Transcript raystown lake - Juniata College

NUTRIENT CONCENTRATIONS in RAYSTOWN LAKE
Smith, Jennifer
1Juniata
1
L. ,
Degagne, Rebecca
College Geology Department
2Juniata
2
S. ,
Mutti, Laurence
College Biology Department
3Juniata
1
J. ,
Yohn, Charles
3
E.
College Department of Environmental Studies
ABSTRACT
RAYSTOWN LAKE
Raystown Lake is a large reservoir (514,000 acre-feet of storage
capacity) located in the Valley and Ridge region of south central
Pennsylvania. The Army Corps of Engineers completed the present
dam in the early 1970s. Primary purposes include flood-control,
fishing, boating and hydropower. Nearly half of the watershed drains
agricultural land. To date, extensive nutrient data for Raystown Lake
is not published. This study was conducted to obtain a horizontal
profiling of concentrations along the length of the lake and to
investigate seasonal patterns. Identification of nitrate and
phosphate sources was another objective. The water quality was
monitored for temperature, transparency, total phosphate, and
nitrate concentrations every 2 to 4 weeks from July to November
2000. Epilimnion water samples were taken at 6 sites along the lake,
chosen according to equal geographic distribution and relationship to
input streams. Total phosphate was tested using persulfate acid
digestion and nitrate was tested using a cadmium reduction method.
Both were analyzed using a spectrophotometer.
Transparency, measured with a seechi disk, ranged from 1.5 to 6.9
meters. Values increased with decreasing distance to the dam. Total
phosphate concentrations averaged 0.18 mg L-1. Values varied
significantly throughout the season (0.01 to 0.38 mg L-1). A slight
increase near mid-lake was the only spatial pattern detected. A
sharp decline in total phosphate occurred at the beginning of
November before fall turnover. Nitrate values averaged 0.71 mg L-1,
ranging from 0.47 to 1.00 mg L-1. Concentrations were highest near
the main river input and decreased toward the dam. This relationship
faded with time. A previous sampling in early summer found this
pattern to be statistically significant, though ours did not. Nitrate
concentrations remained relatively constant throughout the summer
then dropped substantially in late September. As with phosphate,
data suggests a possible local nitrate input near mid-lake.
Observed nitrate patterns differed from the expected trend of
nutrient enrichment toward the river. The sudden and drastic drop in
nutrient values in mid-fall was unanticipated.
Raystown Lake is a man-made reservoir located in south-central Pennsylvania in Huntingdon and
Bedford counties (see Figure 1). Construction on the dam began in 1968 as an Army Corps of Engineers
flood control project and was completed in 1973. The lake was filled to its current level in 1975. Today
the reservoir is largely utilized for recreation as well as hydropower, water management, and wildlife
enhancement. The watershed of Raystown Lake encompasses 2490 km2 (960 mi2). Only a small portion
of this area is developed for residential or industrial use. The majority of the land is either forested
or utilized for agricultural purposes. Land use characteristics of the Raystown Lake watershed are
illustrated in Figure 2. The lake is located between two ridges, which expose some relatively steep
slopes prone to sediment sliding and erosion. The general shape is long, narrow, and sinuous with
numerous alcoves and bays. Current pool level is 240 m (780 ft) above mean sea level. The lake is 48 km
(30 mi) long and has 190 km (110 mi) of shoreline. Total water surface area is 3360 hm2 (8300 acres).
Current storage capacity at recreation-pool level is 630 hm3 (514,000 acre-feet). Hydraulic residence
time for water entering the lake is 230 days. The average depths vary considerably, from
approximately 60 meters at the dam to less than 2 meters where the Raystown Branch of the Juniata
River enters Raystown Lake (Army Corps of Engineers 2000, Williams 1978).
FIGURE 1: Raystown Lake is located in south-central Pennsylvania
in Huntingdon and Bedford counties
INTRODUCTION
Since the Raystown Dam was constructed, water quality of
Raystown Lake has been an issue of concern. Reasons for this include
the high intensity use of the lake for fishing and other recreational
activities as well as general monitoring of the impacts that dam
construction has had on the surrounding environment. This has led to a
number of studies, each of which had different specific interests in
mind. The U.S. Geological Survey conducted a post-impoundment
survey shortly after completion of the dam in the years 1974 to 1976.
This was aimed at defining immediate effects of the impoundment on
the quality of the water. It provided base data, including
stratification behavior, specific conductance, pH, dissolved oxygen
and bimonthly nutrient levels (Williams 1978). Between 1985 and 1987,
the Pennsylvania Department of Environmental Resources performed a
study in order to classify the trophic status of the lake (Schott 1987
from Kulp). The Water Quality and Control Section of the Baltimore
District of the U.S. Army Corps of Engineers conducted a broad
water quality investigation in the years 1994 to 1998. This study
reported temperature, stratification, transparency, dissolved oxygen,
specific conductance, pH, alkalinity, acidity, phosphate, nitrate, and
ammonia nitrogen. It was intended to characterize the lake at its
current status and evaluate consequences the lake has experienced
since its creation. Sampling in this study was infrequent, occurring
only 3 times per year. This information has not yet been published
(Kulp 2000, personal correspondence).
Our study, conducted between the months of July to November
2000, focused specifically on total phosphate and nitrate
concentrations. Nutrient loading is a known indicator of lake quality
and trophic status. Our intent was to characterize the water quality
in terms of nutrients through a substantial increase in sampling
frequency and replication as compared to previous studies.
FIGURE 2: Land-use characteristics of
the Raystown Lake watershed
illustrate the dense proportion
of agricultural and forested land
RESULTS
Sampling We collected epilimnion samples to analyze for nitrate and total phosphate every 2 to 4 weeks for a total
of 6 times from July to November 2000. Samples were collected at 6 sites throughout the lake, chosen to
reflect spatial distribution and relation to major input streams. Sites were located at 0,3,9,13,18, and 22 miles
from the dam. The sampling sites are illustrated in Figure 3. Water depth at the sites ranged from 15 to 60
meters. Epilimnion samples were collected from a depth of 2 meters at all sites. A well-defined thermocline
occurs at approximately 8 meters throughout the lake, so we were confident that these samples were
unaffected by any mixing that occurred with the metalimnion.
We used a kemerer bottle to obtain replicate samples from each site. Samples were kept on ice in glass
sampling bottles until processing, which occurred later that day. Weekly sampling included measurement of
transparency with a seechi disk. The disk was lowered into the water until it was no longer visible, then raised
until it could be seen again. This depth was the recorded transparency. Temperature, dissolved oxygen content,
and conductivity were also recorded on a weekly basis as another part of this study. These readings were taken
at the surface and at every meter until either 29 meters or the bottom was reached.
Laboratory procedures –
Nitrate concentrations were analyzed using a cadmium reduction method followed by spectrophotometric
analysis. The test is used to detect concentrations between 0.0 and 30.0 mg L-1 nitrate. Total phosphate in
unfiltered samples was tested using a persulfate acid digestion method and then analyzed for orthophosphate
concentrations using a spectrophotometer. This method is suggested for concentrations between 0.00 and 2.50
mg L-1. Measures were taken to calibrate the spectrophotometer prior to analyses.
Data analysis –
We performed statistical analyses on all of the data collected. Box plots were used to identify statistical
outliers after each sampling. These outliers were not included in the final averages reported. Statistical
analyses were also performed on final averages to identify significant differences between individual values,
both temporal and spatial. These data sets were tested for equal variance prior to pairwise comparisons. In all
cases, a p-value of 0.05 or lower was considered a significant difference. Regression analyses were used to test
for linear trends.
R1 – mile 0 *
R21 – mile 3
R8 – mile 9
R10 – mile 13
R12 – mile 18
R22 – mile 22
Snyder
Run
TransparencyTransparency decreased linearly with distance from the dam. Seechi values ranged
from 6.9 to 1.5 meters. Average seechi depths at individual sites is shown in Figure 4.
Average seechi depths
transparency, m
METHODS
8.0
7.0
6.0
5.0
4.0
3.0
2.0
1.0
0.0
R1
site (dam to river)
R12
R22
PHOSPHATE - spatial and temporal
R1
R10
Trough
Creek
N
R22
JUNIATA
RIVER
R1
0.40
concentration, mg/L
R8
Shy Beaver
Creek
R10
of decreasing transparencies upstream
RAYSTOWN
DAM
James
Creek
R12
R8
FIGURE 4: Average seechi depths at each site show a strong pattern
R21
*distances in
miles from dam
R21
R21
0.30
R8
R10
0.20
R12
R22
0.10
0.00
RIVER (R22)
DAM (R1)
FIGURE 3: Sites sampled for nitrate and total phosphate were distributed
throughout the lake to reflect spatial distribution and location
relative to input streams
FIGURE 8: A 3-D plot of total phosphate concentrations shows the temporal pattern that
was observed as well as the lack of any significant spatial pattern in the lake
R1
R21
concentration, mg/L
1.0
0.8
R8
0.6
R10
R12
0.4
R22
0.2
RIVER (R22)
0.0
A
B
C
nitrate concentration, mg/L
DAM (R1)
FIGURE 5: A 3-D plot of nitrate concentrations demonstrates the linear pattern observed at the
beginning of the sampling season as well as the temporal pattern that was observed
Site R10 (13 miles from the dam)
1.2
1.0
0.8
0.6
0.4
0.2
0.0
25-Jul
1-Aug
14-Aug
6-Sep
date
29-Sep
nitrate concentration, mg/L
NITRATE - spatial and temporal
nitrate concentration, mg/L
Nitrate values averaged 0.71 mg L-1 for the entire
study, ranging from 0.47 to 1.00 mg L-1. Figure 5 illustrates
both temporal and spatial patterns over the sampling period.
This range was produced due to values varying over time and
slightly by location.
In late July, we observed a significant linear pattern of
decreasing nitrate with distance downstream. Values ranged
from a high of 1.00 mg L-1 near the main river input to a low
of 0.60 mg L-1 near the dam. This spatial pattern faded with
time and was no longer significant by September. Figure 7
depicts this disappearing pattern from June to November.
Over the course of the sampling period, nitrate
concentrations throughout the entire lake seemed to
increase slightly from July through early September (0.60 to
0.94 mg L-1). This typical pattern is shown in Figure 6 as
nitrate concentration at site R10 over time. This increase
was more evident near the dam than at the upstream end of
the reservoir. Levels dropped substantially at all sites in late
September to a nearly uniform concentration throughout the
lake (0.50 to 0.58 mg L-1). This was slightly lower than levels
observed at the beginning of sampling.
Nitrate patterns observed during the time of this
study period correlated more strongly with time than with
distance from the dam.
nitrate concentration, mg/L
Nitrate-
1.2
1.0
0.8
0.6
0.4
0.2
0.0
from Simpson, 1998
June 24
R1
1.2
1.0
0.8
0.6
0.4
0.2
0.0
R10
site (dam to river)
R21
R8
R10
R12
R22
R8
R10
R12
R22
site (dam to river)
November 4
R1
R21
site (dam to river)
observed during the summer; C shows
nitrate concentrations in late fall after
the pattern had disappeared
0.40
0.30
0.20
0.10
0.00
25-Jul
1-Aug
14-Aug
6-Sep
29-Sep
4-Nov
date
FIGURE 9: Concentrations of total phosphate
at Site R10show an overall decrease
over the sampling season
0.40
August 1
B
0.30
0.20
0.10
0.00
R1
R21
R8
R10
site (dam to river)
R12
R22
phosphate concentration, mg/L
0.60
phosphate concentration, mg/L
phosphate concentration, mg/L
Site R10 (13 miles from the dam)
A
0.20
Total Phosphate-
November 4
0.10
0.00
R1
R21
R8
R10
R12
site (dam to river)
FIGURE 10: Total phosphate concentrations did not vary with distance from the
dam, but did exhibit a dramatic decrease over summer and fall
R22
R22
FIGURE 7: A and B show the linear spatial pattern
4-Nov
FIGURE 6: Concentrations at Site R10 show the
general temporal pattern of increasing
nitrate through summer followed
by a substantial drop in the fall
0.50
R12
July 25
R1
1.2
1.0
0.8
0.6
0.4
0.2
0.0
R8
Total phosphate averaged 0.18 mg L-1 for the
entire sampling period. The graph in Figure 8 shows
the temporal and spatial changes we observed.
We discovered the highest concentrations in
July (0.38 mg L-1). A statistically insignificant
increase in total phosphate occurred from August
through September (0.13 to 0.29 mg L-1). A sharp
decrease at all sites to extremely low values (0.01 to
0.04 mg L-1) occurred at the beginning of November
prior to fall turnover.
The general temporal pattern that occurred
at the majority of the sites is illustrated by site
R10 in Figure 9.
Phosphate concentrations varied through time
but did not vary with location. See Figure 10.
DISCUSSION
A prior study conducted on Raystown Lake by Simpson in 1998 investigated oxygen depletion in
the metalimnion. She identified a plume-like pattern developing from the upstream end of the
reservoir and enlarging downstream as summer progressed. Simpson determined that the
plume was largely a result of biotic growth in the lake due to the influx of water from the
river and was not a function of recreational activity, which was one of the tested hypotheses.
A clear influence on these oxygen-consuming organisms is the availability of essential
nutrients. Simpson’s study did not include an investigation of nutrient levels in a manner that
can be correlated to biotic growth patterns throughout the season. This study, though
beginning at a time later than desired, attempted to characterize nutrient availability and
loading in the lake.
The Raystown Branch of the Juniata River is the main input stream into Raystown Lake. It
drains a largely agricultural area in the adjacent Morrison’s Cove. We predicted that this input
would be a significant contributor of nutrients and one of the governing factors in oxygen
related biological activity in the lake. Therefore, we thought it would be valuable to document
spatial and temporal distribution of nutrients in the lake. James Creek, which also drains a
largely agricultural area, was considered another potential source of influence near mid-lake.
TransparencySeechi disc transparencies reported in this investigation were nearly identical to values from
past studies. The definite trend of increasing transparency downstream is due to
sedimentation of suspended materials carried into the lake from the river and elevated
nutrient levels resulting in a higher density of plankton in the epilimnion. The pattern we
observed was probably caused by a combination of sedimentation and increased biotic activity.
Behavior of nitrateA post-impoundment survey from 1974 to 1976 reported average lake-wide values of nitrate
ranging from 0.75 to 0.84 mg L-1 with a noted pattern of increasing concentrations towards the
river input. Total range observed in this study was 0.32 to 1.50 mg L-1. Seasonal fluctuations
showed a consistent pattern of highest values in March and lowest values in
August/September with a gradual change in the times between (Williams 1978). Studies
conducted by the Army Corps of Engineers between 1994 and 1998 reported average
epilimnetic nitrate values of 1.00 to 1.20 mg L-1. A weak geographic pattern was observed with
increasing values towards the main river input and a possible nitrate source near mid-lake. No
seasonal trend was detected (Kulp 2000, personal correspondence). A one day nitrate testing
of the lake conducted in June 1998 reported values of 0.3 to 1.1 mg L-1 with a strong trend of
increasing nitrate concentrations toward the river input (Simpson 1998, unpublished data).
Our study reported values in a similar range as these previous studies (0.47 to 1.00 mg L-1), but
introduced a more complex seasonal trend that had remained undetected prior to this study
due to sampling frequency. This indicates the importance of replication and more frequent and
extended sampling in order to characterize nutrient loading occurring in Raystown Lake.
We observed a geographic pattern of lower nitrate concentrations near the dam and higher
concentrations near the river input that was noted in the study conducted by Simpson. This
was evident in the July 25 sampling (Figure 7B). This trend faded over the end of summer and
early fall and was no longer present by September 6. By September 29, nitrate values at all
sites fell to a rather uniform value that was lower than values observed at the beginning of the
sampling (Figure 5).
The spatial distribution of nitrate values suggests that the major source of nitrate into this
system is the Raystown Branch of the Juniata River. This conclusion is consistent with the
observation that this sub-basin is the largest and most agricultural in the Raystown watershed
(Figure 2). Another source suggested by spatial trends is James Creek, which enters Raystown
Lake near mid-lake, but evidence for this is not clear. Elevated values during the summer in the
upper end of the reservoir may be a reflection of high seasonal inputs of nitrate from
agricultural sources. Increased nitrate use from commercial fertilizers and the spreading of
accumulated manure before or shortly after planting could lead to these values. Higher
amounts of rain and rates of runoff in spring and early summer could also contribute to these
increases.
Lake-wide averages showed a modest increase over the period from late July through
September (Figure 6). This appears to reflect a build-up of unutilized nitrate through this
time period as values of available reactive phosphate were decreasing. Another possible cause
of this apparent increase in nitrate concentrations can be explained through migration of the
original input nutrients downstream. Knowing the hydraulic residence time in the lake is 233
days, it would take approximately 16 weeks to travel half the distance of the lake. This
increase may also be influenced by seasonal contributions to the lake from groundwater
draining highly agricultural land. However, because exact groundwater inputs are unidentified
and not characterized in terms of volume and chemistry at this time, we cannot determine how
much of an influence groundwater could have, if any.
The dramatic and rather sudden decrease in nitrate concentrations experienced in late
September was not anticipated. There was no spatial pattern to this event, and the final levels
were lower than what had been reported at the beginning of the sampling season. In order to
explain this event, it may be necessary to examine the biological interactions between
organisms in the epilimnetic waters and elements essential for life, such as carbon, oxygen,
phosphorus, and nitrogen.
Behavior of total phosphateStudies conducted prior to this investigation recorded reactive phosphate concentrations
ranging from 0.03 to 0.08 mg L-1. There was no clear temporal trend but a vague spatial trend
suggested a phosphate input near mid-lake (Kulp 2000). Average lake-wide values obtained from
the post-impoundment survey range from 0.00 to 0.04 mg L-1 orthophosphate, or reactive
phosphate. This study also investigated total phosphorus, which averaged 0.01 to 0.02 mg L-1
(equivalent to 0.03 to 0.06 mg L-1 total phosphate) and included a total range from 0.00 to 0.07
mg L-1 (equivalent to 0.21 mg L-1 total phosphate). A pattern of increasing concentrations
towards the river input was noted in both orthophosphate and total phosphorus. In both cases,
values obtained from near the river input were lowest in September/October 1974 and in
November 1975. Both years show peaks in March, and total phosphorus produced a second peak
in September 1975 (Williams 1978).
Information obtained in previous studies led us to the conclusion that phosphorus must be the
limiting nutrient in Raystown Lake. This agrees with behavior common in many natural systems.
We presumed that there would not be detectable amounts of reactive phosphate remaining in
the water and therefore, this information would not be useful in attempting to characterize
nutrient trends. Testing for total phosphate would reflect available phosphate as well as the
accumulation of phosphate in microorganisms that remain suspended in the water column. The
decision to analyze the water for total phosphate rather than reactive phosphate was made on
this basis.
Values we obtained in this study were dramatically higher than those reported in 1978. This
may be reflective of a lack of planktonic organisms shortly following the creation of the
reservoir. This indicates that there is nutrient loading taking place in Raystown Lake that is
likely to become an issue of concern in the future.
We found no geographic pattern in total phosphate concentrations (Figure 10A). We did find a
trend suggestive of an overall seasonal pattern of decreasing concentrations (Figure 9). This
decrease in total phosphate probably reflects consumption of the epilimnetic phosphate by
algae and other planktonic organisms as the summer progresses. It may also be indicative of
the loss of phosphorus as it settles out of suspension to the underlying sediments. Data from a
preliminary investigation conducted in midsummer (July 13) not included in this sampling set
produced values of 0.90 to 1.08 mg L-1, supporting this interpretation. This data was not
included in this study due to different sampling depths and locations. These high values were
observed during the warmest months of the summer when productivity was probably at a high.
By the following sampling (July 25), concentrations had dropped significantly to 0.33 to 0.52
mg L-1. We surmise that this drop may be a delayed effect of spring bloom. By fall (November
4), total phosphate was nearly undetectable (Figure 10B).
It is important to note that we have not constrained these conclusions very well. We do not
have discharge data for the tributaries to the lake nor do we know what influence springs in
the area might have on the water chemistry. Adjacent topographic relief and variations in the
hydraulic characteristics of the exposed easterly-dipping sedimentary rocks have the potential
to produce flows from undefined sources below lake level, adding an additional factor in
nutrient patterns. This study also did not investigate in detail biological cycles that may have a
substantial impact on nutrient concentrations.
FURTHER INVESTIGATION
This study provides an in depth view of nutrient
concentrations and patterns in the lake and offers a possible
explanation for our observations. A further, more extensive
investigation would be necessary in order to more precisely
classify the water quality of Raystown Lake in terms of nutrient
concentrations and related productivity. This would be useful to
understand the causes of nutrient behavior as well as exact
sources of phosphate and nitrate.
A very important addition to this study would be to begin
sampling much earlier in the year, perhaps April. This would
provide a more complete description of nutrient behavior and
enable better correlation to the time of year and suggested
inputs. When analyzing samples, filtering should be considered.
This could eliminate microfauna captured in the sample that may
alter the actual nutrient concentrations. Focus on vertical
patterns is another possibility. Such patterns have been noted in
literature and by other studies on Raystown Lake and are an
important characteristic in determining degree of
eutrophication of lakes. Monitoring rainfall and springs in the
area would allow us to determine associated dilution or
concentration effects upon nutrients. Chemical discharge data
for major inputs to the lake, both surface and subsurface, would
be another valuable addition. This would enable better
identification of sources of nutrient input.
REFERENCES
1. 1989, HACH Water Analysis Handbook 2nd Edition, Loveland,
Colorado: Hach Company, p. 400-401, 523-545.
2. Army Corps of Engineers, 2001, Raystown Lake Pennsylvania,
<www.nab.usace.army.mil> 2000, September 19.
3. Kulp, Kenneth P., 2000, Water Quality of Raystown Lake,
Army Corps of Engineers, Unpublished data.
4. Metcalf, Alexander, 2001, Unpublished data.
5. Simpson, Sharon, 1998, Unpublished data.
6. Wetzel, Robert G., 1975, Limnology: Pennsylvania, W.B. Saunders
Company, 743 p.
7. Williams, Donald R., 1978, Postimpoundment Survey of WaterQuality Characteristics of Raystown Lake, Huntingdon and
Bedford Counties, Pennsylvania: United States Geological Survey
Water Investigation 78-42, 53 p.