Statistical Analysis of Sediment Phosphorus from Silver
Lake and Lake Casey in 2002
Iowa Lakes Project
Amy Oelmann
Undergraduate Student
Department of Mathematics
University of Northern Iowa
Mark D. Ecker
Associate Professor
Department of Mathematics
University of Northern Iowa
October 7, 2002
I. Introduction
During the summer of 2002, a group of students and faculty members at the University of
Northern Iowa continued their study of water quality of two lakes in Iowa. Water quality
variables in Silver Lake had been sampled at 20 different sites during the summer of
2001, and those same sites were used for a sediment phosphorus analysis in the summer
of 2002. Water quality variables in Lake Casey had been sampled at 23 different sites in
2001, and those sites were also used in the current sediment phosphorus analysis.
II. Sediment Depth Profile: Silver Lake
Each of the 20 sites in Silver Lake were sampled, with 7 sediment phosphorus
concentrations obtained from each site, each at a different depth. The circles in each map
represent the sites where the data were obtained, and the numbers are the concentration of
phosphorus at that particular site. Figure 1 is the Silver Lake sediment phosphorus depth
profile map from 0 to 2 inches, produced in July of 2002. Table 1 contains the summary
statistics for the 0 to 2 inch sediment depth.
Figure 1
4698700
Silver Lake Depth Profile from 0 to 2 inches
387.78
732.04
4698500
39.69
1278.94
11.95
4698300
Northing (m)
606.08
18.65
770.29
11.95
14.82
41.6
844.89
628.76
1006.12
733.95
1021.8
4698100
76.98
1315.38
45.43
422.26
637500
637600
637700
637800
637900
638000
Easting (m)
The majority of the sites with the largest concentration of sediment phosphorus in Silver
Lake are in the middle of the lake. The samples taken near the perimeter of the lake
2
have a fairly low phosphorus concentration. The highest phosphorus level near the
perimeter of the lake is in the southeast corner, where there is a hog lot that is adjacent to
the lake.
Table 1
*** Summary Statistics for data in: sdp02 ***
*** Silver Lake Depth Profile from 0 to 2 inches ***
Mean:
Std Dev.:
Min:
1st Qu.:
Median:
3rd Qu.:
Max:
Concentration
500.4680
452.9522
11.9500
41.1225
514.1700
788.9400
1315.3800
The mean sediment phosphorus concentration for the 0 to 2 inch depth is 500.468 ug/g.
Some of the samples taken in the lake are much higher than 500. There is a range of
concentrations in the 0 to 2 inch depth level equal to 1303.43, (11.95 to 1315.38).
Figure 2 is the Silver Lake sediment phosphorus levels for a depth of 2 to 4 inches. Table
2 contains the summary statistics for the 2 to 4 inch sediment depth.
Figure 2
4698700
Silver Lake Depth Profile for 2 to 4 inches
4698500
17.69
798.03
4698300
11
1193.96
543.65
1065.79
896.53
4698100
Northing (m)
726.31
637500
637600
637700
637800
Easting (m)
3
637900
638000
There were not as many samples taken at the depth 2 to 4 inches. The largest
concentrations of phosphorus are located towards the middle of the lake. The smaller
concentrations seem to be near the perimeter. Again, the largest concentration around the
perimeter of the lake is in the area where the hog lot is located.
Table 2
*** Summary Statistics for data in: sdp24 ***
*** Silver Lake Depth Profile from 2 to 4 inches ***
Mean:
Std Dev.:
Min:
1st Qu.:
Median:
3rd Qu.:
Max:
Concentration
656.6200
443.4256
11.0000
412.1600
762.1700
938.8450
1193.9600
The mean phosphorus concentration for this depth is slightly larger than the mean from
the surface (0 to 2 inch depth), but there is also less data. The spread again is very large,
with a range of 1182.96, (from 1193.96 down to just 11).
Figure 3 is the Silver Lake sediment phosphorus levels for a depth of 4 to 6 inches. Table
3 contains the summary statistics for the 4 to 6 inch sediment depth.
Figure 3
4698700
Silver Lake Depth Profile for 4 to 6 inches
227.12
4698500
804.72
668.93
4698300
16.73
789.55
727.26
735.87
201.78
936.69
151.52
733.95
4698100
Northing (m)
972.07
637500
637600
637700
637800
Easting (m)
4
637900
638000
More samples were taken at this depth than the last, but many of the 20 sites have not
been sampled. The concentrations are generally higher than in the previous two depth
profiles. Most of the concentrations are in the hundreds, and only one is less than that.
The lowest concentration is located on the west side of the lake, near where the lake
bends.
Table 3
*** Summary Statistics for data in: sdp46 ***
*** Silver Lake Depth Profile from 4 to 6 inches ***
Mean:
Std Dev.:
Min:
1st Qu.:
Median:
3rd Qu.:
Max:
Concentration
580.5158
333.2194
16.7300
220.7850
730.6050
793.3425
972.0700
The mean and the median in the previous two depths were very close to each other, but as
is shows in Table 3, they are very different at the depth of 4 to 6 inches. There is a
difference of 150.0892. The range of concentrations at the depth of 4 to 6 inches is
995.34.
Figure 4 is the Silver Lake sediment phosphorus levels for a depth of 6 to 8 inches. Table
4 contains the summary statistics for the 6 to 8 inch sediment depth.
Figure 4
4698700
Silver Lake Depth Profile for 6 to 8 inches
140.09
4698500
741.6
4698300
573.3
926.53
438.13
722.78
471.52
4698100
Northing (m)
325.62
637500
637600
637700
637800
Easting (m)
5
637900
638000
The concentrations at this level are even larger than before, with the largest still being
near the middle of the lake. The smallest concentration is at the northeast corner.
Table 4
*** Summary Statistics for data in: sdp68 ***
*** Silver Lake Depth Profile from 6 to 8 inches ***
Mean:
Std Dev.:
Min:
1st Qu.:
Median:
3rd Qu.:
Max:
Concentration
542.4463
252.2672
140.0900
410.0025
522.4100
727.4850
926.5300
Table 4 shows the summary statistics for the sediment phosphorus at a depth of 6 to 8
inches. The range is 786.44, which spreads from 140.09 to 926.53. The mean and
median at this depth have a difference of only 20. But the site with the smallest
concentration is much larger than it has been previously. It is now over 100 (140.09
ug/g) while it has been under 17 the three depths before.
Figure 5 is the Silver Lake sediment phosphorus levels for a depth of 8 to 10 inches.
Table 5 contains the summary statistics for the 8 to 10 inch sediment depth.
Figure 5
4698700
Silver Lake Depth Profile for 8 to 10 inches
590.51
4698500
397.34
4698300
422.21
1056.84
346.65
127.67
465.23
107.58
352.39
4698100
Northing (m)
296.93
637500
637600
637700
637800
Easting (m)
6
637900
638000
The results for the 8 to 10 inch depth, concentrations are not much different than seen
previously. The largest concentration is towards the middle of the lake, and the smaller
ones are near the perimeter.
Table 5
*** Summary Statistics for data in: sdp810 ***
Silver Lake Depth Profile from 8 to 10 inches ***
***
Mean:
Std Dev.:
Min:
1st Qu.:
Median:
3rd Qu.:
Max:
Concentration
416.3350
267.7539
107.5800
309.3600
374.8650
454.4750
1056.8400
Table 5 shows that the largest concentration at this depth is 1056.84 ug/g. Figure 5
shows that this site is located near the middle of the lake, but towards the area where the
hog lot runs into the lake. The range at the 8 to 10 inch depth is 949.26..
Figure 6 is the Silver Lake sediment phosphorus levels for a depth of 10 to 12 inches.
Table 6 contains the summary statistics for the 10 to 12 inch sediment depth.
Figure 6
4698700
Silver Lake Depth Profile for 10 to 12 inches
4698500
370.57
4698300
366.74
595.29
190.04
244.33
204.17
4698100
Northing (m)
259.63
637500
637600
637700
637800
Easting (m)
7
637900
638000
Looking at Figure 6, it is obvious that there are not many sites that were sampled at this
depth profile. The sites that have been sampled, however, seem to be having lower
sediment concentrations than previously. The site with the largest concentration is still in
the middle of the lake, but near the location of the hog lot.
Table 6
*** Summary Statistics for data in: sdp1012 ***
*** Silver Lake Depth Profile from 10 to 12 inches ***
Mean:
Std Dev.:
Min:
1st Qu.:
Median:
3rd Qu.:
Max:
Concentration
318.6814
141.5339
190.0400
224.2500
259.6300
368.6550
595.2900
The largest concentration at the depth of 10 to 12 inches is only 595.29 ug/g. This is
much smaller than many of the previous maximum concentrations. The spread at this
depth is from 190.04 to 595.29, which gives a range of 405.25. The range at this depth is
also much smaller than seen previously.
Figure 7 is the Silver Lake sediment phosphorus levels for a depth greater than 12 inches.
Table 7 contains the summary statistics for the greater than 12 inch sediment depth.
Figure 7
4698700
Silver Lake Depth Profile greater than 12 inches
445.15
4698500
253.89
4698300
248.1
729.81
235.41
235.7
192.69
4698100
Northing (m)
214.68
637500
637600
637700
637800
Easting (m)
8
637900
638000
There were only 8 sites sampled at the depths greater than 12 inches, and there were no
concentrations obtained at the far west side of the lake. The phosphorus concentrations
seem to be getting smaller with depth, but the site with the largest concentration is still
located in the middle of the lake, near the hog lot.
Table 7
*** Summary Statistics for data in: sdp12more ***
*** Silver Lake Depth Profile greater than 12 inches ***
Mean:
Std Dev.:
Min:
1st Qu.:
Median:
3rd Qu.:
Max:
Concentration
319.4287
183.1106
192.6900
230.2275
241.9000
301.7050
729.8100
Table 7 shows that the largest concentration was 729.81 ug/g, and this was at the same
site as it was previously. The range at the depths greater than 12 inches is 537.12.
III. Overall Conclusions for Silver Lake
Overall, the average sediment concentration at each depth profile in Silver Lake was well
over 300 ug/g, with a maximum mean value of 656.62. The site in the middle of the lake,
near the Eastern shore, just West of the hog lot area, consistently has some of the highest
concentrations.
IV. Sediment Depth Profile: Lake Casey
Figure 8 is the Lake Casey sediment phosphorus levels for a depth of 0 to 2 inches.
Table 8 contains the summary statistics for the 0 to 2 inch sediment depth.
9
Figure 8
4679600
331.35
191.74
267.29
370.57
4679500
404.99
239.55
255.81
290.23
423.16
350.48
492.01
496.79
600.07
351.44
4679400
Northing (m)
4679700
Lake Casey Depth Profile from 0 to 2 inches
343.79
164.96
314.14
4679300
478.63
185.4
252.94
556200
556400
556600
556800
Easting (m)
Three sites in Figure 8 do not have a phosphorus concentration level. These sites are
where the water flows into the lake. No sediment samples were obtained at these inflow
areas (for any depth). The sites with the largest concentration of sediment data are
towards the middle of the lake. At the surface depth however, the site with the largest
concentration is very close to the site with the lowest concentration.
Table 8
*** Summary Statistics for data in: cdp02 ***
*** Casey Lake Depth Profile from 0 to 2 inches ***
Mean:
Std Dev.:
Min:
1st Qu.:
Median:
3rd Qu.:
Max:
Concentration
340.2670
116.2254
164.9600
255.0925
337.5700
409.5325
600.0700
Table 8 shows that the range of concentrations at this depth is 435.11, (164.96 to 600.07).
The largest concentration is 600.07 ug/g.
10
Figure 9 is the Lake Casey sediment phosphorus levels for a depth of 2 to 4 inches.
Table 9 contains the summary statistics for the 2 to 4 inch sediment depth.
Figure 9
4679600
250.07
169.74
288.31
339.96
4679500
360.05
134.36
203.21
295.97
244.33
314.14
590.51
529.31
807.59
308.41
4679400
Northing (m)
4679700
Lake Casey Depth Profile from 2 to 4 inches
438.46
405.95
437.51
4679300
688.05
222.34
203.21
556200
556400
556600
556800
Easting (m)
Again, there is no data for the three inflow sites, and the largest concentrations are
towards the center of the lake. As you can see in Figure 9, the site with the largest
concentration is the same from the surface sediment phosphorus analysis, but the site
with the smallest concentration has changed.
Table 9
*** Summary Statistics for data in: cdp24 ***
*** Casey Lake Depth Profile from 2 to 4 inches ***
Mean:
Std Dev.:
Min:
1st Qu.:
Median:
3rd Qu.:
Max:
Concentration
361.5740
177.5681
134.3600
238.8325
311.2750
437.7475
807.5900
11
The largest concentration at the depth from 2 to 4 inches was 807.59 ug/g, which is much
larger than the largest concentration at the surface depth. The range is 673.23.
Figure 10 is the Lake Casey sediment phosphorus levels for a depth of 4 to 6 inches.
Table 10 contains the summary statistics for the 4 to 6 inch sediment depth.
Figure 10
4679600
262.5
228.07
246.25
358.13
4679500
283.54
244.33
255.81
261.55
370.57
288.32
369.61
81.76
708.13
192.69
4679400
Northing (m)
4679700
Lake Casey Depth Profile from 4 to 6 inches
483.41
384.91
353.35
4679300
510.19
180.26
207.05
556200
556400
556600
556800
Easting (m)
Figure 10 shows that again the largest concentrations are in the middle of the lake, and
there are no observations for the three inflow sites. However, the other twenty sites have
still been sampled at this depth. The site with the smallest concentration has again
changed, but the site with the largest concentration remains the same.
Table 10
*** Summary Statistics for data in: cdp46 ***
*** Casey Lake Depth Profile from 4 to 6 inches ***
Mean:
Std Dev.:
Min:
1st Qu.:
Median:
3rd Qu.:
Max:
Concentration
313.5215
138.1307
81.7600
240.2650
273.0200
369.8500
708.1300
12
The largest concentration for this depth is 708.13 ug/g, and the range is 626.37. The
smallest concentration at this depth is the smallest seen so far in Lake Casey, with a
concentration of only 81.76 ug/g.
Figure 11 is the Lake Casey sediment phosphorus levels for a depth of 6 to 8 inches.
Table 11 contains the summary statistics for the 6 to 8 inch sediment depth.
Figure 11
4679600
298.84
311.27
220.43
421.25
4679500
294.06
317.97
233.81
203.21
511.14
476.71
438.46
340.92
925.21
305.53
4679400
Northing (m)
4679700
Lake Casey Depth Profile from 6 to 8 inches
313.19
218.51
4679300
293.11
165.92
265.37
556200
556400
556600
556800
Easting (m)
The site with the largest concentration is again at the same site as it has been previously,
but the smallest concentration has changed. Some of the largest concentrations are in the
western part of the lake. There is no data for the three inflow sites, and there is one other
site that has not been sampled at this depth.
13
Table 11
*** Summary Statistics for data in: cdp68 ***
*** Casey Lake Depth Profile from 6 to 8 inches ***
Mean:
Std Dev.:
Min:
1st Qu.:
Median:
3rd Qu.:
Max:
Concentration
344.9953
168.3929
165.9200
249.5900
305.5300
381.0850
925.2100
Table 11 shows that the largest concentration at the depth of 6 to 8 inches is 925.21 ug/g,
and the smallest is 165.92. Therefore that the range is 759.29.
Figure 12 is the Lake Casey sediment phosphorus levels for a depth of 8 to 10 inches.
Table 12 contains the summary statistics for the 8 to 10 inch sediment depth.
Figure 12
4679600
260.59
254.85
254.85
803.77
4679500
334.25
396.39
349.49
361
484.36
342.83
4679400
Northing (m)
4679700
Lake Casey Depth Profile from 8 to 10 inches
339.01
248.16
4679300
377.26
171.65
556200
556400
556600
556800
Easting (m)
Figure 12 shows that there are more sites that have not been sampled, along with the
three inflow sites. The site with the largest concentration of sediment data has changed
for the first time in Lake Casey. It is now in the western part of the lake. The depth from
14
8 to 10 inches is very different than the previous depths. The site with the smallest
concentration has also changed.
Table 12
*** Summary Statistics for data in: cdp810 ***
*** Casey Lake Depth Profile from 8 to 10 inches ***
Mean:
Std Dev.:
Min:
1st Qu.:
Median:
3rd Qu.:
Max:
Concentration
355.6043
150.7004
171.6500
256.2850
340.9200
373.1950
803.7700
The range for the 8 to 10 inch depth profile is 632.12, with the largest phosphorus
concentration being 803.77 ug/g.
Figure 13 is the Lake Casey sediment phosphorus levels for a depth of 10 to 12 inches.
Table 13 contains the summary statistics for the 10 to 12 inch sediment depth.
Figure 13
4679600
240.51
223.29
4679500
426.99
581.91
4679400
409.77
246.25
4679300
Northing (m)
4679700
Lake Casey Depth Profile from 10 to 12 inches
276.85
556200
556400
556600
Easting (m)
15
556800
There are very few sites that have been sampled in Figure 13. The site that has had the
largest concentration for the majority of the depths was not even sampled at the 10 to 12
inch depth profile.
Table 13
*** Summary Statistics for data in: cdp1012 ***
*** Casey Lake Depth Profile from 10 to 12 inches ***
Mean:
Std Dev.:
Min:
1st Qu.:
Median:
3rd Qu.:
Max:
Concentration
343.6529
133.6405
223.2900
243.3800
276.8500
418.3800
581.9100
The largest concentration at the depth of 10 to 12 inches is 581.91 ug/g, which is much
smaller than the previous depth. The range for this depth is 358.62.
Figure 14 is the Lake Casey sediment phosphorus levels for a depth greater than 12
inches. Table 14 contains the summary statistics for the greater than 12 inch sediment
depth.
Figure 14
4679600
465.62
4679500
345.7
623.91
4679400
260.59
250.07
4679300
Northing (m)
4679700
Lake Casey Depth Profile greater than 12 inches
556200
556400
556600
Easting (m)
16
556800
The site with the largest concentration in Figure 14 is the same site as in Figure 13.
There are even less sites sampled at the depths greater than 12 inches, and still no
samples taken at the three inflow sites.
Table 14
*** Summary Statistics for data in: cdp12more ***
*** Casey Lake Depth Profile greater than 12 inches ***
Mean:
Std Dev.:
Min:
1st Qu.:
Median:
3rd Qu.:
Max:
Concentration
389.1780
157.0977
250.0700
260.5900
345.7000
465.6200
623.9100
Table 14 shows that the smallest concentration was 250.07 ug/g, and the largest was
623.91 ug/g. The range for the concentrations at the depths greater than 12 inches is
373.84.
V. Overall Conclusions for Lake Casey
Overall, the average sediment concentration at each depth profile in Lake Casey are
consistently around the 300-level ug/g, with a minimum mean value of 313.52 and a
maximum value of 389.18. The site that contained the largest phosphorus concentration
for most depth profiles is toward the middle of the lake, just south of the inflow area near
the east edge.
VI. Variogram Analysis
Figure 15 is the Variogram Cloud for Silver Lake from the depth 0 to 2 inches. The
variogram reflects the spatial correlation as a function of distance for all possible pairs of
sites. See Carlson and Ecker (2002) for a review of variogram analysis.
17
Figure 15
400000
0
200000
gamma
600000
800000
Variogram Cloud for Silver lake (Depth 0 to 2 inches)
0
100
200
300
400
500
600
distance
Figure 16 is the Empirical Variogram for Silver Lake from the depth 0 to 2 inches.
Figure 16
200000
100000
0
gamma
300000
400000
Empirical Variogram for Silver Lake (Depth 0 to 2 inches)
0
100
200
18
distance
300
400
There is very little evidence of spatial correlation shown in Figure 16. The circles tend
towards a line, but there are a few outliers that make the significance not very strong. A
horizontal line would indicate no spatial correlation. None of the other depth analysis of
phosphorus data (for either Silver Lake or Lake Casey) indicated any evidence of spatial
correlation.
VII. Wetlands
Another part of the project this summer was mapping the wetland area just North of
Cedar Falls. This area included Beaver Creek, and several lakes and ponds that drain into
the Cedar River. Brad Woodle walked along the creek, and around the lakes and ponds
recording points using a GPS system. Then, the points were downloaded onto a
computer, which produced several maps of the area. Figure 17 is the map that was made
showing all of the lakes and ponds in that area. Figure 18 is the same map, including
Beaver Creek.
Figure 17
Lakes and Pond
42.584
Pond
Island 4
Island 3
Island 2
Island 1
42.582
lat
Lakes 4 & 5
42.580
Big Lake
Lake 1
East Lake
Lake 2
42.578
Lake 3
42.576
-92.517
-92.512
long
19
-92.507
-92.502
Figure 18
Everything
42.58
Lakes, Pond, & Islands
lat
42.57
42.56
Beaver Cr eek
42.55
42.54
-92.52
-92.51
-92.50
-92.49
-92.48
long
-92.47
-92.46
-92.45
It appears that the lakes and ponds in the wetland area drain into Beaver Creek, which
will eventually flow into the Cedar River.
VIII. Reference
Carlson, E. and Ecker, M.D. "A Statistical Examination of Water Quality in Two Iowa
Lakes" (2002). American Journal of Undergraduate Research. 1(2), pp. 31-45.
20