Dudley Pond Water Quality Sampling Program March 18, 2012 – Toni Moores
This document is part of an ongoing Surface Water Quality Committee (SWQC) water quality sampling program initiated for Dudley Pond. The objectives of the Program are to gather Dudley Pond water quality data that will be used to: Compare to similar water quality data gathered in the past at Dudley Pond. Track the trophic index of Dudley Pond during the spring, summer and fall of each year. Summary
On March 18, 2012 water quality data was gathered at three sample points and at three depths at each sample point. Water quality parameters such as, pH, oxidation – reduction potential (ORP), dissolved oxygen (DO), dissolved oxygen % saturation and temperature were measured at three depths at each of the three sample points by SWQC members using a YSI Multimeter. Water and Secchi depths were measured at each of the three sample points. Samples were gathered by SWQC members and analyzed by Nashoba Analytical LLC. The water quality in Dudley Pond was found to be low to mid mesotrophic at the points sampled, based on the Carlson Trophic State Index (TSI), which is a function of Secchi depth, Chlorophyll a and Total Phosphorus (TP) concentration. The average TSI range for the three sample points was found to be 39 - 47. The TP concentrations were 20 ppb and the Orthophosphorus (soluble) (OP) concentrations were 10 ppb, which were greater than the April 2011 sampling. When historical water quality is reviewed for March and April samplings starting in 1981 the trend is mixed, but at least water clarity as measured with a Secchi disc is getting better.
Background Historically Dudley Pond has been sampled most frequently at the “deep hole” (Sample Point 25) on Dudley Pond. In order to have data sets that are comparable to historical data, samples were gathered at locations 24, 25 and 27 as indicated in Figure 1. Two of these sample locations correspond to the sample locations used in the Larkin (1978) and IEP (1983) reports. Line-of-site intersections were used to locate sampling points.
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Figure 1 – Dudley Pond Sampling Points Sample Point 24 is located at the intersection of the lines-of-site between Rocky Point – Bayfield Rd and Mansion Beach – “The Chat”. Sample Point 25 is located at the intersection of the lines-of-site between Mansion Beach –Southern point of the Dudley Rd. peninsula and Lowery’s dock – Williams Point. Sample Point 27 is located at the intersection of the lines-of-site between the Dudley Pond outlet – 107 Dudley Rd. and the foot of Maiden Lane – the 20” outfall adjacent 27 Bayfield Rd. Methods Samples were gathered at depths of one foot (Top), at mid depth (Middle) and one foot off the bottom (Bottom) at Sample Points 24, 25 and 27. A water sampler as shown at http://www.aquaticeco.com/subcategories/2912/Water-Sampler?green=12823266105 with a sample volume of 1.0 – 1.5 liters was used to gather samples at various depths. Samples and data were/are to be gathered during March, August and November each year, in an attempt to understand seasonal variations. Sample Handling - Multiple sample catches at each depth were composited from the sampler into a one gallon container in order to get the volume of sample needed for the various analyses. The one gallon container was mixed and aliquots were poured into sample bottles provided by the analytical laboratory. The labels on the sample containers were filled out. One of the sample bottles contained sulfuric acid necessary to “fix”(preserve) the sample for total phosphorus (TP) analysis. All of the
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samples were stored in a cooler containing ice and delivered with chain of custody documents to the analytical laboratory the day after the samples were gathered. Sample Analyses - A SWQC owned YSI 556 Multimeter was used to measure the following parameters at the same time that the samples are collected. Date Time Weather Barometric Pressure Location Depth pH ORP Dissolved Oxygen Dissolved Oxygen % Saturation Temperature Secchi Depth (http://water.epa.gov/type/rsl/monitoring/155.cfm) Nashoba Analytical, LLC was used to perform analyses of samples from Sample Points 24, 25 and 27. Analyses were performed by Nashoba Analytical, LLC for the following parameters.
Total Phosphorus, ppm (as P) (Soluble + Insoluble) Orthophosphate, ppm (as P) (Soluble) Ammonia, ppm (as N) Nitrate/Nitrite, ppm (as N) Total Nitrogen, ppm (as N) Chlorophyll a, ppb Carlson Trophic State Index – TP concentrations, Chlorophyll a and Secchi depths were used to plot the Carlson Trophic State Index (TSI) for the three sample points in Dudley Pond. Since the Secchi Depths for Sample Points 24 and 27 were greater than the depth of the water, it was assumed that the Secchi Depths for all of the sample points were the same as Sample Point 25. In order to obtain an average TP concentration for each sample point the three Total Phosphorus concentrations for the sample point were averaged. In order to obtain an average Chlorophyll a concentrations for each sample location. The average of the chlorophyll a concentration at each sample point was calculated using the average of the chlorophyll a concentrations at the three depths. (see http://www.aslo.org/lo/toc/vol_22/issue_2/0361.pdf for information re Carlson Trophic State index) Results The results of the March 18, 2012 sampling are shown in Figure 2. The analytical data was plotted to determine the TSI and is shown in Figure 3.
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Figure 2 – Dudley Pond Water Quality Data
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Date / Time: March 18, 2012 / 1:30 PM Samplers: Toni, Andy Weather: Sunny 70 F Light Wind Barometric Pressure: 773.8 mm Hg
SP 24-top SP24-mid SP24-bot
SP 25-top SP25-mid SP25-bot
SP27-top SP27-mid SP27-bot
Depth (ft)
1'
3'
6' 1"
1'
12'
24' 1"
1'
3'
6' 10"
pH
7.4
7.38
7.37
7.32
7.16
7.05
5.97
5.99
6.04
ORP
245.6
238
228
210
220
214.6
262.6
246
232
DO
13.54
13.2
13.2
13.9
14.4
11.75
13.33
13.35
14.12
DO%
121
118
117
124
119
95
118.9
118.2
123
Temp
10.33
10.27
10
9.9
6.9
6.1
10.21
9.57
9.17
Secchi Depth
> 6' 1"
Total Phosphorus (as P) OrthoPhosphate (as P) Ammonia (as N) Nitrate/Nitrite (as N) Total Nitrogen (as N) Chlorophyll É‘
0.02 ppm 0.01 ND 0.53/ND ND 2.28 ppb
12' 6"
0.01 ppm 0.01 ND 0.53/ND ND 2.34 ppb
0.01 ppm 0.01 ND 0.52/ND ND 2.06 ppb
0.02 ppm 0.01 ND 0.51/ND ND 2.56 ppb
> 6' 10"
0.02 ppm 0.01 ND 0.51/ND ND 3.43 ppb
0.02 ppm 0.01 ND 0.5/ND ND 3.74 ppb
0.02 ppm ND ND 0.47/ND ND 1.98 ppb
0.02 ppm 0.01 ND 0.49/ND ND 2.41 ppb
0.02 ppm ND ND 0.5/ND ND 4.62 ppb V1 5
Figure 3 – Carlson Trophic State Index Results for March 18, 2012, Sample Points 24, 25 & 27
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Discussion YSI Measurements - Measurements taken with the YSI 556 Multimeter were relatively similar for all parameters from top to bottom at all sample points indicative of relatively homogeneous conditions, suggesting that the March 18, 2012 sampling was done sometime close to the biannual turnover of the Pond. From the DO, DO percent Saturation data and Secchi depth it appears that photosynthesis was occurring throughout most of the water column, at least to the limit of sunlight penetration. The pH was low at Sample Point 27 at all depths. It is not clear why this was the case, because all the other parameters were comparable to Sample Points 24 and 25. Secchi Depths – Secchi depths (a measure of color, turbidity and suspended solids, such as algae) on March 18, 2012 were very large at all sample points compared to Secchi depth taken in past years, which have been a low as 1.5 feet. Large Secchi depths indicate that there is very little color and suspended materials such as algae and soil solids in the water column. Accurate Secchi depths at Sample Points 24 and 27 could not be obtained because the bottom was clearly visible implying the Secchi depth was greater than the bottom depth. Therefore it has been assumed that the Secchi depths at Sample Points 24 and 27 were similar to that at Sample Point 25, which was 12’ 6” feet (3.8 m). Phosphorus Concentrations – On March 18, 2012 the total Phosphorus (TP) and orthophosphorus (soluble) OP concentrations were relatively consistent from sample point to sample point with TP concentrations of 20 ppb and OP concentrations of 10 ppb. Nitrogen Compounds – No ammonia or nitrite was found on March 18, 2012. However nitrate on March 18, 2012 was found at all sample points and at all depths and ranged from 0.47 ppm to 0.53 ppm. Alpha Chlorophyll (Chlorophyll a) – On March 18, 2012 Chlorophyll a, a measure of algae concentration, increased with depth except at Sample Point 24 where it was relative constant with depth. Carlson Trophic State Index – As Dudley Pond is primarily used for recreational purposes a TSI less than 50 is tolerable and less than 40 would be great. The TSI on March 18, 2012 for the three sample points ranged from 39 to 47 indicating that Dudley Pond was low to mid mesotrophic. Spring to Spring Comparison - Since one of the purposes of the Dudley Pond Water Quality Sampling Program was to compare data from various years to understand if there are trends in water quality data, the following table is a comparison of data gathered at Sample Point 25 in April 2011 and March 2012.
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Table 1
4-Apr-11
18-Mar-12
SP 25-top SP25-mid SP25-bot
SP 25-top SP25-mid SP25-bot
Depth (ft)
1
12
24
1'
12'
24' 1"
pH
7.14
7.1
7.07
7.32
7.16
7.05
ORP
257.3
250.3
251
210
220
214.6
DO
12.66
12.12
12.07
13.9
14.4
11.75
DO%
103.2
98.3
97.9
124
119
95
Temp C
6.45
6.38
6.32
9.9
6.9
6.1
Secchi Depth
3.7 m
Total Phosphorus (as P)
ND ND ND 0.29/ND ND 3.87 ppb
0.02 ppm 0.01 ND 0.51/ND ND 3.43 ppb
0.02 ppm 0.01 ND 0.5/ND ND 3.74 ppb
OrthoPhosphate (as P) Ammonia (as N) Nitrate/Nitrite (as N) Total Nitrogen (as N) Chlorophyll ɑ
3.8 m
ND ND ND 0.28/ND ND 3.81 ppb
0.01 ppm ND ND 0.29/ND ND 3.72 ppb
0.02 ppm 0.01 ND 0.51/ND ND 2.56 ppb
As can be seen from Table 1 above the water quality on April 2011 at Sample Point 25 was very similar to the water quality in March of 2012, with a few exceptions. The upper portion of the water column was warmer by approximately 3.5 degrees C on March 18, 2012, which reflects the much warmer 2012 spring. Additionally TP and OP concentrations were greater in March of 2012 than in April of 2011. A number of factors such as the following could have influenced the differences in phosphorus concentrations: a) Warmer water temperatures on March 18, 2012 resulting in more release of phosphorus from bottom sediments. b) An earlier landscaping season in March 2012 resulting in more fertilizer runoff. c) More of the OP was biologically assimilated by April 4, 2011 due to three more weeks of weed growth compared to March 18, 2012. It is not know why the nitrate concentration was marginally greater in March of 2012 than April of 2011. Based on the data in Table 1 above the Trophic State Index range for Sample Point 25 on April 4, 2011 was 32 – 44 and the Trophic State Index range for Sample Point 25 on March 18, 2012 was 41 – 47. It is not clear whether the differences in the TSI ranges for 2011 and 2012 are significant.
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Since two data points (April 2011 and March 2012) do not make a trend, not much can be said about the trend, if any, in the quality of Dudley Pond water. However, as this water quality sampling program continues it is hoped that seasonal data can be compared form year to year at each of the three sampling points to establish trends in water quality for Dudley Pond. To get more of an idea regarding the water quality trend over time of Dudley Pond see the historical data contained in the attachment to this report. Since water quality changes with the seasons, comparisons of water quality parameters measured during similar seasons is necessary. Secchi depths for March and April samplings starting in 1981 indicate that the clarity of the water appears to be getting better over time. Not much can be said about the trend in the concentration of TP for March and April samplings starting with March of 1981. The three data points for Chlorophyll A starting with March 1981 indicate that the concentration of chlorophyll A appears to be decreasing. Can it be said that the water quality in Dudley Pond is getting better? Maybe. A few more data sets will provide a better idea regarding the trend in Dudley Pond water quality.
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Attachment Historical Dudley Pond Water Quality Data Appearing below is historical data regarding Dudley Pond water quality for Sample Point 25; specifically Secchi disc readings, total phosphorus (TP) and chlorophyll A concentrations. It should be recognized that these parameters vary with the seasons, so it is important when comparing historical data it is necessary compare data gathered during the same seasonal dates. Secchi Disc Data (Sample Point 25) - High Secchi readings indicate greater transparency and better water quality. Secchi readings are usually greater in the early spring and late fall. Larkin 1978
1.93m
3/26/81 6/26/81 7/31/81 8/24/81 3/10/82 2007 Summer 11/18/07 4/4/11 9/11/11 11/13/11 3/18/12
2.8 m 3m 1.7 m 2.1 m 2.6 m 1.93m 2.85m 3.66m 2.98m 3.4m 3.8m
Total P Data (Sample Point 25) - Higher concentrations indicate poorer water quality. Over 2011 and 2012 TP concentrations have been found in the early spring. The reason for this is unclear; however, one theory is that during the summer, the rapid growing season for weeds and algae, most of the phosphorus has been consumed and converted into biomass and therefore no phosphorus is measurable. Date Larkin 6/6/78 IEP 3/26/81 IEP 6/26/81 IEP 7/31/81 IEP 8/24/81 IEP 3/10/82 ESS 6/9/05 G&L 1/28/07 G&L 10/14/07 4/4/11 9/11/11 11/13/11 3/18/12
Surface ppb
Mid ppb
Bottom ppb
10 30 <10 20 20
20 <10 30 10
20 70 50 40 30
ND ND ND 20
ND ND ND 20
10 70 ND 20
Avg ppb 43 15 40 17 30 20 20 29 32 3 ND ND 20
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Chlorophyll A Data (Sample Point 25) - Higher concentrations of Chlorophyll a indicate higher algae concentrations and poorer water quality. Lower concentrations of Chlorophyll A are usually seen in the early spring and late fall. 3/26/81 6/26/81 7/31/81 8/24/81 4/4/11
5.54 ppb 1.7 6.8 7.0 3.8
9/11/11 11/13/11 3/18/12
5.39 3.72 3.24
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