Stephan J. Köhler

Photochemical and microbial processing of stream and soil water dissolved organic matter in a boreal forested catchment in northern Sweden

Abstract. Natural organic matter (NOM) from stream and soil water in a humic-rich headwater catchment in northern Sweden (initial total organic carbon (TOC) concentrations 10-40 mg C L–1) was rapidly degraded by light and microbial activity in an incubation experiment. Concentration losses were 33–50% after 12 days of exposure to 69 W m–2 artificial PAR and 16 W m–2 UV radiation. Natural, unshaded mid-day solar radiation in the region (68°N 18°E) during the month of june is 159 W m–2 for PAR. In contrast to microbial organic carbon removal, TOC exponentially decreased upon radiation, which suggests that TOC is more rapidly oxidized by light than by ambient microbes. Further, rapid decline in TOC concentration implies the presence of a dominant pool of photo-labile compounds (p > 95%). A measured mass balance for carbon identified 50–75% of the degraded TOC as carbon dioxide after 12 days of exposure to light. The observed conversion of organic to inorganic carbon was accompanied by increases in pH and alkalinity, suggesting that photo-degradation of NOM potentially contributes to in-stream buffering capacity. The remaining refractory TOC changed in chemical character, including an altered molecular weight distribution with decreased average weight and a change in the proportions of humics as evidenced by absorbance ratios (A254/A420). Extrapolation of the experiment to natural headwater conditions show that photo-degradation is an important in-stream process that should be considered in calculations of carbon turnover in surface waters because of its influence on both TOC amount and character.

Separating the natural and anthropogenic components of spring flood pH decline: A method for areas that are not chronically acidified

The quantitative distinction of the natural and anthropogenic components of episodic pH decline is important but problematic. This paper presents the Boreal Dilution Model as a means of separating the natural effects of acid neutralizing capacity (ANC) dilution and organic acids from those of acid deposition on stream pH during spring flood or other flow episodes in areas that are not chronically acidified. Key model assumptions are that base flow ANC reflects preindustrial ANC, that the contemporary dissolved organic carbon is natural, and that dilution of base cations during spring flood is a measure of natural ANC dilution. The model could help elucidate the natural variation of spring flood chemistry, as well as the response to changes in acidification pressures. An example of the models application to one catchment is presented. This example, together with the correlation between winter SO42− deposition and modeled anthropogenic ANC depression during spring flood in some 27 episodes from 18 catchments, is presented to assess the models plausibility.

In-Lake Processes Offset Increased Terrestrial Inputs of Dissolved Organic Carbon and Color to Lakes

Increased color in surface waters, or browning, can alter lake ecological function, lake thermal stratification and pose difficulties for drinking water treatment. Mechanisms suggested to cause browning include increased dissolved organic carbon (DOC) and iron concentrations, as well as a shift to more colored DOC. While browning of surface waters is widespread and well documented, little is known about why some lakes resist it. Here, we present a comprehensive study of Mälaren, the third largest lake in Sweden. In Mälaren, the vast majority of water and DOC enters a western lake basin, and after approximately 2.8 years, drains from an eastern basin. Despite 40 years of increased terrestrial inputs of colored substances to western lake basins, the eastern basin has resisted browning over this time period. Here we find the half-life of iron was far shorter (0.6 years) than colored organic matter (A420 ; 1.7 years) and DOC as a whole (6.1 years). We found changes in filtered iron concentrations relate strongly to the observed loss of color in the western basins. In addition, we observed a substantial shift from colored DOC of terrestrial origin, to less colored autochthonous sources, with a substantial decrease in aromaticity (-17%) across the lake. We suggest that rapid losses of iron and colored DOC caused the limited browning observed in eastern lake basins. Across a wider dataset of 69 Swedish lakes, we observed greatest browning in acidic lakes with shorter retention times (< 1.5 years). These findings suggest that water residence time, along with iron, pH and colored DOC may be of central importance when modeling and projecting changes in brownification on broader spatial scales.

Selective chlorination of natural organic matter: identification of previously unknown disinfection byproducts.

Natural organic matter (NOM) serve as precursors for disinfection byproducts (DBPs) in drinking water production making NOM removal essential in predisinfection treatment processes. We identified molecular formulas of chlorinated DBPs after chlorination and chloramination in four Swedish surface water treatment plants (WTPs) using ultrahigh resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). Chlorine-containing formulas were detected before and after disinfection and were therefore classified to identify DBPs. In total, 499 DBPs were detected, of which 230 have not been reported earlier. The byproducts had, as a group, significantly lower ratio of hydrogen to carbon (H/C) and significantly higher average carbon oxidation state (COS), double bond equivalents per carbon (DBE/C) and ratio of oxygen to carbon (O/C) compared to Cl-containing components present before disinfection and CHO formulas in samples taken both before and after disinfection. Electrophilic substitution, the proposed most significant reaction pathway for chlorination of NOM, results in carbon oxidation and decreased H/C while O/C and DBE/C is left unchanged. Because the identified DBPs had significantly higher DBE/C and O/C than the CHO formulas we concluded that chlorination of NOM during disinfection is selective toward components with relatively high double bond equivalency and number of oxygen atoms per carbon. Furthermore, choice of disinfectant, dose, and predisinfection treatment at the different WTPs resulted in distinct patterns in the occurrence of DBP formulas.

Riparian soil temperature modification of the relationship between flow and dissolved organic carbon concentration in a boreal stream

Discharge is often strongly correlated to the temporal variability of dissolved organic carbon concentrations ([DOC]) in watercourses. One recently proposed way to model this is the riparian flow-c ...

Long-term dynamics of dissolved organic carbon: Implications for drinking water supply

Surface waters are the main source of drinking water in many regions. Increasing organic carbon concentrations are a cause for concern in Nordic countries since both dissolved and particulate organic carbon can transport contaminants and adversely affect drinking water treatment processes. We present a long-term study of dynamics of total (particulate and dissolved) organic carbon (TOC) concentrations in the River Fyris. This river supplies drinking water to approximately 200000 people in Uppsala, Sweden. The River Fyris is a main tributary to Lake Mälaren, which supplies drinking water to approximately 2 million people in the greater Stockholm area. Utilities responsible for drinking water supply in both Uppsala and Stockholm have expressed concerns about possible increases in TOC. We evaluate organic carbon dynamics within the Fyris catchment by calculating areal mass exports using observed TOC concentrations and modeled flows and by modeling dissolved organic carbon (as a proxy for TOC) using the dynamic, process based INCA-C model. Exports of TOC from the catchment ranged from 0.8 to 5.8 g m(-2) year(-1) in the period 1995-2010. The variation in annual exports was related to climatic variability which influenced seasonality and amount of runoff. Exports and discharge uncoupled at the end of 2008. A dramatic increase in TOC concentrations was observed in 2009, which gradually declined in 2010-2011. INCA-C successfully reproduced the intra- and inter-annual variation in concentrations during 1996-2008 and 2010-2011 but failed to capture the anomalous increase in 2009. We evaluated a number of hypotheses to explain the anomaly in 2009 TOC values, ultimately none proved satisfactory. We draw two main conclusions: there is at least one unknown or unmeasured process controlling or influencing surface water TOC and INCA-C can be used as part of the decision-making process for current and future use of rivers for drinking water supply.

Chemical Equilibrium Modeling of Organic Acids, pH. Aluminum, and Iron in Swedish Surface Waters

A consistent chemical equilibrium model that calculates pH from charge balance constraints and aluminum and iron speciation in the presence of natural organic matter is presented. The model requires input data for total aluminum, iron, organic carbon, fluoride, sulfate, and charge balance ANC. The model is calibrated to pH measurements (n = 322) by adjusting the fraction of active organic matter only, which results in an error of pH prediction on average below 0.2 pH units. The small systematic discrepancy between the analytical results for the monomeric aluminum fractionation and the model results is corrected for separately for two different fractionation techniques (n = 499) and validated on a large number (n = 3419) of geographically widely spread samples all over Sweden. The resulting average error for inorganic monomeric aluminum is around 1 µM. In its present form the model is the first internally consistent modeling approach for Sweden and may now be used as a tool for environmental quality management. Soil gibbsite with a log *Ks of 8.29 at 25°C together with a pH dependent loading function that uses molar Al/C ratios describes the amount of aluminum in solution in the presence of organic matter if the pH is roughly above 6.0.

Acid/base character of organic acids in a boreal stream during snowmelt

In northern regions, spring snowmelt generally results in an episodic decline of surface water pH. Natural organic acids may be an important factor in this type of pH change. We studied the variations in the acid/base character of aquatic organic acids during spring snowmelt in 1997 at Svartberget, a stream rich in total organic carbon (TOC) that is located in the boreal zone of northern Sweden. Snowmelt at Svartberget was accompanied by a drop in stream pH of up to 1.8 pH units, as well as the dilution of base cation and strong acid anion concentrations. Aluminum and TOC increased or decreased during snowmelt, depending on the sampling site within the 50-ha catchment. Although there were distinct differences in pH, TOC, and major inorganic ions in the runoff from three subcatchments, there was very little variation in the acid/base character of TOC. Thus organic acids in the Svartberget catchment share a common set of acid/base properties despite markedly different subcatchment drainage patterns, vegetation, and soils. The dissociation behavior of organic acids at Svartberget could be described with high precision (R2 = 0.91, p < 0.001, and n = 115) by a triprotic acid analog model (pKa1 = 2.5, pKa2 = 4.0, and pKa3 = 5.8), together with the measured site density of organic acids (8.6±0.8 μeq (mg TOC)−1). A Gaussian pKa distribution (μ = 4.20 and σ = 1.43) predicted organic acid dissociation with similar precision (R2 = 0.91, p < 0.001, and n = 94). Variations in site density among the tributary streams could largely be explained by aluminum complexation. Sites with lower measured site densities had greater concentrations of organically bound Al. Thus Al binding reduces the potential release or neutralization of H+ by organic acids.

Natural acidity or anthropogenic acidification in the spring flood of northern Sweden

Abstract Episodic pH decline associated with spring flood runoff in a typical headwater stream in the boreal forest of northern Sweden is driven almost exclusively by natural processes. Despite a drastic decline in acid neutralisation capacity (ANC), a 75-fold increase in hydrogen ion concentration and an associated increase in inorganic monomeric aluminium fractions, anthropogenic deposition (the strong acids SO42− and NO3− associated with anthropogenic deposition) made only a minor contribution (5–8%) to the ANC and pH decline during the spring flood of 1997. Instead the ANC decline from 60 μeq. l−1 during winter baseflow to −26 μeq. l−1 at peak flow, as well as the associated pH decline from approximately 6.4–4.6 was, according to an episodic acidification model, driven almost exclusively by organic acids originating from the soil and dilution by low ionic strength snowmelt water. The natural component of ANC and pH decline during spring flood, such as that reported here, has important implications for the aquatic ecology of the boreal zone in general, and for the Swedish liming program in particular, since government subsidies are used to keep pH above 6.0 throughout spring flood, in the belief that this is the natural pH level prior to the appearance of acid rain in Europe.

Long-term trends in water chemistry of acid-sensitive Swedish lakes show slow recovery from historic acidification

Long-term (1987–2012) water quality monitoring in 36 acid-sensitive Swedish lakes shows slow recovery from historic acidification. Overall, strong acid anion concentrations declined, primarily as a result of declines in sulfate. Chloride is now the dominant anion in many acid-sensitive lakes. Base cation concentrations have declined less rapidly than strong acid anion concentrations, leading to an increase in charge balance acid neutralizing capacity. In many lakes, modeled organic acidity is now approximately equal to inorganic acidity. The observed trends in water chemistry suggest lakes may not return to reference conditions. Despite declines in acid deposition, many of these lakes are still acidified. Base cation concentrations continue to decline and alkalinity shows only small increases. A changing climate may further delay recovery by increasing dissolved organic carbon concentrations and sea-salt episodes. More intensive forest harvesting may also hamper recovery by reducing the supply of soil base cations.