from the atmosphere was measured with an average deposition rate of 0.2 ± 0.1 ng m - 2 h - 1 . Emission of ambient Hg from the soil was observed after soil wetting with the isotope solution and after a storm event. However, the added moisture from the storm event did not affect 198 Hg flux. Results suggest that in this desert environment, where there is limited precipitation, Hg deposited by wet processes is not readily re-emitted and that dry deposition of elemental Hg may be an important process.

Environmental Science & Technology↗

USGS2005

Speciation and transport of newly deposited mercury in a boreal forest wetland: A stable mercury isotope approach

As part of the Mercury Experiment to Assess Atmospheric Loadings in Canada and the United States (METAALICUS) the fate and transport of contemporary mercury (Hg) deposition in a boreal wetland was investigated using an experimentally applied stable mercury isotope. We applied high purity (99.2% ± 0.1) 202 Hg(II) to a wetland plot to determine if (1) the 202 Hg was detectable above the pool of native Hg, (2) the 202 Hg migrated vertically and/or horizontally in peat and pore waters, and (3) the 202 Hg was converted to methylmercury (MeHg) in situ. The 202 Hg was easily detected by ICP/MS in both solid peat and pore waters. Over 3 months, the 202 Hg migrated vertically downward in excess of 15 cm below the water table and traveled several meters horizontally beyond the experimental plot to the lake margin along the dominant vector of groundwater flow. Importantly, at one location, 6% of aqueous 202 Hg was detected as Me 202 Hg after only 1 day. These results indicate that new inorganic Hg in atmospheric deposition can be readily methylated and transported lakeward by shallow groundwater flow, confirming the important role of wetlands as contributors of Hg to aquatic ecosystems.

Water Resources Research↗

USGS2005

Mercury transport in a high-elevation watershed in Rocky Mountain National Park, Colorado

Mercury (Hg) was measured in stream water and precipitation in the Loch Vale watershed in Rocky Mountain National Park, Colorado, during 2001–2002 to investigate processes controlling Hg transport in high-elevation ecosystems. Total Hg concentrations in precipitation ranged from 2.6 to 36.2 ng/L and showed a strong seasonal pattern with concentrations that were 3 to 4 times higher during summer months. Annual bulk deposition of Hg was 8.3 to 12.4 μ g/m 2 and was similar to deposition rates in the Midwestern and Northeastern U.S. Total Hg concentrations in streams ranged from 0.8 to 13.5 ng/L and were highest in mid-May on the rising limb of the snowmelt hydrograph. Stream-water Hg was positively correlated with dissolved organic carbon suggesting organically complexed Hg was flushed into streams from near-surface soil horizons during the early stages of snowmelt. Methylmercury (MeHg) in stream water peaked at 0.048 ng/L just prior to peak snowmelt but was at or below detection (< 0.040 ng/L) for the remainder of the snowmelt season. Annual export of total Hg in Loch Vale streams ranged from 1.2 to 2.3 μ g/m 2 , which was less than 20% of wet deposition, indicating the terrestrial environment is a net sink of atmospheric Hg. Concentrations of MeHg in stream water and corresponding watershed fluxes were low, indicating low methylation rates or high demethylation rates or both.

Colorado↗

USGS2005

Mercury concentrations in water from an unconfined aquifer system, New Jersey coastal plain

Concentrations of total mercury (Hg) from 2 μg/L (the USEPA maximum contaminant level) to 72 μg/L in water from about 600 domestic wells in residential parts of eight counties in southern New Jersey have been reported by State and county agencies. The wells draw water from the areally extensive (7770 km 2 ) unconfined Kirkwood–Cohansey aquifer system, in which background concentrations of Hg are about 0.01 μg/L or less. Hg is present in most aquifer materials at concentrations <50 μg/kg, but is at 100–150 μg/kg in undisturbed surficial soils. No point sources of contamination to the affected areas have been conclusively identified. To determine whether high levels of Hg in ground water are related to a particular land use and (or) water chemistry, water samples from 105 wells that tap the aquifer system were collected by the United States Geological Survey. These included randomly selected domestic wells, domestic and observation wells in selected land uses, and sets of clustered observation wells—including two sets that are downgradient from residential areas with Hg-contaminated ground water. Hg concentrations in filtered samples (Hg f ) were at or near background levels in water from most wells, but ranged from 0.1 to 3.8 μg/L in water from nearly 20% of wells. Hg f concentrations from 0.0001 to 0.1 μg/L correlated significantly and positively with concentrations of other constituents associated with anthropogenic inputs (Ca, Cl, Na, and NO 3 ) and with dissolved organic carbon. Hg f concentrations >0.1 μg/L did not correlate significantly with concentrations of the inorganic constituents. Hg f concentrations near or exceeding 2 μg/L were found only in water from wells in areas with residential land use, but concentrations were at background levels in most water samples from undeveloped land. The spatial distribution of Hg-contaminated ground water appears to be locally and regionally heterogeneous; no extensive plumes of Hg contamination have yet been identified.

New Jersey↗

USGS2003

Mercury in soil near a long-term air emission source in southeastern Idaho

At the Idaho National Engineering and Environmental Laboratory in southeastern Idaho, a 500 °C fluidized bed calciner was intermittently operated for 37 years, with measured Hg emission rates of 9–11 g/h. Surface soil was sampled at 57 locations around the facility to determine the spatial distribution of Hg fallout and surface Hg variability, and to predict the total residual Hg mass in the soil from historical emissions. Measured soil concentrations were slightly higher ( p <0.05) within 5 km of the source but were overall very low (15–20 ng/g) compared to background Hg levels published for similar soils in the USA (50–70 ng/g). Concentrations decreased 4%/cm with depth and were found to be twice as high under shrubs and in depressions. Mass balance calculations accounted for only 2.5–20% of the estimated total Hg emitted over the 37-year calciner operating history. These results suggest that much of the Hg deposited from calciner operations may have been reduced in the soil and re-emitted as Hg(0) to the global atmospheric pool.

Idaho↗

USGS2003

Mercury accumulation in snow on the Idaho National Engineering and Environmental Laboratory and surrounding region, southeast Idaho, USA

Snow was sampled and analyzed for total mercury (THg) on the Idaho National Engineering and Environmental Laboratory (INEEL) and surrounding region prior to the start-up of a large (9-11 g/h) gaseous mercury emission source. The objective was to determine the effects of the source on local and regional atmospheric deposition of mercury. Snow samples collected from 48 points on a polar grid near the source had THg concentrations that ranged from 4.71 to 27.26 ng/L; snow collected from regional background sites had THg concentrations that ranged from 0.89 to 16.61 ng/L. Grid samples had higher concentrations than the regional background sites, which was unexpected because the source was not operating yet. Emission of Hg from soils is a possible source of Hg in snow on the INEEL. Evidence from Hg profiles in snow and from unfiltered/filtered split samples supports this hypothesis. Ongoing work on the INEEL is investigating Hg fluxes from soils and snow.

Idaho↗

USGS2003

Application of ultrafiltration and stable isotopic amendments to field studies of mercury partitioning to filterable carbon in lake water and overland runoff

Results from pilot studies on colloidal phase transport of newly deposited mercury in lake water and overland runoff demonstrate that the combination of ultrafiltration, and stable isotope amendment techniques is a viable tool for the study of mercury partitioning to filterable carbon. Ultrafiltration mass balance calculations were generally excellent, averaging 97.3, 96.1 and 99.8% for dissolved organic carbon (DOC), total mercury (Hg T ), and methylmercury (MeHg), respectively. Sub nanogram per liter quantities of isotope were measurable, and the observed phase distribution from replicate ultrafiltration separations on lake water agreed within 20%. We believe the data presented here are the first published colloidal phase mercury data on lake water and overland runoff from uncontaminated sites. Initial results from pilot-scale lake amendment experiments indicate that the choice of matrix used to dissolve the isotope did not affect the initial phase distribution of the added mercury in the lake. In addition there was anecdotal evidence that native MeHg was either recently produced in the system, or at a minimum, that this ‘old’ MeHg partitions to the same subset of DOC that binds the amended mercury. Initial results from pilot-scale overland runoff experiments indicate that less than 20% of newly deposited mercury was transported in the filterable fraction (<0.7 μm). There is some indication of colloidal phase enrichment of mercury in runoff compared to the phase distribution of organic carbon, but the mechanism of this enrichment is unclear. The phase distribution of newly deposited mercury can differ from that of organic carbon and native mercury, suggesting that the quality of the carbon (available ligands), not the quantity of carbon, regulates partitioning. Further characterization of DOC is needed to clarify the underlying mechanisms.

Science of the Total Environment↗

USGS2003

Mercury and methylmercury contents in mine-waste calcine, water, and sediment collected from the Palawan Quicksilver mine, Philippines

The Palawan Quicksilver mine, Philippines, produced about 2,900 t of mercury during mining of cinnabar ore from 1953 to 1976. More than 2,000,000 t of mine-waste calcines (retorted ore) were produced during mining, much of which were used to construct a jetty in nearby Honda Bay. Since 1995, high Hg contents have been found in several people living near the mine, and 21 of these people were treated for mercury poisoning. Samples of mine-waste calcine contain high total Hg concentrations ranging from 43-660 ??g/g, whereas total Hg concentrations in sediment samples collected from a mine pit lake and local stream vary from 3.7-400 ??g/g. Mine water flowing through the calcines is acidic, pH 3.1-4.3, and total Hg concentrations ranging from 18-31 ??g/l in this water significantly exceed the 1.0-??g/l drinking water standard for Hg recommended by the World Health Organization (WHO). Total Hg contents are generally lower in water samples collected from surrounding domestic wells, the mine pit lake, Honda Bay, and the nearby stream, varying from 0.008-1.4 ??g/l. Methylmercury concentrations in water draining mine calcines range from <0.02-1.4 ng/l, but methylmercury is highest in the pit lake water, ranging from 1.7-3.1 ng/l. Mercury methylation at the Palawan mine is similar to or higher than that found in other mercury mines worldwide. Much of the methylmercury generated in Palawan mine-waste calcines and those in Honda Bay is transferred to water, and then to marine fish and seafood. A food source pathway of Hg to humans is most likely in this coastal, high fish-consuming population.

Conference Paper↗

USGS2002

Mercury and methylmercury contamination related to artisanal gold mining, Suriname

Elemental Hg-Au amalgamation mining practices are used widely in many developing countries resulting in significant Hg contamination of surrounding ecosystems. We have measured total Hg and methyl-Hg concentrations in sediment and water collected from artisanal Au mines and these are the first Hg speciation data from such mines in Suriname. Total Hg and methyl-Hg contents in mine-waste sediment and water are elevated over local uncontaminated baselines. Total Hg (10-930 ng/L) and methyl-Hg (0.02-3.8 ng/L) are highly elevated in mine waters. Increasing total Hg contents in discharged mine waters correlate with increasing water turbidity indicating that most Hg transport is on suspended particulates. Our Hg results are similar to those found in artisanal Au mines in the Amazon basin, where Hg contamination has led to adverse effects on tropical ecosystems.

Geophysical Research Letters↗

USGS2002

Reactivity and mobility of new and old mercury deposition in a boreal forest ecosystem during the first year of the METAALICUS study

The METAALICUS (Mercury Experiment To Assess Atmospheric Loading In Canada and the US) project is a whole ecosystem experiment designed to study the activity, mobility, and availability of atmospherically deposited mercury. To investigate the dynamics of mercury newly deposited onto a terrestrial ecosystem, an enriched stable isotope of mercury ( 202 Hg) was sprayed onto a Boreal forest subcatchment in an experiment that allowed us, for the first time, to monitor the fate of “new” mercury in deposition and to distinguish it from native mercury historically stored in the ecosystem. Newly deposited mercury was more reactive than the native mercury with respect to volatilization and methylation pathways. Mobility through runoff was very low and strongly decreased with time because of a rapid equilibration with the large native pool of “bound” mercury. Over one season, only ∼8% of the added 202 Hg volatilized to the atmosphere and less than 1% appeared in runoff. Within a few months, approximately 66% of the applied 202 Hg remained associated with above ground vegetation, with the rest being incorporated into soils. The fraction of 202 Hg bound to vegetation was much higher than seen for native Hg (<5% vegetation), suggesting that atmospherically derived mercury enters the soil pool with a time delay, after plants senesce and decompose. The initial mobility of mercury received through small rain events or dry deposition decreased markedly in a relatively short time period, suggesting that mercury levels in terrestrial runoff may respond slowly to changes in mercury deposition rates.

Environmental Science & Technology↗

USGS2002

Methylmercury in flood-control impoundments and natural waters of northwestern Minnesota, 1997-99

We studied methylmercury (MeHg) and total mercury (HgT) in impounded and natural surface waters in northwestern Minnesota, in settings ranging from agricultural to undeveloped. In a recently constructed (1995) permanent-pool impoundment, MeHg levels typically increased from inflow to outflow during 1997; this trend broke down from late 1998 to early 1999. MeHg levels in the outflow reached seasonal maxima in mid-summer (maximum of 1.0 ng L −1 in July 1997) and late-winter (maximum of 6.6 ng L −1 in February 1999), and are comparable to high levels observed in new hydroelectric reservoirs in Canada. Spring and autumn MeHg levels were typically about 0.1–0.2 ng L −1 . Overall, MeHg levels in both the inflow (a ditch that drains peatlands) and outflow were significantly higher than in three nearby reference natural lakes. Eleven older permanent-pool impoundments and six natural lakes in northwestern Minnesota were sampled five times. The impoundments typically had higher MeHg levels (0.071–8.36 ng L −1 ) than natural lakes. Five of six lakes MeHg levels typical of uncontaminated lakes (0.014–1.04 ng L −1 ) with highest levels in late winter, whereas a hypereutrophic lake had high levels (0.37–3.67 ng L −1 ) with highest levels in mid-summer. Seven temporary-pool impoundments were sampled during summer high-flow events. Temporary-pool impoundments that retained water for about 10–15 days after innundation yielded pronounced increases in MeHg from inflow to outflow, in one case reaching 4.6 ng L −1 , which was about 2 ng L −1 greater than the mean inflow concentration during the runoff event.

Minnesota↗