Eagle Creek Watershed and Reservoir Results

• Surface Sediment Characterization

• Suspended sediment characterization of Three Watersheds

• Eagle Creek Reservoir Water Chemistry and Water Quality

A.  Surface Sediment Characterization 

General geochemical characteristics of surface sediments from the reservoirs can be used to assess relative differences in reservoir inputs, productivities, and anthropogenic loading. Here, we describe geochemical parameters for Eagle Creek reservoir sediments.

Sediment Characteristics of Eagle Creek Reservoir           

            Particle size data from surficial sediments in Eagle Creek Reservoir show distinct patterns in clay and sand abundance as shown on Figures VII-1 and 2 and in Table VII-1. Both the eastern and western margins have the highest sand content of the six sample groups (see Fig. IV-9 for sample group locations). The eastern margin averages 7.1% ± 10.4% clay and 69.3% ± 28.0% sand. These data indicate that the eastern margin on both the north and south sections of the reservoir experience the highest energy level in the reservoir.  This is supported by the abundance of gravel in these samples.  The western margin samples fall into two distinct groups with average clay abundance at 25.3% ± 17.9%. There are no obvious spatial patterns in the samples from this group that explain this variability.  Both the north inlet and coves have similar clay contents, averaging 20.3% ± 10.6% and 21.8% ± 12.2%, respectively. Higher sand content in the north inlet indicates that water flowing through this region either reaches higher energy levels or has a greater supply of sand-size sediment from the watershed.
 

The Eagle Creek Reservoir sediments contain the finest sediment and show less variability in particle size than the margins and inlets.  Sediments from the northern reservoir (north of 56^th street) contain 36.8% ± 7.2% clay and the sediments from the southern reservoir contain 41.0% ± 9.0%. Throughout the reservoir, we observe a decrease in sand content and an increase in clay content toward the south. Gravel is absent for all samples collected in the middle of the southern reservoir. This pattern indicates decreasing energy levels with distance from the inlet and coves. The variability within the sample groups may be attributed to the morphology of the reservoir bottom, as well as disturbance from human structures and activities. Analysis of the role of sediment resuspension and transport, as well as contribution from biological production are being evaluated.          

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Organic Matter Distribution in Eagle Creek Reservoir 

Organic matter content in sediments is controlled by several factors. First, reservoirs typically exhibit in situ organic matter production from algae, certainly the case in Eagle Creek Reservoir. Second, terrestrial organic matter derived from eroded soils and litter may also contribute to the total organic matter content of sediments. Both of these processes will add organic matter to sediments. Third, sediments can be diluted by input of mineral fractions, thus reducing the content of organic matter but not necessarily the flux rate of organic matter to sediments. Fourth, sediments can be redistributed within a basin after deposition. Finally, degradation of organic matter by bacteria and in-fauna can reduce the net organic matter content of sediments. In conditions of low or no oxygen, however, this rate of decomposition diminishes substantially. 

In the Eagle Creek Reservoir, organic matter ranged from less than 2%, typically on the eastern margins of the reservoir, to over 15% in the deeper reservoir (Fig. VII-3; also Table VII-2). Although the average value for the reservoir sediments is 6.6%, variations due to in situ organic matter production and sediment dilution dominate the signal. The higher values (above ~10%) are typically found in the deeper portions of the reservoir, including the deep hole in the south basin and the quarry/bird sanctuary, and in a region roughly straddling the 56^th Street causeway. Lower values are typically found in the coves and in shallow and more sand-and silt-rich regions of the reservoir. 

The main contributor to the organic matter signal in Eagle Creek Reservoir appears to be in situ algal productivity. This interpretation is supported by observations of intense biological blooms in the water, the texture of the retrieved sediment and by the characteristics of the mineral-rich sediment inputs to the reservoir. The distribution of organic matter is likely primary, driven by productivity in the overlying water mass. The quarry/bird sanctuary is an excellent example, where the high organic matter values are not diluted by sediment input from the northern inlet. Even where mineral sediment input is high, like in the northern reservoir, organic matter values are relatively high, indicating a substantial organic matter flux in these areas. Based on observations of leaf litter in sediments, particularly in the western portion of the reservoir, terrestrial organic matter is likely a contributor to the overall signal. But the overall texture of sediments and the low values of organic matter in the northern inlet indicate that external input of organic matter is not a substantial portion of the total organic matter content observed in the sediments. The generally high values of organic matter in sediments is not likely significantly altered by decomposition, as this process is not likely to keep up with the organic matter flux in this setting. One exception to this might be in the deeper portions of the south reservoir, where the persistence of suboxic conditions during summer stratification might significantly reduce decomposition rates in these areas, resulting in very high organic matter values in the sediments. It is unclear, however, whether the redistribution of finer, organic-rich sediments to the deeper holes of the southern reservoir contribute to these high values.

A correlation exists between mean grain size and organic matter content, with finer sediments correlating with higher organic matter contents (Fig.VII-4). Since the grain size analyses were performed on sediment subsequent to organic matter removal, this relationship indicates that the fine fraction contains and likely preserves a higher proportion of organic matter than the coarse fraction, a finding observed in a variety of other settings (e.g., Schlesinger, 1997).
 

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Phosphorus Distribution in Eagle Creek Reservoir Sediments 

Phosphorus (P) is a limiting nutrient for biological productivity in a number of settings and time scales (e.g., Filippelli, 2002). This nutrient has in fact been invoked as a prime culprit for lake and reservoir eutrophication over the last several decades, and thus examining the mass balance of P in a watershed or reservoir system may greatly enhance our ability to ultimately understand the controls on drinking water quality in natural and engineered systems. To this end, one focus of our analysis was on P distribution in ECR.  To clarify, P can be measured in waters and sediments.  In sediments, P occurs in many geochemical forms as outlined in the next section.  In waters, however, P is restricted to only a few geochemical forms; the most important of which are phosphate (PO₄^3-) and orthophosphate (HPO₄^2-).  The concentration of P, and hence phosphate and orthophosphate, in waters is significantly lower than concentrations in solids.  This leads to the situation where P can be unmeasurable in water samples directly overlying sediments in which P is easily measured. 

Phosphorus has a relatively straightforward cycle. Biologically reactive P is incorporated into terrestrial systems via mineral weathering. After release from minerals, P is often transformed via a series of natural soil-forming processes (Filippelli and Souch, 1999; Filippelli, 2001). Upon erosion from a landscape, the total load of potentially biologically-available P includes dissolved inorganic P (usually orthophosphate), dissolved organic P (usually minor), and weakly surface-bound P associated with particulates (often a substantial component). Thus, all of these P-bearing constituents can be delivered to reservoirs via runoff and inflow. Upon entering a reservoir, biologically-available P can be rapidly incorporated into photosynthetic biomass as a limiting nutrient. This P is transformed into an organically-bound fraction, and will be transported to the reservoir sediments upon organism death or consumption via zooplankton (Fig. VII-5). Thus, organic-rich sediments should be P-rich as well. Another significant component of P for biomass productivity can be introduced to a reservoir internally. In this process, P is regenerated from bottom sediments either by the degradation of organic matter (and thus release of organic P) or by desorption and/or dissolution from mineral surfaces. In low oxygen conditions, this dissolution process can be a substantial contributor of P, due to the dissolution of iron oxyhydroxides, which are typically extremely P-rich (Slomp et al., 1998; Filippelli et al., 2003). In an anthropogenically influenced setting like that encountered in the Eagle Creek Watershed, the net external input of P to the reservoir can be enhanced by loading from human and industrial waste (sewage is high in reactive P, as are the detergents and surfactants used in industrial processes), from excess fertilizer application in agricultural areas, and from increased weathering of organic-rich surface soils during land use change. Furthermore, this increase in external P loading can drive enhanced productivity in a reservoir, leading to lower oxygen contents of deeper waters and thus higher internal P recycling. Finally, as the load of external P to reservoir sediments hits a certain threshold, regardless of reducing external inputs, internal recycling of P from P-rich reservoir sediments can drive eutrophication of a reservoir system. 

            Phosphorus contents of ECR surficial sediments are high compared to background values. For example, soils usually contain about 0.5-2 mg/g P (0.05-0.2 wt % P), whereas the high P values of ECR sediments (typically above 2 mg/g: Fig. VII-6; also Tables VII-2 and VII-3) reflect the dominance of the organic P sink in the reservoir. The relationship between P and organic matter is relatively strong (Fig. VII-7), supporting the assertion that in situ organic matter productivity is strongly affecting the distribution of both of these components. The net C_org:P_total ratio (mol:mol), calculated from the data presented in figure VII-7 is 36, typical of algal matter (Filippelli, 2001).  The spatial distribution of P contents in the reservoir are similar to those for organic matter, reflecting relatively high productivity in discrete areas of the reservoir (particularly north and south of the causeway and within the deep portion of the south reservoir) as well as generally high productivity in the quarry/bird sanctuary and some of the southern inlets. A correlation exists between P and grain size, with finer mean grain sizes correlating with high P contents (Fig. VII-8). Since the grain size analysis was performed on an organic-free basis, the correlation indicates that finer sediments are responsible for retaining a significant portion of the total P signal. 

            On a whole, the high P contents of Eagle Creek Reservoir sediments reflect the history of P loading and high organic matter productivity. Apparent varves are observed in sediments from the deep hole of the south reservoir, which yield an approximate sedimentation rate here of 1 cm/yr. Until chronostratigraphic analyses are performed, we cannot definitively state that these represent annual layers, but this sedimentation rate is consistent with other anthropogenically influenced reservoirs. Using this sedimentation rate, plus the dry bulk density of the sediment and the average content of P in the reservoir sediments, we obtain a P accumulation rate of about 50 µmol P/cm²/yr. This rate is about 10 to 100 times higher than what is found in lake sediments unaffected by urbanization (e.g., Filippelli and Souch, 1999), revealing the extent of P loading in the Eagle Creek Reservoir.  The potential exists for a significant proportion of the total reservoir productivity to be fueled by internal recycling of reactive P from the reservoir sediments themselves.  It is important to recognize that P loading from watersheds was the primary source of elevated P contents in ECR sediments. We will pursue this issue by examining downcore profiles of P distribution and by performing more detailed extraction geochemistry to determine the potential reactivity of P in the sediments.  

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Copper and Lead Distribution in Eagle Creek Reservoir Sediments 

Copper (Cu) and lead (Pb) both have natural sources in the environment. But both of these metals have been extensively exploited as industrial and environmental products, and thus the natural cycling of these elements is usually overwhelmed by their anthropogenic cycles. For Cu, two anthropogenic sources are automotive brake pads (near roadways) and from algaecide applications. For Pb, multiple anthropogenic sources exist:  leaded paint until the early 1970’s, leaded gasoline until the early 1980’s, and lead smelting and recycling facilities (a continuing, but declining source). Although both metals tend not to be mobile in natural environments, wind can redistribute them in association with fine particles, and some potential for remobilization occurs in sediments that are acidic and do not undergo significant sulfate reduction (solid-phase sulfur compounds are extremely efficient at trapping and retaining both Cu and Pb). Both metals are extremely particle reactive in the water column (especially in alkaline waters), and will rapidly adsorb to any number of suspended particulates, including algae, before eventual transport to bottom sediments. Thus, once present in a system, both elements respond similarly and can be used to examine their comparative biogeochemical cycles. 

Both Cu and Pb have unintentional inputs to ECR, mainly from atmospheric deposition and from association with incoming fine particulates, as well as perhaps some direct roadway input from the 56^th Street causeway, I65 and Lafayette Road. But Cu has also been applied to ECR over the past several summers in the form of Cutrine Plus, a commercial algaecide used to reduce in situ production of organisms that contribute to taste and odor compounds in the water. The Cu released from this compound is adsorbed by algae during growth, causing chlorosis and ultimately cell death. The first application of this compound was to the entire reservoir, but since then, subsequent applications have focused on the middle third of the reservoir to maximize efficiency and reduce the risk of widespread anoxia and fish kills associated with large-scale biomass die-offs. A discussion of the use of algaecide both in ECR and in other reservoir systems is presented in Section V.  

In ECR, Pb has a spatial pattern that matches that of both organic matter and P (Fig. VII-9). In fact, the correlation between Pb and P is extremely robust (Fig. VII-10), revealing that their depositional cycles are nearly identical. Because the main output sink for biologically-available P is in the form of organic matter and in association with fine particulates, it is likely that these processes also drive Pb deposition. Although Pb is not a biolimiting nutrient, the particle-reactive nature of this element results in this strong correlation between Pb and P. A correlation also exists between Pb and grain size (Fig.VII-11), whereby sediments with higher percentages of clay have higher Pb concentrations. The total Pb concentration ranges from about 35 to greater than 200 µg/g (Tables VII-2 and VII-3). Interestingly, the causeway region does not exhibit high sediment Pb concentrations, and thus the effects of the roadway are not seen, perhaps due to the phase-out of leaded gasoline 20 years ago and the age of the surface sediments themselves (likely much younger). The quarry/bird sanctuary, distant from sediment input, likely reflects the ambient atmospheric loading of Pb in this region, and these sediments have Pb concentrations that are relatively high compared to the background value of <50 µg/g for surface soils of rural Marion County (Laidlaw, 2001). The sediments of Eagle Creek Reservoir exhibit high Pb content in general, perhaps the result of the double input from atmospheric sources and from the input of Pb-enriched sediments from the watershed.  

The distribution of Cu (Fig. VII-12) does not correlate as well with organic matter and P as does Pb (Figs. VII-13, VII-14, VII-7, VII-10). Although the northern inlet sediments have relatively low Cu concentrations, the reservoir sediments themselves have elevated Cu content. The high Cu values were observed in the middle third and the south portion of the reservoir, whereas the quarry/bird sanctuary exhibits extremely low Cu content (Fig. VII-12, Table VII-2). Given that Pb and Cu geochemistry are similar, this distribution difference must be due to input differences for these two metals. The most obvious cause of the observed Cu distribution is from the Cutrine Plus applications. Subsequent to the initial basin-wide application, these applications have focused on the middle third of the reservoir. This application pattern matches up well with the sedimentary distribution of copper (Fig. VII-12). The high values extending down to the south portion of the reservoir and into the coves may be due to resuspension of Cu-enriched sediments within the reservoir or to the movement of dissolved Cu ‘downstream’ of the application area. The fact that the quarry/bird sanctuary sediments have extremely low Cu values indicates that atmospheric deposition is not a significant source of Cu to this site. Furthermore, the lower correlation between Cu and the other better inter-correlated components (organic matter, P, and Pb) likely reflect the spatially restricted application of Cutrine Plus, with localized sedimentation under the areas of application an additional output for Cu. Thus, the distribution of Cu in the reservoir sediments is likely controlled both by the sedimentary and biogenic processes occurring within the reservoir and the direct sedimentation of Cu in the middle third of the reservoir after algaecide application.
 

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Comparison of Eagle Creek and Geist Reservoir Surficial Sediment Geochemistry 

In the ongoing effort to characterize the dynamics of major watersheds in central Indiana, bottom sediment samples was also collected from Geist Reservoir in late March, 2003, and analyzed for the same geochemical parameters and by the same methods as described above for Eagle Creek Reservoir surficial sediments. Organic matter content and P concentration reveal ranges in Geist Reservoir sediment that are about equal to or slightly below those found in Eagle Creek Reservoir sediment (Fig. VII-15; Tables VII-4 and VII-5). For example, the average organic matter content in Geist Reservoir sediments was 6.8%, compared to 6.6% in Eagle Creek Reservoir sediments. The range at Geist Reservoir, from 0.7-13%, was similar to that found in Eagle Creek Reservoir sediments. Phosphorus was slightly lower in Geist Reservoir sediments, with an average value of 1.4 mg/g and a maximum of 4.0 mg/g, compared to Eagle Creek Reservoir sediments, which displayed an average value of 1.9 mg/g and a maximum value of 4.5 mg/g. Together, the lower values for the biologically reactive components organic matter and P at Geist Reservoir may indicate a slightly lower degree of eutrophication at GR than at Eagle Creek Reservoir. This may be the result of a lower extent of P loading, coupled with a lack of significant anoxic episodes that cause the regeneration of sediment-bound P. More work will be done on this question with cores and detailed sediment geochemical analyses for P, but the organic components of Geist Reservoir bottom sediments are similar to those at Eagle Creek. 

            The Pb values in Geist Reservoir sediments are particularly interesting in comparison to Eagle Creek Reservoir sediments. The concentration of Pb is somewhat lower in Geist reservoir sediments (Fig. VII-15), consistent with a greater distance away from the bubble of high atmospheric Pb loading around downtown Indianapolis (Fig. VII-16; Filippelli et al., 2002). For example, the average Pb concentration in Geist Reservoir sediments is 87 µg/g, slightly higher than typical background values found in soils from rural areas of Marion County (Filippelli et al., 2002) but lower than the average value of 127 µg/g found in Eagle Creek Reservoir sediments. Interestingly, a strong relationship still exists between the spatial patterns of organic matter, P, and Pb (Fig. VII-15), supporting the relationship between these elements in natural biogeochemical cycling related to biological production.  

The Cu values in Geist Reservoir sediments are lower than those found in Eagle Creek Reservoir sediments (Figs. VII-12 and VII-15). For example, the average Cu content of Geist Reservoir sediments is 13 µg/g, with a range from 3-24 µg/g (Table VII-4). This is in contrast to Eagle Creek Reservoir sediments, with an average of 66 µg/g and a range from 16-138 µg/g (Table VII-2). Furthermore, the spatial distribution of Cu correlates well with that of the other components, suggesting that it too plays a predictable part in the biogeochemical cycling related to organic matter production in Geist Reservoir. As noted above, Cu in Eagle Creek Reservoir sediments has a spatially restricted distribution not well correlated to the other parameters. Collectively, these results are further evidence that the elevated Cu values in Eagle Creek Reservoir bottom sediments relative to Geist Reservoir can be linked to application of Cutrine Plus algaecide, a finding not unexpected in light of the typical situation found in algaecide-treated reservoirs (See section V).  

Table VII-6 compares copper concentrations in sediments from a series of lakes and reservoirs mostly in the US. All of the lake and reservoir systems in the table, except for Geist Reservoir, are associated with algaecide treatment. Elevated copper levels are found in all of the treated water bodies. Eagle Creek Reservoir has elevated sediment copper levels relative to levels in untreated Geist Reservoir. Sediment copper levels in Eagle Creek Reservoir are dramatically lower than levels in many of the referenced reservoirs. Algaecide applications in reservoirs should be viewed as a short to intermediate solution to avoid the accumulation of high levels of copper in reservoir sediments.

Table VII-6.  Comparison of Copper Concentrations in Sediments of Indianapolis Reservoirs with Sediment Copper Concentrations of Other Surface Water Bodies Associated with Algaecide Application 

1. Over a 50 year period, the lake was treated with 1.5 million pounds of copper sulfate
2. Over a 58 year period, the lakes were treated with 3.2 million pounds of copper sulfate
3. Over a 55 year period, the lake was treated with 4.4 million pounds of copper sulfate
4. There is no record of algaecide treatment at Geist Reservoir. All other locations have been treated with algaecide.

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B.  Suspended sediment characterization of Three Watersheds 

Watershed Suspended Sediments 

Results of stream water discharge and sediment concentration during low and higher stream flow conditions during the winter of 2003 from select stream segments and subwatersheds in the Eagle Creek, Fall Creek and Cicero Creek Watersheds are presented in Table VII-7. Cicero Creek Watershed was not sampled at higher stream flow conditions due to a lack of a significant higher flow event during the sampling window. While the results presented here are informative, it reflects only watershed contributions during a paired set of observations. Additional sampling is necessary to adequately characterize these complex systems and is scheduled for spring, summer, and fall.
 

Eagle Creek Watershed 

Suspended sediment concentrations for streams from Eagle Creek sampled at low flow conditions ranged from 4.5 to 14.7 g/m³.  At higher flow conditions, suspended sediment concentration increased, ranging from 47.9 to 126.7 g/m³.  A plot of suspended sediments relative to watershed area drained at each sampling location for low and high flow conditions are shown in Figure VII-17.  At low flow, as the area drained by Eagle Creek and its subwatersheds increased, there was no significant increase in the suspended sediment content.  At high flow on the trunk streams in Eagle Creek Watershed, an increase in watershed area resulted in a linear increase in the suspended sediment load.  However, Fishback Creek (ECW2), School Branch (ECW1) and Little Eagle Creek Branch Headwaters (ECW7) exhibited elevated suspended sediment concentrations.  At high flow, Fishback Creek (ECW2) had similar suspended sediment concentration as Eagle Creek (ECW3) despite having a contributing watershed area ~6 times smaller. 

The results of stream water sampling for suspended sediment show that although Fishback Creek and School Branch had smaller watershed contributing areas, these two subwatersheds contribute disproportionately higher sediments directly into Eagle Creek Reservoir.  These results are consistent with rapid surburbanization and accompanying land use changes in these subwatersheds (See section VIII for analyses of land use).  

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C.  Eagle Creek Reservoir Water Chemistry and Water Quality 

For ease of analyses and interpretation of data collected during the Eagle Creek Reservoir study, we have classified sampling locations into major groups.  The groupings include (a) north inlet stations, which are upstream of the reservoir proper and above the restriction associated with the levee that separates the bird sanctuary on the northeast side of Eagle Creek Reservoir (Stations EC 28, 30, 31, 32, and 33); (b) coves (EC 4, 8, 12, 13, 17, and 21) which that have significantly reduced flow during the summer, but may have higher flows during the spring seasons; (c) a downstream location sampled in Eagle Creek below the dam outlet (EC1); and (d) samples from the quarry/bird sanctuary (EC 29, and 34). For the locations of each sample grouping, see Figure VI-7.  Also, we define the northern and southern portions of the reservoir as above and below 56^th Street, respectively. All samples were collected over a five day period from September 13-17, 2002. 

Eagle Creek Reservoir Physical Parameters 

Water Clarity 

            Water clarity measured as the depth of light penetration using a Secchi disc is shown in Table VII-8.  A plot of the light penetrating depth along with the total water column depth for each sampling location is shown in Figure VII-18.  The average depth of light penetration in the reservoir was 78 + 21 cm.  The plot also shows that in general, light penetration in the reservoir increased from the inlet to the outlet as water column depth increased.  The station with the lowest light penetration was 10 cm at EC24 located in the east edge of the northern portion of the reservoir (Fig.VII-19).  The greater penetration of light with distance from the inlet to the outlet suggests settling of sediments brought into the reservoir from the watershed as the flow velocity of water slows in the reservoir.  The average light penetration for the inlets and coves was 74 + 18.1 cm which was not significantly different from light penetration within the reservoir.  Light penetration for stations upstream of the reservoir was lower compared to reservoir water, averaging 20 + 7.1 cm.  The lower light penetration for these stations may be due to relatively shallower water depths for these stations and faster moving water compared to the reservoir. 

            The light level in the water column in the quarry was 135 cm which was higher compared to the reservoir water column. This likely due to the fact that the quarry is physically (partially or wholly) isolated from the reservoir. Thus lower sediment input and greater water depths may account for higher water column clarity.  Water clarity depths were similar to those reported in the Heartland Model Implementation Project (1982) for the two month period August and September, 1980. 

Water Temperature 

            Temperature of reservoir water at 0.25 m averaged 25.4 + 0.7 ^oC.  Water temperature decreased with depth, averaging 24.5 + 0.4, 24.5 + 0.4 and 16 + 0.9 ^oC at depths of 3, 6, and 9 or more meters (Table VII-8). The average temperature for water sampled from the coves was 25.4 + 0.8 ^oC, which was similar to reservoir surface water sampled at 0.25 m.  The average temperature for water sampled upstream of the reservoir was 22.1 + 0.6 ^oC, which was cooler than reservoir surface water.  The temperature of the water downstream of the reservoir outlet was 22.2 ^oC, which was also colder compared to reservoir surface water.  Water from the quarry averaged 24.4 + 0.5 ^oC sampled at depths of 0.25, 3 and 6 m.  The averaged water column temperature in the quarry is similar to that of the reservoir for comparable depths. Temperature profiles for select stations from upstream to downstream of the reservoir are shown in Figure VII-20.  The temperature profiles show that temperature decreases with increase in water column depth, with a pronounced temperature gradient at depths below 6m.  Also, longitudinally from the northern inlet to the outlet, water temperatures decrease with an increase in the total water column depth. 

pH 

            pH for reservoir water sampled at depths of 0.25m averaged 8.1 + 0.3.  The pH of water upstream of the reservoir (northern inlet area) averaged 7.9 + 0.2, while the downstream station beyond the outlet was 7.3. Both upstream and downstream water had pH that was lower compared to reservoir surface water. Water sampled from the coves averaged 8.1 + 0.3, which was similar to reservoir surface water.  Water in the quarry had pH that ranged from 8.6 at 0.25 m to 7.7 at 6 m, with an average of 8.3 + 0.5. The averaged pH for quarry water was higher compared to similar depths from the reservoir. Depth profiles of pH for reservoir water is shown in Figure VII-21.  The pH decreased with depth within the water column averaging 7.6 + 0.2, 7.4 + 0.1 and 6.9 + 0.05 at depths of 3, 6 and 9 m or greater.  Longitudinally from the northern reservoir inlet to outlet, pH decreased with an increase in water column depth.

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Dissolved Oxygen (DO) 

DO for reservoir water sampled at depths of 0.25m averaged 3.6 + 0.4 mg/l.  DO of water sampled upstream of the reservoir (northern inlet area) average 5.5 + 0.7 mg/l, while the downstream station beyond the outlet was 3.3 mg/l. The upstream had a slightly higher DO content compared to the downstream and reservoir surface water. DO of water sampled from the coves averaged 3.5 + 0.6 mg/l, which was similar to reservoir surface water.  Water in the quarry had DO that ranged from 4.9 mg/l at 0.25 m to 2.9 mg/l at 6 m, with an average of 3.9 + 0.9 mg/l (Table VII-8).  The averaged DO for quarry water was lower compared to similar depths from the reservoir.  Depth profiles of DO for reservoir water are shown in Figure VII-22.  The DO decreased with depth within the water column averaging 3.6 + 0.4, 2.3 + 0.7 and 1.0 + 0.2 mg/l at depths of 3, 6 and 9 m or greater.  The DO gradient was pronounced below depths of 3 m.  Longitudinally from the northern reservoir inlet to outlet, DO decreases with an increase in water column depth. 

Specific Conductance 

Specific conductance for reservoir water sampled at depths of 0.25 m averaged 0.378 + 0.01 S.  Specific conductance of water sampled upstream of the reservoir averaged 0.782 + 0.228 S, while the downstream station beyond the outlet was 0.411 S. The upstream and downstream stations had higher specific conductance compared to reservoir surface water. Specific conductance of water sampled from the coves averaged 0.377 + .008, which was similar to reservoir surface water.  Water in the quarry had specific conductance that ranged from 0.419 S at 0.25 m to 0.428 S at 6 m, with an average of 0.422 + 0.005 S.  The specific conductance for quarry water was higher compared to similar depths from the reservoir.  Depth profiles of specific conductance for reservoir water are shown in Figure VII-23.  The specific conductance increased with depth within the water column averaging 0.383 + 0.011, 0.389 + 0.009 and 0.455.0 + 0.002 S at depths of 3, 6 and 9 m or greater.  The specific conductance gradient was pronounced below depths of 6 m. Longitudinally from the northern reservoir inlet to outlet, specific conductance increased with an increase in water column depth. 

Eagle Creek Reservoir Nutrients 

Phosphate 

Phosphate measured as orthophosphate was below the detection limits of 0.1 mg/l for the instrument.  Thus, it was not possibly to discuss phosphates in Eagle Creek Reservoir water. Future sampling will utilize better techniques with lower detection limits.

Nitrates 

Nitrates for reservoir water sampled at depths of 0.25m averaged 0.22 + 0.06 mg/l.  Nitrates in water sampled upstream of the reservoir was below the instrument detection limits of average 0.04 mg/l, while the downstream station beyond the outlet was 0.13 mg/l. The downstream station had lower nitrate compared to reservoir surface water. Nitrates in water sampled from the coves averaged 0.22 + 0.06 mg/l, which was similar to reservoir surface water.  Water in the quarry had no detectable nitrates compared to higher levels for similar depths from the reservoir.  Depth profiles of nitrates in reservoir water is shown in Figure VII-24. The nitrates decreased with depth within the water column averaging 0.25 + 0.005, 0.29 + 0.10 mg/l at depths of 3 and 6 m (Table VII-8).  The only location where nitrates were measured for depths greater than 6 m was at EC6 with concentration of 0.3 mg/l at 9m depth.  Nitrate profiles showed pronounced depth gradients below 6 m.  Longitudinally from the northern reservoir inlet to outlet, nitrates decreased with increase in water column depth. 

Sulfate 

Sulfate in reservoir water sampled at depths of 0.25m averaged 21.3 + 0.8 mg/l.  Sulfate in water sampled upstream of the reservoir averaged 38.3 + 7.3 mg/l, while the downstream station beyond the outlet was 16.8 mg/l. The upstream had higher sulfate compared to the downstream location and reservoir surface water. Sulfate in water sampled from the coves averaged 21.2 + 0.6 mg/l, which was similar to reservoir surface water.  Sulfate in the quarry was nearly similar for all depth with an average of 16.1 + 0.2 mg/l.  The sulfate in quarry water was lower compared to similar depths from the reservoir.  Depth profiles of sulfate for reservoir water is shown in Figure VII-25.  Sulfate decreased with depth within the water column averaging 21.7 + 1.1 and 21.2 + 1.4 mg/l at depths of 3 and 6 m.  At depths below 9 m, EC 6 still had a high level of sulfate (20.2 mg/l) compared to EC2 and EC 3 with sulfate levels of 4.0 and 4.8 mg/l, respectively.  The sulfate gradient was pronounced below depths of 6 m only in the south portion of the southern reservoir.
 

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Eagle Creek Reservoir Major Ion Chemistry 

Cation and anion proportions in water sampled for the Eagle Creek Reservoir are shown in the trilinear diagram (Fig. VII-26). The trilinear diagram was constructed using Rockware Scientific software and shows the relative concentrations of different ions in the water samples.  Prior to constructing the trilinear diagram, the quality of major ion data was assessed by conducting an ionic charge balance.  Samples with a charge balance error of more than 6% or more were excluded from this analysis. In general, water from the reservoir, the northern inlet, the outlet and cove had no dominant cation. Water samples from depths greater than 9 m were dominated by calcium. Water samples from the quarry/bird sanctuary and northern inlet had no dominant anion facies. Water from the reservoir and the reservoir outlet was dominated by bicarbonate ions. The proportions of SO₄ in all water samples (except for water from depths greater than 9 m) were nearly similar. However, the chloride proportions varied from about 20 to 60%. 

There are no obvious mixing relations between water samples. However, there are three groupings (Fig. VII-26). One grouping consists of water sampled from the northern outlet and the quarry/bird sanctuary. The second grouping consisted of all samples from the reservoir sampled at depths less than 6 m and includes samples from the coves. The third group consists of water from depths greater than 9 m. It is important to note that the proportions of both cations and anions for all water samples collected from the coves and reservoir at depths shallower than 6 m have similar cation-anion proportions which is different from water from the northern inlet, the main source of surface water to the reservoir. This can be used to infer that the water in the reservoir is relatively well mixed.

Eagle Creek Reservoir Stable Isotopes 

DIC and d¹³C_DIC   

Dissolved Inorganic Carbon (DIC) in reservoir water sampled at depths of 0.25 m averaged 26.1 + 1.7 mg C/l.  DIC in water sampled upstream of the reservoir average 37.2 + 12.0 mg C /l, while the downstream station beyond the outlet was 40.6 mg C/l. The upstream and downstream locations had higher DIC compared to reservoir surface water. DIC in water sampled from the coves averaged 26.3 + 01.2 mg C/l, which was similar to reservoir surface water.  DIC in the quarry averaged 18.2 + 1.6 mg C/l and was lower compared to similar depths from the reservoir.  Depth profiles of DIC for reservoir water is shown in Figure VII-27.  DIC increased with depth within the water column averaging 26.6 + 1.5, 28.6 + 1.7 mg C/l and 48.5 + 17 mg C/l at depths of 3, 6 and 9 m or greater.  In general, DIC gradients were more pronounced below depths of 6 m and mostly in south of the southern portion of the reservoir. 

δ¹³C_DIC in reservoir water sampled at depths of 0.25 m averaged -7.8 + 0.7⁰/₀₀.  δ¹³C_DIC in water sampled upstream of the reservoir average -8.6 + 2.1⁰/₀₀, while the downstream station beyond the outlet was -11.7⁰/₀₀. In the upstream locations, δ¹³C_DIC was higher compared to the downstream location but lower relative to the reservoir surface water. δ¹³C_DIC in water sampled from the inlets and coves averaged -7.9 + 0.7⁰/₀₀, which was similar to reservoir surface water.  δ¹³C_DIC in the quarry averaged -4.4 + 0.9⁰/₀₀ and was higher compared to similar depths from the reservoir.  Depth profiles of δ¹³C_DIC for reservoir water is shown in Figure VII-28.  δ¹³C_DIC decreased with depth within the water column averaging -8.2 + 0.4, -9.2 + 0.6 and -12.0 + 2.0⁰/₀₀ at depths of 3, 6 and greater than 9 m.  In general, δ¹³C_DIC gradients were more pronounced below depths of 6 m and mostly in the southern portion of the reservoir. 

Implications of Carbon Cycling and Reservoir Nutrient Dynamics 

Depth profiles suggest that most of the nitrate and sulfate depletion is occurring at the bottom of the water column.  In combination with the low oxygen levels at the bottom of the water column, biological activity mainly under anaerobic conditions may be responsible for observed decreases in nitrates and sulfates.  This is likely related to biological activity at the sediment water interface.  We also observe that the lowest nitrate levels are in the southern portion of the reservoir where water depths are greatest and organic matter content is highest.  It is possible that this nitrate consumption is related to organic matter mineralization at the sediment-water interface.  Support for this explanation is provided by DIC concentrations and the isotope ratio of DIC.  In the vertical profiles of DIC and d¹³C_DIC, the highest DIC concentrations occur at the lower water column and correspond to depths with the lowest nitrates. A plot of sulfate and nitrate vs. DIC for reservoir samples is presented in Fig. VII-29.  The figure shows that water samples with the highest DIC concentrations have undetectable nitrates or the lowest sulfate concentrations (samples greater than 9 m, highlighted with red circle in Fig. VII-29).  Although this observation is most evident for the two water samples collected from the deepest depths in the water column, it is possible that organic carbon mineralization could be responsible for the observation.  Alternatively, this observation may simply result from stratification and modified circulation in the deeper water column.  Subsequent sampling events will be designed to explore this question.  

To further explore the role of biological activity at the sediment interface, we examine the relationship between DIC and Ca.  We expect that biological activity at the sediment-water interface will produce higher CO₂.  This CO₂ will decrease the pH, at this interface and the immediate water column as seen in pH profiles in Figure VII-21.  Also, the higher CO₂ may lead to weathering of minerals in the sediments resulting in the release of Ca.  Table VII-8 shows that water from mostly below 9 m had the highest DIC and the highest Ca.  The implication for this observation is that biological interaction at the sediment water interface may release nutrients bound to mineral surfaces and organic matter into the water column.  If the nutrients released from the bottom sediments are brought to the near surface of the water column by circulation, the nutrients may be available to fuel biological activity in the surface water column. 

Reservoir Circulation 

Major inorganic ions can be used to examine physical mixing within the reservoir.  This is because the sources of these ions are mainly from the watershed, although smaller amounts can be produced at the sediment water interface by biological activity.  We assume in the use of the major ions for this analyses that their concentration is affected mainly by watershed supply and dilution from precipitation and within the reservoir by circulation.  We evaluate mixing for all samples related to the reservoir study.  We use the relationship between chloride and sodium for this analysis since chloride is a conservative ion. Figure VII-30A shows that chloride is positively correlated with sodium for all water samples (excluding water from the quarry) at 95% confidence. The plot also shows that relative to stream water input in the inlet of the reservoir, reservoir water is relatively homogeneous. This observation suggests that mixing of water samples can be described by two end members. We further examine the relationship between water within the reservoir (Fig. VII-30B).  A least squares regression analyses between chloride and sodium conducted only for reservoir samples is also significant at 95% confidence. The plot shows that within the reservoir, water can be described by dilution from the inlet to the outlet.  Although this relationship can be used to calculate dilution or mixing, thus the residence time, it is not possible to determine the two end members to use for dilution. Also the use of chloride and sodium could be complicated by the fact that salt used for de-icing may included variable amounts of sodium or magnesium chloride, although we believe the relationship between them may be constant.
 

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