14
HYDROLOGICAL PROCESSES, VOL. 4,387-400 (1990) HYDROLOGY OF A HEADWATER BASIN WETLAND: GROUNDWATER DISCHARGE AND WETLAND MAINTENANCE NIGEL T. ROULET Department of Geography, York University, 4700 Keele St., North York, Onlario, Canada M3J IP3 ABSTRACT The link between groundwater and surface hydrology in a small headwater drainage basin in the zone of glacial deposition of southern Ontario south of the Precambrian Shield was examined for two years. The basin is situated in a discharge zone of a regional aquifer and contains a small treed spring-fed swamp. The swamp exists because of the groundwater and has little effect on the maintenance of streamflow. Groundwater input to the swamp is an order of magnitude larger than precipitation. Groundwater of local and regional origin passes through the swamp by two routes: surface streamlets, where groundwater that emerges at specific seepage points in the swamp is conveyed over the ground surface with little interaction with the swamp itself, and by diffuse seepage in the swamp and through the bed of the stream. While the diffuse seepage input is the smaller component of groundwater it maintains the swamp’s saturation. Groundwater input to the swamp from the specific seepage points and diffuse flow varies little over a year; therefore the saturation of the swamp and baseflow from the basin display little seasonal variation compared to other wetland types. The existence of the valley bottom in the headwater basin alters the seasonal and storm hydrology and is important to biogeochemical transformation of emerging groundwater. KEY WORDS Wetland Spring-fed swamp Groundwater Baseflow Seepage points INTRODUCTION Groundwater discharge is important to streamflow maintenance in headwater environments (Freeze, 1972). Groundwater discharge zones can be seasonally or permanently saturated depending on the source of groundwater, which is controlled by geology, topography, and material permeability (Freeze and Wither- spoon, 1967). As a result of the persistent surface saturation small wetlands can form in groundwater discharge zones. The surface hydrology of these wetlands can be influenced seasonally if associated with local aquifers (eg. Taylor and Pierson, 1985; Whiteley and Irwin, 1986) or groundwater can be the principal hydrological component of the wetland if it is influenced by intermediate to regional scale systems (Siegel, 1988). In the temperate humid glaciated areas of North America small to medium size wetlands called treed spring-fed swamps are associated with groundwater discharge areas (National Wetlands Working Group, 1988).These wetlands are a common element of the landscape and form a link between terrestrial upland and lotic ecosystems. They are also areas where groundwater and surface water hydrology are inseparable. However, the hydrology of these wetlands has received little previous study, therefore, a research project was initiated to examine the hydrology of a treed, spring-fed swamp in a small headwater basin. This project has three objectives: (1) to determine the role of groundwater in maintaining both swamp saturation and stream baseflow; (2) to establish the vectors of groundwater flow through the swamp; and (3) to examine the stormflow generation in the swamp. The paper addressed the first and second objectives. Hill (1990a,b) examines the link between various flow paths in the basin and biogeochemical transformations. 0885-6087/90/040387 ~ 14$07.00 01990 by John Wiley & Sons, Ltd Received 8 February 1990 Revised 30 April 1990

Hydrology of a headwater basin wetland: Groundwater discharge and wetland maintenance

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Page 1: Hydrology of a headwater basin wetland: Groundwater discharge and wetland maintenance

HYDROLOGICAL PROCESSES, VOL. 4,387-400 (1990)

HYDROLOGY OF A HEADWATER BASIN WETLAND: GROUNDWATER DISCHARGE AND WETLAND

MAINTENANCE

NIGEL T. ROULET Department of Geography, York University, 4700 Keele St., North York, Onlario, Canada M3J IP3

ABSTRACT

The link between groundwater and surface hydrology in a small headwater drainage basin in the zone of glacial deposition of southern Ontario south of the Precambrian Shield was examined for two years. The basin is situated in a discharge zone of a regional aquifer and contains a small treed spring-fed swamp. The swamp exists because of the groundwater and has little effect on the maintenance of streamflow. Groundwater input to the swamp is an order of magnitude larger than precipitation. Groundwater of local and regional origin passes through the swamp by two routes: surface streamlets, where groundwater that emerges at specific seepage points in the swamp is conveyed over the ground surface with little interaction with the swamp itself, and by diffuse seepage in the swamp and through the bed of the stream. While the diffuse seepage input is the smaller component of groundwater it maintains the swamp’s saturation. Groundwater input to the swamp from the specific seepage points and diffuse flow varies little over a year; therefore the saturation of the swamp and baseflow from the basin display little seasonal variation compared to other wetland types. The existence of the valley bottom in the headwater basin alters the seasonal and storm hydrology and is important to biogeochemical transformation of emerging groundwater.

K E Y WORDS Wetland Spring-fed swamp Groundwater Baseflow Seepage points

INTRODUCTION

Groundwater discharge is important to streamflow maintenance in headwater environments (Freeze, 1972). Groundwater discharge zones can be seasonally or permanently saturated depending on the source of groundwater, which is controlled by geology, topography, and material permeability (Freeze and Wither- spoon, 1967). As a result of the persistent surface saturation small wetlands can form in groundwater discharge zones. The surface hydrology of these wetlands can be influenced seasonally if associated with local aquifers (eg. Taylor and Pierson, 1985; Whiteley and Irwin, 1986) or groundwater can be the principal hydrological component of the wetland if it is influenced by intermediate to regional scale systems (Siegel, 1988).

In the temperate humid glaciated areas of North America small to medium size wetlands called treed spring-fed swamps are associated with groundwater discharge areas (National Wetlands Working Group, 1988). These wetlands are a common element of the landscape and form a link between terrestrial upland and lotic ecosystems. They are also areas where groundwater and surface water hydrology are inseparable. However, the hydrology of these wetlands has received little previous study, therefore, a research project was initiated to examine the hydrology of a treed, spring-fed swamp in a small headwater basin. This project has three objectives: (1) to determine the role of groundwater in maintaining both swamp saturation and stream baseflow; (2) to establish the vectors of groundwater flow through the swamp; and (3) to examine the stormflow generation in the swamp. The paper addressed the first and second objectives. Hill (1990a,b) examines the link between various flow paths in the basin and biogeochemical transformations.

0885-6087/90/040387 ~ 14$07.00 01990 by John Wiley & Sons, Ltd

Received 8 February 1990 Revised 30 April 1990

Page 2: Hydrology of a headwater basin wetland: Groundwater discharge and wetland maintenance

388 N. T. ROULET

STUDY BASIN AND BACKGROUND HYDROGEOLOGY

The study basin is located in the headwater area of the Duffin Creek Basin, southern Ontario, Canada (43" 47' N, 79" 15' W: area of 307 km2: Figure 1). The central till plain of the Duffin Creek drainage basin rises to contact the Oak Ridges moraine in the northern portion of the basin. The study basin is situated at the moraine-till plain contact point. The maximum elevation of the moraine is 395 m and local relief is approximately 30 m. The moraine is composed of kame and ice contact, outwash deposits which are up to 70 m thick.

The hydrogeology of the Duffin Creek basin is discussed in detail by Sibul et al. (1977) and Howard and Beck (1986). The Oak Ridges aquifer (Figure 1) in the northern portion of Duffin Creek has an area of 21 8 km2 and a thickness of 15 m. The regional water table slopes towards the south and intersects the ground surface near the study basin. The piezometric pressure of the aquifer is above ground at the base of the moraine.

The study basin area is 1.57 km2 and ranges in elevation from 280 m to 340 m (Figure 2a). There are two first order streams that join and flow to the second order outlet and are referred to as streams lN, lS, and 2M respectively. A small treed swamp of 0.031 km2, or 2 per cent of the study basin area, occupies the outlet stream valley. The wetland contains many diffuse saturated zones; some join together to form small streamlets that convey water across the surface of the swamp to the first and second order streams. The swamp has a dense understory of shrubs and saplings and the forest is composed of hemlock, cedar, and a few isolated white pine. The upper 1.5 m of swamp soil is a peaty histosol, which is underlain by fluvio-glacial sand and gravel deposits. The organic soil layer thins toward the swamp's perimeter at the base of the adjacent hillslopes, while the upland soils are grey-brown luvisols (Hill and Warwick, 1987). Beech-maple stands and fallow fields compromise the uplands.

I

Southern extent of the Oak Ridges Aquifer

Surf ic ia l Geology

0 Kame deposits

Lacustrine deposits

Undifferrenriared glacial deposits

Di i f f ins Creek -44'00' Dranage

Figure 1 . Location of the study basin, and elevation and overburden geology of the northern half of the Duffin Creek drainage basin. The southern perimeter of the Oak Ridges aquifer is also indicated. The location of the Duffin Creek Basin relative to the western end of Lake

Ontario is shown in the inset. Contours are in metres

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HYDROLOGY OF A HEADWATER BASIN WETLAND

7 Stilling wells and water level recorders

389

1 /

-----o--- ----___-- - A -

- - Watershed boundarv

-

Figure 2. Upper: The topography of the study basin and location of the treed spring swamp in the valley bottom. The enclosed rectangle indicates the location of the enlargement display below. Lower: Intensive study area on the treed spring swamp showing the streams,

and location of weirs, groundwater wells, and piezometers

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3 90 N. T. ROULET

METHODS

Measurements of the hydrological variables and parameters were made from June to August, 1986 and May, 1987 to May, 1988. The location of the instrumentation discussed below is shown in Figure 2b.

Precipitation was measured using a tipping bucket rain gauge located in a field 500 m east of the basin outlet. In the basin, a network of 10 non-recording rain gauges was monitored weekly. These measurements were supplemented occasionally by the average precipitation measured at three Atmospheric Environment Services (AES) weather stations, all of which were within a 15 km radius of the basin. Regression analysis between weekly basin precipitation and the average of the three weather stations produced an r2 of 0.94 ( N = 16), standard error of the estimate of 0.1 mm, and a slope of 0-99. Snow on the ground was measured twice each winter along two snowcourses in the basin and daily snowfall was recorded at the AES weather stations. At the site of the tipping bucket rain gauge, air temperature, relative humidity, and wind speed were measured hourly for the computation of swamp evaporation using a simplified form of the Penman-Mon- teith combination model (Munro, 1986). Net radiation from the Scarborough AES station, 35 km southwest of the basin, was used and a canopy resistance of 100 s m- and aerodynamic resistance of 5 s m- (Munro, 1987) were assumed for the swamp.

Thin-plate, 53" V-notch weirs were installed on the outlets of streams lN, 1s and 2M. Water level behind the 2M and IN weirs was measured continuously. Water level behind the 1s weir was measured manually in conjunction with streamlet discharge measurements. Smaller, 30" V notch weirs were installed on all I3 streamlets that discharged into streams 1N and 2M. Streamlet instantaneous discharge was measured bi- weekly in 1986, once a week during the summer of 1987, and periodically from September to December 1987.

Groundwater wells and piezometers were installed in two areas of the basin. Area A is located in the source area of stream 1N and Area C is located down valley near the junction of IN and 1s (Figure 2b). Groundwater wells were made from 5 cm diameter, perforated, ABS pipe. The piezometers comprised a 1.25 cm diameter PVC pipe and a 20 cm long slotted point. Two lines of wells and two, 1 m deep piezometers were installed in Area A. The presence of a hard compact silt layer prevented the installation of deeper piezometers in this area. A row of wells and piezometers were installed in Area C from the southern perimeter of the swamp, through stream 2M to the northern perimeter of the swamp. Wells were installed every 10 m and piezometer nests consisting of depths of 1.0, 1.5 and 2.5 m spaced 5 cm apart, were installed every 20 m. The peizometer nest on and at the base of the hillslope had an additional piezometer installed to 3.5 m. After the head in each piezometer equilibrated, they were pumped and the water level in adjacent piezometers was monitored. If a change in level was observed the piezometer was replaced: this occurred in 3 of 28 piezometers. Water levels in the wells and piezometers were measured every four days after installation in 1986, weekly from May to September 1987, and monthly thereafter. A parallel line of wells and piezometers was installed at the beginning of the project to determine the direction of groundwater flow. Once it was established that groundwater flow could be plotted as a two-dimensional flownet, this row of wells and piezometers was used for the measurement of hydraulic conductivity by the Hvorslev water level recovery method (Freeze and Cherry, 1979).

The perimeter of the swamp, areas of groundwater emergence, and the location and elevation of all weirs, wells, and piezometers were surveyed in late November 1986 and May 1988.

RESULTS

Precipitation and stream discharge During the main period of measurement, extending from May 1987 to May 1988, precipitation fell on 50

per cent of the days and was evenly spread throughout the year (Figure 3). However, over 35 per cent of all events were less than 2 mm. Larger events occurred in late May to August and late November and early December. Total precipitation for the study period was 886 mm; 141 mm or 16 per cent fell as snow. From December to late March snow accumulated on the ground, but there were two mid-winter thaws. A maximum ground snowcover of 27 cm occurred in late February. Calculated evapotranspiration, integrated from May 1987 to May 1988 was 554mm. The long-term average evaportranspiration for this area of

Page 5: Hydrology of a headwater basin wetland: Groundwater discharge and wetland maintenance

HYDROLOGY OF A HEADWATER BASIN WETLAND 39 1

I

J ' J ' A ' s ' 0 ' N ' D

1987

- D \ E E L

z *O 0 F- U

a 10 0 W E a

0

J ' F ' M I A ' h

1988

OUTFLOW DISCHARGE

Figure 3. Study basin precipitation and discharge from May 21, 1987 to May 20, 1988

southern Ontario is between 500 and 600mm yr-' (Ontario Ministry of Natural Resources, 1984). Evapotranspiration was in excess of 100 mm mon-' for May, June, July, and August, and decreased through September (56 mm mon- ') and October (25.1 mm mon-I). It was assumed that no evapotranspiration occurred between November and March.

The mean daily basin discharge was 17.0 k 2.8 L s - l and ranged from 14.2 to 41.9 L s - ' (Figure 3). Maximum and minimum instantaneous discharge was 85.5 and 14.2 Ls- ' , respectively. Peaks in flow occurred in January and March and were caused by melting snow, while many smaller flow peaks resulted from rainfall. Mean daily baseflow discharge, computed for days when streamflow was not influenced by either snowmelt or rainfall (determined by a baseflow recession curve), was 16.0 1.0 L s-'. Maximum baseflow (18.3 L s-') occurred during late spring and late autumn and there were two periods of low flow: one in September (14-5 L s - ') and the other in late February-early March (14-2 L s - ' ) . Mean daily discharge from 1N and 1s was 3.1 k 1.0 and 8.0 1.6 L s-', respectively, based on instantaneous measure- ments. These represent 18 per cent and 48 per cent of total basin discharge. The remaining 36 per cent of the basin discharge originated in the 2M portion of the basin. Mean daily basin runoff was 0.9 mm d- ' and ranged from 0.8 to 2.3 mm d- '. Total annual runoff was 328 mm.

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392 N. T. ROULET

I JUNE8.1987 C13

,121

llo{ c10 c11 I -6 .I09 43

9 1

102{

I I

m 0

Vert ical exaggeration 2X 100 L I I

114 SEPT. 24, 1987

114- SEPT. 24, 1987

112-

110-

108-

I

100 4 1 1 I 10 0 10 20 30 40 50

DISTANCE (m)

Figure 4. Groundwater flownels for three dates, 1987. In the upper cell (June 8, 1987) the head (m) relative to an arbitrary datum are indicated. In the middle cell the location of the two points where groundwater discharges to the surface are shown

Page 7: Hydrology of a headwater basin wetland: Groundwater discharge and wetland maintenance

HYDROLOGY OF A HEADWATER BASIN WETLAND 393

E

Groundwater discharge pattern Flownets showing the distribution of hydraulic head in the swamp were produced for each month of the

year. Three flownets representing spring, late summer-early autumn, and mid-winter are presented in Figure 4. The gradient in hydraulic head clearly indicates that the wetland is situated in a groundwater discharge zone. The direction of flow near the stream is vertical and at the slope-wetland contact it switches from horizontal in summer to more vertical in the winter.

The hydraulic conductivity of the organic layer was high, over 10 m d-', and less than 0.05 m d- ' for most of the underlying substrate with the exception of two locations where the conductivity was one to two orders of magnitude greater (Figure 5). The sediments and glacial material a t the latter points lack the fine fraction found under the rest of the swamp (A. R. Hill, personal communication). The areas of high hydraulic conductivity also correspond to the points of permanent groundwater seepage.

Groundwater flow was computed using the measured hydraulic conductivity and the flownets. The mean daily groundwater discharge along the Area C transect (65.5 m in length) for the period May to January was 2.64 f 0.35 m3 d-'. Discharge through the two points of higher conductivity accounted for 98 per cent of this flow (Figure 5). The emergence of groundwater in isolated patches in the swamp is common. In many of the saturated areas near the base of the hillslopes percolating water can be seen, while in several saturated areas of the swamp there are deposits of iron oxide. When the deep groundwater, which is low in dissolved oxygen (Hill, 1990a) emerges reduced iron is oxidized. The maximum daily groundwater discharge from the transect of 3.05 m3 d- ' occurred in September and the lowest of 1.87 m3 d- ' occurred in January. Groundwater discharge was probably lower in February and March since the outflow discharge was smaller (see Figure 3). No flownets were derived for this period. The mean daily groundwater discharge divided by the length of the transect yielded a mean daily depth of runoff through the swamp of 44.3 f 5.8 mm.

The lack of any significant seasonal variation in groundwater produced a reasonable stable water table in the swamp (Figure 6). The swamp can be divided into two zones based on the persistence of the water table intersection of the wetland surface. Wells C9, C12, and A2 (see Figure 7) are located beside saturated zones and represent points where groundwater emerges continuously. The other wells, C7, C8, C10, and A l , represent the water table over most of the swamp. The water table elevation in these wells is relatively constant through the summer, rises slightly in autumn, and drops from December through to spring

Q 104- z l-l

00 007 .O 002

00 002 .a 001

00 469

a 0 1

- 102- Q >

U -1

I- - l l O \

-

Vert ical exaggeration 2X

Page 8: Hydrology of a headwater basin wetland: Groundwater discharge and wetland maintenance

394 N. T. ROULET

1987

u" 0.11 a

1988

l i

5 v) 0.24

1987

0

0

0 * 0.4

1988

Figure 6. The water table elevation relative to the ground surface for five wells in Area C of the swamp (see Figure 2 for location of wells). The solid lines indicate wells where frequent measurements were made, while the dashed lines represent a less intensive sampling regime

snowmelt. The exception was well C7, where the water table continued to rise throughout the winter, presumably due to an increased lateral flux near the edge of the stream. Well A6 represents the water table beyond the swamp's perimeter.

Limited piezometric data were obtained from Area A. The trend in head at a depth of 1.0 m in Area A was similar to that observed near the stream in Area C in the summer and fall, but differed with increased head in December and January (Figure 7). This portion of the basin is a narrow valley which is oriented diagonally to the major axis of groundwater flow. In contrast, Area C is wider and orientated perpendicular to the groundwater flow. The rise in head in Area A may have resulted from local groundwater flow, while during the same period Area C piezometers are being influenced more by intermediate and regional flow.

Transport of water over the surface of the wetland The groundwater that emerges in the swamp sustains surface saturation and provides water to the

streamlets. With the exception of the area where groundwater actually feeds the streamlets, the streamlets do not intersect the water table (Figure 8). The beds of the streamlets are covered with a thin layer of fine sediments. The course of a streamlet changes in time and some streamlets pass underground in old root cavities, reemerging several metres downslope in the swamp.

In order to assess the amount of water conveyed to the stream by streamlets, discharge from IS was subtracted from total basin discharge to obtain the streamflow that originated from the zone where streamlet discharge was measured. The mean 2M-1S instantaneous discharge in 1986 on days when streamlet discharge was measured was 8.4 L s- '. The mean total streamlet discharge for the same period was 5.0 L s I ,

representing 60 per cent of the streamflow. In 1987 mean total 1N and 2M discharge was 9.2 L s- ' , while total streamlet discharge was 4.9 L (52 per cent of the streamflow). The absolute streamlet discharge varied little throughout the summer and fall, but as a proportion of 1N and 2M discharge varied 10 per cent (Figure 9). This proportion increased in early August and October to December. The first increase resulted from a decrease in basin discharge, while streamlet discharge remained constant. The second period resulted from a small increase in streamlet discharge which was related to the change in ground-water flow in early winter (see Figure 5). This analysis indicates that between 10 and 40 per cent of the groundwater reaches the stream as diffuse subsurface flow.

While streamlet discharge showed little temporal variation over the period of study, with the exception of very short lived response to rainfall, the ratio of streamlet to stream discharge increased substantially in the downstream direction (Figure 9). Groundwater conveyance by streamlets in 1N was much smaller (0.16) than that in 2M (0-67).

Page 9: Hydrology of a headwater basin wetland: Groundwater discharge and wetland maintenance

HYDROLOGY OF A HEADWATER BASIN WETLAND 395

0.6-

0.5- 0

>" - 0.4- - E

d d 0.3-

I- a

I-

ZG a 2 0 0.2-

2, 0.1 -

z I- a -I W

0.0

1.0 ' I I 1 1 I I I I I I I . . 0 PI . P2

. 0 . . .. . .

0 . . . .. 0 . .

O 0 O O 0 0

0

0 0 0 0

1

0

0 0

Figure 7. The water table elevation and head relative to the ground surface for three wells and two piezometers in Area A of the swamp (see Figure 2 for location of the wells and piezometers). Well A1 is at the stream head and A6 is on the hillslope. P1 is beside well A1 and

P2 is located on the swamp 30 rn toward the slope

0

- E

0

a: 3 m

- 0.1

2

9 0.2

8 3

c7 P 0

m I a D

0.3

i-

W

0.4

June G July 8 Aug. 18

Water table

I

1 I I !

2 i 4 5 DISTANCE (m)

Figure 8. The water table elevation on three dates during 1987 relative to the water surface of a streamlet (S4)

Page 10: Hydrology of a headwater basin wetland: Groundwater discharge and wetland maintenance

396 N. T. ROULET

Figure 9. Streamlet discharge for the period May to December 1987. Upper: Total instantaneous streamlet discharge (Q,$) and 2M ~ 1s stream discharge (Qse). Middle: The ratio of total instantaneous streamlet discharge to 2M-1.5 stream discharge. Lower: The ratios of

instantaneous streamlet discharge and stream discharge for the 1N basin and the 2M portion of the stream

Page 11: Hydrology of a headwater basin wetland: Groundwater discharge and wetland maintenance

HYDROLOGY OF A HEADWATER BASIN WETLAND 397

In the area where the piezometer network was located, the streamlets and the saturated and groundwater seepage areas were surveyed (Figure 10). The total width of groundwater seepage areas at the base of the hillslope was 15.8 m, while in mid-swamp they were 13 m wide. The 1987 mean daily groundwater discharge from these two areas was approximately 39.2 m3 (calculated as groundwater discharge per linear metre of emergence [see Figure 51 x the width of emergence zone) and the mean daily discharge from the two streamlets, S4 and S5, that drained these emergence zones was 44.0 m3.

DISCUSSION AND CONCLUSIONS

Until recently there have been few process studies of wetland-ground water linkage (ck Carter, 1986; Siegel, 1988) and wetland-groundwater discharge and recharge relationships were speculative. The results of this study show that the treed spring-fed swamp saturation is sustained by a large groundwater input and probably exists because of that input. The results demonstrate that the swamp itself has little impact on stream baseflow. This is contrary to the 'popular' belief of the hydrological function of swamps (see Carter, 1986 for review). Transpiring vegetation has a small effect on the magnitude of groundwater reaching the stream during the dry season. However, Munro (1987) found that treed swamps evapotranspired at near equilibrium rates, and it is likely that the evapotranspiration loss from the non-wetland forests would be not much different. The influence of evapotranspiration can be deduced by comparing changes in the groundwater flow nets, swamp water table elevation, and streamflow over the year. Streamflow decreased slightly through spring and summer and increased in the fall, hydraulic head, water table elevation, or magnitude of precipitation changed little over the entire period. The only significant change was after leaf fall in late September. In contrast, the mid-winter decrease in streamflow was associated with reduced hydraulic gradients at the base of the hillslope.

An annual water balance (May 1987-May 1988) was derived to assess the relative magnitude of basin groundwater flow and groundwater passing through the swamp. Measured precipitation (886 mm) almost equalled the sum of basin runoff (340 mm) and evapotranspiration (554 mm). This indicates that the basin is of sufficient size for the volume of groundwater discharged to be replenished locally, but the chemistry of the groundwater emerging in the swamp (Hill, 1990a, b) suggests that some of the groundwater is from a system with longer residence times. Shallow groundwater (CI- of between 1-2 and 1.8 mg L-' and mean 6 "0 of - 11-56 f 0.34°/0,) emerges at the perimeter of the swamp, while water from a deeper source (Cl- of between 0.6 and 0.9 mg L- ' and mean b "0 of - 12.08 f 0.08°/00) emerges in the central portion of the swamp. Temporal changes in the concentration of NO3 and NH, in the groundwater emerging at the perimeter of the swamp and the cation, dissolved oxygen and reduced iron concentrations of groundwater upwelling beneath the centre of the swamp also indicate a portion of the groundwater is of regional origin. The Oak Ridges aquifer provides the regional groundwater, while the local groundwater is from the basin hillslopes. The implication of both local and regional groundwater discharging in the basin and closure of the basin's surface water balance is that some of the precipitation incident on the basin recharges the groundwater aquifer and later discharges beyond the basin perimeter.

Since all the baseflow derived from groundwater passes through or beneath the swamp before it enters the stream, the water balance can be recalculated using the surface area of the swamp to estimate net groundwater flow through the swamp. This net flow was 17 294 mm yr-' or on average 46-5 mm d-'. The unit area runoff from any spring-fed swamp is a function of the volume of groundwater discharge and the size of the swamp. The surface area of the study basin swamp is very small relative to the volume of groundwater discharge, but calculating groundwater input on a per length of stream basis gives an average of 0.035 L s - l m-l, which is comparable to the input reported by Calles (1985) for a similar basin in Sweden. The groundwater component for a larger (0.35 km') groundwater fed swamp in southwestern Ontario, for the period of June to November, was approximately 800 mm or 4.4 mm d-' (calculated from data presented in Whiteley and Irwin, 1986). If the study swamp was of a similar size the groundwater component passing through the swamp for the same period of the year would be approximately 766 mm. As the area of the swamp increases, however, it is unlikely the groundwater input would be as homogeneous as was observed in this study.

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398 N. T. ROULET

Figure 10. Plain view of a portion of Area C of the swamp, indicating the areas of saturation and zones of groundwater seepage

Page 13: Hydrology of a headwater basin wetland: Groundwater discharge and wetland maintenance

HYDROLOGY OF A HEADWATER BASIN WETLAND 399

The 46.5 mm d - l calculated above is close to the 44.3 mm d-’ calculated from the flownet analysis. Even though these values were derived independently they are both sensitive to errors and the absolute values should be treated with caution. Area C transect was originally selected because it appeared to be representative of the average basin condition. The above comparison confirms this assumption.

The large and relatively constant influx of groundwater to the swamp reduced the temporal extremes of wetland saturation normally observed in swamps. Average change in water table was less than 0.10m. Annual changes of 0.20 m in riverine (Woo and Valverde, 1981) and 0-50 m in perched water table swamps (Taylor and Pierson, 1985) have been observed. Only a change in regional groundwater flow would significantly reduce the water table in the study swamp. Such a change could be produced by reduced aquifer recharge or increased groundwater extraction.

The persistent and large input of groundwater has led to the development of the surface streamlets. Two independent calculations revealed that on average, over half the groundwater entering the swamp was transported over the swamp’s surface, but this proportion ranges from less than 20 per cent to over 70 per cent in the downstream direction. The streamlets are not connected to the groundwater except at seepage points, resulting in a small area of the surface being saturated. The short residence time of the emergent groundwater in the streamlet network is important in biogeochemical processing of elements (Hill, 1990a, b) and stormflow production (Roulet, 1989). While the streamlets’ courses shift over time, the surface area of the swamp covered by streamlets should remain constant if groundwater discharge does not change significantly. Direct seepage through the organic soil decreased in relative importance as a mode of conveyance of groundwater in the lower portion of the basin, but its absolute flux of direct seepage remained constant and is critical to the maintenance of swamp saturation essential for peat development.

The existence of small spring-fed swamps is not unique to southern Ontario. In the northeastern United States, Carter and Novotzki (1 988) found a close correspondence between permanent groundwater discharge zones predicted by a groundwater flow model (F. P. Lyford, U.S.G.S.) and small valley bottom wetlands. Where seasonal groundwater discha.rge was predicted there was an absence of wetlands.

ACKNOWLEDGEMENTS

I would like to thank Adrian Renzetti, Rhonda Bateman, Rosemary Ash, and Neil Comer for their assistance in the field. Dr. Alan Hill and M. Waddington, K. Devito, K. Outerbridge, and two anonymous referees provided helpful criticisms of an earlier draft of this manuscript. Permission to use the basin was kindly granted by the Metropolitan Toronto and Region Conservation Authority. This research was supported by an NSERC Canada Operating Grant, an NSERC Canada Equipment Grant and York University President’s NSERC, and Faculty of Arts Research Grants. This paper was written while the author was on leave with a York University Faculty of Arts Research Fellowship.

REFERENCES

Calles, U. M. 1985. ‘Deep groundwater contribution to a small stream’, Nordic Hydrology, 16, 45 54. Cartcr, V. 1986. ‘An overview of the hydrologic concerns related to wetlands in the United States’, Can. J. Bot., 64, 364-374. Carter, V. and Novitzki, R. P., 1988. ‘Some comments on the relation between ground water and wetlands’, in Hook, D. D. (Ed.) The

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