October 7, 2022 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 This report contains a summary, background, methods, results, and a discussion.
Summary:
•
On September 1, 2022 water quality data was gathered at two depths, top and bottom at Location #25 (Deepest section of the Pond) and at the top, approximately one foot below the surface at sample 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 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 in the morning and delivered to Nashoba Analytical LLC the same day for testing.
• Key water quality parameters found on September 1, 2022 are shown in Table1 below along with the same parameters from previous summer samplings for comparison purposes.
Table 1 Key Dudley Pond Water Quality Parameters
Date TSI Range Chlorophyll a Range ppb
September 1, 2022
Secchi Depth meters
Total Phosphorus Range ppb
ND 27.2 10.0 feet 20 30 ppb
1.2 162.0 11.0 feet ND 20 ppb August 7, 2019
August 7, 2020
ND 37.8 12.7 feet 10 60 ppb July 31, 2018
1.2 10.9 14.0 feet ND 20 ppb
August 1, 2017 30 46 .95 23.1 9.0 feet ND 70 ppb
August 3, 2016 30 44 ND 5.76 9.8 feet 10 60 ppb ND = non Detected
• The Dudley Pond trophic state index (TSI) on September 1, 2022 ranged from oligotrophic to low mesotrophic (20 47, excluding the sub thermocline readings at 21 feet at test point 25. See more on this at Figure 2) The Chlorophyll a concentration w as at good levels except at the bottom of Sampling Point 25, traditionally the highest concentration point in the summer. While it is significantly higher than the other sampling points, we have not used it in assessing the overall water quality of the entire pond. As a result, we will not use this number in our TSI calculation.
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.
To have data sets that are comparable to historical data, samples were
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 w ere gathered at depths of one foot (Top) for all sites and approximately one foot off the bottom (Bottom) at Sample Point 25. 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 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 We labeled all sample bottles appropriately.
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 documen ts to the analytical laboratory the day of the sampling
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.
Date Time
Weather
Barometric Pressure
Location
Depth pH
ORP https://www.ysi.com/File%20Library/Documents/Application%20Notes/A567 ORP Management in Wastewater as 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 In order to obtain a mean TP concentration for sample point 25, the two Total Phosphorus concentrations for the sample point were added and divided by two..
Testing Results
The results of the September 1, 2022 sampling are shown in Table 2 below .
DO% Saturation
Temp C
Secchi Depth
Total Phosphorus (ppm)
Nitrate
Chlorophyll a ppb (ug/l)
TKN (as N) ppm (mg/l)
Note: See addendum report provided by Ben Wetherell of ACASAK for more complete description of results.
water quality at that section of the pond and therefore is not used in calculating the Trophic State Index.
Figure 2
Carlson Trophic State Index Results for September 1, 2022, Sample Points 24, 25 & 27
Sample Point 24
Sample Point 25
Sample Point 27 Secchi Disc (meters) 3.1 m 3.1 m 3.1 m T P (ppb) 20 ppb (20+230)/2 =125 20 ppb Chlor a (ppb) ND (27.2+2.1)/2 = 14.7 ppb 2.2 ppb
From above: Excluding the extremely high numbers below the thermocline at the bottom of Test site # 25, the TSI range for the Pond is approximately 20 47, which is in the oligotrophic to low mesotrophic range. For a further description of these classifications, see Table 3 below:
Discussion:
YSI M easurements 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 6.66 at the bottom at Sample Point 25 to a high of 8.93 at the top of Sample Point 25. There was very little variation in pH readings versus depth at points 24 & 27, likely due to mixing within the water column. There was a variation at point 25 from 6.66 at the bottom at 21 feet to 8.93 at the top, likely due to lack of mixing from the thermocline layer created by temperature variation from warm summer water up top and cooler (more dense) water sinking to the bottom. This pH does bear watching as the 8.93 reading is higher than normal. The greater than normal plant growth due to tape grass spreading may be the cause due to higher oxygen production through photosynthesis , which can raise pH levels. This will bear watching.
• DO Percent Saturation The DO Percent Saturation ranged from a low of 5.4 % at 21 feet at Sample Point 25 to a high of 111.3 % at one foot at Sample Point 25. DO concentration values and DO Percent Saturation values were relatively uniform at sample point 27 with some variation at point 24 It is not unusual to have DO Percent Saturation values in excess of 100%, which is usually due to active photosynthesis producing high concentrations of oxygen in the water column.
• Temperature The temperature ranged from a low of 17.6 C at a depth of 21 feet at Sample Point 25 (below the thermocline) to a high of 25.8 C at a depth of one foot at Sample Point 27. The lack of significant variation in temperature throughout the water column indicates the presence of mixing, except at the 21 feet level at Sample Point 25
• Oxidation Reduction Potential (ORP) The ORP ranged from a low of 13.6 mv at a depth of 21 feet at Sample Point 25 to a high of 112.6 mv at a depth of 1 foot at Sample Point 25 ORP values were relatively uniform vertically at sample points 24 and 27
• Secchi Depths Secchi depth (a measure of color, turbidity and suspended solids, such as algae) on August 7, 2020 w as 10’ 0” (3.05 m ) at Sample Point 25 This measurement was taken in the morning approximately 7:45 AM . It should be noted that an afternoon reading may have been higher with better light. Greater Secchi depths indicate better quality water and lesser Secchi depths indicate poorer quality.
Phosphorus Total Phosphorus (TP) ranged from Non detectable to 30 ppm, with the exception of the 21 foot bottom level at Test point 25 which showed a relatively high concentration. It is worth noting that 10 ppb is the minimum detection level for the analytical method used . As mentioned in previous reports, the deep section of the pond can act almost like a funnel for all kinds of plant matter, etc. Total Phosphorus is a measure of all forms of phosphorous, dissolved or particu late, 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 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.
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 No nitrate w as found at any of the sample points , which is not unusual for Dudley Pond
Total Kjeldahl Nitrogen (TKN) The TKN concentration, if any, was below the minimum detection level at all sample points for the analysis used except the bottom of test point 25 . 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 September 1, 2022, Chlorophyll a, an indirect measure of the algae concentration, ranged from Non detected to 2.2 ppb at test points 24 nd 27 up to 27.2 ug/l at 21 feet and below the thermocline at Sample point 25. With this exception, the Chlorophyll a concentration w as at relatively low concentrations throughout the pond.
In summary, the water quality testing results from the September 1, 2022 sampling indicate overall good water quality ranging from Oligotrophic to low Mesotrophic as seen in Table 3 above.
The next anticipated testing and report will be in Spring 2023.