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Identification of the riparian sources of aquatic dissolved organic carbon
EnvironmentIntcmatienal,Vol.20, No. 1, pp. 11-19, 1994 CopyTight©1994 Elsevier Science Ltd Printed in the USA.All rights reseaved 0160-4120/94 $6.00 +.00
Pergamon
IDENTIFICATION OF THE RIPARIAN SOURCES OF AQUATIC DISSOLVED ORGANIC CARBON Kevin Bishop Department of Forest Ecology, Swedish University of Agricultural Sciences, 901 88 Ume&, Sweden
Catharina Pettersson and Bert Allard Department of Water and Environmental Studies, Link6ping University, 581 88 Link0ping, Sweden
Ying-Hua Lee Swedish Environmental Research Institute, Box 47086, Gothenburg, Sweden
EI 9307-183 M (Received 12 July 1993; accepted 30 November 1993)
The riparian zone has been s u g g e s t e d by several r e s e a r c h e r s as the place where o r g a n i c rich runoff acquires much of its dissolved organic carbon (DOC). The great variation in the hydrochemieal 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 Svartberget 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 contribution 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.
INTRODUCTION
mechanical intervention in the form of drainage and changes relating to vegetation and climate regime. Vegetation and climate are also, of course, subject to natural variation. Progress is being made in understanding the effects of both chemical and hydrological alterations. Whereas the effects of acidic deposition on DOC were unclear in the review by Marmorek et a1.(1987), the results of the HUMEX Experiment (Gjessing 1993) support the contention that inputs of stronger mineral acidity will suppress the dissociation and hence mobility of DOC.
There is an increasing awareness of the role of dissolved organic carbon (DOC) in the chemical viability of surface waters. (Bishop 1991b; Kullberg et al. 1993). This poses the question: How do human activities affect the amount and character of DOC entering the aquatic ecosystem from terrestrial sources? There are two broad categories of human influence on forested areas: chemical alterations in the composition of atmospheric deposition and hydrological alteration of the mass transport system within the catchment. The latter category includes both direct 11
12
Hydrologically, there is now evidence of how the precipitation regime affects DOC output (LOfgren 1991). Analysis of the color of Swedish rivers over the course of two decades has revealed that increased rainfall led to increased concentrations of DOC in runoff. Investigations of the effect of forest drainage on DOC amounts and quality have been more ambiguous, showing increases in DOC output in some cases and decreases in other cases (Lundin 1984). Research at the Svartberget Research Catchment has demonstrated how the long-term effects of drainage on DOC output can vary from stream to stream depending on subtle differences in the geometry of the relationship between flow pathways and the source areas of DOC (Bishop et al. 1993). The interplay between source areas of DOC and the hydrological transport system is of importance for more than just the problem of predicting the effects of drainage. That interplay is the key to the marked seasonal and flow-related changes that are apparent in DOC concentration and character. As such, the relation of source areas to the hydrological transport system is important for all predictions about how anthropogenic influences, both chemical and hydrological, will influence the DOC of surface waters. This is especially true when those predictions attempt to go beyond generalizations about the overall direction of a change initiated by anthropogenic influences to quantitative model predictions. There is some agreement that the immediate geographic source of the DOC in runoff is the riparian zone (Hemond 1990). This has also been demonstrated on the Svartberget Catchment (Bishop et al. 1990a). The riparian zone, however, encompasses a number of distinct hydrochemical environments. At Svartberget, these range from podzolized soil profiles a few meters from the stream, through peat profiles of varying depth overlying mineral soils enriched in DOC, to the streambank vegetation where mosses are partially submerged in the stream. The purpose of this study is to explore the possibilities of resolving the contribution of DOC from specific areas within the riparian zone. This will be done by characterizing the DOC quality and other chemical parameters of the soil solution within different areas of a representative transect through the riparian zone extending away from the stream into soils upslope. These will then be compared to the composition of the runoff entering the stream via the riparian zone. By assuming that runoff is a conservative mixture of several "end-members" from this representative transect with specific chemical compositions, it is then possible to define the ratios in
K. Bishop et al,
which the different source areas contribute to runoff. (A conservative mixture is one in which the concentration of a component is determined solely by the concentration of that component in the different sources and the ratio in which those sources contribute to the mixture, i.e., there is no precipitation, dissolution, or other chemical transformation.) This technique is similar to that applied by Easthouse et al. (1992). That a n a l y s i s , h o w e v e r , was b a s e d on the definition of several source areas within the catchment as a whole rather than focusing on individual soil horizons within the riparian zone as potential source areas. This paper will not assign specific contributions to the different riparian source areas, but instead seeks to investigate the potential of this mapping approach for eventually quantifying the contribution from different areas within the riparian zone. Key questions for this study will be to determine the feasibility of defining discrete source areas along a representative transect through the riparian zone on the basis of DOC amount, character, and other chemical parameters, as well as to adapt the DOC characterization techniques employed to the range of chemical conditions extant within the riparian zone. How well the chosen transect represents the situation along the stream will be left for future studies to establish. It should also be noted that the assumption of conservative mixing of DOC is intended only as a starting point in the analysis of the origins of DOC in runoff. Any eventual model of stream DOC generation from source areas within the riparian zone must examine the assumption of conservative mixing, which may well prove to be an oversimplification. STUDY AREA
This study was conducted on the 50 ha Svartberget Catchment (Fig. 1) in northern Sweden (64 ° 14' N, 10 ° 4 6 ' E.). The catchment is forested with mature spruce and pine on podzol soils. There are two tributaries, Kallk/tllb~tcken and V~tstrab/tcken. An 8-ha mire is the source of the former tributary. Both tributaries were straightened and deepened to a depth of ca. 1 m during the 1930's. Much of the riparian zone within 10 m of each tributary is covered by peat 20 to 80 cm in depth overlaying a mineral soil enriched in DOC. Mean annual temperature in the area is 0°C with an average annual precipitation of 720 mm, of which roughly half is snow. Annual runoff is 330 mm, with a mean pH of 4.4 and a total organic carbon (TOC) concentration of 15 mg/L. (Water samples were not filtered prior to analysis, so TOC rather than DOC
Riparian sources of aquatic DOC
13
/?
~
°
~
Fig. 1. The Svartberget Catchment with locations of stream sampling and gauging on the Kallklillbiicken and V~strablicken tributaries.
values are reported.) There is a strong correlation between flow, acidity, and TOC, as well as a negative correlation between acidity and sulfate concentration (Grip and Bishop 1990). These and other chemical characteristics indicate that DOC is the controlling factor in stream acidity. This has been confirmed by ultraviolet oxidation of DOC with a corresponding loss of acidity quantitatively equivalent to the measured anion deficit in the initial sample. The mire from which Kallkiillbiicken originates is an obvious source of the organic acidity reaching the stream. Simultaneous monitoring of flow and chemistry at the outlet of the mire, as well as just above the confluence of Kallkllllb~tcken and Vlistrabllcken revealed that the output of TOC per unit area of catchment was almost as high along Kallkallbttcken as it was from the headwater mire (Table 1). Previous investigation of the source of the episodic acidity in runoff has shown that during storm events, a simple mixing of shallow groundwater from the mineral soil with r a i n w a t e r
in the proportions indicated by isotope hydrograph separation succeeded in predicting many features of runoff chemistry, including base cations and sulfate (Bishop 1991a). What that mixing model failed to predict was the increase in TOC and associated decrease in alkalinity. Even under summer low flow conditions, the TOC content of runoff is more than twice that seen in shallow groundwater in mineral soils, but that difference is even larger during periods of high flow. This additional TOC and acidity are acquired by runoff in the riparian zone. Hydrological measurements indicate that variation in runoff rate is determined by fluctuations of the water table within a well-defined band of roughly half a meter in width. Overland flow has been observed between 5 m and 15 m from the stream during a few, exceptionally large storm events. Overland flow, however, was not present during a number of runoff events accompanied by increases in TOC from ca. 15 mg/L to 30 mg/L. During most events, the
14
K. Bishop et al.
Subcatchment Mire Kallk&llb~cken below the mire V~strab~icken Entire Catchment*
Specific Output (g/m z) 3.1 2.8 1.9 2.9
*The annual average output of TOC from the catchment between 1985 and 1989 was 5.5 g/m2. Table 1. Total organic carbon outputs, August-November 1990.
water table stayed 10 to 20 cm below the soil surface. In the riparian zone within 5 m of the stream, the water table never reached the surface. This information suggests that the principal source area of TOC is a band of "spate-specific" flow pathways at several decimeters depth within which the water table varies (Bishop et al. 1990b). METHODS One soil profile, 5 m from V~tstrab~tcken, with a relatively shallow (20 cm) peat was chosen to represent the riparian zone. This was sampled at the surface of the peat, at the base of the peat (15-20 cm depth), and in the organic rich mineral soil beneath the peat (25-30 cm depth), as well as at 50 cm and 80 cm depths. The upslope input to this riparian zone was defined by sampling a soil profile in the podzolized mineral soil 25 m upslope from Kallk~tllb~cken. Samples were taken from the mor layer, the bleached A horizon (0-10 cm depth), the Bhs horizon 15-20 cm, the B horizon at 50 cm, and a gleyed zone at 80 cm. A sample of the Sphagnum moss growing on the bank at stream level was also collected from Kallk/tllb/tcken. A bulk sample of 5 dm 3 to 10 dm 3 of solid matter was taken from each sampling location. Centrifugation was used to extract soil solution (Giesler and Lundstr0m 1993). For those samples where characterization of the TOC was conducted, 1 L of soil solution was extracted. The centrifugation of the organic soils was completed within 24 h of removal from the field. The remainder of the extractions were completed within a week. All water samples were stored at 4 ° C in the dark prior to analysis. On the same day as soils were sampled, streamwater was collected from the mire outlet, and on both Kallk~tllb~tcken and V/tstrab/tcken just above their confluence.
Major cations were analyzed using a Perkin-Elmer ICP/AES, while anions were measured using a Dionex Ion chromatograph. TOC as well as inorganic carbon were measured on unfiltered samples with a Shimadzu TOC-5000 analyzer utilizing catalytic combustion. One method of characterizing the TOC in the samples was separation of the humic substances from the nonhumic material using a weak anion exchanger, diethylaminoethyl (DEAE) on a Sehpadex-A25 substrate. A batch method designed for use in the field was employed in this study (Pettersson et al. 1989): 500 mL of water sample was poured into a flask containing 10 g of the DEAE in the hydrogen ion form adjusted to within half a pH unit of the water sample. This mixture was shaken several times, and then the supernatant which contains nonhumic substances was decanted. The ion exchange resin with adsorbed humic substances was then poured into a column. This was eluted with 0.1 M NaOH to desorb the humic substances. When a distinct "plug" of dissolved humic matter left the column, it was collected as the humic fraction and immediately neutralized with concentrated HCI. The second method of TOC characterization was gel filtration (after filtering through a 0.5-1xm filter) to fractionate the organic material according to molecular size. RESULTS AND DISCUSSION Sampling of soils and streamwater was conducted on October 22, 1992. Without a runoff event during the two weeks prior to sampling, the conditions in the stream were characteristic of autumn base flow conditions, with a specific discharge of 0.7 ram/d, a pH on Kallkallb~cken of 5.1, and a TOC concentration of 24 mg/L.
Riparian sources of aquatic D O C
15
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Fig. 2. The equivalents of major anions and cations that were measured in runoff and soil solution, together with the calculated deficit in anions needed to achieve electroneutrality. The values reported for the Kallk~llbicken tributary have been adjusted to remove the input to that tributary from the mire, so that the values represent the chemistry of runoff from the forested catchment draining to Kallk~illb~cken. Fe and A1 are assumed to be trivalent, even though most are organically complexed.
16
K. Bishop et al,
The inorganic chemistry of the samples display some notable features (Fig. 2). The first is the exceptionally high ionic strengths in the solution extracted from the mor, the top of the peat, and the streambank sphagnum, all of which have over 1000 I.teq of positive charge, as opposed to the stream samples which had between 200 Ixeq/L and 400 Beq/L of positive charge. The only soil solutions with measured cations in the same range as runoff were from the podzol soil below the A horizon. In these equivalence calculations, Fe and A1 are counted as trivalent, even though most are complexed by organic substances. Previous measurements of A1 complexation at Svartberget indicated that over 80% of the AI in streamwater was organically complexed (Townsend et al. 1990). With the exception of Fe, AI, and hydrogen ion, other inorganic constituents of the soils below the superficial organic horizons in both the podzol and riparian peat soil have concentrations between half to double those in runoff. The superficial organic horizons, (i.e., the mor layer, the top of the riparian peat, and the s t r e a m b a n k s p h a g n u m ) , h o w e v e r , had concentrations of base cations or chloride that diverged by one hundred to several hundred txeq/L from those seen in runoff. Assuming that the runoff is essentially a mixture of the water from different areas of the catchment and that these samples of soil solution are representative of the water moving through the areas where they were collected, the inorganic chemical data suggests that the superficial soil solutions are relatively minor
contributors to runoff since the concentrations of certain constituents are so much higher than the concentrations in runoff. From the soils below the superficial horizon, though, there is no clear signal that can distinguish the major contributors to runoff chemistry. In the characterization of the proportion of TOC that was associated with humic substances, the sum of the organic carbon extracted from the ion exchange resin and that in the nonhumic fraction which did not adsorb to the ion-exchange resin often failed to equal the TOC measured in the whole sample. This error in the mass balance was correlated to the amount of TOC in the whole sample (Fig. 3). It is believed that the missing TOC was eluted from the ion exchange column prior to the elution of the distinct, colored "plug" of humic material which was all that was collected from the column. Subsequent elutions, where all of the eluate was collected, have alleviated this error. In this paper, therefore, the missing TOC is assumed to be humic material. The total concentration of TOC in runoff varied from 12 mg/L on V~strab~cken, to 21 mg/L from the forested catchment of Kallk/fllbacken below the mire to 36 mg/L from the mire. Most of the TOC in runoff, 90% or more, was in the form of humic substances (Fig. 4). Within the soil solution, a somewhat larger fraction of TOC was in the nonhumic fraction, particularly in the Bhs horizon of the podzol where more than half of the material was in the nonhumic fraction. With only two exceptions, the absolute concentrations of TOC and its two f r a c t i o n s in soil
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TOC in Whole Sample (mg/L) Fig. 3. The difference between the total TOC in samples and the sum of the measured humic and nonhumic fractions produced by the DEAE separation, plotted as a function of the total amount of TOC in the sample. In this application of the DEAE method, only the distinct "plug" of humic material was collected. Prior eluate was discarded. The missing TOC is believed to be humic material adsorbed to the ion exchange resin that was eluted prior to the elution of the major "plug" of humic material.
Riparian sources of aquatic DOC
17
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Fig. 4. The fraction of humic TOC, nonhumic TOC, and that not recovered, which is assumed to be humic. Note that in samples where the total TOC exceeds 100 mg/L, the results have been scaled to fit on the diagram while still showing the relative proportions of each fraction.
solution were more than twice that in the runoff to Kallk~tllb/tcken or Vllstrab/icken. The increased concentrations were particularly apparent in the superficial samples which had from 5 to over 25 times the concentration of TOC in runoff. The soil solutions which had lower concentrations of total TOC than the runoff were from the Bhs horizon in the podzol soil and the groundwater deeper in the same profile. In these soil solutions, the concentration of humic material was particularly low (<3 mg/L), as opposed to the 10 mg/L to 20 mg/L of humic material in runoff. The extremely high concentrations of TOC in the water extracted from the superficial organic horizons raise the question of whether those concentrations represent those of water moving under natural conditions. Lysimeter techniques for collection of water from that horizon give lower values (Zabowski and Ugolini 1990), but it is not clear that lysimeter techniques give a proper representation of this superficial soil solution chemistry either. Still, bearing in mind that the actual concentration of TOC in soil solution from the superficial horizons may be lower than those extracted by centrifugation, it is clear that if conservative mixing is the source of TOC in runoff, that runoff must be a mixture of the low TOC water from below the bleached layer of the mineral soil and
water with more TOC, such as that from the riparian zone. As with the inorganic chemistry, though, the TOC and its fractionation do not provide sufficient information to specify which portion of the riparian zone is the principal contributor. If the superficial horizons do contribute, then much smaller amounts are required than if the riparian contribution comes from below the superficial horizons. The final source of information about source areas in this sampling comes from gel filtration (Fig. 5). Within both the iron podzol and the peat soil profile, there is a tendency for distinct peaks present in the superficial soil horizon to attenuate with depth in the soil profile. Since the primary direction of transport is downward through the upper two decimeters of these soil profiles, it appears that this is a feature of the aging of the TOC. The most distinctive alteration with depth in the soil profile is the loss of a welldefined, high molecular weight peak to the left of the major peak that is present in water from the superficial horizons. The runoff water almost entirely lacks that high molecular weight peak apparent in the superficial soil horizons, again suggesting that little of the water in the stream originates from the superficial soil horizon (in so far as TOC is not altered in the transport from soil to surface water.)
18
K. Bishop et al,
Iron Podzol Soil
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Fig. 5. Gel filtration curves for the samples from stream and soilwater. Note that unlike other values reported for Kallkillbicken, no correction for the contribution from mire water has been made.
This study has demonstrated the heterogeneity of the soil solution chemistry within and adjacent to the riparian zone. This heterogeneity makes it insufficient to simply say that it is the riparian zone where the organic features of runoff chemistry are determined. The present study has explored the potential of identifying sources of runoff chemistry from the soil solution chemistry in distinct areas along a transect through the riparian zone and upslope mineral soils that cover the majority of the Svartberget Catchment. It is questionable whether the chemical composition of the runoff is a process of mixing water from different source areas, especially with regards to the amount and character of the organic substances in solution. Nonetheless, the assumption of mixing provides a starting point for the analysis of runoff origins. The representativity of soil solution extracted by centrifugation in the superficial soil horizons with a large proportion of living plant material also needs to be examined, and the streambank moss presents a special situation because of its presence in the streamwater itself. It will be of
interest to better understand the chemical relationship between those mosses and the stream. The extent to which any transect can be representative of the variation in the riparian zone along the stream also needs to be assessed. Bearing in mind these considerations, the variation of soil solution chemistry that prompts the division of the riparian zone into specific source areas also contributes to the ability of the approach explored here to identify those areas that contribute most to the organic chemistry of runoff. The characterizations of soil solution chemistry in this study suggest that any contribution to runoff from the superficial soil horizons would be small because the TOC, its quality and the inorganic chemistry of those superficial soil solutions diverges far more from the chemistry of runoff than that of soil solutions deeper in the riparian zone. It thus appears most likely that runoff is essentially a mixture of low TOC water from the B horizon of mineral soils and the TOC-rich subsurface soil solution from within the riparian zone. This chemical understanding of the runoff
Riparian sources of aquatic DOC
generatim process coincides with that obtained by previous hydrological studies. CONCLUSIONS
The current set of analyses was not sufficient to conclusively exclude any potential source area, quantify the contribution from different subsurface sources within the riparian zone or test the assumption that runoff chemistry is generated by conservative mixing. The purpose of this study, however, was not to so much to make that quantification, as to explore the potential of the general approach for eventually achieving such a quantification. With refinements in the techniques employed, it should be possible to increase the p o w e r of this approach by using additional methods to better differentiate potential source areas. Techniques for DOC characterization which separate specific fractions (as opposed to, for example spectroscopic characterization) are particularly suitable as input to a mixing model of runoff chemistry. Seasonal and flow related changes in the chemistry of source areas and runoff can also be exploited to provide information about source areas. Thus, our conclusion is that mapping the features of the different chemical environments within the catchment can be expanded upon to better quantify the contribution from specific riparian source areas to aquatic DOC. -This research was supported by grants from the Swedish Environmental Protection Agency and the Swedish Geological Survey. We are also grateful for the assistance ofMagnu s Edstrom and Gunilla Folkesson in the field and laboratory.
Acknowledgment
REFERENCES Bishop, K.H. Episodic increases in stream acidity, catchment flow pathways and hydrograph separation. Ph.D. thesis, Cambridge University; 1991a. Bishop, K.H. Is there more to acidity in organic-rich surface waters that air pollution? An example from northern Sweden. Vatten 47: 330-335; 1991b.
19
Bishop, K.H.; Grip H.; O'Neill, A. The origins of acid runoff in a hillslope during storm events. L Hydrol. I16: 35-61; 1990a. Bishop, K.H.; Grip, H.; Piggott, E. The significance of spatespecificflow pathways in an episodicallyacid stream. In: Mason, BJ., ed. The surface water acidificationprogramme. London: The Royal Society; 1990b: I07-I19. Bishop, K.H.; Lundstrgm, U.S.; Giesler,R. The transferof organic carbon to surface waters. Appl. Geochem, Special Issue No. 2: 11-15; 1993. Easthouse, K.B.; Mulder, J,; Christophersen, N.; Seip H.M. Dissolved organic carbon fractions in soil and stream water during variable hydrological conditions at Birkenes, southern Norway. Water Resour. Res. 28(6): 1585-1596; 1992. Giesler, R.; Lundstr6m, U.S. Soil solution chemistry--Effects of bulking samples. Soil Sci. Soc. Am. J. 57: 1283-1288. Gjessing, I. The effects of the acidification on humic substances in soil and water and the role of humic substances for the acidification of surface water. Environ. Int. 20(3); 1993. (In press) Grip, H.; Bishop, K.H. Chemical dynamics of an acid stream rich in dissolved organics. In: Mason, B.L, ed. The surface water acidification programme. London: The Royal Society; 1990: 75-84. Hemond, H.F. Wetlands as the source of dissolved organic carbon to surface waters. In: Perdue, E.M.; Gjessing, E.T.0 eds. Organic acids in aquatic ecosystems. N e w York: John Wiley & Sons; 1990: 301-313. Kullberg, A.; Bishop, K.H.; Hargeby, A.; Jansson, M.; Petersen Jr. R.C. Biological role of aquatic humus in acid waters. Amble. 22(5): 331-33"/; 1993. L6fgren, S. Naturliga och antropogena killers betydelse f6r de 6kade halterna av kviive och organiskt material i V~sterdal~Iven och KIar~Iven, 1965-1989. Stockholm: Swedish Environmental Protection Agency Report #3902; 199 I. (In Swedish) Lundin, L. Peatland Drainage--Effects on hydrology of the mire Docksmyren. University of Uppsala, Hydrology Division Report; 1984. Marmorek, D.R.; Bernard, D.P.; Jones, M.L.; Rattle, L.P.; Sullivan, TJ. The effects of mineral acid deposition on concentrations of dissolved organic acids in surface waters. Report to the U.S. Environmental Protection Agency from Environmental and Social Systems Analysts Ltd., Vancouver, B.C. Canada; 1987. Pettersson, C.; Arsenie, I.; Ephraim, J.; Boren H.; AI1ard, B. Properties of fulvic acids from deep groundwaters. Sci. Tot. Environ. 81/82: 287-296; 1989. Townsend, G.; Bishop K.H.; Bache, B. AIuminium speciation during acid episodes. In: Mason, B.J., ed. The surface water acidification programme. London: The Royal Society; 1990: 275-278. Zabowski, D.; Ugolini, F.C. Lysimeter and centrifuge soil solutions: seasonal differences between methods. Soil Sci. Soc. Am. J. 54: 1130-1135; 1990.
Pergamon
IDENTIFICATION OF THE RIPARIAN SOURCES OF AQUATIC DISSOLVED ORGANIC CARBON Kevin Bishop Department of Forest Ecology, Swedish University of Agricultural Sciences, 901 88 Ume&, Sweden
Catharina Pettersson and Bert Allard Department of Water and Environmental Studies, Link6ping University, 581 88 Link0ping, Sweden
Ying-Hua Lee Swedish Environmental Research Institute, Box 47086, Gothenburg, Sweden
EI 9307-183 M (Received 12 July 1993; accepted 30 November 1993)
The riparian zone has been s u g g e s t e d by several r e s e a r c h e r s as the place where o r g a n i c rich runoff acquires much of its dissolved organic carbon (DOC). The great variation in the hydrochemieal 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 Svartberget 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 contribution 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.
INTRODUCTION
mechanical intervention in the form of drainage and changes relating to vegetation and climate regime. Vegetation and climate are also, of course, subject to natural variation. Progress is being made in understanding the effects of both chemical and hydrological alterations. Whereas the effects of acidic deposition on DOC were unclear in the review by Marmorek et a1.(1987), the results of the HUMEX Experiment (Gjessing 1993) support the contention that inputs of stronger mineral acidity will suppress the dissociation and hence mobility of DOC.
There is an increasing awareness of the role of dissolved organic carbon (DOC) in the chemical viability of surface waters. (Bishop 1991b; Kullberg et al. 1993). This poses the question: How do human activities affect the amount and character of DOC entering the aquatic ecosystem from terrestrial sources? There are two broad categories of human influence on forested areas: chemical alterations in the composition of atmospheric deposition and hydrological alteration of the mass transport system within the catchment. The latter category includes both direct 11
12
Hydrologically, there is now evidence of how the precipitation regime affects DOC output (LOfgren 1991). Analysis of the color of Swedish rivers over the course of two decades has revealed that increased rainfall led to increased concentrations of DOC in runoff. Investigations of the effect of forest drainage on DOC amounts and quality have been more ambiguous, showing increases in DOC output in some cases and decreases in other cases (Lundin 1984). Research at the Svartberget Research Catchment has demonstrated how the long-term effects of drainage on DOC output can vary from stream to stream depending on subtle differences in the geometry of the relationship between flow pathways and the source areas of DOC (Bishop et al. 1993). The interplay between source areas of DOC and the hydrological transport system is of importance for more than just the problem of predicting the effects of drainage. That interplay is the key to the marked seasonal and flow-related changes that are apparent in DOC concentration and character. As such, the relation of source areas to the hydrological transport system is important for all predictions about how anthropogenic influences, both chemical and hydrological, will influence the DOC of surface waters. This is especially true when those predictions attempt to go beyond generalizations about the overall direction of a change initiated by anthropogenic influences to quantitative model predictions. There is some agreement that the immediate geographic source of the DOC in runoff is the riparian zone (Hemond 1990). This has also been demonstrated on the Svartberget Catchment (Bishop et al. 1990a). The riparian zone, however, encompasses a number of distinct hydrochemical environments. At Svartberget, these range from podzolized soil profiles a few meters from the stream, through peat profiles of varying depth overlying mineral soils enriched in DOC, to the streambank vegetation where mosses are partially submerged in the stream. The purpose of this study is to explore the possibilities of resolving the contribution of DOC from specific areas within the riparian zone. This will be done by characterizing the DOC quality and other chemical parameters of the soil solution within different areas of a representative transect through the riparian zone extending away from the stream into soils upslope. These will then be compared to the composition of the runoff entering the stream via the riparian zone. By assuming that runoff is a conservative mixture of several "end-members" from this representative transect with specific chemical compositions, it is then possible to define the ratios in
K. Bishop et al,
which the different source areas contribute to runoff. (A conservative mixture is one in which the concentration of a component is determined solely by the concentration of that component in the different sources and the ratio in which those sources contribute to the mixture, i.e., there is no precipitation, dissolution, or other chemical transformation.) This technique is similar to that applied by Easthouse et al. (1992). That a n a l y s i s , h o w e v e r , was b a s e d on the definition of several source areas within the catchment as a whole rather than focusing on individual soil horizons within the riparian zone as potential source areas. This paper will not assign specific contributions to the different riparian source areas, but instead seeks to investigate the potential of this mapping approach for eventually quantifying the contribution from different areas within the riparian zone. Key questions for this study will be to determine the feasibility of defining discrete source areas along a representative transect through the riparian zone on the basis of DOC amount, character, and other chemical parameters, as well as to adapt the DOC characterization techniques employed to the range of chemical conditions extant within the riparian zone. How well the chosen transect represents the situation along the stream will be left for future studies to establish. It should also be noted that the assumption of conservative mixing of DOC is intended only as a starting point in the analysis of the origins of DOC in runoff. Any eventual model of stream DOC generation from source areas within the riparian zone must examine the assumption of conservative mixing, which may well prove to be an oversimplification. STUDY AREA
This study was conducted on the 50 ha Svartberget Catchment (Fig. 1) in northern Sweden (64 ° 14' N, 10 ° 4 6 ' E.). The catchment is forested with mature spruce and pine on podzol soils. There are two tributaries, Kallk/tllb~tcken and V~tstrab/tcken. An 8-ha mire is the source of the former tributary. Both tributaries were straightened and deepened to a depth of ca. 1 m during the 1930's. Much of the riparian zone within 10 m of each tributary is covered by peat 20 to 80 cm in depth overlaying a mineral soil enriched in DOC. Mean annual temperature in the area is 0°C with an average annual precipitation of 720 mm, of which roughly half is snow. Annual runoff is 330 mm, with a mean pH of 4.4 and a total organic carbon (TOC) concentration of 15 mg/L. (Water samples were not filtered prior to analysis, so TOC rather than DOC
Riparian sources of aquatic DOC
13
/?
~
°
~
Fig. 1. The Svartberget Catchment with locations of stream sampling and gauging on the Kallklillbiicken and V~strablicken tributaries.
values are reported.) There is a strong correlation between flow, acidity, and TOC, as well as a negative correlation between acidity and sulfate concentration (Grip and Bishop 1990). These and other chemical characteristics indicate that DOC is the controlling factor in stream acidity. This has been confirmed by ultraviolet oxidation of DOC with a corresponding loss of acidity quantitatively equivalent to the measured anion deficit in the initial sample. The mire from which Kallkiillbiicken originates is an obvious source of the organic acidity reaching the stream. Simultaneous monitoring of flow and chemistry at the outlet of the mire, as well as just above the confluence of Kallkllllb~tcken and Vlistrabllcken revealed that the output of TOC per unit area of catchment was almost as high along Kallkallbttcken as it was from the headwater mire (Table 1). Previous investigation of the source of the episodic acidity in runoff has shown that during storm events, a simple mixing of shallow groundwater from the mineral soil with r a i n w a t e r
in the proportions indicated by isotope hydrograph separation succeeded in predicting many features of runoff chemistry, including base cations and sulfate (Bishop 1991a). What that mixing model failed to predict was the increase in TOC and associated decrease in alkalinity. Even under summer low flow conditions, the TOC content of runoff is more than twice that seen in shallow groundwater in mineral soils, but that difference is even larger during periods of high flow. This additional TOC and acidity are acquired by runoff in the riparian zone. Hydrological measurements indicate that variation in runoff rate is determined by fluctuations of the water table within a well-defined band of roughly half a meter in width. Overland flow has been observed between 5 m and 15 m from the stream during a few, exceptionally large storm events. Overland flow, however, was not present during a number of runoff events accompanied by increases in TOC from ca. 15 mg/L to 30 mg/L. During most events, the
14
K. Bishop et al.
Subcatchment Mire Kallk&llb~cken below the mire V~strab~icken Entire Catchment*
Specific Output (g/m z) 3.1 2.8 1.9 2.9
*The annual average output of TOC from the catchment between 1985 and 1989 was 5.5 g/m2. Table 1. Total organic carbon outputs, August-November 1990.
water table stayed 10 to 20 cm below the soil surface. In the riparian zone within 5 m of the stream, the water table never reached the surface. This information suggests that the principal source area of TOC is a band of "spate-specific" flow pathways at several decimeters depth within which the water table varies (Bishop et al. 1990b). METHODS One soil profile, 5 m from V~tstrab~tcken, with a relatively shallow (20 cm) peat was chosen to represent the riparian zone. This was sampled at the surface of the peat, at the base of the peat (15-20 cm depth), and in the organic rich mineral soil beneath the peat (25-30 cm depth), as well as at 50 cm and 80 cm depths. The upslope input to this riparian zone was defined by sampling a soil profile in the podzolized mineral soil 25 m upslope from Kallk~tllb~cken. Samples were taken from the mor layer, the bleached A horizon (0-10 cm depth), the Bhs horizon 15-20 cm, the B horizon at 50 cm, and a gleyed zone at 80 cm. A sample of the Sphagnum moss growing on the bank at stream level was also collected from Kallk/tllb/tcken. A bulk sample of 5 dm 3 to 10 dm 3 of solid matter was taken from each sampling location. Centrifugation was used to extract soil solution (Giesler and Lundstr0m 1993). For those samples where characterization of the TOC was conducted, 1 L of soil solution was extracted. The centrifugation of the organic soils was completed within 24 h of removal from the field. The remainder of the extractions were completed within a week. All water samples were stored at 4 ° C in the dark prior to analysis. On the same day as soils were sampled, streamwater was collected from the mire outlet, and on both Kallk~tllb~tcken and V/tstrab/tcken just above their confluence.
Major cations were analyzed using a Perkin-Elmer ICP/AES, while anions were measured using a Dionex Ion chromatograph. TOC as well as inorganic carbon were measured on unfiltered samples with a Shimadzu TOC-5000 analyzer utilizing catalytic combustion. One method of characterizing the TOC in the samples was separation of the humic substances from the nonhumic material using a weak anion exchanger, diethylaminoethyl (DEAE) on a Sehpadex-A25 substrate. A batch method designed for use in the field was employed in this study (Pettersson et al. 1989): 500 mL of water sample was poured into a flask containing 10 g of the DEAE in the hydrogen ion form adjusted to within half a pH unit of the water sample. This mixture was shaken several times, and then the supernatant which contains nonhumic substances was decanted. The ion exchange resin with adsorbed humic substances was then poured into a column. This was eluted with 0.1 M NaOH to desorb the humic substances. When a distinct "plug" of dissolved humic matter left the column, it was collected as the humic fraction and immediately neutralized with concentrated HCI. The second method of TOC characterization was gel filtration (after filtering through a 0.5-1xm filter) to fractionate the organic material according to molecular size. RESULTS AND DISCUSSION Sampling of soils and streamwater was conducted on October 22, 1992. Without a runoff event during the two weeks prior to sampling, the conditions in the stream were characteristic of autumn base flow conditions, with a specific discharge of 0.7 ram/d, a pH on Kallkallb~cken of 5.1, and a TOC concentration of 24 mg/L.
Riparian sources of aquatic D O C
15
Anions
Cations
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[ ] Ca+Mg
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Deficit
Fig. 2. The equivalents of major anions and cations that were measured in runoff and soil solution, together with the calculated deficit in anions needed to achieve electroneutrality. The values reported for the Kallk~llbicken tributary have been adjusted to remove the input to that tributary from the mire, so that the values represent the chemistry of runoff from the forested catchment draining to Kallk~illb~cken. Fe and A1 are assumed to be trivalent, even though most are organically complexed.
16
K. Bishop et al,
The inorganic chemistry of the samples display some notable features (Fig. 2). The first is the exceptionally high ionic strengths in the solution extracted from the mor, the top of the peat, and the streambank sphagnum, all of which have over 1000 I.teq of positive charge, as opposed to the stream samples which had between 200 Ixeq/L and 400 Beq/L of positive charge. The only soil solutions with measured cations in the same range as runoff were from the podzol soil below the A horizon. In these equivalence calculations, Fe and A1 are counted as trivalent, even though most are complexed by organic substances. Previous measurements of A1 complexation at Svartberget indicated that over 80% of the AI in streamwater was organically complexed (Townsend et al. 1990). With the exception of Fe, AI, and hydrogen ion, other inorganic constituents of the soils below the superficial organic horizons in both the podzol and riparian peat soil have concentrations between half to double those in runoff. The superficial organic horizons, (i.e., the mor layer, the top of the riparian peat, and the s t r e a m b a n k s p h a g n u m ) , h o w e v e r , had concentrations of base cations or chloride that diverged by one hundred to several hundred txeq/L from those seen in runoff. Assuming that the runoff is essentially a mixture of the water from different areas of the catchment and that these samples of soil solution are representative of the water moving through the areas where they were collected, the inorganic chemical data suggests that the superficial soil solutions are relatively minor
contributors to runoff since the concentrations of certain constituents are so much higher than the concentrations in runoff. From the soils below the superficial horizon, though, there is no clear signal that can distinguish the major contributors to runoff chemistry. In the characterization of the proportion of TOC that was associated with humic substances, the sum of the organic carbon extracted from the ion exchange resin and that in the nonhumic fraction which did not adsorb to the ion-exchange resin often failed to equal the TOC measured in the whole sample. This error in the mass balance was correlated to the amount of TOC in the whole sample (Fig. 3). It is believed that the missing TOC was eluted from the ion exchange column prior to the elution of the distinct, colored "plug" of humic material which was all that was collected from the column. Subsequent elutions, where all of the eluate was collected, have alleviated this error. In this paper, therefore, the missing TOC is assumed to be humic material. The total concentration of TOC in runoff varied from 12 mg/L on V~strab~cken, to 21 mg/L from the forested catchment of Kallk/fllbacken below the mire to 36 mg/L from the mire. Most of the TOC in runoff, 90% or more, was in the form of humic substances (Fig. 4). Within the soil solution, a somewhat larger fraction of TOC was in the nonhumic fraction, particularly in the Bhs horizon of the podzol where more than half of the material was in the nonhumic fraction. With only two exceptions, the absolute concentrations of TOC and its two f r a c t i o n s in soil
25 ~E o
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100
150
200
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TOC in Whole Sample (mg/L) Fig. 3. The difference between the total TOC in samples and the sum of the measured humic and nonhumic fractions produced by the DEAE separation, plotted as a function of the total amount of TOC in the sample. In this application of the DEAE method, only the distinct "plug" of humic material was collected. Prior eluate was discarded. The missing TOC is believed to be humic material adsorbed to the ion exchange resin that was eluted prior to the elution of the major "plug" of humic material.
Riparian sources of aquatic DOC
17
X1.4
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80
60
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Humic (assumed) [ ] Hydr0philie
Fig. 4. The fraction of humic TOC, nonhumic TOC, and that not recovered, which is assumed to be humic. Note that in samples where the total TOC exceeds 100 mg/L, the results have been scaled to fit on the diagram while still showing the relative proportions of each fraction.
solution were more than twice that in the runoff to Kallk~tllb/tcken or Vllstrab/icken. The increased concentrations were particularly apparent in the superficial samples which had from 5 to over 25 times the concentration of TOC in runoff. The soil solutions which had lower concentrations of total TOC than the runoff were from the Bhs horizon in the podzol soil and the groundwater deeper in the same profile. In these soil solutions, the concentration of humic material was particularly low (<3 mg/L), as opposed to the 10 mg/L to 20 mg/L of humic material in runoff. The extremely high concentrations of TOC in the water extracted from the superficial organic horizons raise the question of whether those concentrations represent those of water moving under natural conditions. Lysimeter techniques for collection of water from that horizon give lower values (Zabowski and Ugolini 1990), but it is not clear that lysimeter techniques give a proper representation of this superficial soil solution chemistry either. Still, bearing in mind that the actual concentration of TOC in soil solution from the superficial horizons may be lower than those extracted by centrifugation, it is clear that if conservative mixing is the source of TOC in runoff, that runoff must be a mixture of the low TOC water from below the bleached layer of the mineral soil and
water with more TOC, such as that from the riparian zone. As with the inorganic chemistry, though, the TOC and its fractionation do not provide sufficient information to specify which portion of the riparian zone is the principal contributor. If the superficial horizons do contribute, then much smaller amounts are required than if the riparian contribution comes from below the superficial horizons. The final source of information about source areas in this sampling comes from gel filtration (Fig. 5). Within both the iron podzol and the peat soil profile, there is a tendency for distinct peaks present in the superficial soil horizon to attenuate with depth in the soil profile. Since the primary direction of transport is downward through the upper two decimeters of these soil profiles, it appears that this is a feature of the aging of the TOC. The most distinctive alteration with depth in the soil profile is the loss of a welldefined, high molecular weight peak to the left of the major peak that is present in water from the superficial horizons. The runoff water almost entirely lacks that high molecular weight peak apparent in the superficial soil horizons, again suggesting that little of the water in the stream originates from the superficial soil horizon (in so far as TOC is not altered in the transport from soil to surface water.)
18
K. Bishop et al,
Iron Podzol Soil
Shallow Peat Soil
Streambank Sphagnum
Mori, : !PeatTo
Streamwater
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Time(min,)
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30
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Fig. 5. Gel filtration curves for the samples from stream and soilwater. Note that unlike other values reported for Kallkillbicken, no correction for the contribution from mire water has been made.
This study has demonstrated the heterogeneity of the soil solution chemistry within and adjacent to the riparian zone. This heterogeneity makes it insufficient to simply say that it is the riparian zone where the organic features of runoff chemistry are determined. The present study has explored the potential of identifying sources of runoff chemistry from the soil solution chemistry in distinct areas along a transect through the riparian zone and upslope mineral soils that cover the majority of the Svartberget Catchment. It is questionable whether the chemical composition of the runoff is a process of mixing water from different source areas, especially with regards to the amount and character of the organic substances in solution. Nonetheless, the assumption of mixing provides a starting point for the analysis of runoff origins. The representativity of soil solution extracted by centrifugation in the superficial soil horizons with a large proportion of living plant material also needs to be examined, and the streambank moss presents a special situation because of its presence in the streamwater itself. It will be of
interest to better understand the chemical relationship between those mosses and the stream. The extent to which any transect can be representative of the variation in the riparian zone along the stream also needs to be assessed. Bearing in mind these considerations, the variation of soil solution chemistry that prompts the division of the riparian zone into specific source areas also contributes to the ability of the approach explored here to identify those areas that contribute most to the organic chemistry of runoff. The characterizations of soil solution chemistry in this study suggest that any contribution to runoff from the superficial soil horizons would be small because the TOC, its quality and the inorganic chemistry of those superficial soil solutions diverges far more from the chemistry of runoff than that of soil solutions deeper in the riparian zone. It thus appears most likely that runoff is essentially a mixture of low TOC water from the B horizon of mineral soils and the TOC-rich subsurface soil solution from within the riparian zone. This chemical understanding of the runoff
Riparian sources of aquatic DOC
generatim process coincides with that obtained by previous hydrological studies. CONCLUSIONS
The current set of analyses was not sufficient to conclusively exclude any potential source area, quantify the contribution from different subsurface sources within the riparian zone or test the assumption that runoff chemistry is generated by conservative mixing. The purpose of this study, however, was not to so much to make that quantification, as to explore the potential of the general approach for eventually achieving such a quantification. With refinements in the techniques employed, it should be possible to increase the p o w e r of this approach by using additional methods to better differentiate potential source areas. Techniques for DOC characterization which separate specific fractions (as opposed to, for example spectroscopic characterization) are particularly suitable as input to a mixing model of runoff chemistry. Seasonal and flow related changes in the chemistry of source areas and runoff can also be exploited to provide information about source areas. Thus, our conclusion is that mapping the features of the different chemical environments within the catchment can be expanded upon to better quantify the contribution from specific riparian source areas to aquatic DOC. -This research was supported by grants from the Swedish Environmental Protection Agency and the Swedish Geological Survey. We are also grateful for the assistance ofMagnu s Edstrom and Gunilla Folkesson in the field and laboratory.
Acknowledgment
REFERENCES Bishop, K.H. Episodic increases in stream acidity, catchment flow pathways and hydrograph separation. Ph.D. thesis, Cambridge University; 1991a. Bishop, K.H. Is there more to acidity in organic-rich surface waters that air pollution? An example from northern Sweden. Vatten 47: 330-335; 1991b.
19
Bishop, K.H.; Grip H.; O'Neill, A. The origins of acid runoff in a hillslope during storm events. L Hydrol. I16: 35-61; 1990a. Bishop, K.H.; Grip, H.; Piggott, E. The significance of spatespecificflow pathways in an episodicallyacid stream. In: Mason, BJ., ed. The surface water acidificationprogramme. London: The Royal Society; 1990b: I07-I19. Bishop, K.H.; Lundstrgm, U.S.; Giesler,R. The transferof organic carbon to surface waters. Appl. Geochem, Special Issue No. 2: 11-15; 1993. Easthouse, K.B.; Mulder, J,; Christophersen, N.; Seip H.M. Dissolved organic carbon fractions in soil and stream water during variable hydrological conditions at Birkenes, southern Norway. Water Resour. Res. 28(6): 1585-1596; 1992. Giesler, R.; Lundstr6m, U.S. Soil solution chemistry--Effects of bulking samples. Soil Sci. Soc. Am. J. 57: 1283-1288. Gjessing, I. The effects of the acidification on humic substances in soil and water and the role of humic substances for the acidification of surface water. Environ. Int. 20(3); 1993. (In press) Grip, H.; Bishop, K.H. Chemical dynamics of an acid stream rich in dissolved organics. In: Mason, B.L, ed. The surface water acidification programme. London: The Royal Society; 1990: 75-84. Hemond, H.F. Wetlands as the source of dissolved organic carbon to surface waters. In: Perdue, E.M.; Gjessing, E.T.0 eds. Organic acids in aquatic ecosystems. N e w York: John Wiley & Sons; 1990: 301-313. Kullberg, A.; Bishop, K.H.; Hargeby, A.; Jansson, M.; Petersen Jr. R.C. Biological role of aquatic humus in acid waters. Amble. 22(5): 331-33"/; 1993. L6fgren, S. Naturliga och antropogena killers betydelse f6r de 6kade halterna av kviive och organiskt material i V~sterdal~Iven och KIar~Iven, 1965-1989. Stockholm: Swedish Environmental Protection Agency Report #3902; 199 I. (In Swedish) Lundin, L. Peatland Drainage--Effects on hydrology of the mire Docksmyren. University of Uppsala, Hydrology Division Report; 1984. Marmorek, D.R.; Bernard, D.P.; Jones, M.L.; Rattle, L.P.; Sullivan, TJ. The effects of mineral acid deposition on concentrations of dissolved organic acids in surface waters. Report to the U.S. Environmental Protection Agency from Environmental and Social Systems Analysts Ltd., Vancouver, B.C. Canada; 1987. Pettersson, C.; Arsenie, I.; Ephraim, J.; Boren H.; AI1ard, B. Properties of fulvic acids from deep groundwaters. Sci. Tot. Environ. 81/82: 287-296; 1989. Townsend, G.; Bishop K.H.; Bache, B. AIuminium speciation during acid episodes. In: Mason, B.J., ed. The surface water acidification programme. London: The Royal Society; 1990: 275-278. Zabowski, D.; Ugolini, F.C. Lysimeter and centrifuge soil solutions: seasonal differences between methods. Soil Sci. Soc. Am. J. 54: 1130-1135; 1990.