Spatial Trends in the Physical and Bulk Sediment Chemistry Composition of Sediments in Andover Lake, Connecticut

Transcription

1 Spatial Trends in the Physical and Bulk Sediment Chemistry Composition of Sediments in Andover Lake, Connecticut Author Jennifer N. Vinci Advisor Dr. James A. Hyatt A report submitted to the department of Environmental Earth Science in conformity with the EES 480 requirements. Eastern Connecticut State University 83 Windham Street Willimantic, CT

2 ABSTRACT Examination of fourteen sediment cores (to 1.5m in length) from Andover Lake, CT, dammed in 1927, reveal spatially variable trends in physical properties and bulk sediment chemistry within three well defined stratigraphic Units. Basal Unit I varies with location and consists of coarse grained gravel and sands to sandy silts that often retain buried soil horizons indicating a terrestrial setting prior to damming. These sediments are draped by up to 0.88 m of stratified sand and silt (Unit II) at locations near primary inflows to the lake indicating rapid deposition soon after damming. This facies is absent in cores collected from the deep, north end of the lake. Fine-grained and massive lacustrine muds (Unit III) with high moisture, organic and inorganic contents cap the sequence at all locations. At the shallow south end of the lake, however the uppermost 0.13 m of Unit III contain discrete layers of sand. Moisture and carbon concentrations are higher for Unit III than Units II or I, although buried soil A-horizons in Unit I also have high concentrations. Bulk sediment chemistry concentrations by aqua-regia digestion and ICP-AES are also highest in Unit III, except for Cr, Mg, and Ni. 210 Pb dating for a single core at the deep north end of the lake, indicates a pronounced increase in mass accumulation beginning between 1964 and 1972, a time when power line construction across the primary inflow likely increased sediment delivery to the lake. Cross lake correlations, based on Al concentrations in the 210 Pb dated core suggest that sediment rates in the shallow south end of the lake has always exceeded that in the deep north end of the lake. Increasing rates of mass accumulation as indicated by 210 Pb results from at the deep north end of the lake are likely associated with the progradation of a small sandy delta at the shallow south end of the lake thereby, bringing the source of sediment closer to the 210 Pb coring location. 1

3 TABLE OF CONTENTS ABSTRACT TABLE OF CONTENTS... 2 LIST OF FIGURES... 3 LIST OF TABLES... 3 INTRODUCTION... 4 PREVIOUS RESEARCH AT ANDOVER LAKE... 5 RESEARCH METHODS... 7 Field Methods... 7 Laboratory Methods RESULTS Core Descriptions Core Core Core Core Core Core Core Core Core Core Core Physical Trends with Depth in the Sediment Column Chemical Trends with Depth in the Sediment Column DISCUSSION Impact of Buried Soil Horizon Spatial Trends in the Lake Pb Chronology and Mass Sediment Loading to the Lake CONCLUSIONS / SUMMARY ACKNOWLEDGEMENTS REFERENCES

4 LIST OF FIGURES Figure 1: Location Map of Andover Lake in relation to Rt. 6 and Rt Figure 2: Digital Elevation Model of Andover Lake including four major bottom types, and submerged stream bed. Figure from Hyatt, Figure 3: Coring location for all core collected from Andover Lake between 2000 and Figure 4: Cross sectional profile of submerged stream channel showing location of core collection for JVAND03-01 to JVAND Figure 5: Core collection equipment; (a) unused core barrel with aluminum core catcher installed (b) percussion core set up including: slide hammer, boom, and winch for removal..9 Figure 6: Vertical extruder apparatus for piston cores, and subsampling, polyethal bag..10 Figure 7: Photographic logs for core collected in Andover Lake between August 2000 and August (l) shows coring locations. Cores JVAND-01-03, collected from the same location as (k) are not shown 11 Figure 8: Physical logs for core: (a) (b) (d) (e) (f) (g) (h) (i) (j) (k) , including texture, moisture, organic carbon and inorganic carbon Figure 9: Chemical logs for core: (a) (b) (c) (d) (e) (f) (g) (h) (i) (j) (k) , including several elemental concentrations Figure 10: 3-Unit model of deposition for a transect across Andover lake, and corresponding Al values 26 Figure 11: Plots correlating 210-Pb and several elemental concentrations for piston core JVAND Filled dots indicate measured 210-Pb samples. (Modified from Hyatt, 2003).28 Figure 12: Air photographs of Andover Lake indicating power line construction between (a) 1952 and (b) Table 1 Table 2 Table 3 LIST OF TABLES Core Information..7 Physical mean and standard deviation for Units I, II and III within all Core.. 16 Sediment Accumulation Rates

5 INTRODUCTION Andover Lake in Andover, Connecticut (Figure 1) is a complex ecosystem with a variety of habitats that sustain numerous aquatic species. As such, the health of the lake is very important to organisms in the lake as well as to people living around the lake who value it s natural beauty. Many factors influence the health of the ecosystem including sediment on the lake bed. N 0m 500m Figure 1: Location Map of Andover Lake in relation to Rt. 6 and Rt. 316 The physical and chemical characteristics of lake sediments commonly vary with location. For example, coarse sediment and associated chemical constituents carried in suspension by waters entering the lake are often deposited rapidly near shore, as the competence and capacity of the flow decreases (Greve, 2001). In contrast, fine grained sediment which commonly have elevated nutrient and trace element concentrations, often are focused to deep water locations in the lake. Therefore, paleoenvironmental records contained within lake sediments, as well as the potential impact of lake sediments on present ecosystems varies spatially. Several previous undergraduate research studies have characterized nutrient levels, chemical concentrations, and physical properties of sediment cores collected at differing locations within Andover Lake. However, there has been no attempt thus far to develop a lake wide model of deposition that considers all available cores collected in the lake. The purpose of this study therefore, is to examine sediment cores collected from several locations in the lake in order to: 1. Analyze and characterize the spatial variability of lake bottom sediments in 14 cores collected throughout the lake. 2. Define lake wide physical and chemical stratigraphic trends 3. To relate these changes to new 210 Pb dating for one core collected in the summer of

6 PREVIOUS RESEARCH AT ANDOVER LAKE Andover Lake occupies approximately 156 hectares and has a watershed of 2,502 acres. In common with many small dammed lakes, Andover Lake has a relatively small volume of water when compared with the size of its watershed (Knoecklein, 1996). The lake was created in 1927 by the damming of a small tributary to the Hop River. This dam, at the north end of the lake, provides a controlled spillway which is used each winter to lower lake levels by approximately 1-1.5meters. The lake is maintained by the Andover Lake Management Association (ALMA), and the Andover Lake Property Owners Association (ALPOA). Previous research at the lake, funded in whole or in part by ALMA, has focused on water column characteristics (Knoecklein 1996, 1997, 1998, 1999, 2000, 2001, 2002), and the nature of the lake sediments (Hyatt 2001, Carlson 2001, McCann 2002). As well, Tokarz (2002) has evaluated the extent to which watershed land use has influenced nutrient loading to the lake. Work by Knoecklein ( ) examining water quality in the lake was initiated following large algal blooms in 1996 and These studies examine the hydrology and physical parameters of the water column including turbidity, temperature, dissolved oxygen and a variety of chemical parameters. Annual turbidity of the lake fluctuates about a progressively increasing mean until late fall months, and then drops as temperatures cool. Knoecklein (1996) found similar trends at three sampling stations in the lake. He reported little variation in oxygen levels to depths of two meter throughout the lake. However, at locations deeper than 3 meters bottom waters became anoxic for a longer portion of the year in 1996 and 1997 than in According to Knoecklein (2001) phosphorus levels in deep water samples from Andover Lake also show a significant increase during in comparison to previous years. Initial work by Hyatt (2001) focused on bathymetric mapping of the lake bed and constructing a digital elevation model (DEM) for the lake that identified four major bottom types (Figure 2). The first bottom type consists of soft sediments with finegrained mud gyttja which typically has a high organic content. Hardy sandy and /or rocky bottom conditions were noted primarily around the wave-worked margins of the lake. Weedy bottom conditions occur primarily in the south end of the lake. The final bottom condition was confirmed by grab sampling, consists of brush covered bottom conditions (Hyatt, 2001). Several studies of lake sediments have been undertaken by Dr. Drew Hyatt and students from Eastern Connecticut State University. These studies have examined the physical and chemical characteristics of sediment cores collected at several locations in the lake (Figure 2). Carlson (2001) identified three sedimentary units at the deep north end of the lake. This included pre-lake, transitional sediment deposits, and modern lake sediments deposited since the lake was dammed. Chemical analysis indicates ratios of Fe:P less than 10 which, in the presence of low dissolved oxygen concentrations release phosphorus to the water column (Scheffer, 1998). Although significant changes in physical properties occur with changing water depth, no such trend in chemistry was identified by Carlson (2001). A significant increase in total organic carbon did occur with depth, although no systematic change was evident with location in the lake (Carlson, 2001). 5

7 Figure 2: Digital Elevation Model of Andover Lake including four major bottom types, and submerged stream bed.. McCann (2002) examined sediments at the shallow south end of the lake. He reported a difference in basal pre-lake sediment due to differing pre-lake conditions at each location. Sediments were much coarser at the bottom than reported by Carlson (2001), and McCann (2002) identified several fining upward sequences. Chemical analysis indicated enriched concentrations within modern sediments related to pre-lake sediment at depth. McCann (2002) attributed this to increased deposition of fines which have a greater surface area, thus increasing the quantity of absorbed and precipitated elements. He also reported Fe:P ratios greater than ten which, unlike Carlson s (2001) findings, suggests that there is sufficient iron present in sediments to bond phosphorus, at least under oxic conditions 6

8 RESEARCH METHODS Field Methods A total of 14 cores have been collected at Andover Lake (Figure 3). Of these cores, 11 were obtained by previous students using percussion coring techniques in August 2000, July 2001, and June 2002 (Table 1). Three additional cores were collected for this study from the north end of the lake on August 18, 2003 using piston and percussion coring techniques (Cores JVAND03-01 to 03). These cores specifically sampled lake sediments deposited within a relict stream channel that drained a small swamp which existed before the lake was dammed in Figure 4 depicts a cross section for the lake bottom mapped using a Lowrance X khz echo sounder at the time of sampling. A percussion core 35cm in length was collected using equipment illustrated in Figure 5a in order to examine the stratigraphy of the site. However, only piston core JVAND is analyzed in detail. The piston and percussion cores were collected within 3m of each other using equipment illustrated in Figure 5b. Core # Date Collected Table 1: Core Information Core Length (cm) Water Depth (cm) Bottom Type 1 Source Aug Soft Seds Hyatt July Soft Seds Hyatt (pers. com 2003) July Weeds Hyatt (pers. com 2003) July Soft Seds Hyatt (pers. com 2003) July Brush Hyatt (pers. com 2003) July Brush Hyatt (pers. com 2003) July Soft Seds Carlson July Soft Seds Carlson June Soft Seds McCann June Soft Seds McCann June Soft Seds McCann 2002 JVAND03-01 Aug Soft Seds This Study JVAND03-02 Aug Soft Seds This Study JVAND03-03 Aug Soft Seds This Study 1 Bottom types are based on comparison of GPS location of core (Figure 3) and map of bottom types (Figure 2). Depth to lake bottom for all cores was measured using a tape measure and diskshaped weight. Coring locations were determined with a TRIMBLE geoexplorer global positioning system. Data files were differentially corrected and are considered accurate within +/- 3m. 7

9 Figure 3: Coring location for all core collected from Andover Lake between 2000 and Figure 4: Cross sectional profile of submerged stream channel showing location of core collection for JVAND03-01 to JVAND

10 (a) W S B (b) Figure 5: Core collection equipment; (a) unused core barrel with aluminum core catcher installed (b) percussion core set up including: slide hammer (S), boom (B), and winch (W)for removal. 9

11 Laboratory Methods All cores were stored in a cold room at 5 o C prior to analysis. Laboratory methods described below apply specifically to cores JVAND03-01 through JVAND03-03 although similar techniques were used for analysis of all cores. Both piston cores were extruded at 1cm depth intervals (Figure 6). Each 1 cm sample was placed in a previously numbered and tarred polyethal sample bag. The sample was homogenized within the bag and extruded through a slit into clear pyrex vials. Samples were then dried at 105 o C for 24 hours to determine moisture content as a fraction of the total wet mass of the sample. Figure 6: Vertical extruder apparatus for piston cores, and subsampling polyethal bag. Subsamples from the 2003 piston cores were submitted to MyCore Scientific Inc. for 210 Pb analysis and to Chemex Inc. for trace element analyses by inductively coupled plasma (ICP) atomic emission Spectroscopy (AES) techniques ( Duplicate samples were also analyzed for moisture content, bulk density, and organic/inorganic carbon by loss on ignition techniques (Dean, 1974). The percussion core was split lengthwise using HCL washed tools so as not to contaminate the sediments. Half of the core was designated as the working half for observation and subsampling and detailed analysis similar to that described above. All cores in Table 1 were logged for texture, Munsell color and sedimentary characteristics. Texture was measured visually using a low (3X) power microscope and grain size chart noting maximum, minimum and modal grain size. Each core was sampled above and below abrupt contacts or within subtle transition zones in the cores. The remaining halves of the percussion cores were dried, shaved and digitally photographed in order to construct photographic logs for all core (Figure 7). 10

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13 RESULTS The following provides (1) a brief description for 11 of the 14 cores listed in Table 1, (2) a summary of physical characteristics, and (3) a summary of chemical characteristics for the cores. Core logs are grouped by location in the lake with group 1 including cores collected from the southern shallow end of the lake (core to 03, and , 03) and group 2 including cores collected the deeper northern end ( , 04, 05, 06, 07, and ). Previous studies (McCann, 2002 and Carlson 2001) have recognized three sediment units in Andover Lake. These include basal Unit I, interpreted as pre-lake (i.e. deposited prior to damming in 1927); a transitional Unit II, thought to reflect early deposition and mixing that occurred as lake levels first increased; and finer grained sediment (Unit III) deposited in the modern lake. Core Descriptions The following descriptions are based on the reexaminations of all core using unit names identified above. It should be noted, however, that several of the deep core, ( , 05, 06,07, ) lack a well developed Unit II, in part reflecting the recognition of submerged soil horizon at the top of unit I. Core (100cm), (Figure 7a): Basal Unit I (> 12 cm thick) is composed of very coarse grained brown (7.5YR 4/4) gravel that fined upwards from a large cobble >7cm in diameter at the bottom of the core. The upper Unit I contact is abruptly overlain by 20cm of coarse sediment (Unit II), with stratified beds <2cm thick. Fine grained layers are black (10YR2/1) while the coarse grained sands are yellowish brown (10Y3/4). A cobble (7 x 5 x 3cm) is located at a depth of 10cm. Very few organics are present in either unit II and I. Unit II is transitionally overlain by about 58 cm of fine-grained clay (Unit III) that darkens upwards to (5YR 2.5/1). Unit III is massive and contains roots, birch bark and undecomposed wood chips with concentrations at 26cm and 17cm. Core (119cm), (Figure 7b): Unit I consists of coarse pebbly sand ranging from dark brown (10YR3/3) to dark yellowish brown (10YR 4/4) These sands are dense and lack organic fibers although a wood chip fragments occurs immediately above a sharp contact with Unit II at108cm. Unit II consists of stratified pebbly-sand and clayey mud. Layers range from 1-5cm in thickness; the sands are dark-yellowish brown (10YR 4/4) and the organic rich muds are dark brown (10YR 2/2). An accumulation of subangular 2cm clasts occurs at 62-64cm. These sediments are overlain by 52cm of organic rich, silty and black (10YR 2/1) muds (Unit III). Organic fragments include twigs, leaf debris, and wood chips. Sandy pulses are dispersed throughout the top 30 cm. Core ( 150cm), (Figure 7c): Only 3 cm of Unit I is present, consisting of muds and poorly decomposed black (7.5YR 2/0) leaves and wood fragments. These materials are abruptly overlain by stratified sands and silt with layers ranging from 2-5cm thick. This Unit II is 88cm thick are varies from dark brown (10YR 3/3) to black (10YR 2/1). Two coarse sandy zones occur at 82 and 134 cm, both 3-4cm thick. An accumulation of mildly decomposed 2-3cm long twigs, is located at 102cm depth. These sands are abruptly overlain by 57 cm of mostly massive mud (Unit III), with small sandy layers increasing and becoming coarser in the uppermost 13cm of the core. Unit III lightens 12

14 upwards from black (10YR 2/1) to very dark brown (10YR 2/2) within the upper sandier section of the unit. Organic fragments including roots and leaves are abundant increasing in concentrations upwards. Core ( 67cm), (Figure 7d): This core did not intersect Unit I. Unit II (>36cm thick) is composed of weakly stratified coarse sands and finer-grained silts. Sand layers average 10cm thick and generally fine upwards. Sands within Unit II range from weak red (2.5YR 4/2) to very dark gray (5Y3/1) while the remainder of the unit is dark brown (10YR 3/1). Unit II is abruptly overlain by 31 cm of very fine grained muds with some small wood and root fragments (Unit III). These organic fragments become more abundant upwards in the core. A 5 cm layer of coarse sand bed with a sharp erosive base and normal grading occurs at 10-15cm depth. These sands are dark yellowish brown (10YR 4/4) where organic rich muds in Unit III above and below this sand layer are black (10YR 2/1). Core (118cm), (Figure 7e): Unit I consists of fine grained sand, including some small gravel. Root traces are present throughout, as are mottled zones between 102 and 113cm. These mottles range in color from gray (2.5 Y 3.5/1) to black (5Y 2.5/2). The mottled silty sand grades upward to clay-rich very dark brown (10YR 2/2) sediments with terrestrial root fragments. The contact between Unit I and overlying lacustrine mud and fine sands (Unit II) is marked by the gold mica chips. Unit II has a higher clay content than Unit I and it becomes darker upwards grading from yellowish brown (10 YR 5/4) at the bottom to very dark grayish brown (2.5 Y 3/2) at the top. A 7 cm thick, fine grained sand layer occurs between 66 and 73 cm depth and wood chips occur at 63 and 70 cm depth. Unit III abruptly caps the sequence and deposits become very dark brown (10YR 2/2) to black (10YR 2/1) and wood and aquatic plant reminents are abundant. Core (62 cm), (Figure 7f): Basal Unit I is 34 cm thick and contains fine-grained sands and muds. Unit I may be subdivided into lower massive sandy sediments that are light brown (2.5YR 5/3) with clasts up to 1.5cm in diameter. The upper part of Unit I is black (10YR 2/1) and contains silty-mud with very fine terrestrial roots thought to be indicative of soil development. Lake transitional sediments (Unit II) are absent. As such Unit I is abruptly overlain by 28cm of mostly dark brown (10YR 2/2). Massive mud with some organics of rootlets and wood chips. Core (40cm), (Figure 7g): Unit I (>26cm thick) consists of fine-grained dark brown sands (10YR 2/2) which fine upwards and are capped by 2 cm of fine silty sand containing a 1cm diameter pebble. These sands and silts grade upwards into black (10YR 2/1) fine silt that contain hairlike roots that likely reflect soil development. Gradually overlaying Unit III is 13 cm of black (10YR 2/1) massive muds that contains roots throughout. Some sand is present in small layers (0.5 cm thick) increasing in concentration towards the top of the core. Core (116 cm), (Figure 7h): Unit I (98cm thick) contains two subunits. Unit IA is grayish brown (7.5 YR 2.5/1), 16cm thick coarse, and consists of subrounded gravel (clasts <2cm) which fines upwards and contains layer of mud (3 cm thick) and 63 cm of medium to fine sands ranging from brown (7.5YR 4/2) to black (10YR 2/1). This is overlain by Unit IB a medium grained (0.5-1cm thick) weak red (2.5YR 4/2) sandy unit that gradually grads into black (10YR 2/1), with abundant terrestrial fragments and leaf 13

15 roots (34 cm long) running vertically. Unit I is overlain by 18 cm of fine grained, lacustrine mud containing some wood, twig with undecomposed bark and roots ranging from black (10YR 2/1) to very dark brown (10YR 2/2) at the bottom of the unit. Core (63cm, (Figure 7i): Basal Unit I (52 cm thick) contains medium grained sands that decrease in organics, grain size, and increase in silt content upwards (Unit IA) and ranges from dark grayish brown (2.5YR 4/2) to brown (10YR 2/2). A layer of coarsening upwards sand (5cm thick) containing roots and twigs is located at 51-55cm depth. This is overlain by clayey mud (Unit IB) with wood chips and twigs (<3cm long). Unit IB is, in turn, is overlain by 11 cm of black(10yr 2/1) massive mud (Unit III) with some twigs and bark. Core (71cm), (Figure 7j): Unit I (35cm thick) consists of alternating layers of light gray (5Y 4/1), gray (2.5Y 4/1), and greenish (5Y 5/4) sands containing dark red (10 R 3/6) circular forms with abundant mica, and organic rich blotches of mottled black (10YR 2/1) sand, and a large pebble (2x1x1cm). Unit IA is overlain by 22cm of dark brown (10YR 3/3) sands and muds with abundant undecomposed twigs with preserved bark (Unit IB), and overling massive, fine grained black (10YR 2/1) lacustrine mud (Unit III). Core (88cm), (Figure 7k): Basal Unit I (75cm thick) contains stratified finegrained basal sands interbedded with dark brown (10Yr 3/2) very fine-grained mud (Unit IA) with four fining upward sequences beginning at depths of 64cm, 45cm, and 34cm depth. Unit IA is overlain by very dark brown (10YR 2/2) mud with few twigs and an abundance of rooty hair-like organics, and weakly dispersed sands (Unit IB). These sediments are capped by 13 cm of black (10YR 2/1), very fine-grained massive mud (Unit III) containing undecomposed twigs located at the bottom of the unit. Physical Trends with Depth in the Sediment Column In addition to the descriptive data presented above, core logs (Figure 8a-k, Table 2) indicate that sandier sediments have lower moisture content than do fine grained and/or organic rich sediments in all core. Moisture contents in Unit III are higher than Units II and I, which contain coarse grained sediments. For example, core collected within the shallow bay, has moisture content mean values of 50.1% in Unit III 23.3 % in Unit II and % within Unit I. Similarly, core , collected in the deep, northern end of the lake also has decreasing moisture contents, with depth (e.g core has mean moisture contents of in Unit III, and in Unit I). In some cases moisture content increases at depth due to the presence of a more organic-rich buried soil A-horizon. For example, core has a mean moisture content of 52.7 in Unit IB which is high in relation to core which do not contain the buried soil zone. Organic and inorganic carbon show similar trends, and in fact Organic carbon may contribute to high moisture contents (Table 2, Figure 8b,c,g,i,j,k). For example, core shows a decrease in organic carbon with depth; with mean organic carbon concentrations of 25.1 in Unit III, and 6.11 in Unit I. Most core that contain Unit II ( , , ) have decreasing mean Organic carbon concentrations with depth from Unit III to Unit II, but increase in Unit I, where submerged soil zones are present. Inorganic carbon also displays the similar decreasing trends; in core , mean inorganic concentrations are 3.09 in Unit III, and decrease to 0.63 in Unit I. 14

16 Chemical Trends with Depth in the Sediment Column Elemental chemistry varies with depth in each core, and to a certain extent, between cores. Examination of all core depicted in Figure 9a-k indicates that most core have higher element concentrations (e.g examine trends for P and Pb in Figure 9c) in Unit III than in underlying Unit II or I. For example, mean concentrations of P for core are 755 ppm in unit III, 313 ppm in Unit II and 230 ppm in basal Unit I. P varies slightly with each core due to the presence, or absence of the buried soil horizon. For example in core , P trends differ from other cores. With the exception of the very top of where P concentrations are high, P values remain high with depth. This likely reflects the presence of fine-grained sediments within lower units. There are some exceptions; Pb in Core decreases upwards throughout Unit III (as discussed in a later section). In most cases, elemental concentrations are more variable in Unit II and I than in Unit III. The standard deviation for Al concentrations from Core are in Unit III in Unit II and in Unit I. In general there is a large contrast between high concentrations in Unit III and lower concentrations in underlying sediments (i.e. Unit I or II) for shallow core collected in the south bay. This contrast is weaker for mid-lake core ( to 05) and is strong again for deep core at the north end of the lake. While general trends hold for most elements there are some exceptions. In particular, Cr and to a lesser extent Mg and Ni have similar or higher concentrations in Unit II and I than in Unit III. Also, Core and differ chemically from others. Elemental concentrations for Unit III in core , collected from a weedy part of the lake bed are lower than Unit I likely because of the presence of a buried, organic rich soil horizon in Unit I. Core also shows elevated concentrations for the buried soil horizon at the top of Unit I and higher concentration for almost all elements at depth. DISCUSSION Impact of Buried Soil Horizon Analysis of sediment core reveal considerable spatial variability in the physical (Figure 7, 8) and chemical (Figure 9) character of sediments with depth and with location in the lake. A 3-unit depositional model, developed by Carlson (2001) and adapted by McCann (2002), generally holds for the entire lake. However, core analysis in this study underscores the importance of identifying buried soil horizons in order to estimate the thickness of sediments that have accumulated in the lake since damming. These buried soil horizons mark the upper boundary of pre-lake (Unit I) sediments separating them from overlying transitional deposits (Unit II) and modern lake sediments (Unit III). The contacts between these boundaries while subtle, may be recognized by: (1) a significant change in color, with darker colors indicative of higher organic contents in the buried soil horizon; (2) the beginning of wood chips in overlying sediments due to the dispersing of wood by rising lake waters; (3) in some cases elevated organic, inorganic and moisture concentrations and/or (4) increased elemental concentrations. In particular Al, Cu, Fe, Mn, V, Zn, Pb and P concentrations often increase above the soil horizon. However, while many elements increase upwards at the 15

17 base of Unit III (i.e. above the buried soil horizon) concentrations decline or remain similar in the upper part of this unit. Table 2: Physical mean and standard deviation for Units I, II and III within all Core Moisture (% wet wt) Organic Carbon (LOI % dry wt) Inorganic Carbon (LOI % dry wt) Unit core # samples mean std mean std mean std Unit bay Unit road Unit delta Unit weeds Unit softsed Unit bite Unit softN Unit brush Unit wall Unit flplane Unit stream Unit bay Unit road Unit delta Unit weeds Unit softsed Unit bay Unit road Unit delta na na na na Unit weeds Unit softsed Na na na na na na na Unit bite Unit softN Unit brush Unit wall Unit flplane Unit stream

18 17

19 18

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24 23

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26 Spatial Trends in the Lake Figure 10 summarizes a 3-unit model of deposition for the entire lake. The physical character of cores are summarized with changes in texture logs (Figure 10), while chemical variations are represented by concentrations in Al, (Figure 10b) the element that explains the greatest amount of variability based on multivarient principal composition analysis (Hyatt, 2003, personal communication). A number of general trends may be recognized when examining all core. Unit I is coarser, and has a more variable texture than units II and III throughout the lake basin. Unit II, where present, is also coarse grained although alternating fine and coarse sandy beds are present. Unit III, while finer grained, does contain discrete sand layers for core collected within the shallow south end of the lake (Figure 7 a, b, c). The transition for Unit I (pre-lake) to Unit III or II is marked by either (1) a change in color and texture for cores collected in the shallow south end of the lake (e.g. Figure 7a, b, c, d, e); (2) a dramatic reduction in grain size particularly for cores collected in the deep north end of the lake (e.g Figure 7 f, g, h, i, j,k) and (3) for deep core ( , 04, 04, 06, 07, ) fine grained lake muds are draped on top of an organic rich soil A-horizon associated with pre-lake Unit I. The thickness of Units II and III vary with location in the lake (Figure 10). Unit II at the shallow south end of the lake ranges in thickness from 30 cm ( ) to 80 cm ( ), and ranges from 35cm ( ) to 50 cm ( ) mid-lake, and disappears in the northern end of the lake ( ). This strongly suggests that there was rapid deposition of sandy sediments near the primary inflow (Erodoni Creek) soon after damming, followed by decreased deposition after this initial phase of infilling. An important observation, also present in the shallow core, relates to the presence small 1-5cm thick sand layers within the upper parts of shallow core at the south end of the lake ( , and ). This sandy facies was not observed in core collected in the deep northern end of the lake. The presence of finer grained sediment at the bottom of Unit III in the shallow core (i.e. Unit IIIA) suggests that sedimentation rates decreased following the initial influx of sands associated with the deposition of Unit II. However, a sandy upper facies (i.e. Unit IIIB) indicates a subsequent increase in the deposition of sand, presumably introduced by renewed runoff from nearby Erodoni Creek. Lake-wide variations in chemistry is complex and is only discussed qualitatively here with reference to Al concentrations along the lake transect (Figure 10b). In general, Al concentrations increase in the bottom portions of Unit III, although the magnitude of this increase varies from core to core. In contrast, the upper part of Unit III is characterized by decreasing Al concentrations for most core. Again, this increasing and then decreasing trend upward through Unit III is more obvious in some core than others. For example, cores and display a sharp spike of Al concentration within the upper boundary of Unit II. This change in Al concentrations is subsequently used with 210 Pb results to assess spatially varying rate of mass sedimentation. 210 Pb Chronology and Mass Sediment Loading to the Lake 210 Pb is a naturally occurring part of the uranium-238 decay series. When 210 Pb decays, gamma photon rays are emitted, which can be measured with a detector designed 25

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28 for low energy photon penetration. 210 Pb in combination with analysis and interpretation of sediment stratigraphies may be used to calculate sediment accumulation rates and to assess impacts of changes seen within sedimentation. (MyCore Scientific, 2003). Sediments store Pb and it s daughter and grand-daughter isotopes. In fact, these isotopes are generally mobilized together. 210 Pb has a short half life of 22.5 years, it is well suited for dating historical sediments, particularly when average accumulations rates are 1-20 years for each one cm section (MyCore Scientific, 2003). Samples from JVAND piston core were analyzed for 210-Pb in order to date the sediments and to identify trends in mass sedimentation at the coring site. These results, are summarized in Figure 11 and are compared to variation in organic content and selected elemental concentrations for the same core. The bottom of the core, which reflects deposition soon after damming in 1927, was dated at 1932, which compares well with the known date of damming. In fact, the presence of sand grains and changes in organic carbon (Figure 11b) suggest that sediment below 40 cm depth were deposited is association with transition to the lake (i.e. they may be part of a thin Unit II). Mass sedimentation rates decreased from 1932 to sometime between 1964 and 1972 and increased thereafter beginning sometime between It is interesting to note that the depth at which mass sedimentation rates change from decreasing to increasing trends in Figure 11a corresponds to the depth where several elements also change (e.g. Al, Fe, Cu, Pb, V and Zn). Figure11a indicates that this change occurs around a depth of 26.5 to 30.5 cm for core JVAND03-02, or 66.3% to 76.3% of the depth of Unit III.. This trend for Al, evident in the 210 Pb dated piston core, is also present in some other core. For example cores , 01 and from the southern end of the lake have Al values that begin to decline at depths of 30, 32, and 14 cm depth (Figure 10). However, sample intervals for Unit III chemical analyses in most cases are too coarse to confidently identify this spike (e.g see cores , , - 04, -05 and in Figure 10). In order to estimate accumulation rates for different locations in the lake the relative depth of the Aluminum spike between 1964 and 1972 from JVAND or % of the thickness of Unit III is applied to other core in Figure 10. Using this approach, it is possible to identify variations in sediment accumulation throughout the lake for two time periods: (1) from the time of damming between Unit I and overlying sediments, as identified in the core by the contact between these units; and (2) for the post 1964 to 1972 period of increased sedimentation rates, as identified by the depth of the inferred Al spike (Table 3). Initial mass sediment accumulation rates were higher in the southern deltaic region of the lake. Rates were 2.0 to 2.7 times higher for core , than , and , and up to 6.2 times higher in the south end of the lake when compared with the north end of the lake. A change can be seen following the initial accumulation. Although mass loading, as indicated by 210 Pb (Figure 11), increased in the north end of the lake following the transition, accumulation rates in Table 3 are either similar (core , ) or decrease for most sites ( , , ) (Table 3). Only core indicates increasing accumulation rates increased by 40-66% when compared to initial accumulation rates. Accumulation rates in core decreased by 15 to 30 % when compared with the initial accumulation rates (i.e 1927 to 1964/72), and similarly decreased by % in core Despite this decrease, 27

29 sediment accumulation rates are still higher at the south end of the lake both before and after To summarize, the key change in accumulation rates were as follows. Initially ( /72) sedimentation at the south end of the lake occurred primarily because of the rapid deposition of a thick Unit II with no equivalent deposition at the north end of the lake. Following this initial pulse of sedimentation, rates likely declined, although Table 3 is not able to distinguish these changes. Between 1964 and 1972 additional sand began to enter the south end of the lake from Erdoni Creek, although total sedimentation was less than the initial period ( i.e. when Unit II was deposited). Also, beginning in Pb results indicate that mass accumulation rates increased at the north end of the lake. The timing of this increase is likely related to an increase in sand in core at the south end of the lake. Air photographs (Figure 12) indicate that this change is probably connected to powerline construction between 1952 and The continued rise in 210 Pb mass loading in the deep, north end of the lake is the result of a prograding delta which causes the source of sediment to move closer to the deep end of the lake throughout time. Attempts to estimate sediment accumulation based on correlation of the 210 Pb records suggest declining sediment accumulation after This is at odds with 210 Pb data and requires further investigation. a.) b.) c.) d.) e.) f.) g.) h.) Figure 11: Plots correlating 210-Pb and several elemental concentrations for piston core JVAND Filled dots indicate measured 210-Pb samples. (modified from Hyatt, 2003). 28

30 Thickness of Unit III (cm) Depth Range for correlated point (cm). Range in accumulation rate 1964/72 to 2003 (cm/year) Range in accumulation rate 1927 to 1964/72 Table 3: Sediment Accumulation Rates na Figure 12: Air photographs of Andover Lake indicating power line construction between (a) 1952 and (b)

31 CONCLUSIONS / SUMMARY The purposes of this study, as outlined in the introduction are to analyze and characterize the spatial variability of lake bottom sediments in 14 cores collected throughout the lake, to define lake wide physical and chemical stratigraphic trends, and to relate these changes to new 210 Pb dating for one core collected in the summer of The most important findings of this study are as follows: 1. Three sedimentary units can be found within core from Andover Lake. Basal Unit I, present in most core, often is capped by an organic rich buried soil A-horizon. Unit II, while present and thick at the shallow, south end of the lake, is not present in several of the deep core, ( , 05, 06,07, ), and Unit III at the north end of the lake, the modern lake sediments, is present in all core. 2. Several measures indicate that sedimentation has varied with location in the lake. These include variable thickness and/or presence of units, changes in physical characteristics, differing chemical concentrations, rates of mass loading, and sediment accumulation for different cores. 3. Rapid deposition of sandy sediments near the primary inflow stream, soon after damming deposited sandy Unit II sediments in the shallow south end of the lake. No such deposition occurred in the deep north end of the lake. 4. A second, substantial change in sedimentation occurred in both the shallow and deep ends of the lake beginning around This is marked by increased concentrations of sand within the upper part of Unit III. These sands likely reflect increased runoff and sediment delivery to the lake by Erdoni Creek because of the construction of a power line across the stream at the south end of the lake. The times of construction ( ) based on air photographs crudely coincides with increasing 210 Pb mean sedimentation rates at the north end of the lake. 5. Attempts to use 210 Pb data from of core JVAND03-02 to evaluate the spatial variability of sedimentation in the lake have mixed results. These analyses suggest higher accumulation rates at the south end of the lake, then occur at the north end of the lake both from the time of damming to , and from to present. However, these results suggest that accumulation for most core decreased after This is not consistent with 210 Pb data. 6. Progradation of a delta from the south end of the lake northward likely explains the continued rise of 210 Pb mass accumulation rates in the deep end of the lake. 30

32 ACKNOWLEDGEMENTS I would like to take this opportunity to thank several individuals and organizations, without whom, this study would not have been possible. First I would like to thank the Andover Lake Property Owners Association and Andover Lake Management Association for logistical support, as well as permission to conduct fieldwork on Andover Lake. I would also like to thank property owners Raymond Gagne, and Precilla Bronke for access to the lake via their property. I would especially like to thank Dr. Hyatt for his outstanding patience and commitment to his students, as well as my research. His support and guidance far surpasses any expectation, or experience I have encountered during my undergraduate career. Thank you, Dr. Hyatt, for providing me with such an amazing opportunity! 31

33 REFERENCES Carlson, H Characteristics of Lake Sediments Across a Submerged Valley, Andover, Lake, Ct. Unpublished. 39pp. Greve, A., Sphar, N.E. et al Identification of Water-Quality Trends Using Sediment Cores from Dillon Reservoir, Summit County Colorado. Water Resources Investigations Report: U.S.G.S. Hyatt, J Mapping and Analyzing the Spatial Variability of Sediment in Andover Lake, Ct. Unpublished. 34pp. Knoecklin, G., Kortmann, R. PhD Andover Lake 1996 Monitoring Report. Unpublished. Ecosystems Consulting Services. Inc. 38pp. Knoecklin, G Andover Lake Perimeter Monitoring Study. Unpublished. Northeast Aquatic Research, LL. 28pp. Knoecklin, G Andover Lake Perimeter Monitoring Study. Unpublished. Northeast Aquatic Research, LL. 28pp. Knoecklin, G Andover Lake Perimeter Monitoring Study. Unpublished. Northeast Aquatic Research, LL. 28pp. Knoecklin, G Andover Lake Perimeter Monitoring Study. Unpublished. Northeast Aquatic Research, LL. 28pp. Knoecklin, G Andover Lake Assessment of Five Years of Lake Monitoring Data. Unpublished. Northeast Aquatic Research, LL. 31pp. McCann, B Characteristics of Lake Sediments of the South Bay, Andover Lake, Ct. Unpublished. 29pp. MyCore Scentific, Inc The Dating Game: Aging Environmental Samples Using 210 Pb. Retrieved Scheffer, M Ecology of Shallow Lakes. Chapman and Hall. 357pp. Tokarz, W Using Geographic Information Systems (GIS) to Model Total Phosphorous (TP) and Total Nitrogen (TN) Levels in Andover Lake, Connecticut, Based on Land-use Patterns. Unpublished. 37pp. 32