April 27, 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 Test points #24 and #27 (see Map at Figure 1) again showed minimal variation of results from 1 feet depth testing to bottom testing (only 5-6 feet), which may lead to testing only one depth in the future at those locations. • •
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On April 9, 2021, water quality data was gathered at two depths at the three sample points historically utilized, versus three depths in the past. 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 8:15AM & 9:30AM and delivered to Nashoba Analytical LLC. for other testing parameters. Key water quality parameters found on April 9, 2021 are shown in Table1 below along with the same parameters from four previous Spring samplings for comparison purposes. We decided to not test in the spring of 2020 due to consistently good results in prior spring testing as we shifted to twice annual testing versus previous three times per year due to consistently good results and unnecessary additional cost.
Table 1 – Key Dudley Pond Water Quality Parameters Date
TSI Range (See figure 2)
Secchi Depth Feet
37 - 53 -
Chlorophyll a Range ppb 2.01 – 4.58 -
13’ 5” -
Total Phosphorus Range ppm 10 - 54 -
4/9/2021 Not tested in spring 2020 3/27/19 3/29/18 3/26/17 3/23/16
33 - 43 22 - 36 21 -41 30-40
.9 – 2.3 ND – 1.0 .59 – 1.1 .89 - 5.13
11’ 2” 19’ 0” 12’ 0 15’ 0”
ND ND 0 – 10 ND
ND = Non-Detect •
The Dudley Pond trophic state index (TSI) on April 9, 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 13.5 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. Samples and data were gathered in November 2020 and April 2021 in attempting to understand seasonal variations. 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 April 9, 2021 sampling are shown in Table 2 below.
Table 2 – April 9, 2021 Dudley Pond Water Quality Data Date Barometric pressure
Testing & sampling began at approx. 8:15 AM, completed appr. 9:45 AM
4/9/2021 767.7 (mmHg)
Near Chateau LOCATION # ON POND 24 24 TOP BOT Depth (Feet) 1 5 pH 7.51 7.50 ORP (mv) 126.9 127.5 DO (mg/l) 11.52 11.53 DO% Saturation 106.8 106.2 Temp C 11.9 11.6 Secchi Depth >5.0 Total Phosphorus (as P) ppb Nitrate TKN (as N) ppm (mg/l) Chlorophyll a ppb (ug/l)
10 .29 ND 2.0
10 .29 ND 2.5
Middle/Deep section 25 25 TOP BOT 1 22 7.31 7.10 119 118.3 11.62 9.8 109.0 81.5 12.4 7.2 13' 5" 10 .21 ND 3.2
54 .25 ND 4.6
Outlet 27 TOP 1 7.69 117.3 11.97 112.4 12.4 >6.0 10 .24 ND 2.1
27 BOT 6 7.80 119.5 12.33 115.0 12.2
10 .23 1.0 2.3
Site notes: DUD-024 Per previous years’ tests, no significant diff. in results between top & bottom levels. At DUD-025 thermocline has started to develop at 16 feet depth. Dissolved oxygen concentrations at bottom already below 5 mg/L. Note testing showed lower dissolved oxygen levels at the very bottom than the discrete measurement that was taken just above the bottom For DUD-027, although very shallow, there was a small but noticeable difference between surface & bottom dissolved oxygen (see graph below). The clear water and new bottom vegetation were producing oxygen at the bottom.
Figure 2 – Carlson Trophic State Index Results for April 9, 2021, Sample Points 24, 25 & 27
Secchi Disc (meters) TP (ppb) Chlor a (ppb)
Sample Point 24 4.11 m (10 ppb +10 ppb)/2 = 10 ppb (2.0 ppb + 2.5 ppb)/2 = 2.3 ppb Red
Sample Point 25 4.11 m (10 ppb + 54 ppb)/2 = 32 ppb (3.2 ppb + 4.6)/2 = 3.9 ppb Yellow
Sample Point 27 4.11 m (10 ppb +10 ppb)/2 = 10 ppb (2.0 ppb + 2.3 ppb)/2 = 2.2 ppb Blue
From above: The TSI range for the Pond is approximately 37 – 53, which is in the oligotrophic to high mesotrophic range. However, it should again be noted that the high end was due to the Total Phosphorous reading in the deepest section 25. All 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
Discussion: YSI Measurements - Measurements taken with the YSI 556 Multimeter varied from location to location and by depth. • pH – The pH for all three sample points ranged from 7.1 at the bottom at Sample Point 25 to a high of 7.8 at the bottom at Sample Point 27. There was very little variation in pH readings versus depth at all points, likely due to mixing within the water column. • DO Percent Saturation – The DO Percent Saturation ranged from a low of 81.5 % at 22 feet at Sample Point 25 to a high of 115.0 % at six feet at Sample Point 27. DO concentration values and DO Percent Saturation values were relatively uniform at all sample points except Sample Point 25, the deepest section of the pond. The vertical uniformity in dissolved oxygen at the other two test points indicates mixing in the water column at those points. It is not unusual to have DO Percent Saturation values more than 100%, which is usually due to active photosynthesis producing high concentrations of oxygen in the water column. This is more common in warmer water temperatures with more active photosynthesis taking place. Cold water, however, holds more oxygen in the water than warm water does. • Temperature – The temperature ranged from a low of 7.2 C at the bottom at Sample Point 25 to a high of 12.4 C at a depth of one foot at Sample Points 25 & 27. The lack of significant variation in temperature throughout the water column indicates the presence of mixing at locations 24 & 27 (with maximum depths of 5 and 6 feet, respectively). Location 25 showed a drop at approximately 16 feet from approximately 12.4 Celsius to 7.2 Celsius. This may have reduced mixing at this lower level and affected water testing results. • Oxidation Reduction Potential (ORP) – The ORP ranged from a low of 117.3 mv at a depth of one foot at Sample Point 27 to a high of 127.5 mv at a depth of 5 feet at Sample Point 24. ORP values were relatively uniform vertically at all sample points.
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Secchi Depths – Secchi depth (a measure of color, turbidity and suspended solids, such as algae) on April 9, 2021 was 13’ 5” (4.1 m) at Sample Point 25. This measurement was taken in the morning approximately 8:30AM. 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.
Phosphorus – Total Phosphorus (TP) was detected at 10 ppb at all test points except the bottom of point 25, the deepest point, at 54 ppb. (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 ranged from .21 to .29. mg/L at all six test points, which is low. 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 except the bottom of location 27 at 1.0 mg/L. 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 April 9, 2021 Chlorophyll a, an indirect measure of the algae concentration, ranged from 2.01 ug/L at the bottom of test point 24 to 4.58 ug/L at the bottom of point 25. 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 April 9, 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 measured at the bottom of the deepest section of the pond. All other locations tested indicated water quality in the Oligotrophic to low Mesotrophic range. The next testing and report will be in August 2021.