DRAFT August 7, 2019 Report Dudley Pond Water Quality Sampling Program 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 state index of Dudley Pond during the spring, summer and fall of each year. Summary On August 7, 2019 water quality data was gathered at three depths at three sample points. 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. As Dudley Pond is primarily used for recreational purposes, a Trophic State Index (TSI) less than 50 is tolerable and less than 40 is great. A TSI range of 35 – 55 for August 7, 2019 is not particularly good. The TSI range, which is based on a combination of Secchi Depth, Chlorophyll a and Total Phosphorus concentrations, was high (not good) despite an unusually good Secchi Depth (transparency) on August 7, 2019. The moderately high TSI was because of elevated concentrations of Total Phosphorus. High concentrations of Total Phosphorus and a high TSI range are indicators of poor water quality. A thermocline (layering caused by temperature differences) on August 7, 2019 was found at approximately 12 feet, which is typical for the summer samplings; however, with the Secchi depth of 12’ 7” on August 7, 2019, there was probably limited photosynthesis below the thermocline. An elevated ORP, no odor and marginally aerobic conditions in the deep hole (> 20 feet) at Sample Point 25 are thought to be the result of limited photosynthesis below the thermocline. There was no measurable Total Kjeldahl Nitrogen found at any of the sample points on August 7, 2019, which is unusual for a summer sampling. The reason for this is unknown. The table below presents parameters making up the TSI range as well as the TSI range resulting from August samplings. Lower numbers for TSI, Chlorophyll a and Total Phosphorus indicate better water quality. High numbers for Secchi depths also indicate better quality water.
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Date
Trophic State Index (TSI) * Range
Chlorophyll a * range 7.0 ppb 4.41 – 6.3 ppb 2.9 – 17.29 ppb
Secchi Depth
Total Phosphorus* range 30 ppb ND ND ND - 20 ppb ** ND 10 – 20 ppb ND – 20 ppb ND – 20 ppb 20 – 40 ppb
August 24, 1981 49 - 54 2.1’ Sept. 11, 2011 44 - 48 9.83’ August 5, 2012 48 - 52 6.75’ 4.65 – 13.03 ppb August 5, 2013 43 - 48 10.75’ 7.48 – 10.83 ppb August 3, 2014 47 - 53 8.24’ 1.19 – 2.54 ppb August 3, 2016 30 - 44 9.8’ 1.19 – 3.86 ppb August 1, 2017 30 - 46 9’ 1.2 – 3.1 ppb July 31, 2018 31 - 39 14’1” ND – 3.4 August 7,2019 35 - 55 12’ 7” * Above the thermocline which is usually at about 12 feet **Contaminated sample On August 7, 2019 data and samples were gathered from Sample Point 25 first, between 12:15 and 12:45 PM. Just before data and samples were gathered at Sample Point 24 there was a violent thunderstorm between 1:00 and 2:00PM and refuge was sought in the Dudley Chateau. Following the storm, data and samples were gathered from Sample Point 24 and subsequently from Sample Point 27. It is possible that the reason for the elevated concentrations of Total Phosphorus (TP) that were found at Sample Points 24 and 27 were due to phosphorus compounds contained in the runoff from the storm, which entered the Pond. This situation could also possibly explain the unusually high concentration of TP at a depth of one foot at Sample Point 24, which was sampled first just after the thunderstorm resulting in a TP concentration of 100 ppb. So, the high TP concentrations that negatively impacted the TSI may have been a result of the rainstorm. 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 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), 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://pentairaes.com/watersampler-1.html 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, 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 onegallon 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 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. 3
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 (https://en.wikipedia.org/wiki/Secchi_disk) 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 Kjeldahl Nitrogen (TKN), ppm (as N) Chlorophyll a, ug/l (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 a mean TP concentration for each sample point the three Total Phosphorus concentrations for the sample point were added and divided by three. In order to obtain a mean Chlorophyll a concentrations for each sample location, the mean chlorophyll a concentration at each sample point was calculated using the sum of the chlorophyll a concentrations at the three depths divided by three. For the purpose of calculating means non-detect (ND) was assumed to be zero. (see http://www.aslo.org/lo/toc/vol_22/issue_2/0361.pdf for information re Carlson Trophic State index) Thermocline http://en.wikipedia.org/wiki/Thermocline – Temperature readings were taken at two-foot intervals of depths at Sample Point 25 in an attempt to determine if there was a discontinuity in the temperature vs. depth curve (thermocline). Results The results of the August 7, 2019 sampling are shown in Figure 2. The analytical data was plotted to determine the TSI and is shown in Figure 3. Thermocline data is displayed in Figure 4.
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Figure 2 – August 7, 2019 Dudley Pond Water Quality Data
Figure 3 – Carlson Trophic State Index Results for August 7, 2019, Sample Points 24, 25 & 27
Sample Point 25 Thermocline 30
Temperature C
25 20 15
Temp C
10 5 0 0
5
10
15
20
25
Depth f
Figure 4 – Dudley Pond August 7, 2019 Thermocline Plot – Temp. C vs. Depth f. 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 5.18 to 8.74. The low pH of 5.18 was found a depth of 22 feet at Sample Point 25 and is thought to be due to acids resulting from faculatative bacterial decomposition of biomass. The high pH of 8.74 was found at 3 feet at Sample Point 27 and is thought to be due to carbon dioxide, a weak acid, being consumed due to photosynthesis. There was not much variation in pH top to bottom at each of the sample points indicating vertical mixing in the water column, except at depths below 12 feet. DO and DO Percent Saturation – The dissolved oxygen (DO) ranged from a high of 10.2 mg/l at the bottom (5’ 6” feet) at Sample Point 24 to a low of 1.0 mg/l at 22 feet at Sample Point 25. The DO percent saturation ranged from a high of 126.3% at the bottom of Sample Point 24 to a low of 9.9% at 22 feet at Sample Point 25. It is very unusual to find any DO present at a depth of 22 feet at Sample Point 25 during the early August sampling because of active facultative and anaerobic decomposition of bottom biomass. The DO mg/l and DO Percent Saturation data did not vary much with depth in the water column in the upper levels of the water column and is the result of active photosynthesis and is corroborated by elevated pH values. The elevated DO values at the 3 and 5 foot depths are thought to be due to photosynthesis occurring from suspended algae and weeds as well as benthic (bottom) algae. Temperature – The temperature ranged from 15.6 C at the bottom of Sample Point 25 to 27.7 C near the surface (one foot) at Sample Point 27. The bottom temperature at Sample Point 25 was considerably less than the surface temperature due to the thermocline (very little mixing below the thermocline at 12 feet).
Oxidation Reduction Potential (ORP) – On August 7, 2019 the ORP ranged from a low of +39.7 mv at one foot at Sample Point 24 to +165.3 mv at 22 feet at Sample Point 25 to. This is unusual. In past August samplings the ORP has been high ( +100 to +200 mv) in the upper water column and low ( - 50 to -100 mv) at the bottom (22 feet) of Sample Point 25. The reason for this is not known; however, a possible reason is presented in the thermocline discussion below. Secchi Depths – Secchi depth (a measure of color, turbidity and suspended solids, such as algae) was 12’ 7” (3.83 m). 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. A Secchi depth of 12’ 7” (3.83 m) is the second best that has been found in an early August sampling. The reason for this unusually good Secchi disc value is not known; however, it may be due to low concentrations of suspended solids in the water column.
Phosphorus Concentrations – The concentrations of phosphorus compounds were sampled because phosphorus is considered the growth rate-limiting nutrient for fresh water biomass (algae and weeds). Total Phosphorus is composed of three types of phosphates; ortho-phosphate, condensed phosphate and organic phosphates. Ortho-phosphate, which is the bio-available form of phosphorus, was not found during the August 7, 2019 sampling. The Total Phosphorus (TP) concentration ranged from a low of 20 parts per billion (ppb) at one foot at Sample Point 25 to a high of 100 ppb at one foot at Sample Point 24. All sample points at all depths had measurable Total Phosphate, which is unusual for an August sampling. The TP value at 22 feet at Sample Point 25 was not used to calculate the Trophic State Index because it is not thought to be representative of the water above the thermocline. The Total Phosphate value of 100 ppb at one foot at Sample Point 24 is thought to be an anomaly and was not used to calculate the Trophic Index because a Total Phosphorus concentration of 100 ppb would correspond to a Secchi Disc value of 1’ 6” and a Chlorophyll a value of 60 ppb. The reason(s) for the elevated Total Phosphorus concentrations is unknown, but may be the result of a rainstorm that occurred during sampling. Nitrogen Compounds – The concentrations of nitrogenous compounds were sampled because nitrogenous compounds are biomass nutrients. Total Nitrogen (TN) is the sum of nitrate-nitrogen
(NO3-N), nitrite-nitrogen (NO2-N), plus Total Kjeldahl Nitrogen (TKN), which consists of ammonianitrogen (NH3-N) and organically bound nitrogen. TKN was not found at any of the sample points,
which is unusual for a summer sampling of Dudley Pond. The reason for a lack of measurable TKN on August 7, 2019 is not known. Nitrate and nitrite were not present at any of the sample points, which is not unusual for a summer sampling at Dudley Pond. Alpha Chlorophyll (Chlorophyll a) – Chlorophyll a, an indirect measure of the algae concentration, ranged from a high of 37.8 ppb (ug/l) at the bottom of Sample Point 25 to a low of non-detect at 3 feet at Sample Point 24. The Chlorophyll a concentration increased at Sample Point 25 from 2.8 ppb at a depth one foot, to 3.4 ppb at 12 feet and 37.8 ppb at the bottom (22 feet). Historically the Chlorophyll a concentration in the deep hole (bottom of Sample Point 25) has been much higher than other sample points. The reason for high Chlorophyll a concentrations at the bottom of Sample point 25 is not known. The average of the Chlorophyll a concentration at each sample point, excluding the concentration at 22 feet, was used to determine the Trophic State Index at each sample point. The average Chlorophyll a concentrations found during the August 7, 2019 sampling were not unusual, which leads one to believe that the good Secchi disc values were not due to low algae concentrations, but possibly due to low suspended solids concentrations.
Carlson Trophic State Index (TSI) – As Dudley Pond is primarily used for recreational purposes a TSI less than 50 is tolerable and less than 40 is great. The TSI on August 7, 2019 for the three sample points ranged from 35 to 55, indicating that the Dudley Pond TSI was in the mesotrophic to low eutrophic range. Despite good transparency (Secchi Depth) the TSI range for August 7, 2019 was not very good primarily due to elevated Total Phosphorus concentrations. Only Chlorophyll a and Total Phosphorus concentrations in samples taken at depths of 12 feet or less were used in the determination of the Carlson Trophic Index for August 7, 2019. Thermocline – Figure 4 is a graphic presentation of water temperature versus depth at Sample Point 25. As can be seen in Figure 4 there is a discontinuity in the slope of the temperature vs. depth curve and this discontinuity occurs around 12 feet. This discontinuity in the temperature slope indicates the depth at which the thermocline (thermo layering) exists. http://en.wikipedia.org/wiki/Thermocline Above the thermocline the water column is mixed and below the thermocline the water column is not mixed, keeping accumulated materials and biomass decomposition by-products such as phosphorus and nitrogen compounds out of the photosynthetic (upper) portion of the water column. The fact that the thermocline existed at about 12 feet on August 7, 2019 should have precluded DO from above the thermocline from mixing with the water column below the thermocline. However, DO, DO % Saturation, and particularly ORP data plus a lack of sulfide odor indicate that oxidizing potential was present below the thermocline at 12 feet. This may be due to the fact that light was penetrating to 12’ 7” (Secchi disc), resulting in photosynthetic oxygen being generated below the 12-foot thermocline, causing ORP readings to be unusually high at 22 feet.