July 31, 2018 Report Dudley Pond Water Quality Sampling Program Author - 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 state index of Dudley Pond during the spring, summer and fall of each year. Summary • •
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On July 31, 2018 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. Based on a number of parameters shown below the water quality found on July 31, 2018 was the best for a summer sampling since the SWQC has been gathering data. The exceptionally low Trophic State Index (TSI) range, which is based on Secchi Depth, Chlorophyll a and Total Phosphorus concentrations, for July 31, 2018 was driven primarily by a very good Secchi disc measurement (larger Secchi depths indicate better transparency). As Dudley Pond is primarily used for recreational purposes, a TSI less than 50 is tolerable and less than 40 is great. A TSI range of 31 – 39 for July 31, 2018 is exceptionally good. A thermocline (layering caused by temperature differences) on July 31, 2018 was found at approximately 12 feet, which is typical for the summer samplings; however, with the Secchi depth of 14’ 1” on July 31, 2018, resulted in photosynthesis below the thermocline, which the SWQC has not found in the past, resulting in some interesting chemistry in the deep hole (> 20 feet) at Sample Point 25. The table below presents trophic state related data 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. Date
TSI *
Chlorophyll a * range 7.0 ppb 4.41 – 6.3 ppb 2.9 – 17.29 ppb
August 24, 1981 49 - 54 Sept. 11, 2011 44 - 48 August 5, 2012 48 - 52 4.65 – 13.03 ppb August 5, 2013 43 - 48 7.48 – 10.83 ppb August 3, 2014 47 - 53 1.19 – 2.54 ppb August 3, 2016 30 - 44 1.19 – 3.86 ppb August 1, 2017 30 - 46 1.2 – 3.1 ppb July 31, 2018 31 - 39 * Above the thermocline which is usually at about 12 feet **Contaminated sample
Secchi Depth 2.1’ 9.83’ 6.75’ 10.75’ 8.24’ 9.8’ 9’ 14’1”
Total Phosphorus* range 30 ppb ND ND ND - 20 ppb ** ND 10 – 20 ppb ND – 20 ppb ND – 20 ppb
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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.
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, in an attempt to understand seasonal variations.
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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. 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 Kjeldahl Nitrogen, 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)
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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 July 31, 2018 sampling are shown in Figure 2. The analytical data was plotted to determine the TSI and is shown in Figure 3. Thermocline data is plotted in Figure 4.
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Figure 2 – July 31, 2018 Dudley Pond Water Quality Data
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Figure 3 – Carlson Trophic State Index Results for July 31, 2018, Sample Points 24, 25 & 27
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Sample Point 25 Thermocline 30
Temperature C
25 20 15 10 5 0 0
5
10
15
20
25
Depth in Feet
Figure 4 – Dudley Pond July 31, 2018 Thermocline Plot – Temp. C vs. Depth ft. 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 6.41 to 8.5. The low pH of 6.41 was found a depth of 20 feet at Sample Point 25 and is thought to be due to bacterial decomposition of biomass and the absence of photosynthesis. The high pH of 8.5 was found at 3 feet at Sample Point 24 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 10.72 mg/l at 3 feet at Sample Point 24 to a low of 3.38 mg/l at 20 feet at Sample Point 25. The DO percent saturation ranged from a high of 136.7% at the bottom of Sample Point 24 to a low of 35% at 20 feet at Sample Point 25. It is very unusual to find any DO present at a depth of 20 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 16.9 C at the bottom of Sample Point 25 to 29 C near the surface (one foot) at Sample Point 24. The bottom temperature at Sample Point 25 was
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considerably less than the surface temperature due to the thermocline (very little mixing below the thermocline at 12 feet). Oxidation Reduction Potential (ORP) – The ORP ranged from +256.7 mv at 20 feet at Sample Point 25 to a low of +223.3 mv at one foot at Sample Point 27. Based on past years early August sampling data ORP values have ranged from 210 mv to -193 mv. Negative ORP values have been typical at Sample Point 25 at 20 feet where ORP values are usually negative and the samples have had a definite sulfide odor, indicative of active anaerobic decomposition and essentially no photosynthesis (no oxygen). The reason why the DO concentration at 20 feet on July 31, 2018 was high resulting elevated ORP values is discussed below. Secchi Depths – Secchi depth (a measure of color, turbidity and suspended solids, such as algae) was 14’ 1” (4.29 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 14’ 1” (4.29 m) is the best that has been found in an early August sampling. The reason for this unusually good Secchi disc value is not known, but may be related to the very good Secchi disc value obtained during the March sampling of this year when the Secchi disc depth was 19’.
Phosphorus Concentrations – The concentrations of phosphorus compounds were measured 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 July 31, 2018 sampling. The Total Phosphorus (TP) concentration, which includes orthophosphate, ranged from a low of non-detect at various sample points to 20 parts per billion (ppb) at various depths at the three sample points. The TP value at 20 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. Nitrogen Compounds – The concentrations of nitrogenous compounds were measured 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 found at all sample points and all depths
and ranged from 0.5 parts per million (ppm) at three feet at Sample Point 24 to 0.71 ppm at the bottom at Sample Point 25. TKN concentrations were relatively uniform with depth at all sample points with the exception of at 20 feet at the bottom of Sample Point 25, where the concentration was 0.71 ppm, which is probably due to the presence of ammonia. A nitrate concentration of 0.06 mg/l was found at the bottom of Sample Point 25 and ammonia was present at a concentration of 0.17 ppm with an ORP value of + 256.7 mv, which indicates that some oxidation of ammonia to nitrate was probably occurring. De-nitrification of nitrate to nitrogen gas requires an ORP range of – 50 to + 50 mv range, so it was expected that de-nitrification was not possible. However, if the water column was to become less transparent, with a Secchi disc depths of less than the thermocline depth (< 12 feet) and no photosynthesis (oxygen) and no mixing was occurring below the thermocline, the dissolved oxygen would disappear and the ORP in the deep hole at Sample Point 25 would become negative, as is usually the case during the summer. The nitrate that was produced would then be reduced (de-nitrified) by facultative bacteria to insoluble nitrogen gas. This de-nitrification occurs because the ubiquitous facultative bacteria need the oxygen moiety of nitrate to continue aerobic degradation of bottom sludge. Once the facultative bacteria run out of oxygen from nitrate they
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will utilize the oxygen moiety from any sulfate ion present producing hydrogen sulfide (rotten egg odor) as the ORP decreases further into anaerobic conditions. The reduction of sulfate to hydrogen sulfide is why the water samples from the deep hole usually smell like rotten eggs during the summer samplings. Reducing nitrate to nitrogen gas, which bubbles out of solution, is nature’s way of removing nitrogen nutrients from the water column. Alpha Chlorophyll (Chlorophyll a) – Chlorophyll a, an indirect measure of the algae concentration, ranged from 10.9 ppb (ug/l) at the bottom of Sample Point 25 to 1.2 ppb at one foot at Sample Point 24. The Chlorophyll a concentration increased at Sample Point 25 from 1.7 ppb at a depth one foot, to 3.1 ppb at 12 feet and 10.9 ppb at the bottom (20 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 20 feet, was used to determine the Trophic State Index at each sample point. 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 July 31, 2018 for the three sample points ranged from 31 to 39, indicating that the Dudley Pond TSI was in the upper oligotrophic to low mesotrophic range. The TSI range for July 31, 2018 was remarkably good and the best that the SWQC has measured for a mid-summer sampling of Dudley Pond. 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 July 31, 2018. 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 at about 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 a thermocline existed at 12 feet on July 31, 2018 should have precluded DO from above the thermocline from mixing with the water column below the thermocline. However, DO, DO % Saturation and ORP data taken on July 31, 2018 indicates that oxygen was present below the thermocline at 12 feet. This may be due to the fact that light was penetrating to 14’ 1” (Secchi disc), resulting in photosynthetic oxygen being generated below the 12 foot thermocline, causing DO, DO % Saturation and ORP readings to be unusually high at 20 feet.
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