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Introduction In southwestern Nova Scotia, some lakes have been adversely affected by acid precipitation, causing a reduction in pH and stress to fish habitat. The effects of this acidification have been heavily studied in terms of ecology and water chemistry, and have been extensively monitored and modelled (Clair et al., 2007). Because of the concern of acid rain’s effects on lakes, sulphur dioxide (SO2) emissions have been reduced by over 40% of 1980 levels (Jeffries et al., 2003). Reductions in SO2 emissions have resulted in decreasing sulphate (SO4

2-) concentrations in precipitation; SO42- is the

primary agent of acidification in Canada (Jeffries et al., 2003). Despite these efforts, there have been to date no measurable improvements in water chemistry (Clair et al., 2007). This observation, in combination with modelling results showing that Nova Scotia will be the slowest to recover to pre-acidification levels relative to other Atlantic provinces (Clair et al., 2003), highlights the long-term nature of this issue. A study by Ginn et al. (2007) used diatoms from lake cores to infer a record of pH changes in 51 lakes since the pre-industrial period. In this method, a proxy for pH is established based on the abundance of species of diatoms and calibrated to deep-water surface-sediment samples from a large number of present-day lakes covering a range of environmental conditions. Ginn et al. (2007) found that lakes in Nova Scotia responded differently to the effects of acid rain: lakes in the Kejimkujik area were the most affected, whereas lakes in the area of Yarmouth, Bridgewater, and Cape Breton Highlands were affected to a lesser extent, and less consistently. Ginn et al. (2007) also found that although the lakes were acidified as a result of acid precipitation, many of the lakes were naturally acidic even in pre-industrial times. Ginn et al. (2007) speculated that the differing responses may be a result of lakes in Kejimkujik having a relatively lower initial pH, thus being more acid

sensitive. The Ginn et al. (2007) study, however, did not take into account bedrock or surficial materials, which may act as buffering agents. In fact, previous studies have generally ignored the effects of geology on lake-water chemistry, describing the bedrock as ‘un-buffering’ granite, metasediments and slate (e.g. Clair et al., 2007; Ginn et al., 2007; Whitfield et al., 2006). In this study we used lake-water chemistry and lake-sediment geochemistry to show the effects of surficial and bedrock geology on the buffering capacity of lakes. These results identify areas of natural buffering capacity, and suggest that geology should be more closely considered in projects studying the effects of acid rain. Regional Setting and Bedrock Geology Most of the bedrock (Fig. 1) in the study area is non-carbonate: the South Mountain Batholith (granite) and the Meguma Supergroup (metasedimentary units with some minor carbonate) are units with low potential for buffering capacity (Keppie, 2000), and some units are the source of acidity (Feetham et al., 1997). There are numerous occurrences of carbonate bedrock in Nova Scotia, however, and in the surrounding Maritimes Basin, which provided a source for carbonate-bearing glacial sediments to be deposited in the province. The Torbrook Formation contains limestone (Keppie, 2000) and outcrops southeast of the Annapolis Basin. Small, un-mapped limestone units may also exist in the Lake George area, based on the occurrence of a large limestone erratic found in that area (C. White, personal communication, 2007). More importantly, the Scots Bay Formation contains limestone, and although only located on land near Scots Bay (Keppie, 2000), thick deposits are located offshore in the Fundy Basin (Wade et al., 1996).

Utting, D. J. and Goodwin, T. A. 2008: in Mineral Resources Branch, Report of Activities 2007; Nova Scotia Department of Natural Resources, Report ME 2008-1, p. 105-111.

Lake-sediment Geochemistry and its Influence on Lake Alkalinity, Southwest Nova Scotia D. J. Utting and T. A. Goodwin

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The Windsor Group contains thick units of limestone, and in northern Nova Scotia, the Pictou and Cumberland groups contain units of limestone (Keppie, 2000). Collectively, these three groups are part of the Maritimes Basin, which is also found offshore on the Magdalen Shelf (Wheeler et al., 1996). During the Quaternary, glacial events resulted in deposition of varying thicknesses of sediment derived from bedrock found both locally and outside the province. The Escuminac glacial phase (Fig. 2) resulted in non-local tills (e.g. Lawrencetown, Saunierville, Hants tills) to be deposited in the province and this material forms the core of drumlins (Stea, 2004). An earlier glacial phase also deposited the calcareous Red Head Till (composed of Fundy Basin material) in the Yarmouth-Digby area. These deposits are overlain by material derived from local bedrock sources, but the drumlins in the area are cored by red calcareous tills (Stea and Grant, 1982). The Escuminac phase

tills are primarily derived from sedimentary strata that contain carbonate units on the Magdalen Shelf and northern Nova Scotia (Stea, 2004), with some erratics from the Cobequid Highlands (Stea and Pe-Piper, 1999). Tills from later glacial events (e.g. Scotian glacial phase) tend to be thinner and more locally derived, and typically mask underlying units. Because most of these local tills are composed primarily of granite and metasediments, they likely offer little buffering capacity. Soils in the southwest of Nova Scotia are primarily humo-ferric podzols, with lesser distributions of gleysols, mesisols and gray luvisols (Soil Classification Working Group, 1998). Locally, there is more soil development in the Yarmouth and Bridgewater areas, which Ginn et al. (2007) speculated may release base cations or have some other masking effect on acidity. Proximity to the ocean provides elevated marine aerosols (Ca2+, Mg2+, Na+, K+, SO4

2-, Cl-; Whitfield et al., 2006). The contribution of marine

Figure 1. Bedrock units and location of features discussed in text.

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SO42- in lakes can be assessed based on the

concentration of Cl-, primarily derived from marine sources, and allows for a seasalt correction (Clair et al., 2007). Because seasalt-corrected levels of SO4

2- show the same regional pattern as the non-seasalt-corrected levels (Clair et al., 2007), the effects of marine sources of SO4

2- are less important in explaining the acidification of lakes, even though up to 20% of SO4

2- is likely derived from marine sources (Clair et al., 2007). Methods Lake Geochemistry A lake-sediment survey of over 3000 lakes was conducted in the 1970s by the Nova Scotia Department of Mines and Energy and the Geological Survey of Canada as part of the National Geochemical Reconnaissance program (MacDonald, 1998). Lake sediment was selected as a sample medium because it reflects input from a combination of bedrock and surficial materials. Lake-sediment samples could contain calcium carbonate eroded from bedrock sources within the catchment basin, or from glacial sediments derived from bedrock units located outside the catchment basin. Sampling was completed using a helicopter on floats in 1977 and 1978. The deepest part of the lakes (based on surface darkness) were

preferentially selected as sample sites, based on a perception that these sites would be more representative of the lake basin. Samples were collected using a gravity corer, mounted to the helicopter’s fuselage, which was dropped into the lake and retrieved by a hand-operated winch. Samples were prepared and analyzed at the Technical University of Nova Scotia in Halifax using Atomic Absorption Spectrometry (AAS) following aqua regia digestion. No test for carbonate content was performed, but calcium (ppm) was measured. Calcium carbonate is the likely source for most of the calcium detected in the samples. Lake Water Chemistry Environment Canada has monitored lake water chemistry in Nova Scotia since 1983 (n=74) (Clair et al., 2002; Environment Canada, 2008). Water samples have been analyzed using the Gran titration method, providing the sum of carbonate plus organic buffering. This allows for calculation of negative buffering capacity, which indicates excess acidity (Clair et al., 2007). Interpretation In lakes where both the sediment geochemistry and water chemistry were measured (n=22) (Table 1), there is a good correlation (R2=0.79) between calcium concentration in the sediment and the alkalinity of the water: lakes with higher concentrations of calcium in their sediments have water with higher buffering capacity (Fig. 3). This positive correlation also shows that the calcium in the lake sediment is primarily derived from calcium carbonate, as the carbonate (CO3

2-) provides the buffering capacity. Using the observed trend line to generalize the dataset, lakes with a calcium concentration of less than 935 ppm have negative Gran alkalinity (i.e. have excess acidity), and as a corollary, lakes with greater than 935 ppm have excess buffering capacity. This level, however, does not take into account site-specific sources of SO4

2- from marine salts, transportation corridors and urban pollution, and therefore can only be considered as a general, regional indicator of depleted or elevated buffering capacity.

Figure 2. Escuminac glacial phase (after Stea, 2004).

108 Mineral Resources Branch

Despite these limitations, this threshold is used over the larger lake sediment geochemistry dataset (n=3388) to identify areas of depleted (<935 ppm) or elevated (>935 ppm) buffering capacity (Fig. 4). Low levels (1-935 ppm) are primarily found over the South Mountain Batholith in the Kejimkujik area, and southwards (underlain by Meguma Supergroup rocks). The area of low buffering capacity coincides with low pH conditions in lakes (Ginn et al., 2007). This zone is flanked to the east and west by drumlin fields, and generally higher concentrations of calcium in lake sediments. Many of these drumlins are cored by tills derived from non-local sources that likely have high calcium carbonate contents. This suggests the drumlins are a significant source of buffering capacity to nearby lakes.

Summary This study compared lake-sediment geochemistry with lake-water chemistry, and found a positive correlation between the concentration of calcium in sediment and the alkalinity of the water. This finding shows that lake-water chemistry is strongly influenced by the lake-sediment geochemistry, and it allows us to use the larger lake-sediment geochemistry database to identify areas of high and low natural buffering capacity. Areas with high buffering capacity tend to be near drumlins, which are typically cored by till derived from bedrock that includes carbonate. These results show that the area where the lowest pH measurements have been observed in the province (Kejimkujik) is also the area of the lowest buffering capacity. Regional

Lake Name

Latitude

Longitude

pH

Gran Alkalinity (mg L-1)

Ca (ppm)

Halfway Brook 44.9475 -62.465 5.7 0.09 1340

Grafton 44.385 -65.185 6 0.56 1160

Cobrielle 44.3175 -65.235 5.4 -0.76 1060

Pebbleloggitch 44.3025 -65.351 4.5 -1.9 220

Peskowesk 44.325 -65.3 4.9 -1.14 120

Big Dam (West) 44.4587 -65.287 5.1 -0.64 760

Big Dam (East) 44.45 -65.265 6.1 0.42 840

Frozen Ocean 44.45 -65.352 4.9 -.0.78 780

Channel 44.4356 -65.312 4.8 -1.14 1000

Peskawa 44.325 -65.375 4.7 -1.32 200

Beaverskin 44.3124 -65.3351 5.5 -0.57 460

Mountain 44.3254 -65.265 5.3 -0.55 1000

Back 44.2925 -65.275 5.5 -0.34 1600

Kejimkujik 44.375 -65.25 4.9 -0.98 540

Trefry 43.8301 -66.0475 6.6 2.82 1940

George 44 -66.05 5.9 -0.01 360

Killams 44 -66.08 6.2 0.54 980

Allens 43.95 -66.15 6.6 5.88 2760

Darlings 43.96 -66.13 6.4 3.93 2000

Bird 43.975 -65.95 6.7 2.86 1900

Kesse 44.032 -66.01 6.3 1.25 1160

Tedford 44.1 -66.02 6.4 1.22 1260

Lower Cornings 44.05 -66.08 6 0.5 1140

Table 1. Water chemistry data from T.A. Clair (Environment Canada), as reported in Ginn et al. (2007).

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geology needs to be included in models and policy surrounding the recovery of lakes from acid rain. References Clair, T. A., Ehrman, J. M., Ouellet, A. J.,

Brun, G., Lockerbie, D. and Ro, C.-U. 2002: Changes in freshwater acidification trends in Canada’s Atlantic Provinces: 1983-1997; Water, Air, and Soil Pollution, v. 135, p. 335-354.

Clair, T. A., Dennis, I. F. and Crosby, B. J. 2003: Probable changes in lake chemistry in Canada’s Atlantic Provinces under proposed North American emission reductions; Hydrology and Earth System Sciences, v. 7, p. 574-582.

Clair, T. A., Dennis, I. F., Scruton, D. A. and Gillis, M. 2007: Freshwater acidification research in Atlantic Canada: a review of results and predictions for the future; Environmental Reviews, v. 15, p. 153-167.

Environment Canada 2008: Envirodat Water Quality Database; http://map.ns.ec.gc.ca/envirodat/root/main/en/extraction_page_e.asp

Feetham, M., Ryan, R. J., Pe-Piper, G. and O’Beirne-Ryan, A. M. 1997: Lithogeochemical characterization of the Beaverbank unit of the Halifax Formation, Meguma Group, and acid drainage implications; Atlantic Geology, v. 33, p. 133-141.

Ginn, B. K., Cumming, B. F. and Smol, J. P. 2007: Assessing pH changes since pre-industrial times in 51 low-alkalinity lakes in Nova Scotia,

y = 308.72x + 935.28R2 = 0.7854

0

500

1000

1500

2000

2500

3000

-3 -2 -1 0 1 2 3 4 5 6 7

Gran Alkalinity

Ca

(ppm

) Lak

e Se

dim

ent

Series1Linear (Series1)

Figure 3. Graph of Ca (ppm) in lake sediments vs Gran alkalinity of lake water.

110 Mineral Resources Branch

Canada; Canadian Journal of Fish and Aquatic Science, v. 64, p. 1043-1054.

Jeffries, D. S., Brydges, T. G., Dillon, P. J. and Keller, W. 2003: Monitoring the results of Canada/U.S.A. acid rain control programs: some lake responses; Environmental Monitoring Assessment, v. 88, p. 3-19.

Keppie, J. D. (compiler) 2000: Geological map of the Province of Nova Scotia; Nova Scotia Department of Natural Resources, Minerals and Energy Branch, Map ME 2000-1, scale 1:500 000.

MacDonald, M. A. 1998: Overview of regional geochemical and geophysical data for the South Mountain Batholith: implications for

future mineral exploration; in Minerals and Energy Branch, Report of Activities 1997; Nova Scotia Department of Natural Resources, Report ME 1998-1, p. 75-92.

Soil Classification Working Group, 1998: The Canadian System of Soil Classification (third edition); Agriculture and Agri-Food Canada, Publication 1646, 187 p.

Stea, R. R. 2004: The Appalachian Glacier Complex in Maritime Canada; in Quaternary Glaciations - Extent and Chronology, Part II, eds J. Ehlers and P. L. Gibbard; Elsevier B.V. Amsterdam, The Netherlands, p. 213-232.

Stea, R. R. and Grant, D. R. 1982: Pleistocene geology and till geochemistry of southwestern

Figure 4. Map of calcium concentrations from lake geochemistry samples: samples below 935 ppm in red; above

935 ppm in green.

Report of Activities 2007 111

Nova Scotia (sheet 7); Nova Scotia Department of Natural Resources, Minerals and Energy Branch, Map ME 1982-10, scale 1:100 000.

Stea. R. R. and Pe-Piper, G. 1999: Using whole rock geochemistry to locate the source of igneous erratics from drumlins on the Atlantic Coast of Nova Scotia; Boreas, v. 28, p. 308-325.

Wade, J. A., Brown, D. E., Traverse, A. and Fensome, R. A. 1996: The Triassic-Jurassic Fundy Basin, eastern Canada: regional setting, stratigraphy and hydrocarbon potential;

Atlantic Geology, v. 32, p. 189-231. Wheeler, J. O., Hoffman, P. F, Card, K. D.,

Davidson, A., Sanford, B. V., Okulitch, A. V., and Roest, W. R. (Compilers) 1996: Geological map of Canada.; Geological Survey of Canada, Map 1860A, scale 1:5 000 000.

Whitfield, C. J., Aherne, J. Watmough, S. A., Dillon, P. J. and Clair, T. A. 2006: Recovery from acidification in Nova Scotia: temporal trends and critical loads for 20 headwater lakes; Canadian Journal of Fish and Aquatic Science, v. 63, p. 1504-1514.