(Received 19 June 1990; accepted after revision 3 September 1990)
ABSTRACT
Cook, J.M., Edmunds, W.M. and Robins, N.S., 1991. Groundwater contribution to an acid upland lake (Loch Fleet, Scotland) and the possibilities for amelioration. J. Hydrol., 125: 111-128.
The Loch Fleet catchment lies in an upland region in the centre of the outcrop of the Cairnsmore of Fleet granite. It is a recently acidified lake (pH = 4.4) which has been the subject of a liming experiment to restore fisheries. In the present study, hydrogeological and geochemical techniques were used to determine the contribution of ground water to the loch and its role in buffering the lake water chemistry.
Diffuse groundwater seepage was detected by infrared linescan survey, and overflowing ground water (2 m above the level of the loch) was encountered in a shallow borehole. This ground water has an alkaline geochemistry (pH = 7.2, HCO~ = 142mg1-1) determined by secondary vein calcite and hydrolysis of silicate minerals. The net gains or losses of various constituents in the ground water and in the loch outflow have been determined relative to rainfall inputs. Na, K, Ca, Mg, HCO~, SO4, C1, Si, Sr, Fe, Mn, Li and F all show net gain in the ground water; NO3, A1, Zn and B show a net loss. In the acidic loch outflow, Ca, Mg, Si, Sr, Ba, Fe, Mn, A1, Zn and Li show a net gain over rainfall inputs; most of these elements derive from ground water, enhanced by evapotranspiration by a factor of 1.8.
The chemical results have been used to determine that ground water contributes around 3.51 s- to the loch, compared with an estimated 3-41 s -1 derived from hydrograph analysis. This con- stitutes 5% of the mean loch outflow, which was sufficient to buffer the loch at around pH = 6.0 until the late 1960s. Titrations of ground water with loch water show that as little as 0.061 s -1 (1656 m ~ year-1) of additional ground water would be required to restore the loch to conditions suitable for a self-sustaining fish population. Twice this flux (3310 m ~ year- ~) would restore the loch to the conditions pertaining in the pre-industrial era. These targets could be achieved at an economic cost, it is suggested, by induced abstraction of ground water in the upper reaches of the catchment without any harmful ecological effect.
INTRODUCTION
A c i d s u r f a c e w a t e r s o c c u r w h e r e n o n - c a r b o n a t e a n d p o o r l y p e r m e a b l e
b e d r o c k i n h i b i t t h e n e u t r a l i z a t i o n o f r a i n f a l l ac id i ty . T h o s e a r e a s o f B r i t a i n
w h i c h a r e l i k e l y to g i v e r i s e to a c i d s u r f a c e w a t e r s h a v e b e e n i d e n t i f i e d by
E d m u n d s a n d K i n n i b u r g h (1986) b a s e d u p o n g e o l o g i c a l c r i t e r i a . A c i d (and
ac id i f ied) l a k e s a n d s t r e a m s h a v e b e e n i d e n t i f i e d in m a n y of t h e s e a reas ,
especially in Scotland and Wales where hard rock lithologies occur (Harriman, 1989; UKAWRG, 1989; Hornung et al., 1990).
Acidification of waters is a natural process related to the depletion of base cations by the weathering of rocks and minerals on a geological timescale. Over the past two centuries, and especially in recent decades, the process has been rapidly advanced by anthropogenic activity giving rise to increases in input acidity estimated to be well above one order of magnitude. Strong evidence now exists from palaeoecological studies of lakes from several areas of Britain that acidification has been primarily due to the dissolution of atmospheric pollutants (Battarbee et al., 1985; Jones et al., 1986). Before the industrial revolution, many acid lakes were more alkaline (around pH = 6.0) with apparently stable ecosystems controlled by the weathering regime.
Relatively alkaline ground waters are to be found in most lithologies within a few tens of metres of the surface, even in non-carbonate terrains. River baseflow is invariably derived from shallow ground water whose composition is determined by the contact between acidic rain and bedrock, resulting in acid neutralization by silicate mineral hydrolysis. An intermediate buffering step in the soil horizons may also be significant and the soils may also release further H ÷, but the ultimate sink for acidity is the bedrock geology. The baseflow index, even in hard rock areas, may be quite high (Gustard et al., 1986) indicating the importance of shallow groundwater contributions. The importance of baseflow contributions in regulating the surface water acidity has been recognized in recent studies. Modelling studies (Whitehead et al., 1986) have demonstrated the likely effect of different percentages of baseflow on H ÷ and A13÷ concentrations in a conifer-forested catchment in Canada. Detailed field studies of catchments in Canada (Bottomley et al., 1984), North Wales (Reynolds et al., 1986; Robson and Neal, 1990; Neal et al., 1990), and Scotland (Kleissen et al., 1990) have all shown that soil-derived waters form a relatively small component of the total stream flow; they have also shown that ground water is recognizable by its high alkalinity and has a considerable importance in buffering the effects of acid rain inputs.
The present study gives the results of geochemical investigations of Loch Fleet, Galloway, situated on the Cairnsmore of Fleet granite. The main objective was to determine whether there was a groundwater contribution to the loch and, if so, how much. These studies were carried out as part of a wider multidisciplinary investigation (Howells and Brown, 1987) aimed at restoring conditions suitable for restocking the loch with fish. Work by Flower et al. (1987) had shown that lakes in Galloway situated on granite were generally susceptible to acidification and that this process had been accelerated by forestry activities. Loch Fleet is abnormal in relation to other lochs on the grani te since it only became acidified in the early 1970s, as opposed to the early twentieth century for the other lakes. It is an upland lake with high rainfall, granitic bedrock and with no long-term afforestation and therefore might have been expected to have been acidified much earlier. It is possible that this delay may have been due to the constant groundwater contribution to the loch. The
GROUNDWATER CONTRIBUTION TO AN ACID UPLAND LAKE 113
importance of groundwater chemistry in buffering the composition of lake waters in North America has been recognized by Kenoyer and Anderson (1989); groundwater contributions of around 10% had a significant effect on regulating the acid inputs to Crystal Lake on a sand aquifer in Wisconsin.
In this paper the wider implications of the groundwater hypothesis are considered in relation to the possible exploitation of ground water in acidified terrain as an alternative amelioration process to liming.
Loch Fleet - - location and geological sett ing
Loch Fleet is situated 340 m above sea level in a remote region of SW Scotland with 2150mm mean annual rainfall (Fig. 1). The lake covers 17 ha within a catchment of 111 ha which rises to a maximum elevation of 470 m (Howells, 1986). The vegetation is mostly rough moorland but 10% was planted with Sitka spruce and Lodge Pole pine in 1961. The remainder of the catchment comprises moorland with Molinia and CaUuna. The volume of the loch is estimated at 1.06 × 10~m 3 with a total mean outflow of 2.2 × 106m 3 year -1 and an average retention time of 173 days.
The loch is situated near the centre of the exposed Cairnsmore of Fleet granite pluton of Caledonian age (380-360 Ma). The granite (Parslow, 1968) is concentrically zoned: (1) an outer coarse-grained biotite granite; (2) an inner coarse-grained bioti te-muscovite granite; (3) a central core of fine-grained bioti te-muscovite granite. The latter zone underlies the Loch Fleet catchment. Despite the petrographic differences, the chemical composition of the granite is fairly uniform. The granite contains numerous aplite, pegmatite and quartz veins although these are generally concentrated at the granite margin. Three types of fracture are present within the granite: flow foliation related to the original fabric in the pluton, shear foliation (not important) and joints, usually vertical or steeply dipping and emphasized by weathering. The granite is overlain by a thin, stony, sandy loam with peaty surface layers.
The hydrogeochemistry of the area is summarized in a map of groundwater baseflow alkalinity using unpublished data from BGS sources (Fig. 1). The alkalinity in baseflow to streams draining the granite is typically below 10 mg HCO3 l-1 and contrasts with that of corresponding waters draining Ordovician or Silurian metasediments of the country rocks.
Methods
The hydrogeological survey of the loch catchment was assisted by an infrared linescan survey to locate any groundwater seepages. The survey was carried out at an air temperature of - 3 °C to provide a good contrast with groundwater inflows.
Three narrow diameter cored boreholes were drilled at sites marked on Fig. 2 using a lightweight portable drilling rig. Only one of these yielded water, which overflowed at 1 × 10-21s -1 (Cook et al., 1987).
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GROUNDWATER CONTRIBUTION TO AN ACID UPLAND LAKE 115
Borehole
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Fig. 2. Loch Fleet catchment showing results of the infrared linescan survey, borehole localities and sites sampled in the present study.
Samples of rain, stream, loch outflow and ground water were collected at monthly intervals between May 1985 and July 1986 to establish the natural geochemical conditions. Petrographic studies were also performed on the borehole core material.
116 J.M. COOK ET AL.
All water samples were collected in polyethylene bottles which had previously been acid washed in 10% v/v high purity HC1 and rinsed with distilled, deionized water before use. Samples were filtered through 0.45#m cellulose acetate membrane filters in the field and acidified to 1% v/v with high purity HC1. Additional filtered but unacidified samples were collected for the determination of nitrate and halogens. Rain samples were collected in two FVC raingauges sited in the catchment, and the monthly bulked samples decanted into polyethylene bottles. Water samples for stable isotope analysis were also collected in 30ml glass bottles.
The concentrations of most major ions and trace elements were determined by inductively coupled plasma-optical emission spectroscopy (ICP-OES) after a twenty-fold preconcentration by evaporation under infrared lamps, carried out in laminar flow hoods to minimize contamination. Automated colorimetry was used to determine C1, Br, I, F and NO3-N, and alkalinity was determined by t i tration in the field. The stable isotopes of oxygen (~180) and hydrogen (~i2H) were determined by mass spectrometry.
RESULTS
Hydrogeological studies
The study of groundwater flow in granites and crystalline rocks in Scotland has been restricted mainly to the Strath Halladale granite complex of Caithness (Robins, 1990). This has indicated that the intergranular porosity of fresh granite is always < 1%, and storage and flow of ground water is effective- ly limited to cracks and joints acting as secondary porosity. The majority of groundwater flow circulates within near-surface superficial deposits in weathered granite (Brereton et al., 1987).
At Loch Fleet, subvertical jointing is evident in exposures of the granite in the vicinity of the loch but there is apparently no preferred orientation. The frequency of open joints ranges from 0.5 to 3 m. The greatest density of open joints occurs along the northern tr ibutary to the loch known as the Altiwhat (Fig. 2). Here, the rock is also weathered and a distinct linear gulley follows an apparent line of weakness believed to be a fault or shear zone within the granite. Bathymetry (Anderson and Battarbee, 1985) indicates that this feature continues towards the centre of the loch. Similar, though weaker, sub-parallel features are apparent on the hillside to the north of the loch, although they are not readily identifiable on air photographs.
It is considered likely that most groundwater flow is shallow and that it occurs in the vicinity of the Altiwhat. The piezometric surface is likely to follow a subdued version of surface topography, depth to water being shallow on lower ground and perhaps 5-20 m beneath the catchment divides. Ground- water flows from the high fluid potential of the perimeter of the catchment (upland) to the low fluid potential to the south and west of the loch catchment (lowland). The piezometric surface is intersected by the topographical
GROUNDWATER CONTRIBUTION TO AN ACID UPLAND LAKE 117
depression of the Loch Fleet catchment so that ground water may emerge as spring seepage or baseflow to the loch.
The infrared linescan survey around the perimeter of the loch identified the location of springs and seepages whose volume was then determined either by timing a known volume or by visual estimation. The strongest groundwater inflows were found along the eastern side; they were less along the northwest and least of all in the southwest (Fig. 2). These findings are compatible with the prevailing flow of ground water, with fluid potential declining to the southwest. The seepages had temperatures of around 1-2°C and inflows commonly occur at, or near, the water's edge. Most of the sources were estimated to yield < 0.51 s- 1 and the estimated sum of all the detected likely groundwater sources is within the range 2-4 1 s ~ ; seepages from beneath the loch away from the shore were not detectable by this method. Recession curves of the loch outflow indicate a low flow or baseflow in the range 3-41 s- 1 (Fig. 3).
The temperature of the water issuing from the borehole is 7.5°C which, allowing for altitude, indicates a moderate depth of circulation and limited shallow storage. This temperature indicates that the water has been influenced by the natural geothermal gradient and/or that it has retained solar heat from the previous summer; winter recharge has a temperature of around 2°C. Interflow within the peat soils is affected by air temperature giving a tem- perature at or near 0°C during the linescan exercise. Mixing of granite and soil water, plus cooling of the ground water near the surface, resulted in seepages of only 1-2°C.
In order to characterize the physical and chemical characteristics of both the granite and the water circulating within it, three fully cored shallow exploratory boreholes were drilled in the catchment. The first borehole was drilled at the centre of the suspected fault zone along the Altiwhat, some 85 m from the shore. However, it encountered very broken and highly weathered granite unsuitable for narrow diameter coring with a light portable rig and the borehole had to be abandoned at a depth of 4 m; nevertheless, this indicated the likelihood of a fault zone. Two other boreholes were successfully completed to
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118 J.M. COOK ET AL.
10 m depth: one on the western margin of the fault zone, some 18 m from the edge of the loch, and the other on the western shore (Fig. 2).
Only the borehole (BH2) on the western margin of the Altiwhat struck ground water. This borehole overflowed at a rate of 1 x 10 _21 s -1, the overflow resulting from the intersection of piezometric surface in the fissures with ground level. This artesian flow (some 2 m above the height of the loch) issues from fractures which were seen in the core between 2.9 and 3.0 m below ground level. A brief constant discharge test was attempted using a bladder pump but pumping at a rate of only 1 x 10 - l l s -1 rapidly dewatered the borehole. The other successful borehole (BH3) did not intersect any significant open fractures and water stood in this hole at a depth of 0.7m below ground level, at about the level of the loch, implying some hydraulic contact with the loch water. No flow could be induced by pumping of this borehole.
Geochemistry
The rainfall chemistry measured by us for the present study is reported fully in Cook et al. (1987) and average values are used here for calculating catchment solute contributions; the chemical variation of rainfall in this area has been discussed in more detail by Howells (1986). However, several elements which may signify an industrial aerosol source (B, Zn, Pb, V) were detected in our analyses. The rainfall input is a significant proportion of the total solute input to the catchment for most elements (see below).
The mean compositions of the main inflow (7U2) and outflow (OD) from the Loch are compared in Table 1. In addition to those elements detected in rainfall, A1, Li, F, Be and Y, were found in both the inflow and outflow to the loch. The inflow stream sampled not only has the largest flow of any tributary but also the highest baseflow. Several smaller streams also feed the loch but the construction of a full elemental budget was not an objective in this study. The loch water has a residence time of the order of six months and it is possible to make some qualitative observations of the difference between the main inflow and outflow.
The concentrations of Si, Sr, A1, Li and Y were higher in the inflow stream relative to the outflow, whereas Ba, Fe, Mn and B concentrations were higher in the outflow than in the inflow. In the former case it is likely that the elements are derived from a geological source whereas, in the second group, sources other than ground water must be contributing.
The chemistry of ground water (Table 1) discharging from the overflowing borehole (BH2) was monitored over a 30 week period January to July 1986. With the exception of Fe, the coefficients of variation were < 5%. The variation over this period is illustrated in Fig. 4 for Na, C1, Mg and Mn, and it can be seen that there is little or no response (dilution) during heavy rainfall, indicating a significantly larger reservoir of ground water. The variation of Fe is considered to be due to fluctuating redox conditions. The Eh was measured on the over- flowing water together with dissolved oxygen, giving results of + 115 mV and
GROUNDWATER CONTRIBUTION TO AN ACID UPLAND LAKE 119
TABLE 1
Mean chemical composit ion of n samples of ground waters from BH2 over the period J a n u a r y - April (1986); the mean composi t ion of n samples of loch inflow, loch outflow and rainfall over the period Ju ly 1985 to April 1986 are also shown
Const i tuent Ground water Loch Loch Rainfall (BH2) inflow outflow
n 32 11 11 10 Tempera ture (°C) 7.5 4.8 2 - pH 7.2 4.8 4.4 4.8 Na 6.4 3.8 3.8 2.2 K 0.35 0.23 0.23 0.14 Ca 36.1 1.3 0.85 0.20 Mg 7.4 0.60 0.55 0.26 HCO 3 142 3.0 0 0 SO 4 6.8 4.1 4.3 2.4 C1 8.1 5.7 6.4 4.0 NO3-N < 0.04 0.15 0.21 0.24 Si 6.4 1.19 0.38 < 0.05 Sr 0.086 0.0052 0.0051 0.0022 Ba 0.002 0.022 0.32 0.0079 Fe 0.116 0.085 0.110 0.0035 Mn 0.802 0.031 0.159 0.0026 A1 < 0.003 0.233 0.192 0.0056 Zn < 0.002 0.026 0.038 0.0096 B 0.006 0.0059 0.0082 0.0071 Li 0.0169 0.0021 0.0019 < 0.0005 F 0.54 0.0605 < 0.05 < 0.05 Br 0.030 0.045 0.040 0.022 I 0.003 0.013 0.0043 0.0024 Cu < 0.0002 0.0009 0.0011 0.0009 Be 0.00022 0.00008 0.00009 < 0.00005 Ni 0.015 < 0.002 0.003 < 0.002 Pb < 0.005 < 0.005 0.003 0.008 Y 0.00060 0.00015 0.00009 < 0.00005 Co < 0.0005 < 0.003 < 0.0005 < 0.0003 La < 0.0005 < 0.0004 < 0.0005 < 0.0004 V < 0.0002 0.0003 < 0.0002 0.0004
All resul ts are in mg 1-1. Loch inflow and outflow are spot samples and the rainfall data are from bulked month ly samples.
0.6 mg 1-1, respectively. These values are sufficiently low to ascribe the changes in Few (0.01-0.45 mg 1-1) to mixing of oxygen-free and oxygenated ground water, which would also account for the deposition of Fe(OH)3 observed around the discharge area. The existence of reducing ground water implies that the water has circulated to a sufficient depth and time for all oxygen to be consumed. The stable isotope results indicate that the ground water is closely related to rain and surface waters as the results lie on or close to a UK meteoric line, with s l o p e 5 2 H -- 6.4 5180 - 0.7. The ground water from BH2 was monitored for a
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GROUNDWATER CONTRIBUTION TO AN ACID UPLAND LAKE 121
TABLE 2
Comparison of BGS and Scottish Power (formerly SSEB) data for the Loch Fleet ground water (BH2). Although not strictly corresponding to the date of liming, these results can be regarded as pre- and post-intervention
Mean concentration (early 1986)
Mean concentration (June 1986 to March 1989)
BSG data SSEB data SSEB data (January 6 to April 14) (February 5 to June 24)
n 32 20 69 pH 7.2 7.1 7.3 Na 6.4 6.2 6.1 K O.35 - O.32 Ca 36.1 35.6 34.3 Mg 7.4 7.1 6.5 HCO 3 142 138 135 SO 4 6.8 6.5 6.2 C1 8.1 7.7 7.9 NO~ < 0.04 0.09
fu r the r th ree years by Scot t i sh Power dur ing which t ime no s ignif icant change in the chemis t ry was found. M e a n va lues of Na, C1 and Mg are shown in Fig. 4 and o the r ma jo r cons t i t uen t s are compared in Table 2. No change in chemis t ry was de tec ted dur ing or a f te r the l iming in Apri l 1986.
P e t r o g r a p h i c e x a m i n a t i o n of g ran i t e and vein ma te r i a l was made on core ma t e r i a l f rom BH2 and BH1. The p lag ioc lase fe ldspar is s t rong ly al tered, especia l ly the cen t res of phenocrys t s , and bio t i te is s t rong ly chlori t ized, also b r e a k i n g down to ser ic i te and f ine-grained a l t e r a t i on products ; muscov i t e is l i t t le a l tered. Much of the mine ra l a l t e r a t i on re la tes to p ro longed hydro ther - mal reac t ion , but the same processes ope ra te a t the presen t -day low- t e m p e r a t u r e condi t ions of wea ther ing . Thus, p lag ioc lase will be more r eac t ive t h a n K-fe ldspar and the calc ic c o m p o n e n t of the p lag ioc lase is more r eac t ive t h a n albite, r e su l t ing in the p re fe ren t ia l a l t e r a t i on of the fe ldspar cores.
The core ma te r i a l is t r ave r sed by la te quar tz veins and some l a rge r infilled f rac tures . These f r ac tu re s con ta in ca lc i te in addi t ion to f ine-grained serici te , whi te mica and quar tz . Sulphide mine ra l i za t ion (chalcopyr i te ) was also found and gra ins have been oxidized qui te s t rongly , p robab ly as a resu l t of wea ther ing .
GROUNDWATER CONTRIBUTION TO THE LOCH FLEET CATCHMENT
Ground wa te r overf lowing a t 7.5°C at a he igh t of 2 m above the level of the loch a t the n o r t h e r n end of the c a t c h m e n t is c lear ev idence of a hydrau l i c g rad ien t wi th in the g ran i t e and conf i rms the t e n t a t i v e conc lus ion of the in f ra red l inescan t h a t g round w a t e r mus t be con t r ibu t ing basef low to the loch.
122 J.M. COOK ET AL.
The chemistry of the ground water indicates considerable reaction between rainwater and rock. The groundwater pH and alkalinity indicate a high degree of neutralization of rainfall acidity during sub-surface flow, although saturation with calcite is not achieved (SIcALCITE = --0.7).
In order to estimate the extent of reaction with the granite, the composition of infiltrating rainfall was adjusted to allow for evaporation and then compared with that of the ground water. A corrected rainfall value was obtained by multiplying the rainfall results by 1.36, which is the ratio of bromide, a conser- vative element, in the ground water to that in the rainwater. This corrected rainfall value was then subtracted from the value for the ground water to provide the net gains or losses (Table 3). The same procedure was carried out for the loch water where the factor was 1.82; this is cgnsistent with a higher rate of evaporation. Bromide provides a more reliable conservative parameter than chloride for this purpose since it is likely that some chloride might be contributed from minerals within the granite (Edmunds et al., 1985; Hamilton- Taylor et al., 1988).
Although most elements are added to the ground water during its circula- tion in the granite, NO3 Ba, A1, Zn and B are removed. Iodide appears to remain inert both in the ground water and the loch.
T A B L E 3
N e t g a i n o r loss of e l e m e n t s in (a) t h e g r o u n d w a t e r , a n d (b) t h e l och o u t f l o w r e l a t i v e to r a i n f a l l
a f t e r c o r r e c t i o n fo r e v a p o t r a n s p i r a t i o n , e x p r e s s e d in m g 1 1 a n d g eq 1-1
B o r e h o l e O u t f l o w
m g l 1 p e q l ~ m g 1 - 1 t~eql - I
N a + 3.4 150 - 0.2 8.7
K + 0.16 4 - 0.02 0.5
C a + 35.8 1790 + 0.49 24.5
M g + 6.6 540 + 0.08 6.6
H C O 3 + 142 2320 -
SO 4 + 3.6 75 0 0
C1 + 2.7 76 - 0.9 25.3
N O 3-N - 0.3 - - 0.23 3.7
Si + 6.4 + 0.38
S r + 0.083 1.9 ÷ 0.0011 0.03
B a + 0.008 + 0.018 0.3
Fe ÷ 0.111 4 + 0.104 3.7
M n ÷ 0.798 29 + 0.154 5.6
A1 - 0.008 + 0.182 20.2
Zn - 0.013 - - 0.021 0.6
B - 0.004 - - 0.005
Li + 0.017 2.5 + 0.0019 0.3
F + 0.535 28.2 - -
I 0 0
GROUNDWATER CONTRIBUTION TO AN ACID UPLAND LAKE 123
The principal gain in the ground water is in bicarbonate which is balanced by distribution of Ca and Mg. This is linked with the presence of calcite in the rocks and, assuming carbonate is the only source of Mg and Ca, implies that there is a Ca:Mg ratio in the calcite of 3.3:1. Calcite is also likely to be the main source of Sr. Other elements, such as Na, K, Si, Li, F and C1, are likely to be derived by congruent or incongruent silicate reactions, although any A1 dissolved during the hydrolysis would be rapidly reprecipitated as secondary minerals at the groundwater pH. The 3.6mg1-1 gain of sulphate denotes the extent of sulphide oxidation. Fe is produced during this oxidation but, as discussed above, the Fe 2÷ may have been partially reprecipitated. The rather high Mn (0.8mg1-1) may be derived from the sulphide oxidation or from substitution in the calcite. The excess of Li and F indicates that there has been some congruent or incongruent reaction of biotite (Edmunds et al., 1985), and this could also account for the net increase of chloride. Several other elements are also found in the granite ground waters as a result of mineral-water interaction. Be (0.22 #g 1-1) is probably derived from the reaction of silicates, as is Y, which usually correlates well with A1 (Neal et al., 1986).
The loch outflow, in contrast with the ground water, contains only small gains over rainfall and these are likely to stem from the groundwater contribu- tions. Ca, Mg, Sr, Fe, Mn, A1 and Li are all likely to be derived from mineral sources. An estimate of the groundwater contribution to the loch can be made
- 8 O~ E g) v -m
0
o -~
-+=- -6 o_
o S r 0.. ,-* E 0 0 0
Mg o $ 4 - - 4 B ..~ "0
0 ~= _~.
.E 3 - L O C H - o ,-- O U T F L O W
' o • 2- - 2 B ..~ co
o (D
f i [ 0 0 L j 2fO , 4'0 6'0 8'0 100
% Groundwater contribution
Fig . 5. M i x i n g d i a g r a m b e t w e e n r a i n a n d g r o u n d w a t e r c o m p o s i t i o n s u s i n g d a t a for Sr a n d M g u s e d to d e r i v e t h e p e r c e n t a g e g r o u n d w a t e r c o m p o s i t i o n to L o e h F lee t .
124 J.M. COOK ET AL.
using a mixing diagram with ground water and rainwater as the end-members (Fig. 5). The average concentrations of Mg and Sr (x 100) in ground water corrected for evapotranspiration are used as end-members of the mixing series. It is assumed that these are the only two sources of Mg and Sr and that both elements behave in a conservative manner.
The range of Mg and Sr concentrations in the loch outflow are indicated on the mixing lines. These concentrations correspond to a groundwater contribu- tion of between 2 and 8% (average 5%). Although the catchment is 'flashy' the retention time in the loch is 173 days (Howells and Brown, 1987) and during this time the ground water mixes thoroughly with rain and runoff. Average annual flow out of the loch is ~ 701 s-l, of which an average of 5% is ground water, suggesting that the total baseflow input is ~ 3.51 s-1.
D I S C U S S I O N A N D C O N C L U S I O N S
The onset of acidification of Loch Fleet has been shown to be a relatively recent process taking place in the late 1960s, some half a century or more later than other lakes in Galloway. This delayed effect has been attributed primarily to buffering by a significant groundwater contribution. The discharge of alkaline ground water from shallow depths has contributed some 5% of the total flow to the loch, an amount that has probably remained constant in previous centuries, sufficient to buffer the pH ~ 6.0 under natural conditions.
This natural buffering mechanism has been distributed, in the main, by the deposition of strong acids and it is logical to consider whether it might be feasible to reverse the process by hydrological management, particularly by induced circulation and abstraction of ground water.
Various mechanisms have been suggested to counter acidification. Apart from hydrological management, other techniques which have been considered (Howells and Dalziel, 1988) include: (1) land-use m a n a g e m e n t - - replacing conifers with broadleaved trees, correcting mineral deficiencies in soils and restoring moorland agricultural economy; (2) f isheries m a n a g e m e n t - -
replacing lost stocks, introducing more tolerant species, providing refuges from acid pulses; and (3) aquat ic ecosystem m a n a g e m e n t - - encouraging chemical and biological processes which generate alkalinity, introducing tolerant species etc. So far, little full-scale testing or few economic studies of any of these techniques have been carried out.
The only method that has so far been tried has been l iming, and this has been the prime purpose of the Loch Fleet experiment, where the main economic objective has been to restore fisheries (Howells and Brown, 1987). In the short term this has proved to be an effective measure. Some evidence of adverse effects, notably of the damage to the acid tolerant upland ecosystems has, however, recently been reported (Weatherley, 1988; Ormerod et al., 1990). The scale of acidification in general is widespread and it becomes very difficult to treat whole landscapes or every ecosystem. Treating the loch or tr ibutary streamwaters to permit the return of fish nevertheless seems to be a realistic
GROUNDWATER CONTRIBUTION TO AN ACID UPLAND LAKE 125
intervention until such time (up to 100 years) when it might be realistic to expect a return to near-natural conditions, assuming that there is, in the meantime, a further rapid reduction of atmospheric acid deposition.
The option for using ground water as a possible intervention treatment has not been explored so far and no field experiments have been carried out. The 'natural ' chemistry of Loch Fleet and its ground water may be used to demon- strate the likely effect of adding ground water to the loch above the quantities already being contributed entirely under natural gradients.
A sample of Loch Fleet outflow (pH = 4.4) was ti trated with the alkaline ground water (pH = 7.6, HCO3 = 156mg1-1) taken from the overflowing borehole (BH2), to demonstrate the likely recovery from acidic conditions (Fig. 6). The t i tration curve of this weak base-strong acid reaction is essentially the reverse of the natural acidification process and demonstrates the rapid change in pH over the poorly buffered interval from pH = 6.0 to pH = 4.4. Although the t i trat ion curve has been carried out with water from Loch Fleet, it will not simulate exactly the in-situ conditions that may also control pH change - - notably interactions with bottom sediments, biota and land-use changes in the catchment. Changes in the buffering will also be dependent on open-system or
pH
7-
6-
5-
Groundwater volume (m 3 yr 1)
0 5 10 15 I i i _ ~ I I_ _ ~ _ _ l _ _ _ _ _ _ ]
/
A C I D I F I C A T I O N P R O C E S S
p H F O R B R O W N T R O U T
\ RESTORATION OF LOCH FLEET ENT DAY USING SHALLOW GROUNDWATER
~ L O C H A C I D I T Y p H 4 . 4 W I T H 1 5 6 m g 1-1 H C O 3
Volume % groundwater (above that a l ready enter ing the Ioch) needed for restorat ion of the Ioch
Fig. 6. Results of titration of Loch Fleet with local ground water to demonstrate the likely effects of pH restoration using groundwater addition.
126 J.M. COOK ET AL.
closed-system behaviour with regard to CO2 released from ground water or from bottom sediments. Nevertheless, it may be used as a basis for estimating the approximate quantities of ground water required to restore the conditions in the loch.
The minimum target conditions for restoring brown trout to Loch Fleet (Howells and Dalziel, 1988) are pH >5.0, Ca = ~ 2mgl 1 and total A1 < 100 mg 1 1, although a higher pH should be aimed at for a self-sustaining fish population. This minimum condition could be met with an addition of 1.5% ground water of the same chemical composition (0.061 s 1 or 1656 m 3 year 1) in excess of that already entering the loch. A more reliable target would be to reproduce the likely natural in situ conditions pertaining before the industrial era (~ 1800) as inferred from the palaeoecological data. An addition of 3.5% ground water (0.141 s-1 or 3310 m 3 year-1) would be required for this objective.
The key question then arises as to how this additional small supply might be obtained and how it may best be managed to produce the required improve- ment. The second part of this question is more easily dealt with. Under natural conditions, it appears from the present study that the sdpply of ground water is greater on the northern side of the catchment. This provides baseflow to the Altiwhat stream and seepage to the loch. The small but significant alkalinity of the stream has probably also been important in making this a main fish spawning site. The amount of groundwater storage must be considerable and the residence time must also be quite large since no change in chemistry was detected over a 3 year period, even following the liming of the catchment. It is clear that this region of the catchment would provide a good place for release and regulation of the ground water, which is then mixed natural ly by circula- tion in the loch over the retention time of 173 days.
In order to regulate the streamwater chemistry, it would be necessary to induce groundwater flow above its present-day natural discharge of 41 s- 1. This would require new boreholes to induce and intercept ground water flowing through the fractured granite where secondary carbonate minerals are present at shallow depths (below 5 m). Drilling within the Altiwhat some 100 m or so away from the loch and at least 20 m above its level would therefore be a favourable site for between two and four small diameter (10 cm) boreholes to depths of around 20 m. Not all the boreholes would intercept a conductive fracture system and only a 50% success rate might be envisaged. Although present borehole flow is artesian near the loch, it is likely that ground water from higher in the catchment would need to be pumped. The cost of such a scheme would include drilling (e.g. £50m-1), borehole site completion costs plus the cost of a small pump and generator. Site maintenance would also be needed. The overall costs would therefore be of the order of £6000 capital plus maintenance.
The groundwater addition technique is now inappropriate for the Loch Fleet catchment since liming experiments have already been undertaken. The procedure illustrated is nevertheless an alternative and generally inexpensive amelioration technique for upland surface waters elsewhere. Groundwater
GROUNDWATER CONTRIBUTION TO AN ACID UPLAND LAKE 127
circulation is now recognized (see Introduction) as an important component of many catchments, often thought to be 'impermeable'. Therefore, hydrological management may be applicable quite widely, al though regulation during changing flow regimes with changing acidity also needs to be considered. Moreover, by enhancing the natural buffering mechanism of lakes and streams, harmful ecological side effects produced by liming over large surface areas of the catchment can be avoided. The geochemical/geological conditions in many upland areas may be favourable for treatment, al though the hydrogeological si tuation may limit the volumes of water that can be obtained. Thin limestones, marbles, calc-silicate or metavolcanic rocks are likely to contain calcite, and secondary calcite veining occurs quite widely in non-carbonate lithologies in Britain and elsewhere. As a result of silicate mineral hydrolysis, near-neutral ground waters may be found at depths of several metres, al though in the absence of calcite the volumes of ground water with sufficiently high alkalinity may be difficult to obtain. As a further al ternative it might be possible to import ground water from an adjacent catchment, but at greater expense. It remains for the definitive site experiments to be carried out, but it is considered that ground water should be added to the list of possible viable alternatives for amelioration of acidified waters.
ACKNOWLEDGEMENTS
The authors are grateful to K. Paterson of the South of Scotland Electrici ty Board (Scottish Power) for help with sampling, collaboration over chemical analysis and for providing some data on groundwater chemistry; to the Scottish Area Chief Scientist for British Coal for providing staff and equipment to undertake the linescan survey. We are indebted to David Kinniburgh and to other colleagues at BGS Wallingford for assistance with the project and subsequent discussion. This paper has benefited from a review by Colin Neal. The paper is published by permission of the Loch Fleet Management Committee and the Director, British Geological Survey (NERC).
REFERENCES
Anderson, N.J. and Battarbee, R.W., 1985. Loch Fleet: bathometry and sediment distribution. Working Paper No. 10, Palaeoecology Unit, Geography Department, University College, London.
Battarbee, R.W., Flower, R.J., Stevenson, A.C. and Rippey, B., 1985. Lake acidification in Galloway; a palaeoecological test of competing hypotheses. Nature, 314: 35(L352.
Bottomley, D.J., Craig, J. and Johnston, L.M., 1984. Neutralisation of acid runoff by groundwater discharge to streams in Canadian Precambrain Shield watersheds. J. Hydrol., 75: 1-26.
Brereton, N.R., McEwen, T.J. and Lee, M.K., 1987. Fluid flow in crystalline rocks: relationships between groundwater spring alignments and other surface lineations at Altnabreac, United Kingdom. J. Geophys. Res., 92: 7797-7806.
Cook, J.M., Edmunds, W.M. and Robins, N.S., 1987. Groundwater contribution to Loch Fleet, Galloway. Hydrogeology Research Report 87]4.
Edmunds, W.M. and Kinniburgh, D.G., 1986. The susceptibility of UK groundwaters to acidic deposition. J. Geol, Soc. London, 143: 707-720.
128 J.M. COOK ET AL.
Edmunds, W.M., Kay, R.L.F. and McCartney, R.A., 1985. Origin of saline groundwaters in the Carnmenellis granite (Cornwall, England): natural processes and reaction during hot dry rock reservoir circulation. Chem. Geol., 49: 287-301.
Flower, R.J, Battarbee, R.W. and Appleby, P.G., 1987. The recent palaeolimnology of acid lakes in Galloway, South-west Scotland: diatom analyses, pH trends and the role of afforestation. J. Ecol., 75: 797424.
Gustard, A., Jones, P. and Sutcliffe, M.F., 1986. Base Flow Index, Scotland (1:625 000). Institute of Hydrology, Wallingford.
Hamilton-Taylor, J., Edmunds, W.M., Darling, W.G. and Sutcliffe, D.W., 1988. A diffusive ion flux of non-marine origin in Cumbrian lake sediments. Geochim. Cosmochim. Acta, 52: 223-227.
Harriman, P., 1989. Patterns of surface water acidification in Scotland. In: Acidification in Scotland. Scottish Development Department, pp. 72-79.
Hornung, M., le Grice, S., Brown, N. and Norris, D., 1990. The role of geology and soils controlling surface water acidity in Wales. In: R.W. Edwards, A.S. Gee and J.H. Stoner (Editors), Acid Waters in Wales. Kluwer, Dordrecht, pp. 55-66.
Howells, G.D. (editor), 1986. The Loch Fleet Project. A report on the pre-intervention phase (1) 1984-1986. Publication of Central Electricity Generating Board (with South of Scotland Elec- tricity Board, North of Scotland and Highlands Electricity Board and British Coal).
Howells, G. and Brown, D.J.A., 1987. The Loch Fleet Project, SW Scotland. Trans. R. Soc. Edinb., Earth Sci., 78: 241-248.
Howells, G.D. and Dalziel, T.R.K., (Editor), 1988. The Loch Fleet Project: a report of the interven- tion phase (2). 1986-1987. Central Electricity Research Laboratory, Leatherhead.
Jones, V.J., Stevenson, A.C. and Battarbee, R.W., 1986. Lake acidification and the land use hypothesis: a mid post-glacial analogue. Nature, 322:157 158.
Kenoyer, G.J. and Anderson, M.P., 1989. Groundwaters' dynamic role in regulating acidity and chemistry in a precipitation-dominated lake. J. Hydrol., 109: 287-306.
Kleissen, F.M., Wheater, H.S., Beck. M.B. and Harriman, R., 1990. Conservative mixing of water sources: analysis of the behaviour of the Allt a' Mharcaidh catchment. J. Hydrol., 116: 365-374.
Neal, C., 1988. pCO 2 variations in streamwaters draining an acidic and acid sensitive spruce forested catchment in mid-Wales. Sci. Total Environ., 76: 279-283.
Neal, C., Smith, C.J. and Edmunds, W.M., 1986. Geochemical controls on trace element behaviour in Welsh upland streams. Proceedings of the Fifth Symposium on Water Rock Interaction, Reykjavik, pp. 397-400.
Neal, C., Robson, A. and Smith, C.J., 1990. Acid neutralization capacity variations for the Hafren Forest stream, mid-Wales: inferences for hydrological processes. J. Hydrol., 121: 85-101.
Ormerod, S.J., Weatherley, N.S., Merrett, W.J., Gee, A.S. and Whitehead, P.G., 1990. Restoring acidified streams in upland Wales: a modelling comparison of the chemical and biological effects of liming and reduced sulphate deposition. Environ. Pollut., 64: 1-19.
Parslow, G.R, 1968. The physical and structural features of the Cairnsmore of Fleet granite and its aureole. Scott. J. Geol., 4:91 108.
Reynolds, B., Neal, C., Hornung, M. and Stevens, P.A., 1986. Baseflow buffering of streamwater acidity in five mid-Wales catchments. J. Hydrol., 87:167 185.
Robins, N.S., 1990. Hydrogeology of Scotland. HMSO, London. Robson, A. and Neal, C., 1990. Hydrograph separation using chemical techniques: an application
to catchments in mid-Wales. J. Hydrol., 116: 345-363. UKAWRG, 1988. Acidity in United Kingdom fresh waters. Second report of the U.K. Acid Waters
Review Group, HMSO, London. Weatherley, N.S., 1988. Liming to mitigate acidification in freshwater ecosystems: a review of the
biological consequences. Water, Air, Soil Pollut., 39: 412-437. Whitehead, P.G., Neal, C. and Neale, R., 1986. Modelling the effects of hydrological changes on
