Dissolved Organic Carbon in Seepage Water – Production and Transformation during Soil Passage

Dissolved organic carbon (DOC) in seepage water can combine with organic pollutants, with Al and heavy metal ions and transport them through the soil profile with a potential to contaminate groundwater. We studied the production of DOC in aerobic decomposition experiments at 8 °C and moisture close to field capacity in soils from two sites with different microbial activities (spodic dystric Cambisols with moder (SLB) and mor‐moder (SLS) layers) using ¹³C‐depleted plants of differing decomposability (Epilobium angustifolium and Calamagrostis epigeios). Additionally, we investigated the DOC transformation during soil passage in decomposition experiments and in the field for the sites SLB and SLS. For SLS, decomposition of Epilobium resulted in a cumulative CO₂ production of 14% of the added C within 128 days. Priming effects were negligible. CO₂ production for the experiments using Calamagrostis was less with 11% for SLB and 10% for SLS. Cumulative DOC production was markedly high in the Epilobium decomposition experiment, being 25 g m^–2, out of which 11 g m^–2 were Epilobium‐derived (2% of the added C). For the Calamagrostis experiments, cumulative productions of DOC and Calamagrostis‐derived DOC (0.1% of the added C for SLS and SLB) were much less. During the soil passage, much of the DOC was removed by sorption or decomposition processes. Field studies at SLS and SLB using ¹³C natural abundance showed that ¹³C distribution of soil organic matter increased with depth, probably mainly due to a discrimination of C isotopes by decomposing microorganisms. DOC, however, showed a depletion of ¹³C from –28γ PDB to –29γ (SLB at 40 cm) or –28 to –30γ (SLS at 20 cm) with depth, owing to preferential decomposition of ¹³C‐enriched substances or preferential adsorption. This study indicates that DOC production is strongly affected by litter composition and that significant changes in DOC composition may occur during its passage through a soil depth of 40 cm.     

Origin of Dissolved Organic Carbon Studied by UV‐vis Spectroscopy

Dissolved organic carbon (DOC) distributions in water from Lake Ipê, MS, Brazil, were investigated. The samplings were performed monthly (surface, 1 m depth, and bottom) from June 1999 to June 2000. Absorbance at 285 nm and DOC concentrations in mg dm^—3, p(DOC), were highly correlated for the three depths. 77% of the surface, 85% for 1 m and bottom samples presented a variation between 20 dm³ g^—1 cm^—1 and 50 dm³ g^—1 cm^—1 of A(285 nm)/p(DOC), that characterizes the dissolved organic matter in lake water as essentially fulvic. The ratio A(254 nm)/p(DOC) was also sensitive for fulvic matter, and an A(250 nm)/A(365 nm) = 4 ratio was characteristic of strongly colored waters. The ratios A(436 nm)/p(DOC) for the three depths also showed a significant correlation. The predominance of fulvic acid is explained by environmental characteristics such as the tropical climate, temperatures above 18 °C, and the lake environment. It was demonstrated that the variation in the water carbon content due to different compartments in the lake can be monitored by UV‐vis spectroscopy ratios.     

Sources of particulate and dissolved organic carbon in the St Lawrence River: isotopic approach

We investigate sources of both dissolved and particulate organic carbon in the St Lawrence River from its source (the Great Lakes outlet) to its estuary, as well as in two of its tributaries. Special attention is given to seasonal interannual patterns by using data collected on a bi‐monthly basis from mid‐1998 to mid‐2003. δ¹³C measurements in dissolved inorganic carbon, dissolved organic carbon (DOC) and particulate organic carbon (POC), as well as molar C : N in particulate organic matter (POM), are used to bring insight into the dynamic between aquatic versus terrigenous sources. In addition, ¹⁴C activities of DOC were measured at the outlet of the St Lawrence River to its estuary to assess a mean age of the DOC exported to the estuary. In the St Lawrence River itself, aquatically produced POC dominates terrestrially derived POC and is depleted in ¹³C by approximately 12‰ versus dissolved CO₂. In the Ottawa River, the St Lawrence River's most important tributary, the present dataset did not allow for convincing deciphering of POC sources. In a small tributary of the St Lawrence River, aquatically produced POC dominates in summer and terrestrially derived POC dominates in winter. DOC seems to be dominated by terrestrially derived organic matter at all sampling sites, with some influence of DOC derived from aquatically produced POC in summer in the St Lawrence River at the outlet of the Great Lakes and in one of its small tributaries. The overall bulk DOC is relatively recent (¹⁴C generally exceeding 100% modern carbon) in the St Lawrence River at its outlet to the estuary, suggesting that it derives mainly from recent organic matter from topsoils in the watershed. Copyright © 2005 John Wiley & Sons, Ltd.     

Determination of dissolved organic matter in streamwater using visible spectrophotometry

Absorbance at 360 nm was measured on 44 filtered streamwater samples of different dissolved organic matter (DOM) contents. A regression equation of DOM on absorbance predicted DOM with a standard error of estimate of 1.26 mgl^?1, Use of a published equation relating dissolved organic carbon (DOC) to absorbance gave DOC values for the samples which were consistent with measured DOM. The method offers considerable potential for rapid quantification of dissolved organic matter concentrations in streamwater.     

A coupled hydrology–biogeochemistry model to simulate dissolved organic carbon exports from a permafrost‐influenced catchment

We outline the development of a simple, coupled hydrology–biogeochemistry model for simulating stream discharge and dissolved organic carbon (DOC) dynamics in data sparse, permafrost‐influenced catchments with large stores of soil organic carbon. The model incorporates the influence of active layer dynamics and slope aspect on hydrological flowpaths and resulting DOC mobilization. Calibration and evaluation of the model was undertaken using observations from Granger Basin within the Wolf Creek research basin, Yukon, northern Canada. Results show that the model was able to capture the dominant hydrological response and DOC dynamics of the catchment reasonably well. Simulated DOC was highly correlated with observed DOC (r² = 0.65) for the study period. During the snowmelt period, the model adequately captured the observed dynamics, with simulations generally reflecting the timing and magnitude of the observed DOC and stream discharge. The model was less successful over the later summer period although this partly reflected a lack of DOC observations for calibration. The developed model offers a valuable framework for investigating the interactions between hydrological and DOC processes in these highly dynamic systems, where data acquisition is often very difficult. © 2015 The Authors Hydrological Processes Published by John Wiley & Sons, Ltd.     

Dynamics and fluxes of organic carbon and nitrogen in two Guiana Shield river basins impacted by deforestation and mining activities

Deforestation and mining activities have proven to be very damaging to rivers because these activities disturb the environmental characteristics of rivers. Thus, the concentrations of dissolved organic carbon (DOC), particulate organic carbon (POC), particulate nitrogen (PN), and Chlorophyll‐a (Chl‐a) were measured monthly during 2 hydrological years in the Maroni and Oyapock Rivers to assess the dynamics and fluxes of organic carbon and nitrogen in these 2 Guiana Shield basins, which have been strongly (Maroni) and weakly (Oyapock) impacted by deforestation and mining activities. The 2‐year time series show that DOC, POC, PN, and Chl‐a concentrations vary seasonally with discharge in both rivers, indicating a hydrologically dominated control. Temporal patterns of DOC, POC, and PN indicate that these variables show maximum concentrations in rising waters due to the yield of organic matter and nitrogen accumulated in soils, which are incorporated into the rivers during rainfall. However, the Chl‐a concentrations were at a maximum during low‐water stages. The C/N and C/Chl‐a ratios also showed a seasonal trend, with lower values during the low water periods due to an increase in algal biomass. During high water, the POC in both rivers is the result of terrestrial organic matter, whereas during low‐water autochthonous organic matter can reach up to 34% of the POC. The mean annual fluxes of TOC and PN were higher (4.56 × 10⁵ tonC year^?1 and 1.77 × 10⁴ tonN year^?1, respectively) in the Maroni River than those (1.84 × 10⁵ tonC year^?1 and 0.54 × 10⁴ tonN year^?1, respectively) in the Oyapock River. However, the specific fluxes of DOC, POC, and PN from both basins were nearly the same. Although gold mining activities are performed in both basins, there is no conclusive evidence regarding the impact of these activities on the dynamics of organic matter and particulate nitrogen in the Maroni and Oyapock Rivers.     

Sediment‐water Fluxes of Nutrients and Dissolved Organic Carbon in Extensive Sea Cucumber Culture Ponds

Sediment samples were collected from three seawater aquaculture ponds, and soil characteristics, sediment oxygen consumption (SOC), dissolved organic carbon (DOC) and nutrient fluxes were measured using chamber incubations at laboratory. The three ponds were each representing a specific monoculture or polyculture model of sea cucumber. Total organic carbon (TOC) and total nitrogen (TN) contents in the dry sediment ranged from 0.14 to 0.26% and 0.022 to 0.037%, respectively. Total phosphorus (TP) contents in the sediment were more spatially and temporally variable. SOC ranged from 15.29 to 45.86 mmol m^–2 d^–1 and showed significant differences among the three ponds (p < 0.05). TOC, total carbon (TC) contents, and SOC of the sediment in the pond polycultured with jellyfish increased with culture time, indicating that jellyfish farming enhanced the accumulation of organic matter in the sediments to some extent. Sediment showed net nitrate and ammonium uptake in most ponds and months, and significant differences were found among months (p < 0.05). Dissolved inorganic phosphate (DIP) was released from the sediments in all ponds with low flux rates. DOC was released from the sediment in all ponds and ranged from 0.67 to 1.74 g DOC m^–2 d^–1. The results suggested that non‐artificial‐feeding sea cucumber culture ponds could not only yield valuable seafood products, but also effectively remove nutrients from the aquaculture systems and consequently alleviate nutrient loadings of the nearby coast.     

Analysis and characterization of natural organic matters in freshwaters

Several analytical techniques have been studied for the analysis and characterization of humic and fulvic compounds of freshwaters. DOC analysis, thermogravimetry, UV absorbance, fluorescence, cascade ultrafiltration, freezing concentration techniques are particularly discussed. The effects of several important factors, e.g. nature and concentration of organic matters, pH, and concentrations of several important factors, e.g. nature and concentration of organic matters, pH, and concentrations of inorganic constituents (particularly Fe(III), Si(IV), Ca(II), HCO ₃ ⁻ ) on the analytical results, were studied. Determination of A/DOC, F/DOC and fractionation by cascade ultrafiltration were found to be useful means to characterize the nature of organic matter. But it must be noted that the results of these techniques can be strongly affected by the concentration of fulvic compounds. Adsorption on freshly precipitated CaCO₃ could be one of the important processes of elimination of organic matter.     

Assessing DOC export from a Sphagnum‐dominated peatland using δ13C and δ18O–H2O stable isotopes

Dissolved organic carbon (DOC) originating in peatlands can be mineralized to carbon dioxide (CO₂) and methane (CH₄), two potent greenhouse gases. Knowledge of the dynamics of DOC export via run‐off is needed for a more robust quantification of C cycling in peatland ecosystems, a prerequisite for realistic predictions of future climate change. We studied dispersion pathways of DOC in a mountain‐top peat bog in the Czech Republic (Central Europe), using a dual isotope approach. Although δ¹³C_DOC values made it possible to link exported DOC with its within‐bog source, δ¹⁸O_H2O values of precipitation and run‐off helped to understand run‐off generation. Our 2‐year DOC–H₂O isotope monitoring was complemented by a laboratory peat incubation study generating an experimental time series of δ¹³C_DOC values. DOC concentrations in run‐off during high‐flow periods were 20–30 mg L^?1. The top 2 cm of the peat profile, composed of decaying green moss, contained isotopically lighter C than deeper peat, and this isotopically light C was present in run‐off in high‐flow periods. In contrast, baseflow contained only 2–10 mg DOC L^?1, and its more variable C isotope composition intermittently fingerprinted deeper peat. DOC in run‐off occasionally contained isotopically extremely light C whose source in solid peat substrate was not identified. Pre‐event water made up on average 60% of the water run‐off flux, whereas direct precipitation contributed 40%. Run‐off response to precipitation was relatively fast. A highly leached horizon was identified in shallow catotelm. This peat layer was likely affected by a lateral influx of precipitation. Within 36 days of laboratory incubation, isotopically heavy DOC that had been initially released from the peat was replaced by isotopically lighter DOC, whose δ¹³C values converged to the solid substrate and natural run‐off. We suggest that δ¹³C systematics can be useful in identification of vertically stratified within‐bog DOC sources for peatland run‐off.     

Organic and inorganic matter in Louisiana coastal waters: Vermilion, Atchafalaya, Terrebonne, Barataria, and Mississippi regions

Chromophoric dissolved organic matter (CDOM) spectral absorption, dissolved organic carbon (DOC) concentration, and the particulate fraction of inorganic (PIM) and organic matter (POM) were measured in Louisiana coastal waters at Vermilion, Atchafalaya, Terrebonne, Barataria, and Mississippi River locations, in 2007-2008. The range of CDOM was 0.092 m⁻¹ at Barataria in June 2008 to 11.225 m⁻¹ at Mississippi in February 2008. An indicator of organic matter quality was predicted by the spectral slope of absorption coefficients from 350 to 412 nm which was between 0.0087 m⁻¹ at Mississippi in May 2008 and 0.0261 m⁻¹ at Barataria in June 2008. CDOM was the dominant component of light attenuation at Terrebonne and Barataria. Detritus and CDOM were the primary components of light attenuation at Vermilion, Atchafalaya, and Mississippi. DOC ranged between 65 and 1235 μM. PIM ranged between 1.1 and 426.3 mg L⁻¹ and POM was between 0.3 and 49.6 mg L⁻¹.     

The freshwater dissolved organic matter fluorescence–total organic carbon relationship

The fluorescent properties of dissolved organic matter (DOM) enable comparisons of humic‐like (H‐L) and fulvic‐like (F‐L) fluorescence intensities with dissolved organic carbon (DOC) in aquatic systems. The fluorescence‐DOC relationship differed in gradient, i.e. the fluorescence per gram of carbon, and in the strength of the correlation coefficient. We compare the fluorescence intensity of the F‐L and H‐L fractions and DOC of freshwater DOM in north Shropshire, England, featuring a river, wetland, spring, pond and sewage DOM sources. Correlations between fluorescence and DOC varied between sample sites. Wetland water samples for the F‐L peak gave the best correlation, r = 0·756; the lowest correlation was from final treated sewage effluent, r = 0·167. The relationship between fluorescence and DOC of commercially available International Humic Substances Society standards were also examined and they generally showed a lower fluorescence per gram of carbon for the F‐L peak than the natural samples, whereas peat wetland DOM gave a greater fluorescence per gram of carbon than river DOM. Here, we propose the strength of the fluorescence–DOC correlation to be a useful tool when discriminating sources of DOM in fresh water. Copyright © 2007 John Wiley & Sons, Ltd.     

Spatial Variability of Nitrate and Ammonium in Pleistocene Aquifer of Central Yangtze River Basin

It becomes increasingly important and challenging for nitrogen pollution prevention to identify key controls for spatial variability of nitrogen in groundwater that could be affected by multiple factors, including anthropogenic input, groundwater flow, and local geochemistry. This study characterized spatial variability of both nitrate and ammonium in the Pleistocene aquifer of central Yangtze River Basin and assessed the effect of various factors in controlling nitrate and ammonium levels based on multiple statistical approaches (correlation, geostatistics, multiple liner regression). The results indicate that nitrate is mostly influenced by Cl⁻ that represents anthropogenic input, while Eh representing local redox state is a secondary variable influencing nitrate concentrations. The groundwater with elevated nitrate concentrations are estimated to occur mainly in areas with higher-permeability near-surface sediments which can facilitate more anthropogenic nitrate transport and less nitrate removal owing to more oxidized state. Ammonium is mostly correlated to Eh, followed by dissolved organic carbon (DOC), but only DOC improves significantly the accuracy of co-kriging prediction model. The groundwater with elevated ammonium concentrations are estimated to occur mainly in areas with more organic-rich sediments within or around the aquifer which can facilitate more ammonium release owing to natural organic matter consumption accompanying strong reducing conditions. The regional groundwater flow is not a factor significantly controlling nitrate or ammonium levels owing to flat topography and sluggish lateral flow.     

UV spectrometry for measuring particulate organic matter in natural water

Ultraviolet (UV) spectra were measured to characterize the dissolved and particulate organic matter in a bog and to investigate the seasonal fluctuation of this organic matter. The optical density spectra in the UV region of dissolved organic matter (DOM) were proportional to ^–4, where is the wavelength of incident light. There was also small absorption on the ^–4 spectra. The optical density at a wavelength of 280 nm was proportional to the concentration of dissolved organic carbon (DOC). The spectra and the dependency of optical density on DOC did not vary throughout the investigation period. The UV spectra of acid extractions of particulate organic matter (POM) showed two absorption peaks at the wavelengths of 220 nm and 335 nm. These peaks are well represented by the sum of two Lorentz functions. The ratio of the absorbances,A ₂₈₀/A ₃₃₅, was dependent on the nitrogen/carbon ratio of POM and the chlorophyll fraction in particulate organic carbon (POC), and not dependent on POC itself. The dependency of POC absorption at 280 nm varied with this ratio which fluctuated seasonally.     

Temporal and spatial variations in the discharge and dissolved organic carbon of drip waters in Beijing Shihua Cave,China

To detect the causal relationship between cave drip waters and stalagmite laminae, which have been used as a climate change proxy, three drip sites in Beijing Shihua Cave were monitored for discharge and dissolved organic carbon (DOC). Drip discharges and DOC were determined at 0 to 14‐day intervals over the period 2004–2006. Drip discharges show two types of response to surface precipitation variations: (1) a rapid response; and (2) a time‐lagged response. Intra‐annual variability in drip discharge is significantly higher than inter‐annual variability. The content of DOC in all drip waters varies inter‐ and intra‐annually and has good correlation with drip water discharge at the rapid response sites. High DOC was observed in July and August in the three years observed. The flushing of soil organic matter is dependent upon the intensity of rain events. The DOC content of drip water increases sharply above a threshold rainfall intensity (>50 mm d^?1) and shows several pulses corresponding with intense rain events (>25 mm d^?1). The DOC content was lower and less variable during the dry period than during the rainy period. The shape of DOC peak also varies from year to year as it is influenced by the intensity and frequency of rainfall. The different drip sites show marked differences in DOC response, which are dominated by hydrological behaviour linked to the recharge of the soil and karst micro‐fissure/porosity network. The results explain why not all stalagmite laminae are consistent with climate changes and suggest that the structure of the rainy season events could be preserved in speleothems. Copyright © 2008 John Wiley & Sons, Ltd.     

Sunlight Degradation of Aquatic Humic Substances Abbau von Huminstoffen unter dem Einfluß von Sonnenlicht

Coloured bog water and an aqueous solution of fulvic acid were exposed to natural sunlight. Up to 67% reduction in colour and 32% reduction in DOC concentration were measured after 12 days exposure. Colour bleaching and reduction in ultraviolet light absorbance were independent of added preservatives. The high molecular weight fraction of dissolved organic matter was most reduced in concentration while substances with low molecular weight increased. The reduction in absorbance (λ = 254 nm) per DOC was most significant for substances with high molecular weight. The final concentration of DOC was highest in samples where microbial activity was suppressed.     

Fluxes and seasonal changes in composition of organic matter in the English Channel

Dissolved and particulate organic matter (DOM and POM) have been investigated along a transect between Cherbourg and the Isle of Wight. In addition, the relative contribution of different sources of POM have been assessed by the use of lipid biomarkers (e.g. fatty acids). Seawater samples were collected at two depths (subsurface and above the bottom) at five stations located on the transect during five cruises (from September 1994 to July 1995). Particulate organic carbon (POC) and dissolved organic carbon (DOC) concentrations vary between 30–530 μg l⁻¹ and 0.5–2.7 mg l⁻¹, respectively, for all the cruises. Fluxes of POM and DOM have been estimated at 0.6×10¹² g yr⁻¹ and 6.5×10¹² g yr⁻¹ of carbon, respectively. General fluxes of water and therefore of DOC and POC are oriented eastward. However, around the Isle of Wight a westward oriented flux exists due to a gyre located in the area. The major DOC and POC fluxes occur in the central part of the Channel where the water column is deepest. Seasonal variations of different sources of POM (algal, bacterial and terrigenous) have been examined for the five cruises. The fresh algal organic fraction is relatively important in September in coastal waters with a predominance of diatom species on the English side, whereas it has a low or undetectable contribution during winter months. The bacterial fraction generally varies in concert with the algal component. It is low during the winter period and more important in bloom or post-bloom conditions, as for example in May. Terrestrial organic matter is restricted to coastal areas in September, and is present at low levels in May and July. Nevertheless, in November and February, terrigenous inputs have been clearly identified for the whole transect even in central waters.     

Dynamic stream network intermittence explains emergent dissolved organic carbon chemostasis in headwaters

Dissolved organic carbon (DOC) concentrations vary among headwaters, with variation typically decreasing with watershed area. We hypothesized that streamflow intermittence could be an important source of variation in DOC concentrations across a small watershed, through (a) temporal legacies of drying on organic matter accumulation and biotic communities and (b) spatial patterns of connectivity with DOC sources. To test these hypotheses, we conducted three synoptic water chemistry sampling campaigns across a 25.5‐km² watershed in south‐eastern Idaho during early spring, late summer, and late fall. Using changepoint analysis, we found that DOC variability collapsed at a consistent location (watershed areas ~1.3 to ~1.8 km²) across seasons, which coincided with the watershed area where variability in streamflow intermittence collapsed (~1.5 km²). To test hypothesized mechanisms through which intermittence may affect DOC, we developed temporal, spatial, and spatio‐temporal metrics of streamflow intermittence and related these to DOC concentrations. Streamflow intermittence was a strong predictor of DOC across seasons, but different metrics predicted DOC depending on season. Seasonal changes in the effects of intermittence on DOC reflected seasonal changes from instream to flowpath controls. A metric that captured spatial connectivity to sources significantly predicted DOC during high flows, when DOC is typically controlled by transport. In contrast, a reach‐scale temporal metric of intermittence predicted DOC during the late growing season, when DOC is typically controlled by instream processes and when legacy effects of drying (e.g., diminished biological communities) would likely affect DOC. The effects of intermittence on DOC extend beyond temporal legacies at a point. Our results suggest that legacy effects of intermittence do not propagate downstream in this system. Instead, snapshots of spatial patterns of intermittence upstream of a reach are critical for understanding spatial patterns of DOC through connectivity to DOC sources, and these processes drive patterns of DOC even in perennial reaches.