Organic carbon in the boreal spring flood from adjacent subcatchments

Environment International, Vol. 22, No. 5, pp. 535-540,1996 Copyright 01996 Ekvis Science Ltd Printed in the USA. All rights reserved 0160-4120/96 S15.00+.00

Pergamon

ORGANIC CARBON IN THE BOREAL SPRING FLOOD FROM ADJACENT SUBCATCHMENTS Kevin Bishop Department of Forest Ecology, Swedish University of Agricultural Sweden

Sciences, S-901 83 Ume&

Catharina Pettersson Department of Water and Environmental Studies, Linkt)ping University, S-581 83 Linkbping, Sweden

EI 9507-369 M (Received 26 April 1996; accepted 28 April 1996)

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

INTRODUCTION

Several authors have described a characteristic relationship between TOC and flow where sharp, initial increases in TOC with rising flow are followed by a more gradual decline in TOC (Grieve 1991; Denning et al. 1991). They have suggested that this pattern can be explained as a combination of rising flow activating new sources of TOC, followed by a progressive depletion ofthose sources as a discharge event progresses. That hypothesis has been modeled and found to be consistent with data from the Snake River in Colorado, USA (Homberger et al. 1994).

The spring flood is the major hydrological event in the boreal region where 2550% of the annual runoff can occur within several weeks. That period is thus of considerable importance for understanding the hydrochemical outputs from boreal catchments. Regional monitoring of watercourses in Sweden indicates that total organic carbon (TOC) is less subject to dilution than other solutes during spring flood (Lofgren 1992), which makes this period of particular importance for TOC cycling. 535

K. Bishop and C. Pettersson

536

Mire Perennial Stream intermittent Stream L 200m ,

Fig. 1. The Svartberget Catchment showing the location of sampling sites, and the MK Reach of the Kallklillbilcken Subcatchment below the mire.

In this study, the TOC output from three subcatchments (two forested and one mire) within a boreal catchment were examined. The runoff from these subcatchments differed from one another in average annual, volumeweighted TOC concentrations. This study looked at the TOC dynamics of these subcatchments during the 1993 spring flood to see whether they are consistent with the hypothesis that TOC output is controlled by a combination of new, but capacity-limited sources tapped by rising flows during the spring flood. STUDY AREA

The study was conducted on the 50 ha Svartberget Catchment (Fig. 1) in northern Sweden (64” 14’N, 19” 46’ E). The mean annual temperature is 0°C. Mean annual precipitation over the last decade has been 720 mm, with a mean runoff of 325 mm. Half of the runoff occurs during the snow-free half of the year (June to November), and a third of the runoff occurs during three-four weeks of spring flood in April or May.

Except for an open, S-ha mire, the catchment is afforested with Norway Spruce (Picecr abies) in lower, wetter areas, and Scats Pine (Pinus sylvestris) on higher, better-drained areas. The podzol soils, which cover much of the catchment, have developed on several meters of glacial till. The podzols give way to riparian peat soils near the two tributaries, Kallkallbacken and Vastrabacken. Both tributaries were ditched to about the same depth (ca. 1 m) during the 1930s. Much of the 1O-mto 20-m wide riparian zone along the length of both tributaries is covered by peat 20 to 80 cm in depth overlaying a mineral soil enriched in TOC (Bishop 1994). The depth of the riparian peat differed considerably along these two tributaries. The frequency of ‘deep’ peat (40 to 80 cm in depth 12 m away from the stream) was twice as high along Kallklllbacken as along Vastrabacken. This difference in the structure of the riparian soils has been used to explain the higher mean annual concentration of TOC in runoff from the forested portion of the Kallkallbacken Subcatchment relative to that from Vastrabacken (Fig. 2).

537

Organic carbon in the boreal spring flood from adjacent subcatchments

Mire

KallkAl b&ken below Mire

?? Non-Humic

V&tra backen

TOC

HTOC

Fig. 2. The volume-weighted mean concentration of TOC, including the non-humic fraction, from the subcatchments defined by the Mire (Site M), V%strab%cken(Site V), and KallkilllbSicken below the mire (Reach MK) during 1993. These values are based on weekly sampling.

T

75

%

KallklllbScken

below Mire

0 Vistrabi%zken

Subcatchment. To facilitate comparisons of the forested areas draining to each tributary, the amount of runoff and organic carbon output from the Kallkiillblcken Subcatchment below the mire (i.e., along the forested stream reach between sites M and K) was calculated from the differences in flow and chemistry at Sites M and K. Water samples during spring flood were taken in the afternoon as the daily peak in flow was approached. The samples were not filtered, so TOC rather than dissolved organic carbon (DOC) is reported here. (The difference between TOC and DOC that passes through a 0.45~pm filter is typically less than 5% at Svartberget.) The humic fraction of TOC was isolated in a batch extraction (Pettersson 1993) using the weak anion exchange resin, Diethylaminoethyl Sephadex-A25 (Pharmacia Fine Chemicals). The supernatant from this extraction was decanted after 20 min, and its TOC concentration was deemed to be the non-humic fraction of TOC. To estimate the amount of runoff in the stream that was fresh snowmelt, isotope hydrograph separation was used. The separation was based on oxygen isotope ratios measured in the snowpack and stream prior to and during the spring snowmelt. The results, in which these isotope concentrations are used to divide runoff into snowmelt and water displaced from the catchment, have been reported previously (Bishop 1995a), and are included in this paper for reference. RESULTS

k Q b

h <

s

it?

EE

5

z

cu

s

I m

6 H

Fi

Fig. 3. The cumulative daily mean runoff from subcatchments during the 1993 spring flood.

METHODS Runoff was measured continuously at Sites M, V, and S during the spring flood, and the runoff at Site K was calculated by difference (Fig. 1). Sampling of runoff chemistry was conducted at three sites (M, K, and V) that correspond to three subcatchments: the 15 ha Mire Subcatchment (Site M) which drains 8 ha of mire and 7 ha of surrounding moraine soils; the 9 ha, forested Vastrabacken Subcatchment (Site V), and the 41 ha Kallkallbacken Subcatchment, which includes the Mire

The 1993 spring flood commenced on April 23rd (Fig. 3) and continued for three weeks, during which less than 2 mm of rain fell. During that period, there were 99 mm of runoff. This amounted to 28% of the total runoff during 1993. Subcatchment budgets for the output of TOC and water during the spring flood were calculated over this same three-week period (Table 1). During that period, the percentage of the year’s output of water was highest on Vastrabicken and the Mire (ca. 30%), but lower from the forested Kallklllblcken Subcatchment below the mire. The percentage of annual TOC output was comparable to that of water from the forested subcatchments, but less than the output of water from the mire. The proportion of snowmelt in runoff was highest in the mire runoff (40-60%; Bishop et al. 1995a). It remained under about 40% in the Vistrabacken Tributary and in runoff from the Kallkallbacken Subcatchment below the mire (Fig. 6).

K. Bishop and C. Pettersson

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Table 1. Output of water and TOC during the 1993 spring flood: Amount per unit area and proportion of annual subcatchment output. Kallkiillbbken below the mire (MK Reach)

Mire

40

Amount

% of

Amount

% of

Amount

% of

per area

1993

per area

1993

per area

1993

Water

mm

124

31%

82

23%

112

29%

TOC

g/ha

22

20%

20

21%

25

33%

-8-

T

17 Apr

Kallkdlb&zken Kallkdlblcken below Mire Vhtrabiicken

+ - d -

-x

24 Apr

1 May

8 May

15 May

22 May

Fig. 4. Subcatchment output of TOC concentrations during the 1993 spring flood.

* - A -

20% T

0%

VzLstrab8cken

1

17 Apr

/ 24 Apr

I

Kallkdlblcken Viistrablcken

I

1I

1 May

8 May

15 May

I 22 May

Fig. 5. The non-humic fraction of TOC during the 1993 spring flood.

-e- A -

Kallkiillbicken Vlstrablcken

50%

The dynamics of TOC concentration differed between the different sites, as well as from the patterns seen during the remainder of the year. On Vastrabacken (Site V), where TOC concentrations are lowest most of the year (Fig. 2), the TOC concentration more than tripled during the early part of spring flood to almost 30 mg/L (Fig. 4). Although the TOC concentrations subsequently declined, they remained higher than 15 mg/L (i.e., twice the late winter value) until after the spring flood had past. On Kallkallbacken (Site K), the TOC concentration also increased during the initial stage of spring flood before settling back to a value just over 20 mg/L for the remainder of spring flood. When the TOC concentration in runoff from the forested portion ofthe Kallkallbacken Subcatchment below the mire (the MK Reach) is considered, however, the TOC concentrations are more similar to those in runoff from the forested Vastrablcken Subcatchment in terms of both the rapid increase and gradual decline, as well as the absolute levels. These reached a maximum of 30 mg/L before receding to about 20 mg/L. In runoff from the Mire (Site M), however, the pattern was different. Instead of an initial increase in TOC, there was a halving of the late winter TOC concentration during the spring flood to 15 mgL. In fact, throughout the spring flood, the lowest TOC concentrations were found in the runoff from the Mire, despite that site having the highest concentrations, 29 mg/L on average, during 1993. The proportion of non-humic TOC during the spring flood at each site changed slightly from just less than 10% to about 15% (Fig. 5). This increase occurred after the peak in the spring flood runoff had passed. DISCUSSION

25%

0%” 21 Apr

1 28 Apr

1 May

8 May

11 May 16 May

Fig. 6. The fraction of snowmelt in runoff as estimated by isotope hydrograph separation (Bishop et al. 1995b).

The large amount of TOC in the spring flood runoff indicates the importance of soil processes in the chemistry of the spring flood. The extensive contact of runoff with the soil, despite large amounts of water inputs and frozen soils, is also reflected in the large proportion of non-snowmelt water displaced from the catchment during the spring flood.

Organic carbon in the boreal spring flood from adjacent subcatchments

The decline in TOC at the Mire, in contrast to increases in the forested areas, suggests a difference between the processes that regulate TOC output from the Mire and the forested areas. In the Mire, the winter source(s)ofTOCyieldedhighconcentrations(>3OmgL) at low flow. That source was either depleted or complemented by less TOC-rich sources of runoff during the spring flood. On the forested Vastrablicken and the MK Reach of Kallklllb2icken below the mire, there was an initial increase in TOC during the spring flood, which could indicate a new, TOC-rich source of runoff. That initial peak was then followed by steady decline which is consistent with depletion of the new source area. The decline was larger on V&&rabacken,where a larger proportion of the annual runoff occurred during the spring flood than on Kallkiillb%cken below the mire. The recession in TOC concentration at both forested sites, however, was not sufficient to return TOC to pre-spring flood levels, despite such a large portion of the annual runoff having occurred in the space of three weeks. Thus, if the decline represents the progressive depletion of a new TOC source, it is a large source. With the data available, a change in flow paths as the spring flood progresses cannot be excluded as an alternative explanation for the gradual TOC decline on the receding limb of the spring flood hydrograph. A new source of TOC in the forested catchments that could be activated during the spring flood is the superficial flow paths through the organic-rich, upper-soil horizon (i.e., the forest mor layer) over much of the catchment. This mor layer has a high concentration of TOC in observations during the summer and autumn (Bishop et al. 1995b). During snow-free periods in the forested areas, though, these superficial soils do not have a direct flow pathway to the watercourse. During those periods, water infiltrates vertically downwards through the TOC-rich mor layer into mineral soil horizons where sorption and degradation processes lead to lowered TOC concentrations (Cronan and Aiken 1985). It is only after the vertically infiltrating soil water reaches the water table that lateral flow downslope towards the stream becomes the predominant flow direction (Bishop et al. 1990). Deeper flow pathways through mineral soil have also been identified as those responsible for winter low flow in the forested areas of the catchment (Grip and Bishop 1990). During the spring flood, large snowmelt inputs onto frozen soils with reduced permeability could bring runoff from the mor directly to the stream, thus bypassing the sorption and degradation processes in the mineral

539

soil. The similarities in the concentrations of TOC in runoff from the Vastrabiicken Subcatchment and Kallkiillbiicken below the mire during the initial phases of the spring flood (but not at other times of the year) are consistent with the hypothesis of superficial flow pathways that operate during the spring flood. Such superficial flow would bypass the differences deeper in the riparian soils of the two forested subcatchments that lead to differences in runoff TOC concentrations at other times of the year. At the Mire, superficial flow is expected to be important even during the winter, which contributes to the high, late-winter concentrations of TOC. Spring flood on the Mire, however, has no new, TOC-rich water sources to tap. Hence, there is no initial concentration increase of TOC in runoff from the Mire, only a decline as large amounts of low-TOC snowmelt deplete the available TOC in the same superficial source that supplied the winter flows. The non-humic fraction of TOC does not change markedly, although there is a tendency for a slight rise late in the spring flood (Fig. 5). Since the non-humic fraction was found to be higher in the mor layer during the snow- free period (unpublished data), this does not argue for a switch to superficial sources on the forested subcatchments early on in the spring flood. The establishment of the character of superficial TOC sources during the spring flood, though, is necessary for a more satisfactory use of TOC character to test the hypothesis of new, superficial water sources during the spring flood. CONCLUSION Despite the large and rapid input of snowmelt with significant amounts of meltwater onto frozen soils, the concentration of TOC increases in runoff from the forested subcatchments during spring flood. Superficial flow paths in the forested areas unique to the spring flood period may be an important source of TOC in runoff from these areas. The subsequent decline after initial increases is consistent with the hypothesis of a new source that is subsequently depleted as the spring flood progresses. At the Mire, there is not an initial increase in TOC, but rather an immediate decrease (albeit from high, late winter concentrations of about 35 mg/L). This could be understood in terms of a system that already uses the soil surface as its main flow pathway for winter low flows. Therefore, there are no new sources of TOC to draw upon when the spring flood comes to the Mire. For further testing of the

540

hypothesis that superficial flow paths are the primary source of spring flood TOC, direct observation of the hypothesized superficial flow paths and their chemistry is desirable.

REFERENCES Bishop, K.H. Return flow in till hillslopes: Final report of a project funded by The Swedish Geological Survey. Report Series No. 25. Swedish Univ. of Agricultural Sciences, Dept. of Forest Ecology: Ume& Sweden; 1994. Bishop, K.H.; Grip, H.; Piggott, E. Spate-specific flow pathways in an episodically acid stream. In: Mason, B.J., ed. The surface water acidification programme. Cambridge, Cambridge University Press; 1990: 107-120. Bishop, K.H.; Lee, Y.H.; Pettersson, C.; Allard, B. Methylmercury output from the Svartberget Catchment in Northern Sweden during spring flood. Water Air Soil Pollut. 80: 445-454; 1995a. Bishop, K.H.; Lee, Y.H.; Pettersson, C.; Allard, B. Terrestrial sources of methylmercury in surface waters: The importance of the riparian zone on the Svartberget Catchment. Water Air Soil Pollut. 80: 435-444; 1995b.

K. Bishop and C. Pettersson

Cronan, C.S.; Aiken, G.R. Chemistry and transport of soluble humic substances in forested watersheds of the Adirondack Park New York USA. Geochim. Cosmochim. Acta. 49: 1697-1706; 1985. Denning, AS.; Baron, J.; Mast, M.A.; Arthur, M. Hydrologic pathways and chemical composition of runoff during snowmelt in Loch Vale watershed, Rocky Mountain National Park, Colorado, USA. Water Air Soil Pollut. 59: 107- 123; 199 1. Grieve, I. A model of dissolved organic carbon concentrations in soil and stream waters. Hydrol. Proc. 5: 301-307; 1991. Grip, H.; Bishop, K.H. Chemical dynamics of an acid stream rich in dissolved organics. In: Mason, B.J., ed. The surface water acidification programme. Cambridge, Cambridge University Press; 1990: 75-83. Hornberger, G.M.; Bencala, K.E.; McKnight, D.M. Hydrological controls on dissolved organic carbon during snowmelt in the Snake River near Montezuma, Colorado. Biogeochem. 25: 147165; 1994. Mfgren, S. Samordnad vattendragskontroll i Norrbottens ltht (Coordinated monitoring of running waters in Norrbotten County). Ltisstyrelsen i Norrbottens L&t Report Series, 1992. Available from: Swedish Environmental Protection Agency, S106 48, Stockholm. Pettersson, C. Properties of humic substances from groundwater and surface waters. Ph.D. Thesis. Linkoping University; Linkoping, Sweden; 1993.