Catharina Pettersson

Terrestrial Sources of Methylmercury in Surface Waters: The Importance of the Riparian Zone on the Svartberget Catchment

The runoff of methylmercury (MeHg) from forested catchments to surface waters has been identified as a potentially significant input of MeHg to the aquatic ecosystem. Little, however, is known of the processes which control the transfer of MeHg from soils to surface waters. This study investigated the potential terrestrial sources of MeHg in runoff by sampling profiles of soil solution chemistry and determining the flux of water through those profiles into two tributaries on the Svartberget Catchment in northern Sweden. One study profile was from the podzol soil that covers most of the catchment area. The other profiles were taken in the riparian zone of each of the two tributaries. Soil solution was extracted from the soils by centrifugation.High catchment soil solution concentrations of MeHg (>1 ng l−1) occur in the surface layers of the soil, but overland flow on the catchment is rare. MeHg concentrations in the podzol profile dropped to less than 0.2 ng l−1 in the mineral soil just 5 cm below the mor layer. In the riparian soil profiles sampled in October, MeHg concentrations were higher (ca. 0.4 ng l−1), but in a July sampling the concentrations in the riparian profiles were comparable to those in the podzol (i.e. < 0.2 ng l−1). Very high concentrations of MeHg were found in the streambank sphagnum mosses (>2 ng l−1) partially submerged within the stream.The concentrations of MeHg observed under the podzol soil were insufficient to sustain the concentration of MeHg in runoff from the forested subcatchments where podzol profiles cover more than 70% of the surface area. The only sources of additional MeHg that lie along major runoff flow pathways are the riparian soils and mosses on the streambanks. It is therefore hypothesized that output of MeHg from the forest areas of the catchment is controlled by the biogeochemical processes in the riparian zone.

Effects of UV-A irradiation on dissolved organic matter in humic surface waters

Exposure of sterile filtered humic surface water to low intensity UV-A radiation for 89 h significantly affected the organic matter in these systems. A UV-induced degradation of organic matter was demonstrated by a decrease in UV absorbance, dissolved organic carbon (DOC) and fulvic acid content in irradiated replicates, compared to dark controls. Furthermore, qualitative changes in the DOC were observed. The absorbance ratio 254365 nm increased in the irradiated replicates, indicating a decreased size of UV-absorbing organic matter. A UV-induced production of several low molecular weight (LMW) organic acids was also observed. A highly sensitive (μg L−1 level) capillary zone electrophoresis (CZE) method was used for identification and quantification of these organic acids. Oxalic, malonic, formic, and acetic acid were the major carboxylic acids produced, together accounting for about 0.18 mg C L−1. This organic fraction could have a profound effect on the surface water environment, being highly available substrates for aerobic microorganisms as well as being potentially important for the function and distribution of trace metals.

Degradation of humic substances by UV irradiation

Abstract Aqueous solutions of humic substances (humic and fulvic acids) were irradiated with light at λ = 254 nm followed by analyses of total organic carbon (TOC), ultraviolet (UV) absorbance and molecular weight distribution. A degradation of the dissolved humic compounds by up to 50% after 6 h was observed. The average molecular weight decreased (from 1800 to approx. 300) as well as the UV absorbance of the remaining organic fraction. Low molecular weight organic acids were formed during the irradiation and tentatively identified by capillary electrophoresis (formic, acetic, succinic, and oxalic acids). The low molecular organic acids continued to degrade by further irradiation; the loss of TOC is assumed to be due to CO 2 formation. Indications of a large-scale photo degradation process in nature was observed as a seasonal variation of the ratio humic substances/TOC related to the length of the day. Weekly sampling and analysis of UV absorbance and TOC in the Kalix River in northern Sweden indicate a higher humic fraction of the TOC during winter (dark period) than during summer.

Identification of the riparian sources of aquatic dissolved organic carbon

Abstract The riparian zone has been suggested by several researchers as the place where organic-rich runoff acquires much of its dissolved organic carbon (DOC). The great variation in the hydrochemical environments within the riparian zone, however, makes quantification of the contribution from specific source areas within the riparian zone desirable. This paper explores the feasibility of identifying those sources by comparing the quality of DOC and other chemical parameters of runoff with soil solutions from different locations within the catchment. The study was located on the Svarberget Catchment in northern Sweden. Two soil profiles, one a riparian peat and the other an iron podzol, as well as streambank moss, were sampled as potential sources of aquatic DOC. Soil solutions in these samples and runoff were characterized by their inorganic chemistry, separation of the humic/fulvic component of DOC, and gel filtration to determine the molecular size distribution of DOC. The results show that the superficial organic horizons differ markedly in a variety of ways from the chemistry of runoff. The most likely origin of runoff is a mixture of upslope water from the B horizon with subsurface soil solution from the riparian zone. This is in agreement with what is known about flow pathways at Svartberget, but the information collected in this initial study was insufficient to quantify the contribbution from specific sources. Potential, however, is seen to increase the power of this approach by using more techniques to differentiate the chemistry of specific source areas and exploit information provided by both seasonal and flow-related variations in the chemistry of runoff.

On the composition and properties of humic substances isolated from deep groundwater and surface waters

Fulvic and humic acids were collected from groundwater and surface waters by an isolation procedure utilizing DEAE-cellulose, a weak anion exchange resin. DEAE-cellulose was selected since it can adsorb himics without any pH adjustment of the sampled water (in the pH range 4–8), thus making the method well suited for isolation of groundwater humic substances in the field. Characterization of general parameters (elemental composition, molecular weight, acidity, absorptivity, 14C-age) was performed on these humic substances as well as on humics isolated by other procedures. The humic weights and were divided into three categories on the basis of calculated atomic ratios, molecular weights and absorptivity: humic acids (high molecular weight) and fulvic acids (low molecular weight) from groundwater and surface waters, respectively. The similarities of the humics within each group, despite variations in age and origin, support the contention that humic substances have more properties in common than they have differences.

Naturally produced adsorbable organic halogens (AOX) in humic substances from soil and water

Abstract The present study has shown that surface water, groundwater and soil, even in areas far from industrial activities, contain measurable amounts of adsorbable organic halogens (AOX). Although the concentrations are relatively low, the pools of AOX in the environment are considerable. In surface water, the ratio between AOX and total organic carbon (TOC) varied between 730 and 8600 μg/g. In soil, the AOX content ranged from 210 to 1400 μg AOX per g organic matter. The fact that organohalogens (230–370 μg/g) could be detected in fulvic acids isolated from old groundwaters (1300, 4600 and 5200 yrs) indicates a large natural production of organohalogens. This hypothesis was further strengthened by mass balance calculations for a raised bog in Sweden. The pool of AOX in this bog is at least 300 times larger than the present annual deposition. The total pool of AOX in peat in Sweden was estimated to be at least 300,000 metric tons. Potential artefacts in the analytical procedures used are discussed.

Methylmercury Output from the Svartberget Catchment in Northern Sweden During Spring Flood

The problem of mercury (Hg) accumulation in the aquatic food chain is widespread in Sweden. The methylmercury (MeHg) in runoff from catchments may be an important component of the MeHg load in surface waters. The spring flood in northern Sweden constitutes a major portion of the annual catchment runoff. This brief, but large event, therefore, may be of significance for the annual output of MeHg from soils to surface waters in this region.

Organic carbon in the boreal spring flood from adjacent subcatchments

Abstract A large portion of runoff and associated hydrochemical transport in boreal regions occurs during the spring flood. It has been hypothesized that the dynamics of total organic carbon (TOC) concentrations during the spring flood can be explained by new catchment source areas of TOC activated during periods of high flow that are subsequently depleted. This hypothesis was examined in the 1993 spring flood on the Svartberget Catchment in northern Sweden where the runoff from three subcatchments—two forested and one mire—could be isolated. Twenty-eight percent of the 1993 runoff occurred during the two-week period of the spring flood. A similar proportion of the annual TOC output came from the forested subcatchments, but less TOC (20% of the annual output) came from the mire. Snowmelt comprised about half of the runoff from the Mire Subcatchment, but only a third of the runoff from the forested subcatchments. The TOC concentrations in runoff from the Mire Subcatchment decreased about 50% from later winter values to a minimum of 15 mg/L. The TOC concentrations in runoff from the forested subcatchments, however, increased markedly during the early phase of the spring flood before starting to decline. These patterns are consistent with superficial flow pathways activated in the forested areas during spring flood, and a superficial flow pathway in the mire that is active both before and during the spring flood. Without more information on the hypothesized superficial flow paths, the possibility of a progressive change in flow paths as spring flood recedes can be considered an alternative explanation of the observed TOC dynamics.

Subcatchment Output of Mercury and Methylmercury at Svartberget in Northern Sweden

The subcatchments of two tributaries and the headwater mire of the 50 ha Swartberget catchment were studied. Monthly sampling was conducted during 1993 on the two tributaries and at the outlet of a mire. This was complemented by more intensive sampling during spring flood and a rain-driven episode at the end of July. Samples were analyzed for total methylmercury (MeHg), total mercury (Hg-tot) and TOC. The MeHg and TOC content of water were also fractionated into humic and non-humic components. Outputs budgets based on continuous flow monitoring and monthly volume weighted average concentrations of MeHg/ Hg were calculated for the subcatchments of the two tributaries and the mire. There was a pronounced decline in MeHg concentrations at all sampling locations during the spring flood. A clear pattern of the seasonal variation in the MeHg outputs was evident at all three sampling locations. Minimum MeHg concentrations occurred during spring flood and increased during the summer to peak levels in the autumn before tapering off during the winter (except for at the mire). The mire had larger increases in MeHg concentrations during the summer and did not decline during the winter. The warmth and increased soil humidity may promote the biogeochemical processes, including methylation, demethylation which make MeHg available for export by runoff from the soil. Variations in Hg-tot concentrations were differed from MeHg in streams that there were increases in Hg-tot concentrations with flow during both spring flood and the July rain event in runoff from the forested tributaries.

Properties of fulvic acids from deep groundwaters

Abstract Fulvic acids have been recovered from three deep groundwaters and one surface water and characterized with respect to elemental composition, molecular weight, acid capacity and 14 C-age. The corresponding waters are also characterized in detail. All of the fulvic acids, with ages varying from 0 to 5250 years, are very similar. The onlu significant difference between fresh and old materials are lower content of oxygen and nitrogen and higher COOH/OH-ratios for the materials from the old waters. Dating of the water using 14 C-analysis of the dissolved organic material is probably better than using the carbonate as the 14 C-source.