B9: INFLUENCE OF GEOLOGY ON THE ACIDIFICATION STATUS OF ADIRONDACK LAKES AND STREAMS
Richard April
Department of Geology
Colgate University
Hamilton, NY 13346
Robert Newton
Smith College
Northampton, MA 01063
INTRODUCTION
The effects of acid deposition on surface water quality in the Adirondack Mountains have been studied intensively since the 1970's. Findings show that the acidification status of streams and lakes is strongly influenced by the geology and hydrology of watersheds. Deposition input quantity and quality, the mineralogy and depth of surficial materials, the hydrological properties of soils, groundwater flow paths, wetland processes, snowmelt, etc., all contribute to the final chemical composition of surface waters. This field trip will explore the processes and conditions that influence the acid-base status of surface waters as we visit lake watersheds in the eastern and central Adirondacks that display a wide range of geochemical, geologic and hydrologic characteristics.
As it turns out, for reasons to be described later, most acidic lakes and streams (pH 5) are found in the western and southwestern Adirondacks, and also at high elevation and remote areas of the high peaks region. With the 2002 NYSGA/NEIGC meeting and field trips based out of Lake George, it was not feasible to design a one-day trip to encompass all of these areas; they were simply too far away, or too inaccessible. However, the stops selected for this field trip will illustrate how both geology and hydrology play a pivotal role in determining the acid-base status of surface waters contained within watersheds. Lakes and ponds to be visited include Clear Pond, Harris Lake, Arbutus Lake, and Echo and Little Echo Ponds.

B9: INFLUENCE OF GEOLOGY ON THE ACIDIFICATION STATUS OF ADIRONDACK LAKES AND STREAMS

STOP 1. Clear Pond
Clear Pond is a 73 hectare drainage lake occupying a watershed located just a few miles north of the Blueridge Highway (county route 2). The area of the watershed is 12.06 km2 and the lake surface elevation is 582 m. A number of fish species are present, including brook trout, cutlips minnow, common shiner, bluntnose minnow, white sucker, pumpkinseed, and banded killfish. The watershed sits at the southern edge of the high peaks region and is underlain by bedrock consisting primarily of metanorthosite with some granitic gneiss. Thin section examination of two bedrock samples (CL 102 and CL-108) revealed the first to be a garnet-biotite-quartz-oligoclase gneiss (metapelitic) with 5% garnet, 5-10% red-brown biotite, 40-50% quartz and 40-50% oligoclase. The second, CL-108, was a gabbroic anorthosite with 90% plagioclase (∼An45) and 10% hypersthene.
Areas of thick (3 m) till and stratified drift cover approximately 33% of the watershed, with the remaining area covered by thin (<3 m) glacial deposits and bedrock outcrops. (Some nice till cuts can be seen on the right side of Elk Lake Road as we drive into Clear Pond.) Tills in the watershed contain on average 68% sand, 28% silt, and 4% clay. Quartz and plagioclase are the major minerals present in both the soil and glacial sediments. Vermiculite, mixed layer mica-vermiculite and kaolinite constitute the bulk of the clay mineral fraction. Heavy minerals make up 16.2% of the soil, with major constituents as follows: garnet 36.9%, pyroxene 34.4%, opaques 10.1 %, epidote 8.6%, and hornblende 3.8%.
Comparative values of the chemistry of Clear Pond in 1982-84 and 2002 are shown in Table 1. This is a circum-neutral lake with an ANC of about 100 µeq/L. Compared to other lakes we have studied (Driscoll and Newton, 1985; Table 2), Ca concentrations are relatively high but do not approach values found in some carbonate influenced lakes. More importantly, the high Ca concentrations are coupled with high SiO2 suggesting that the Ca is ultimately derived from silicate minerals rather than carbonates. Although there has been a significant decline in SO4-2 (20 µmol/L) over the past 20 years, ANC has remained relatively constant. The nearly one-third drop in SO4-2 suggests that we are seeing the effect of reduced SO2 emissions over this period of time. The lack of response in ANC is due to a corresponding decrease in base cations. Ca concentrations have declined by about 10% in twenty years and may reflect decreases in the flux of particulate matter to the watershed.
Questions to consider at this stop are:
1) Is Clear Pond a typical Adirondack drainage lake? Does the preponderance of metanorthosite over granitic gneiss influence the chemistry of the lake water?
2) Is it possible to tie the decline in SO4-2 in Clear Pond over the past 20 years to reductions in SO2 emissions? Has sulfate (and nitrate) accumulated in soils over time, and if so, how long will sulfate (and nitrate) continue to leach out of soils into stream and lake water?
3) Does the increase in NO3- reflect the fact that Knox emissions have remained relatively steady over the past 20 years, whereas SO4-2 has declined? Has the proportion of sulfuric and nitric acid in deposition changed over time?
4) Do lower Ca values reflect a decrease in the influx of Ca from particulate matter, or does it suggest lower weathering rates in the watershed? Ca depletion on exchange sites has been measured in the Adirondacks (and the Catskills, Lawrence et al., 1999) and it has been suggested that high acid loadings in the Adirondacks have resulted in significant depletion of base cations in the soils. Because ANC values for most Adirondack lakes have not risen over time, is it possible that base cation depletion in soils will delay recovery of Adirondack lakes?

Cross Hudson River

STOP 2. Harris Lake (and Woodruff Pond)
Harris Lake is a 116 hectare drainage lake occupying a watershed located just north of route 28N. The area of the watershed is 183 km2 and the lake surface elevation is 473 m. Over a dozen fish species inhabit the lake, including northern pike, common shiner, white sucker, brown bullhead, rock bass, small mouth and largemouth bass, and yellow perch. The watershed straddles a thin belt of metasedimentary rocks comprising calcitic and dolomitic marble, calcsilicate rock, quartzite and interlayered gneisses (Geologic Map of New York, NYS Museum). Harris Lake has not been included in any of our recent studies, so we cannot provide more detailed information concerning the geologic and mineralogic characteristics of the bedrock and glacial deposits within the watershed. However, in walking the southern shoreline one observes outcrops of calcsilicate rocks, containing calcium carbonate. In a class mapping exercise in 1990, Colgate students identified outcrops of calcsilicates on the northeastern shoreline, and also found outcrops of granitic gneiss in the northern areas of the watershed and on the easternmost shore of the lake.
Harris Lake is classified as a salt-impacted lake using the ALSC classification. This reflects the proximity of the highway and other anthropogenic influences within this watershed. Throughout the Adirondacks, developed lakes with cottages and their associated septic systems, roads, etc. have relatively high concentrations of Cl-. We stop at Harris Lake to consider the effect of the presence of carbonate-bearing rock on the chemistry of the lake water. As Table 3 shows, Ca values are high, but compared with other Adirondack lakes the values do not seem so unusually high to suggest that the chemistry of Hanis Lake is controlled solely by the dissolution of carbonate minerals. In fact the lake does not have a high enough Ca/Si ratio (2) to qualify as a carbonate controlled lake. Perhaps a better indicator of the impact of carbonate minerals in this watershed is the high ANC of about 200 µeq/L, which is one of the higher values observed in the Adirondack lakes we have studied.
Only Woodruff Pond, a drainage lake that sits a stone's throw to the south of Harris Lake and whose watershed also sits within the calcsilicate - marble belt, has a higher ANC (and Ca concentration; Table 2). Three bedrock samples from Woodruff Pond were thin sectioned and analyzed with the petrographic microscope. The first was an amphibolite with 40% hornblende, 10% hypersthene, 50% plagioclase and 1-2% biotite. The second was a quartz-diopside calcsilicate with 30-40% quartz, 20-30% diopside, 20% phlogopite, 5-10% calcite and a trace of tremolite. The third was a quartzite with 60% quartz, 20% hornblende, 5% garnet, 5% hypersthene, and 2% calcite. Areas of thin (<3 m) till and stratified drift cover most of Woodruff watershed; calcsilicate boulders are common. Quartz, plagioclase, K-spar, hornblende and pyroxene are the major minerals present in both the soil and glacial sediments. Vermiculite, mixed layer mica-vermiculite, kaolinite and talc constitute the bulk of the clay mineral fraction. Heavy minerals make up 21.1 % of the soil, with the major constituents as follows: pyroxene 23.5%, opaques 23.5%, hornblende 20.3%, garnet 11.9%, and epidote 5.8%.
On the basis of the thickness of the surficial deposits one might expect Woodruff Pond to have low ANC. In fact, Woodruff Pond has the highest ANC (410-480 µeq/L) observed of all lakes in the Regional Integrated Lake Watershed Acidification Study (RILWAS; Driscoll and Newton, 1985). The relationship between base flow discharge and sum of the base cations (SBC) for several of the RILWAS lakes is shown in Figure 10. All lakes except Woodruff Pond fall between end members Woods Lake, with a watershed dominated by thin till (average depth 2.3 m), and Panther Lake, with a watershed dominated by thick till (average depth 24.5 m) (April and Newton, 1985). Woodruff Pond has a base flow discharge that is almost as low as Woods Lake, yet it has an ANC of 410 µeq/L and SBC of 600 µeq/L. Woodruff Pond watershed provides a dramatic example of the influence that carbonate minerals have on surface water chemistry. Although the water moves rapidly along shallow flow paths, there is sufficient time for carbonate minerals to react. The dissolution of calcium carbonate is rapid and base cations, in this case Ca, are readily supplied to the drainage water. Hence, flow path is not the major determinant in regulating the acid-base status of Woodruff Pond. Rather, the presence of carbonate in the till, and also in the underlying bedrock, is the chief controlling factor.
Harris Lake watershed probably falls somewhere in between and can be considered a carbonate-influenced (but not controlled) lake. Although carbonate is present in the watershed materials, the entire watershed is not within the calcsilicate-marble belt. Indications are that at least half of the watershed area is underlain by granitic gneiss, and a1though some of the till in the watershed may contain carbonate minerals, much of the glacial cover is likely 'granitic' in composition and devoid of carbonate minerals. Water in contact with carbonate-bearing outcrops along the shore may contribute to increasing lake water Ca concentrations and ANC. However, the relative contribution of this water to the overall chemistry of Harris Lake, although significant, is probably small. We still have much more to do in Harris Lake watershed, but it serves as a good example of an area where surface water chemistry is influenced, somewhat, by the presence of carbonate minerals.
Questions to consider at this stop are:
1) How do we measure the influence of carbonate minerals on lake water chemistry? Is it the carbonate in the bedrock, or the carbonate in the till, or both, that is most important in raising ANC and pH?
2) Does the Ca to Si ratio in surface water give us an indication of the amount of carbonate mineral weathering taking place? That is, can carbonate influenced water be differentiated from non-carbonate influenced water by calculating the molar ratio of Ca2+ to H4SiO4?
3) With ANC values of 200 µeq/L or more, are lakes like Harris and Woodruff essentially insensitive to changes caused by acid deposition?
4) Can we artificially create carbonate-influenced lakes by liming techniques? And is it best to lime the lake, or the watershed?

STOP 3. Marble Outcrop

Marble outcrops along side of Rt 28N

Adirondack Visitor Center -- Lunch Stop

STOP 4. Arbutus Lake - Huntington Wildlife Forest
Arbutus Lake is a 49 hectare drainage lake occupying a watershed located in the Huntington Wildlife Forest. The area of the watershed is 3.75 km2 and the lake surface elevation is 513 m. The watershed is underlain by granitic gneiss, but small outcrops of amphibolite and biotite-rich gneiss have been identified. The watershed has been gauged at the lake outlet since October 1991 with a V-notch weir. Huntington Forest has also participated in the National Atmospheric Deposition Program (NADP) and the National Trends Network (NTN) since Oct. 31, 1978 (see Figures 12 and 13 for trends in sulfate and pH). In May, installation of an EPA CASTNET dry deposition monitoring system was completed. Time permitting, we will take a short hike to view these instrumentation set-ups.
Areas of thin (<3 m) till cover approximately 80-90% of Arbutus Lake watershed; bedrock outcrops are numerous. Tills contain on average 73% sand, 24% silt,and 2% clay. Quartz, K-spar and plagioclase are the major minerals present in both the soil and glacial sediments. Vermiculite, mixed layer mica-vermiculite and kaolinite constitute the bulk of the clay mineral fraction. Heavy minerals make up about 16% of the soil, with major constituents as follows: hornblende 33.6%, opaques (ilmenite and magnetite) 29.5%. pyroxene 20.1 %, garnet 7.2%, and apatite 5.4%.
Comparative values of the chemistry of Arbutus Lake in 1982-84 and 2002 are shown in Table 4. This is a moderately sensitive water body with an ANC that ranges between 60 - 80 µeq/L. Ca concentrations are relatively high, and may reflect the presence of amphibolitic bedrock, and the presence of carbonate-bearing boulders in the till, especially up in the area around Archer Creek. Unlike Clear Pond, the 20 µeq/L reduction in SO4-2 here has resulted in a concomitant rise of 20 µeq/L in ANC.
Arbutus Lake watershed is a good example of a system in which the combination of thin till, short flow paths, and short residence times, which usually gives rise to surface waters with low ANC and high sensitivity, is modified by the presence of minerals in the watershed that weather at moderately fast rates (e.g., hornblende, pyroxenes, and garnet) to provide alkalinity to drainage waters. In June of 1989-1991, Colgate students mapping and collecting water samples in Arbutus watershed measured the ANC of all running inlet streams in the watershed. Most streams had ANC values that were in the range of 20-80 µeq/L, but several streams had ANC's that were as high as 160-180 µeq/L. The highest values were measured following a particularly dry spring, and lowest values were measured following a rainy period of several days. During periods of low precipitation, or drought, base flow becomes a proportionately greater fraction of the drainage water reaching the lake and ANC values in Arbutus Lake can rise to their highest levels. Highest ANC values also seemed to be concentrated in the northern part of the watershed, near or within the Archer Creek subcatchment. We are planning to conduct future field studies in this part of the watershed to better characterize the composition of the bedrock, till, and soils.
Questions to consider at this stop are:
1) How does one measure the contribution of mineral weathering to the ANC of surface waters? Can the relative rates of mineral weathering in soils and till be measured, and if so, how do we measure or calculate the relative contribution of each mineral to ANC?
2) Why did a reduction in SO4-2 result in an increase in ANC here and not at Clear Pond?
3) Several calcsilicate belts run east-west in the vicinity of the Arbutus watershed. Just a few miles to the east we noted the influence these rocks have on the chemistry of Harris Lake and Woodruff Pond. How much calcsilicate float is incorporated in the till of Arbutus watershed, and to what degree does this float influence the composition of soil and till in Arbutus? One clue, of course, is to look at the heavy mineral content of the till and soil, which includes significant quantities of hornblende and pyroxene.
4) A second possibility is to look at the bulk chemistry of till samples from the watershed. For example, XRF analyses of the 2 mm (sand) fraction of 13 till samples from the Big Moose watershed, a watershed underlain by granitic gneiss and containing many acid sensitive lakes, has a mean CaO value of 1.64 wt. %. The mean CaO value for till collected from watersheds underlain by metanorthosite or calcsilicates (including samples from Clear Pond and Woodruff Pond) is 4.13 wt. %. Till from Arbutus Lake watershed has a CaO content of 2.54 wt. %, an intermediate value. Does this indicate that source rocks for the till were both granitic gneiss and calcsilicates, and that the bulk chemistry reflects a mixture of the two? MgO values also suggest the same, as shown below.
Till from watersheds in: | wt. % Cao | Wt.%MgO
Granitic gneiss | 1.64 | 0.72
Metanorthosite/calcsilicates | 4.13 | 2.02
Arbutus Lake | 2.54 | 1.49

STOP 5. Echo and Little Echo Ponds
Little Echo Pond is a 0.8 hectare brown water seepage lake surrounded by thick (∼8 m) peat (sphagnum bog) deposits. Its watershed is estimated to be about 0.11 km2. Echo Pond, a stone's throw to the NNE, is a slightly larger seepage lake but without peat deposits edging the shoreline. The lake surface elevation of both ponds is 481 m. No fish species are present in Little Echo Pond; no fish population data is available for Echo Pond. The two Ponds sit in an outwash area at the eastern edge of the anorthositic massif, in the Fish Creek Ponds recreational area. Thin section examination of one bedrock sample taken in the vicinity (LE-105) revealed a typical Adirondack gabbroic anorthosite with 85% plagioclase (∼An45, 7% magnetite, 3% green clinopyroxene, and 5% garnet. Outwash deposits of quartz, K-spar, and plagioclase are composed of 97% sand and 3% silt. Heavy minerals constitute 12.8% of the sand fraction and include 46.3% opaques, 20.1% hornblende, 10.6% pyroxene, 5.6 % garnet, and 2.3% zircon. What little clay is in the soil consists of vermiculite, mixed layer mica-vermiculite, kaolinite, and possibly a trace of talc. Values of the chemistry of Little Echo Pond in 1982-84 and 2002, and Echo Pond in 2002 are shown in Table 5. What is immediately striking is the difference in water chemistry between these two seepage lakes located not more than 100 m apart. Little Echo is an acidic pond with an ANC that averages about of -50 µeq/L (range from ALSC data is-79,6 to -7.4 µeq/L). The pH is 4.3, the SBC and dissolved Si concentrations are low, and the DOC is high. Concentrations of organic anions are high and are comparable to SO42- (on an equivalence basis) (Driscoll and Newton, 1985). The kettle hole that formed Little Echo Pond and the surrounding peatland has almost completely filled in. There is at least one ephemeral channel that drains the lake-peatland system during periods of high water. This lake is virtually isolated from the groundwater in the surrounding outwash aquifer system due to the extremely low hydraulic conductivity of the peat and bottom sediments, both of which form a nearly impermeable barrier around the lake-edge and bottom. Recharge of lake water must depend primarily on direct precipitation, and from runoff through the upper "active" zone of the peat. Although much of the acidity of the pond can be attributed to atmospheric deposition of H2SO4, it appears that the peat deposits and the sphagnum mat surrounding the pond release substantial quantities of organic acids that contribute to the overall acidity. Sulfate concentrations in both the seepage lakes are much lower than in the drainage lakes both in the 1982 and 2002 data (Table 5 and Table 2). This is due to sulfate reduction occurring in the bottom waters of these lakes. The high DOC and long residence time of waters in seepage lakes provide conditions conducive to sulfate reduction. Measured profiles of dissolved oxygen show anoxic conditions in the deeper water of Little Echo Pond. Sulfate reduction is an ANC producing reaction making these lakes less acidic than they would have been had the water column been fully oxygenated. Silica concentrations are very low in seepage lakes. This is likely due to a combination of a reduced input of groundwater and the removal of silica by the action of diatoms. The long residence time of water in seepage lakes makes it likely that diatoms will remove a substantial amount of silica from the water column. Echo Pond is marginally classified as a "flow-through" seepage lake. There is sufficient groundwater contribution in this class of lake to provide enough ANC to neutralize acidic deposition. However, with a pH of 6.4, an ANC of 40 µeq/L, and relatively low SBC compared with drainage lakes we have studied (Table 2), the chemistry of Echo Pond is still greatly influenced by direct precipitation. Despite the good hydraulic connection between the groundwater and the lake, groundwater inflow is limited. This is likely due to the low hydraulic gradient within the groundwater system. Therefore, the influence of direct precipitation is still relatively high especially during rainy periods and snowmelt. We don't have much time series chemical or hydrologic data from Echo Pond, so, at best, our comments should be taken as suggestions based on work completed at similar seepage lakes. Questions to consider at this stop are: 1) Why doesn't Echo Pond have a surrounding peat layer? 2) Would Little Echo Pond be acidic if there was no acid rain? 3) How important is sulfate reduction in generating alkalinity? 4) Can we determine the relative proportions of groundwater and precipitation entering Little Echo and Echo Ponds?