November 17, 2021 Report Dudley Pond Water Quality Assessment Program Program Manager – Norm Marowitz This document is part of an ongoing Surface Water Quality Committee (SWQC) water quality program 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 state index (TSI), a measure of water quality, for Dudley Pond during the spring, summer and fall of each year. We will be testing twice per year, varying which seasons we will test in any particular year, based on test results or other pertinent factors. This report contains a summary, background, methods, results, and a discussion. Summary As described in the previous report, test points #24 and #27 (see Map at Figure 1) have been showing minimal variance in results from 1 feet depth testing to bottom testing (only 5-7 feet). As a result, we only tested one depth at these two shallower points for TSI, Chlorophyll-a and Total Phosphorous. • •
• •
On October 22, 2021, water quality data was gathered at two depths at the deepest section, Test point #25, while data was gathered at one depth at each of Test points #24 and #27. Water quality parameters such as, pH, oxidation – reduction potential (ORP), (although this parameter seems to be used less and less by industry experts for Lake & Pond monitoring), dissolved oxygen (DO), dissolved oxygen % saturation and temperature were measured at two depths at each of the three sample points by Ben Wetherill (ACASAK) using a YSI Multimeter. Water depths and Secchi depths were measured at each of the three sample points. Samples were gathered between 7:37AM & 8:25AM and delivered that morning to Nashoba Analytical LLC. for other testing parameters. Key water quality parameters found on October 22, 2021 are shown in Table1 below along with the same parameters from previous Fall samplings for comparison purposes. We decided to not test in the Fall of 2020 due to consistently good results in prior Fall testing as we shifted to twice annual testing versus previous three times per year. At this point we plan on alternating between Fall and Spring testing while testing in the summer each year as it tends to have the highest level of biological activity.
Table 1 – Key Dudley Pond Water Quality Parameters Date
TSI Range
Oct 22,2021 Not tested this Fall Nov 6, 2019 Nov 8, 2018 Nov 9, 2017 Nov 14, 2016 Nov 16, 2015 Nov 12, 2014 Nov 15, 2013 Nov 11, 2012 Nov 13, 2011 ND = non-Detected
20 - 55 22 - 57 20 - 44 20 - 42 40 - 45 26 - 43 47 - 48 49 - 51 42 - 46 42 - 44
•
Chlorophyll a Range ppb 0.8 – 11.7 11 - 15 ND – 1.0 0.42 – 1.0 ND 0.39 – 3.29 4 – 6.3 4.24 – 9.79 2.2 – 4.6 2.6 – 6.6
Secchi Depth meters 3.96 2.62 4.75 3.7 4 4.39 2.73 2.12 3.4 3.4
Total Phosphorus Range ppb ND - 30 33 – 66 ND – 30 ND – 20 ND – 20 ND ND ND ND ND – 10
The Dudley Pond trophic state index (TSI) on October 22, 2021 ranged from oligotrophic to high mesotrophic (37 – 53). (see more on this at Figure 2) The Chlorophyll a concentration was at good overall levels, although a little higher on the bottom of Sampling Point 25, traditionally the highest concentration point, but still at a level considered fairly good by traditional standards (see Chlorophyll a discussion in lower section) Transparency was very good at 13.0 feet, and it should be noted that the low angle of the sun in the morning impacted the Secchi
disk readings which would most likely have been deeper. The high end of the TSI was attributed to a higher level of phosphate detected at the bottom of test point 25, the deepest section. This number skews the TSI range, and we believe it is due to the accumulation of dead algae and other matter over time as it is the only significant drop in depth and can act almost like a funnel as water moves around the pond. We do not believe this gives a fair representation of the pond’s overall water quality, but we will continue to monitor it for any significant changes over time. 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.
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 Williams Point – “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) and approximately one foot off the bottom (Bottom) at Sample Points 24, 25 and 27. A water sampler as shown at http://pentairaes.com/water-sampler-1.html with a sample volume of 1.0 – 1.5 liters was used to gather samples at various depths. Sampling - Multiple sample catches at each depth were composited from the sampler into a one-gallon container 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. Bottles were pre-labeled by location as Top or Bottom.
One of the sample bottles contained sulfuric acid necessary to “fix” (preserve) the sample for total phosphorus (TP) analysis. All the samples were stored in a cooler containing ice and delivered with chain of custody documents to the analytical laboratory the day of the sampling or the after the samples were gathered. Sample Analyses - A YSI 556 Multimeter was used by ACASAK (Ben Wetherill) to measure the following parameters at the same time the samples were collected, except the Secchi depth, which was measured using a Secchi disk. Date Time Weather Barometric Pressure Location Depth pH ORP https://www.ysi.com/File%20Library/Documents/Application%20Notes/A567-ORP-Management-in-Wastewateras-an-Indicator-of-Process-Efficiency.pdf Dissolved Oxygen Dissolved Oxygen % Saturation Temperature Secchi Depth https://en.wikipedia.org/wiki/Secchi_disk Analyses were performed by Nashoba Analytical, LLC, Ayer, Massachusetts for the following parameters: Total Phosphorus (TP), parts per billion (ppb) (as P) (Soluble + Insoluble) Nitrate Total Kjeldahl Nitrogen (TKN) ppm (as N) (organic nitrogen + ammonia) Chlorophyll a, ug/l (ppb) http://www.ohiowea.org/docs/Wed0900Lab_Chlorophyll_Analysis.pdf 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 disk testing for Sample Points 24 and 27 were visible to the very bottom, it was assumed that the Secchi Depths for all the sample points were the same as Sample Point 25, the deepest section of the pond. To obtain a mean TP concentration for each sample point the two Total Phosphorus concentrations for the sample point were added and divided by two. To obtain a mean Chlorophyll a concentration for each sample location, the mean chlorophyll a concentration at each sample point was calculated using the sum of the chlorophyll a concentration at the two depths divided by two.
Testing Results The results of the October 22, 2021 sampling are shown in Table 2 below.
Table 2 – October 22, 2021 Dudley Pond Water Quality Data
Date
Barometric pressure LOCATION # ON POND
10/22/202 1
Testing & sampling began at approx. 7:37 AM, completed appr. 8:25 AM
767.7 (mmHg) 24
24
25
25
27
27
TOP
BOT
TOP
BOT
TOP
BOT
1
5
1
21
1
6
pH
7.25
7.16
7.23
7.01
7.16
7.28
ORP (mv)
42.3
35.2
83.4
68.2
91.7
66.9
DO (mg/l)
8.25
7.87
7.91
7.23
8.29
8.19
DO% Saturation
85.9
82.0
82.5
75.3
86.2
85.2
Temp C
17.2
17.2
17.3
17.2
17.2
17.2
Secchi Depth
>6.0
Depth (Feet)
Total Phosphorus (ppm)
30
13' 0" n/a
ND
>7.0 ND
ND
n/a
Nitrate
ND
“
“
“
“
“
Chlorophyll a ppb (ug/l)
11.7
“
1.4
n/a
1.4
“
TKN (as N) ppm (mg/l)
ND
n/a
ND
ND
ND
“
Site Notes:: 1. As with all previous samples, there was no significant difference between surface and bottom water at DUD-024. 2. For DUD-027, there are no significant differences between the bottom and the surface, except for ph. Even though it is very shallow, there is a consistent small difference between surface and bottom pH readings. The surface consistently has a slightly lower pH value than the bottom, meaning that there is probably a higher DO:CO2 ratio at the bottom in the tape grass. 3. The highest dissolved oxygen values generally correspond to the lowest temperatures and higher dissolved oxygen values generally correlate with higher pH values. Cold water naturally holds more dissolved oxygen than warm water, and pH is reduced by increasing CO2 in water. 4. The thermocline at DUD-025 has completely disappeared for the season. There is only a very small 0.2°C temperature drop below 20 feet. This corresponds to a very large jump in turbidity and increase in conductivity, as the sensor most likely entered an area of stagnant suspended sediments and organic matter. Surprisingly, dissolved oxygen levels remain quite high, even in this bottom layer, so mixing has been extensive. As mentioned in the previous report, the bottom was very soft and hard to distinguish. 5. Secchi Depths – Secchi depth (a measure of color, turbidity, and suspended solids, such as algae) on October 22, 2021 was 13’ 0” (3.96 m) at Sample Point 25. This measurement was taken in the morning approximately 7:37AM. It should be noted that an afternoon reading may have been higher with a better angle of sunlight. Greater Secchi depths indicate better quality water and lesser Secchi depths indicate poorer quality.
Figure 2 – Carlson Trophic State Index Results for October 22, 2021, Sample Points 24, 25 & 27
Secchi Disc (meters) TP (ppb) Chlor a (ppb)
Sample Point 24 3.96 m 30 ppb 11.7 ppb Red
Sample Point 25 3.96 m (0 ppb + 0 ppb)/2 = 0 ppb (1.4 ppb + 1.4)/2 = 1.4 ppb Yellow
Sample Point 27 3.96 m 0 ppb .8 ppb Blue
From above: The TSI range for the Pond is approximately 20 – 55, which is in the oligotrophic to high mesotrophic range. However, it should be noted that the high end was due to the Total Phosphorous and Chlorophyll a reading at test point 24 near the Chateau. The other test points indicated water quality in the Oligotrophic to low Mesotrophic range. For a description of these classifications, see Table 3 below:
Table 3
Phosphorus – Total Phosphorus (TP) was detected at 30 ppb at test point 24 near the Chateau. (10-30 ppb is the level found in uncontaminated lakes, plant growth may be stimulated from 30-100 ppb with 100 being the maximum sustainable level to avoid accelerated eutrophication). It is worth noting that 10 ppb is the minimum detection level for the analytical method used. Total Phosphorus is a measure of all forms of phosphorous, dissolved or particulate, that are found in a sample. It is an essential element for plant life, however, at higher levels, it can speed up eutrophication (reduction in dissolved oxygen). There are many sources of phosphorus in aquatic systems. These sources can be natural, such as waterfowl waste, atmospheric deposition, and plant decomposition; or they can be human induced, such as fertilizer, pet waste, agricultural and urban runoff, industrial and domestic sewage, or faulty or overloaded septic systems. While tested levels have generally been low, we suspect levels “hiding” in the soil in the lake from plant decomposition and other sources may be why we have experienced high levels of plant growth. This a good reason to remove aquatic vegetation at reasonable levels to avoid continued build-up at the bottom and decomposition. For information on managing phosphorous levels in lakes and ponds, see https://www.des.nh.gov/organization/commissioner/pip/factsheets/bb/documents/bb-20.pdf Nitrate – Levels were not detected at any of the test points. As a reference point, nitrate below 10 mg/L is considered safe for drinking. Total Kjeldahl Nitrogen (TKN) – The TKN concentration was below the minimum detection level at all sample points. Acceptable levels are up to 6 mg/L. The TKN method in chemistry is for the quantitative determination of nitrogen contained in organic and inorganic compounds (ammonia (NH3) & ammonium (NH4)). It is usually used to gain knowledge of the total nitrogen content of a sample.
Alpha Chlorophyll (Chlorophyll a) – On October 22, 2021 Chlorophyll a, an indirect measure of the algae concentration, ranged from non-detected at sample points 25 and 27 to 11.7 ug/L at test point 24. For reference: < 3 ug/L Excellent 3 – 7 ug/L Good 7 - 15 ug/L
Less than desirable
> 15 ug/L
Nuisance
In summary, the water quality testing results from the October 22, 2021 sampling indicate good water quality ranging from Oligotrophic to high Mesotrophic as seen in Table 3 above. The higher end of this range was caused by the higher Phosphorous and Chlorophyll a measured at test point 24 near the Chateau. All other locations tested indicated water quality in the Oligotrophic to low Mesotrophic range. It is difficult to know exactly what causes these variations at different sections of the pond, however, we know that both growing vegetation and drift of floating vegetation tends to accumulate in the Chateau area of the pond. The next testing and report will be in April, 2022.