Contrasts in storm event hydrochemistry in an acidic afforested catchment in upland Wales

Journal

Journal of Hydrology 170 (1995) 159-179

Contrasts in storm event hydrochemistry in an acidic afforested catchment in upland Wales C. Soulsby Department

of Geography,

University of Aberdeen,

Aberdeen AB9 ZUF, UK

Received 5 August 1994; revision accepted 23 November 1994

Abstract The hydrochemistry of stream water in an acidic afforested catchment in the Welsh uplands was monitored routinely between 1985 and 1990. Nineteen storm episodes were sampled intensively during this period. Although the general storm response of the stream can be characterised by increased concentrations of H +, Al and dissolved organic carbon (DOC), and a dilution of Ca and SiO,, the detailed hydrochemistry of individual acid episodes exhibited marked contrasts. The minimum pH reached during specific episodes ranged from 4.1 to 5.0, and peak dissolved Al concentrations varied from 9 to 44 pmol 1-l. The reasons for such differences in the hydrochemical response can be identified for each individual episode by examining the complex interactions between (1) the quantity and quality of event precipitation, (2) antecedent patterns of weather and atmospheric deposition and (3) the hydrological processes which dominate the storm runoff response. The dynamic nature of catchment hydrology was found to exert a particularly strong influence on the hydrochemistry of specific acid episodes.

1. Introduction Storm events are associated with the episodic acidification of stream water in acidsensitive catchments in upland Wales (Ormerod and Jenkins, 1994). Hydrochemical changes during such episodes are usually considered to be relatively consistent, with both stream water acidity and inorganic Al concentrations increasing, and Ca concentrations decreasing, with discharge (Neal et al., 1992). These changes in water quality are particularly acute in afforested catchments and have been implicated in the degradation of macroinvertebrate communities and salmonid fisheries (Weatherley and Ormerod, 1991). The generation of acid, Al-rich storm runoff has been explained by the dominance of hydrological pathways in the acidic surface horizons of the catchment soils (Soulsby and Reynolds, 1993). In contrast, 0022-1694/95/%09.50 0 1995 - Elsevier Science B.V. All rights reserved SSDZ 0022-1694(94)02677-7

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C. Soulsby / Journal of Hydrology 170 (1995)

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basellows are sustained mainly by groundwater in slope drifts and bedrock which is well buffered and Ca rich (Reynolds et al., 1986). Despite this general picture, a recent review of data from experimental catchments throughout Europe highlighted the fact that the hydrochemistry of acid episodes within a particular catchment can be highly variable (Davies et al., 1992). This reflects the interaction of: (1) antecedent conditions, both in terms of atmospheric deposition and climate, (2) event characteristics, including both precipitation mechanisms and deposition quality, and, often of greatest importance, (3) the hydrological pathways involved in storm runoff generation. The Llyn Brianne Acid Waters Project in upland Wales included a storm event sampling programme in the afforested LIl catchment between 1985 and 1990 (Gee, 1990). This catchment has been studied extensively: the general ecological and hydrochemical characteristics of the stream have been described (Stoner et al., 1984; Bird et al., 1990), the land use, geology and soil cover are known (Reynolds and Norris, 1990), and some of the key hydrological processes generating storm runoff have been identified (Soulsby, 1992; Soulsby and Reynolds, 1993). Nineteen storm events, often with contrasting hydrological and hydrochemical characteristics, were monitored during the study. This paper outlines and explains some of these differences in storm event chemistry.

2. Site description The LIl catchment is located in the headwaters of the Afon Tywi 3.5 km from the west coast of Wales (Fig. l(a)). The catchment covers 2.5 km* and spans an altitudinal range from 320 to 500 m (Fig. l(b)). Commercial afforestation occurred between 1961 and 1964, with Sitka spruce (Pica sitchensis) being the dominant species planted. Mean annual precipitation is over 2000 mm, with rainfall being highest in winter when frontal systems dominate. The climate is mild, with mean monthly temperatures ranging from 3.2”C in February to 14.8”C in August. On average, snowfall occurs on 31 days per year. The catchment is underlain by Lower Palaeozoic mudstones and shales. These are dominated by base-poor siliceous minerals, though small mineralised veins can contain sources of calcite (Mackie and Smallwood, 1987). The solid geology is covered by superficial drifts; coarse angular shale covers the steeper slopes and fine-textured colluvium dominates concave slopes (Pyatt, 1977). Soil distribution reflects topography and parent material: brown podzolic soils and stagnopodzols cover the freely draining steep slopes, whereas peat and stagnohumic gley soils dominate the more gently sloping interfluves (Fig. l(c)). The soils are acidic (pH 3.5-4.5) with a low base saturation (less than 20%). All but the steepest slopes were ploughed prior to afforestation, and the peat and gley soils were intensively drained.

3. Field monitoring programme and laboratory analysis Precipitation

in the LIl catchment

was measured

by a network

of rain gauges, and

C. Soulsby 1 Journal of Hydrology 170 (1995)

159-179

161

(b)

??

.

Gaugingstation Throughfall and stemflow

ccdectors

Stagaohumic gky plot 0 Stagnopodzolplor A

3 4 5 6 9 11

Fig. 1.Maps showing (a) location and (c) catchment soils.

Brown podzolk Intergrade between 3 & 5 Ferric stagnopodzol Ironpan stagno odzol Cambic stagno Eumic gley Peat

of Llyn Brianne and LIl, (b) catchment

topography

and instrumentation,

bulk precipitation chemistry and meteorological conditions were monitored at an Automatic Weather Station (AWS) 4 km to the south (Fig. l(a)). Flows from the catchment were measured between 1985 and 1990 using a pressure transducer to record stage in a rated natural section. Stream water spot samples were collected at fortnightly intervals during this period for chemical analysis. An automatic water sampler was deployed at the gauging site for storm event monitoring. Sampling intensity was pre-programmed and ranged from 15 min to several hours. The frequency of sampling was set to be highest on the rising limb of the hydrograph and at the storm peak. The sampler was triggered by a tipping bucket rain gauge at

C. Soulsby / Journal of Hydrology 170 (1995) 159-179

162

rainfall intensities exceeding 4 mm h-l. Event samples were returned to the laboratory within 1 week for analysis. Throughfall and soil water samples were also collected in the catchment (Fig. l(b)); details of the sites and sampling methods, together with a full description of analytical techniques used in the laboratory, have been given by Soulsby and Reynolds (1992, 1993).

4. Chemistry of atmospheric inputs, soil water and groundwater Precipitation at Llyn Brianne is generally acidic, with a mean pH of 4.98. The prevailing westerly weather systems dictate that it is usually dominated by ions of marine origin (Table 1). Pollutant ions such as non-marine SO4 and NO3 are also important, particularly during easterly winds (Donald et al., 1990). When it passes through the tree canopy as throughfall, precipitation is acidified and enriched in most solutes, mainly as a result of the washing out of dry and occult deposition (Hornung et al., 1990). Soil water in the catchment is acidic and Ca deficient, and atmospherically derived solutes dominate the ionic composition (Table 1). Acidity is highest in the upper horizons, where high dissolved organic carbon (DOC) concentrations reflect a source of organic acidity. Aluminium concentrations are highest in the mineral horizons where Hf ions are consumed in weathering and cation exchange reactions which mobilise inorganic Al ions (Neal et al., 1989; Soulsby and Reynolds, 1992). Soil water chemistry is spatially variable; for example, 0 horizon water in stagnohumic gley soils is acidic and Al bearing, whereas overland flow from peat has a similar pH but much lower Al concentrations. Soil water chemistry also exhibits marked temporal variation owing to the interactive effects of atmospheric deposition, biological processes and soil-water interactions (Soulsby and Reynolds, 1994). Table 1 Mean chemistry

of bulk precipitation, pH

Al (pmol-’

throughfall,

1)

Ca (pequiv

soil water and groundwater

1-l)

Cl (pequiv

in the LIl catchment

so4

1-l)

(pequiv

1-i)

DOC (mg 1-l)

SiOl-Si (mg 1-l)

Precipitation Throughfall

4.98 4.37

1 2

41 66

89 344

53 181

1.2 6.1

0.2 0.2

Gley soil water 0 horizon E horizon B/C horizon

3.69 3.93 4.08

37 62 63

50 35 26

306 215 259

176 126 122

8 5 4

2.53 2.31 2.00

Peat soil water Overland flow

3.8

4

18

298

190

13

0.20

5.25

12

223

264

94

4.8

2.83

4.6

36

52

249

215

1.3

1.79

Perennial groundwater Transient groundwater

C. Soulsby / Journal of Hydrology I70 (1995)

163

159-I 79

Groundwater chemistry is also spatially variable; samples taken from a perennially saturated zone in fine-textured drift had higher pH levels and Ca concentrations than acidic, Al-rich groundwater sampled from a zone of transient saturation in shattered bedrock (Table 1).

5. General characteristics of stream water chemistry The average stream water chemistry in the LI 1 catchment is acidic and Al bearing, Table 2 Arithmetic mean chemistry (,uequiv 1-l) of all flows (weighted flow in the LIl catchment 1985-1990 All flows 4.79 (4.46) 4. I-6.4

mean in parentheses),

storm

stormflow

and base-

Baseflows

flows

PH

Mean Range

Al (pm01 1-l)

Mean Range

17 (22) 0.2-44

24 0.1-18

Ca

Mean Range

66 (58) 15-165

51 18-98

99 66-135

Mg

Mean Range

62 (63) 17-124

61 17-97

85 56-113

Na

Mean Range

208 (213) 71-400

208 71-382

236 126-324

K

Mean Range

7 (9) 2-35

10 3-25

4 3-6

Mean Range

146 (146) 49-344

153 49-259

146 58-198

Mean Range

241 (246) 85-457

232 84-403

294 197-400

Mean Range

19 (26) 4-100

so4

Cl

NO3

4.36 4.1-5.1

39 7-100

5.57 4.9-6.2 7 9-39

9 5-14

Mean Range

3 (6) 0.7-38.4

9 0.7-31

1.8 0.7-3.6

Mean Range

2.03 (1.33) 0.79-2.99

0.9 0.7-0.9

1.69 1.4-2.0

DOC (mg 1-l)

Mean Range

1.97 (2.29) 0.2-8.1

2.68 1.2-6.9

1.02 0.6-2.0

Mn (pm01 1-l)

Mean Range

2 (5)
5 2-8

3.6 0.1-6

Fe (pm01 1-l)

Mean Range

1 (2) < 0.05-8

1.5 0.4-4

0.9 0.1-1.6

NH4 SiOl-Si

(mg 1-l)

a

.---

. _I.

.

.

Fig. 2. Variation

.1-1111 . 1..

.

I

.I

of pH and dissolved

flew0

.

PH

~._

‘-:

. .

_I

.__.__

lb 0

Al(tot)

._._

..~

~_.

Al with flow in LIl in (a) spot samples and (b) spot and event samples.

._

Al(tot)

.

C. Soulsby 1 Journal of Hydrology 170 (1995) 159-179

165

and the ionic composition is dominated by atmospherically derived solutes (Table 2). The chemical composition of storm flow is consistent with soil water sources of storm runoff generation, being enriched in H +, Al, DOC, Fe and Mn, whereas baseflow chemistry implies the dominance of groundwater sources that are enriched in Ca, Mg and SiOZ. Ions that are primarily derived from the atmosphere (Na, Cl and Sod) or that are strongly influenced by biological activity (NOs, NH4 and K) showed no consistent variation with flow. These characteristics are similar to those described for other afforested catchments in upland Wales (Reynolds et al., 1989; Stevens et al., 1989). The major sources of storm runoff have been identified in the LIl catchment, and variation in hydrological pathways explains much of the flow-related variation in storm hydrochemistry. Overland flow from the extensive area of peat contributes acidic storm runoff by saturation overland flow (Soulsby, 1992). Lateral flow in the upper soil horizons of the stagnohumic gley and brown podzolic soils provides a further major source of acid storm runoff which is also Al bearing (Chappell et al., 1990; Soulsby and Reynolds, 1993). The stagnopodzols which cover the remainder of the catchment are characterised by lateral flow at the soil-bedrock interface, where slower subsurface storm flow contributes Al-rich acid runoff to the recession limb of the hydrograph (Soulsby and Reynolds, 1992). After storm events, these rapid hydrological pathways gradually decrease in importance and well-buffered groundwater provides an increasingly dominant source of stream flow (Robson and Neal, 1990). Despite the usefulness of this conceptual model of catchment hydrology, it becomes apparent, when the storm event chemistry data are examined in detail, that the flowrelated variation in stream water chemistry is complex. This is evident when the concentration-flow relationships for individual species are plotted both using the spot sample data alone and then combined spot and storm event data. The spot sample plots for pH and Al exhibit relatively simple curvilinear relationships (Fig. 2(a)). These break down when storm event samples are included in the analysis, and the data show significantly increased scatter (Fig. 2(b)). This reflects the substantial variability in the responses of individual storm events and the hysteresis of individual chemical species during the storm hydrograph (Webb and Walling, 1992).

6. Storm event hydrochemistry 6.1. Characteristics

of 19 storm episodes

The variability in the hydrochemical response of the LIl catchment becomes increasingly clear when the changes observed during the 19 monitored storm events are examined individually (Table 3). In general, pH drops with increasing flow (Fig. 3(a)); this feature is seen in both pre-event flows and peak flows, though some variation exists. The exception to this was the largest event sampled (Event 5) where pH actually rose from 4.4 before the event to 4.7 at the peak discharge of

influence.

4 Mar. 86 20 Mar. 86 3 Nov. 86 13 Nov. 86 29 Dec. 86 25 Mar. 87 30 May 87 27 Jul. 87 3 Feb. 88 10 Jul. 88 18 Aug. 88 11 Nov. 88 14 Mar. 89 1 Apr. 89 7 Apr. 89 11 Apr. 89 26 Jun. 89 9 Nov. 89 11 Feb. 90

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

a Snowmelt

Date

60.4a 13.9 10.9 37.9 101 55.8a 16.0 22.5 22.0 38.8 40.0 33.9 55.0 13.8 ll.8a 31.2 34.5 30.5 29.0

(mm)

Precipitation

of 19 storm events monitored

No.

Table 3 Characteristics

E 648 201 189 583 1636 308 263 174 450 450 802 331 657 149 102 520 336 513 368

P 17 80 103 201 200 198 20 33 239 118 133 43 291 42 40 101 29 314 197

Flow (m3 s-l)

5.4 4.8 4.8 5.2 4.4 4.4 5.5 5.5 4.5 4.6 5.1 5.1 4.7 4.9 4.6 5.0 6.2 5.0 4.7

P

PH

in the LIl catchment:

4.1 4.3 4.5 4.4 4.7 4.2 4.7 4.5 4.5 4.2 4.3 4.4 4.3 4.6 4.5 4.3 5.0 4.2 4.4

E 4 14 20 22 17 27 4 4 24 15 7 1 21 19 21 13 2 6 29

P 39 22 25 23 9 32 22 17 27 21 18 44 25 27 22 27 14 27 35

E 97 57 61 55 43 56 96 95 50 74 83 95 52 73 59 68 140 97 51

P

$quiv

data summarize

Al (pm01 1-l)

hydrochemical

47 45 50 50 18 48 71 53 49 49 34 39 41 49 51 46 85 54 45

E

1-l)

pre-event

153 125 122 121 95 171 189 149 128 213 131 149 124 147 145 147 190 155 108

P 259 152 130 112 49 189 161 118 136 178 96 299 99 124 136 109 143 116 127

E

(P) conditions

226 226 310 254 198 198 198 198 310 198 254 254 268 293 237 254 302 336 395

P

Cl (pequiv

254 197 282 226 85 169 169 113 282 169 141 169 310 243 229 392 189 223 457

E

1-l)

1.2 0.9 1.0 1.0 1.1 2.5 0.6 1.2 1.6 3.6 0.7 1.0 1.1 1.1 2.3 1.0 1.5 1.0 1.8

P

(mg 1-l)

DOC

2 5 g =; o 2 5 z ? 2

8.1 3.6 1.5 5.3 5.6 5.9 2.4 3.3 2.7 2.9 4.0 5.6 1.1

: g

E $ ’

g

3.3 4.0 3.3 3.2

3.4 2.5

Eel

and those at the event peak (E)

C. Soulsby / Journal of Hydrology 170 (1995) 159-179

a

167

PH

m

b

Al(tot)

-1

40

-.---

35

----_L---___--_-----___

4 . __._.

20 0

_...

.

0

200

400

-

Fig. 3. Relationhip between monitored storm events.

_

stream

600

iom lIe+V iw

600

Pra-wanl

A

mm

1400

1600 1600

Peek

flow and (a) pH, (b) dissolved

Al and (c) Ca before and during

the 19

168

C. Soulsby / Journal

of Hydrology 170 (1995) 159-I 79

1600 1 s-l. The low antecedent pH reflects the high pre-event flows, which were greater than the peak flow of some of the other events. The high pH at the peak flow probably reflects the direct influence of rainfall on stream water chemistry, a process which is not apparent in most of the smaller events (see below). Aluminium concentrations in both pre-event and peak flows generally increase with discharge (Fig. 3(b)). However, the response shows increased scatter, especially at high flows. Like pH, the largest event was exceptional, having very low Al concentrations, again consistent with a picture of rapid rainfall movement to the stream. Calcium exhibited a more consistent response and was highest in low pre-event flows, whereas concentrations at the storm peak decreased with event magnitude, most notably in the largest event (Fig. 3(c)). Concentrations of DOC increased with flow in most individual episodes, but antecedent levels and the subsequent increase varied owing to the biological controls on DOC release (Chapman et al., 1993). The major anions SO4 and Cl showed no clear relationship with flow. Although the general responses of pH, Al and Ca are consistent with a conceptual hydrological model that predicts groundwater sustaining baseflows and soil water dominating storm runoff, the variability observed during individual events is indicative of the complexity of the processes governing storm flow chemistry. Clearly, both the magnitude of individual events and antecedent climatic conditions strongly influence the magnitude and nature of the storm response. In addition, atmospheric deposition before and during the event can also have a major effect on stream chemistry both during the episode and afterwards. For example, high levels of seasalt deposition can produce high Cl concentrations in storm runoff (Events 13, 16 and 19), whereas SO4 concentrations can increase following periods of easterly winds (Events 1 and 12) when pollutant deposition is greatest (Langan, 1989; Donald et al., 1990). High inorganic anion concentrations in stream water can influence the severity of individual acid episodes if a ‘mobile anion’ effect occurs in the catchment soils (Hendershot et al., 1991). An empirical relationship was observed between Al (the dominant exchangeable cation in the catchment soils) and the total inorganic anion concentration in storm runoff (r = 0.841; P < 0.001). The relationship between H+ and total inorganic anion concentrations was weaker (r = 0.435; P < 0.05). The increase in Al with total inorganic anion concentrations in storm runoff is consistent with the soil cation exchange reactions described by the mobile anion effect, and probably explains some of the scatter in Al concentrations at peak flow in Fig. 3(b). However, this process seldom operates over the time-scale of an individual event; indeed, only Events 1 and 16 exhibited a clear rise in the concentration of one or more mobile anions that was matched by a rise in Al concentrations. Thus, water stored in the catchment appears to be conditioned by mobile anion deposition prior to events. The weaker relationship with H+ ion reflects the complex controls on stream pH, including the unclear influence of organic acids (Driscoll et al., 1989).

6.2. Responses of spec$c

storm events

Detailed examination of the response of individual storm episodes illustrates more fully the complex

chemical species during specific interaction of the factors that

169

C. Soulsby / Journal of Hydrology 170 (1995) 159-l 79

regulate storm event chemistry. Four events of particular to emphasise the extreme variability that can occur.

interest

are examined

here

6.2.1. 14 March 1989 (Event 13): a moderate winter event with wet antecedent conditions The soils of the LIl catchment were wet during February and March 1989 as a result of monthly precipitation totals being 20% and 41% above average, respectively. A rapidly moving frontal system produced 55 mm of rainfall on 14 March; this resulted in a marked catchment response. Pre-event flows were high owing to the wet antecedent conditions (Fig. 4). Stream pH dropped 0.4 units over 8 h as flows increased. Concentrations of Al were initially high and increased further during the recession. Chloride, the dominant anion, increased at the hydrograph peak but rapidly returned to pre-storm levels. In contrast, SO4 and NO3 declined at the peak but subsequently increased. Initially, Na increased slightly but then declined, whereas concentrations of both Ca and Mg decreased. Concentrations of SiOZ declined markedly at peak flow but DOC increased. Because of increased drainage of soil waters, owing to the wet antecedent conditions, Al concentrations were high and pH was low prior to the event. The low pre-event DOC concentrations and high Si02 imply that drainage from the deeper soil horizons was dominant. The lowering of pH and the increase in Al at the storm peak show that increasing volumes of storm runoff were derived from the soil zone; the rise in DOC and decrease in SiOZ indicate the importance of hydrological pathways in the upper organic soil horizons. The increase in Cl concentrations at the storm peak reflects high sea-salt concentrations in the frontal rain of 14 March, when Cl concentrations were 5 16 pequiv 1-l (see Table 1). The rise in Cl is consistent with a direct influence of rainfall on stream water chemistry, probably as a result of rapid overland flow from saturated peat. However, the Cl increase is modest, and recent tracer studies in the catchment have shown that even when ‘new’ event water has a marked impact on stream chemistry, pre-event ‘old’ water accounts for over 75% of storm runoff during such moderate events with wet antecedent conditions (Soulsby, 1995). 6.2.2.28 December 1986 (Event 5): a large winter event with wet antecedent conditions The highest flows sampled during the study period were generated by a rainfall event over 28-30 December 1986, when 101 mm fell on the LIl catchment. This high rainfall occurred on a wet catchment (precipitation totals for November and early December were 109% and 40% above average, respectively) and produced a highmagnitude event (Fig. 5). High antecedent flows were acidic with relatively high Al levels. As flows began to rise, the pH initially declined and Al concentrations increased as expected. However, as flows increased further, pH began to increase and Al concentrations fell. Concentrations of Cl, Sod, Na, Ca, Mg and Si02 were halved at the event peak, whereas concentrations of DOC also declined. Many features of the hydrochemistry of this event, including the pH increase, decline in Al and exceptionally low concentrations of all other major solutes, were unprecedented during the study period. These chemical changes are consistent with

C. Soulsby / Journal of Hydrology 170 (1995)

170

159-l 79 PH

Flow and rainfall lcor- 8

PH.5

‘Ooo’b SC0

-4.9 -4.8 -4.7 .4.6 .4.5 4.4 -4.3

ax--

-4.2

loo0-i 0

-~5,

. 10

~-15

-4.1

~-~ 1 20

’ 25303540

’ 45

50’ 7

hours

-

Fbw

??

wa)

-Fkw

A

SO4

-

Cl

??

NW

SiandDOC

Fig. 4. Hydrochemical response of LII from 0O:OOh on 14 March (c) dissolved Al; (d) anions; (e) cations; (f) Si and DOC.

1989. (a) Precipitation

and flow; (b) pH;

C. Soulsby / Journal of Hydrology

170 (1995) 159-179

Flow and rainfall Z:“\i,

i_:-;::::!:__& -;::i 0

15

IO

5

-now

20

.

25

30

35'

oO-22

hours

I-_RowI

rain

anions 1600

c

1

1400

14W

1200 IWO g 600 2

400 60°

-240

-flow

1600 1200

A

base cations 8

15 20 hours

10

so4

=

Cl

25

??

30

NO3

35"

)

Si and DOC

8

-

-120 -100

m -

6W-

. 0

ACa

-

Mg.Na

x

K

-2.7

IWO-

0

0.7

.

5

IO

-flRow

Fig. 5. Hydrochemical response of LIl from 17:00 h on 29 December pH; (c) dissolved Al; (d) anions; (e) cations; (f) Si and DOC.

15.

*

20

SI

25

-

30

+ 35°.3

DDC

1986. (a) Precipitation

and flow; (b)

172

C. Soulsby / Journal of Hydrology

170 (1995) 159-179

the response of a saturated catchment which is generating a substantial amount of storm runoff by responsive hydrological pathways, particularly overland flow; these allow rainfall to be transferred rapidly into the stream network (Harriman et al., 1990). Thus, ‘new’ event water makes an increasingly important contribution to the storm hydrograph and the dilute chemical signal in precipitation is transferred to surface waters. Robson et al. (1993) hypothesised the same mechanism for the conductivity response observed in the Afon Hafren at Plynlimon during similar large storm events. 6.2.3. 26 June 1989 (Event 17): a moderate summer event with dry antecedent conditions The early summer of 1989 was exceptionally dry at Llyn Brianne, with precipitation totals for May and early June being 52% below average. This dry period ended with 34.5 mm of rain which fell in two phases; the initial phase was light but the second, heavier burst had a peak intensity of 7 mm h-l (Fig. 6). The extremely low antecedent flows had a pH of 6.4 and high Ca concentrations. After 5 h the low-intensity rain caused flows to increase: pH declined slightly on the first hydrograph rise before dropping more abruptly as flows peaked again following the second burst of rain. Concentrations of Al were initially very low, before increasing and peaking on the recession limb. Chloride, S04, Ca, Mg and Na concentrations all increased slightly after the first hydrograph rise, before declining dramatically as flows peaked. Two DOC peaks were observed, with the highest corresponding to the second hydrograph, whereas SiOZ concentrations fell. The baseflows prior to this event were highly buffered. The soils were dry, with no seepage from the extensive peat area being observed. The soil surface was very dry, with the spruce needles covering the litter horizons exhibiting hydrophobic characteristics (Soulsby, 1993). Only drainage ditches that cut through fine drift deposits still flowed, indicating that deeper groundwater sustained baseflows. The slight increase in some solutes observed during the first hydrograph (after 10 h) is consistent with the flushing of soluble material accumulated over the preceding dry period (Walling and Foster, 1978). Vegetation surfaces are the likely source of this material, and high solute concentrations were observed in throughfall at this time (Soulsby, 1991). The dry soil surface appears to have allowed direct movement of incoming throughfall into the channel network by overland flow (Muscutt et al., 1990). Despite the relatively high peak discharge, the minimum pH of 5.0 is anomalously high for storm episodes and the peak Al concentration was exceptionally low. Although the maximum hourly rainfall intensity recorded for the event was not particularly high, there is strong circumstantial evidence that much of the 7 mm h-’ peak fell in an intense short burst. The maximum rate of rise (in 15 min) of the storm hydrograph was the highest observed in the 19 events monitored. Moreover, Soulsby (1991) recorded very high instantaneous flows in forest ditches in LIl during this event. These data suggest that the hydrograph peak was generated primarily by rapid overland flow from hydrophobic peat soils as a result of high rainfall intensities, an effect also observed in other catchments (McEwen, 1987). In contrast to the saturation overland flow hypothesised as being important in the previous two

C. Soulsby 1 Journal of Hydrology 170 (1995)

01 0 b-i0

15

20

25

36

35

159-179

113

c

40’

ba!secatkm6

Si and DOC

“il, 26

I I

-200 -150 I

r

LC-

! 3

I

2

1.5

.

-.!b$,

1

0.5

0

5

10

.

Ca

15

-

20

h&j

??

25

Na

30

x

K

35

40°

4b0

-IkwAcil

Fig. 6. Hydrochemical response of LIl from 12:00 h on 26 June 1989. (a) Precipitation dissolved Al; (d) anions; (e) cations; (f) Si and DOC.

root and flow; (b) pH; (c)

114

C. Soulsby / Journal of Hydrology 170 (1995)

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events, it appears that infiltration-excess overland flow was generated following this dry summer spell. This implies a major ‘new’ event water contribution to peak runoff during this particular event. The increasing concentrations of Al and SiOZ on the recession limb of the hydrograph are consistent with slower subsurface hydrological pathways contributing an increasingly dominant source of runoff as the overland flow response subsides. 6.2.4. 4 March 1986 (Event 1): snowmelt event The most severe acid episode observed during the study period was a rainfallinduced snowmelt event in March 1986 (Fig. 7). Falls of snow in January and February accumulated as anticyclonic conditions resulted in cold weather. The resulting easterly air streams had high pollutant loadings. A sample of snow taken on 27 February had extremely high concentrations of pollutant ions, particularly in the surface layers (Table 4). Very little thawing occurred subsequently, and very low and stable stream flows resulted. Rain gauges recorded 60.4 mm of precipitation for an event on 4 March 1986, but this included the contribution of both snowmelt and rainfall (associated with the passage of a southerly warm front) and it is therefore approximate. Air temperatures increased abruptly from -4°C to 6.6”C with the passage of the front, hence melt rates were rapid. Baseflows prior to the event were low and a steady rise in discharge over 7 h ensued, Stream pH dropped to 4.1 (the lowest pH recorded during the study) at peak discharge. Concentrations of all other solutes increased as soon as flows began to rise. Concentrations of Al peaked, at very high levels, 3 h before maximum flow, and all base cations also peaked on the rising limb. Sulphate and Cl exhibited a similar response; however, NO3 concentrations remained high for 2 h after the peak flow. Dissolved organic carbon concentrations also increased rapidly, reaching 3.4 mg 1-i immediately before the highest flow. In contrast, Si02 concentrations increased only slowly over the whole event. The high-pH, Ca-bearing antecedent flows reflect the dominance of deep groundwater sources as low temperatures resulted in the freezing of seepage water from the surface of the peat soils in the upper catchment. The rapid increase in anion and base cation concentrations is consistent with the rapid and direct movement of water from the melting snow pack into the LIl stream. The preferential removal of solutes from melting snow is a well-known phenomenon (Jenkins et al., 1993). These may subsequently be transferred directly to streams by overland flow from frozen or saturated soil, or by preferential flow in macropores in the upper soil horizons. It is extremely unlikely that the upper layers of the catchment soils were completely frozen during this event, as soil temperatures recorded at a depth of 10 cm at the AWS remained above freezing throughout February and early March. The early peak of Al was a particularly interesting feature of this event, as Al peaked on the recession limb during most episodes. This difference may reflect the displacement of Al-rich soil water by melting snow (Johannessen et al., 1980). Alternatively, Al may have been mobilised by rapid cation exchange processes (promoted by the high anion concentrations in melt water) along preferential flow paths in the upper soil horizons (Luxmoore et al., 1990). A final explanation is that Al may have been released

C. Soulsby / Journal of Hydrology 170 (1995) 159-l 79 flow and rainfall

Al(tot) 700,

175

PH

anions

Pd 742

lw

700,

350

d 6oc-

300

500.

250

400.

200

300.

150 iW 50 D

-6ow

A

base cation9

so4

=

Cl

??

NO3

Si and DOC 700

m9

I

‘

3.5

2.5 2 1.5 loo /

_‘I

0t0

Z

mm_

1 I

c-7

5

10

15

20

3.5

hours

-flow

Fig. 7. Hydrochemical response of LIl from 0O:OOh on 4 March dissolved Al; (d) anions; (e) cations; (f) Si and DOC.

.

si

1986. (a) Precipitation

??

cm and flow; (b) pH; (c)

C. Soulsby / Journal of Hydrology 170 (1995) 159-I 79

176 Table 4 Chemical 1986

composition

Surface Subsurface

(pequiv 1-l) of surface and subsurface

snow at the Llyn Brianne AWS, 27 February

PH

Cl

so4

NO3

Na

K

Ca

Mg

3.7 4.1

422 56

361 57

300 61

286 27

15 7

130 14

63 9

from the bed of the LIl stream network as a result of the effect of acidic stream water with high anion concentrations (Tipping and Hopwood, 1988). DOC also increased rapidly at the onset of melting, implying that the organic soil horizons were involved in storm runoff generation. Silicon did not exhibit its usual decrease with flow, but increased steadily throughout the event, reaching peak concentrations on the hydrograph recession limb; this suggests the increasing dominance of water from the soil mineral horizons as the event progressed. Clearly, the hydrology and hydrochemistry of snowmelt is complex: ‘new’ event water in the form of melting snow certainly contributes to storm runoff generation during this particular event. However, even in environments prone to frozen soils, ‘new’ water seldom dominates the storm hydrograph during snowmelt (Davies et al., 1992) and the response of Al, DOC and Si02 all suggest that substantial volumes of pre-event water were displaced during this particular episode.

7. Conclusions The storm event data examined in this paper demonstrates that the hydrochemical response of the LIl catchment is complex and highly dynamic. Given the interlinkages between the characteristics of individual precipitation events, antecedent conditions and the dominant storm runoff processes, the detailed hydrochemistry of each individual event usually has unique features. The four events examined in detail show that the hydrological processes which dominate storm runoff generation vary markedly and can have a major influence on the hydrochemistry of individual episodes. The wet antecedent conditions that precede most storm events in LIl dictate that storm runoff processes generally cause the displacement of ‘old’ pre-event water which, as in the March 1989 storm, subsequently dominates the storm hydrograph (Soulsby, 1995). This is consistent with the results of isotopic (Renshaw, 1993) and natural (Robson and Neal, 1990) tracer studies in other catchments at Llyn Brianne and elsewhere in upland Wales. In some cases, however, ‘new’ event water also makes an increasing contribution to storm runoff generation. During high-magnitude events such as the December 1986 storm, increasingly saturated conditions result in the rapid transmission of event precipitation into the stream network. The same phenomenon can occur when dry antecedent conditions, such as those prior to the June 1989 event, dictate that infiltration-excess overland flow dominates the catchment storm response. In common with other catchments in Europe (Davies et al., 1992), the most extreme acid episode in LIl was associated with a major snowmelt event, in

C. Soulsby / Journal of Hydrology 170 (1995) 159-l 79

177

March 1986. Clearly, the high pollutant loading of meltwater was reflected in the stream hydrochemical response, and the high concentrations of inorganic anions probably contributed to a marked pH and Al response. Again, ‘new’ event water from melted snow had an important influence on the catchment’s hydrochemical response, even though the displacement of pre-event water probably dominates the storm hydrograph. Examination of the storm event data from the LIl catchment reveals that spot samples from even fairly long investigations (more than 5 years) can give an extremely misleading view of the hydrochemistry of acidic streams. Essentially, such sampling regimes collect baseflow data and include very few samples representative of storm flows. There is a danger that data derived from such routine sampling programmes might provide unrepresentative calibration data for hydrochemical simulation models that are used to predict the impact of environmental perturbations (Whitehead et al., 1990). In some cases, such data may provide an inappropriate basis for making such predictions. Few hydrochemical studies monitor stream water chemistry sufficiently to provide a full understanding of the behaviour of storm episodes. Clearly, it is desirable to incorporate both spot and event sampling in monitoring programmes. However, practical and technical constraints usually mean that the choice of events for sampling is generally arbitrary. It therefore becomes very difficult to interpret how individual events relate to the longer-term fluctuations in the hydrochemistry of streams. Recent attempts to use continuous monitoring of several chemical species have been valuable in showing how chemical changes during particular episodes relate to conditions before, during and after the event (Robson et al., 1993). Reliable continuous monitoring techniques or improved sampling strategies are clearly a prerequisite to a more comprehensive understanding of the processes involved in producing contrasts in storm hydrochemistry in acidic catchments.

Acknowledgements This study was part of the Llyn Brianne Acid Waters Project funded by the National Rivers Authority, Department of Environment, and Welsh Office. Staff from the NRA and the Catchment Research Group at UC Cardiff collected and analysed samples. Catherine Shaw of the Institute of Hydrology helped retrieve the data. The author is also grateful to the Carnegie Trust for the Universities of Scotland for financial support, and to two anonymous referees for their helpful comments.

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