August 5, 2013 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 index of Dudley Pond during the spring, summer and fall of each year. Summary
On August 5, 2013 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 August 5, 2013 was much better than than August 24, 1981, better than the water quality found during the August 5, 2012 and about the same as the water quality on September 11, 2011.
Date TSI * August 24, 1981 49 - 54 Sept. 11, 2011 44 - 48 August 5, 2012 48 - 52 August 5, 2013 43 - 48 * Above the thermocline (~ 12 feet) **Contaminated sample
Chlorophyll a * 7.0 4.41 – 6.3 ppb 2.9 – 17.29 ppb 4.65 – 13.03** ppb
Secchi Depth 2.1 9.83’ 6.75’ 10.75’
Total Phosphorus* 30 ppb ND ND 20** ppb
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 Mansion Beach – “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://www.aquaticeco.com/subcategories/2912/Water-Sampler?green=12823266105 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. 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 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 an average TP concentration for each sample point the three Total Phosphorus concentrations for the sample point were averaged. In order to obtain an average Chlorophyll a concentrations for each sample location, the average of the chlorophyll a concentration at each sample point was calculated using the average of the chlorophyll a concentrations at the three depths. (see http://www.aslo.org/lo/toc/vol_22/issue_2/0361.pdf for information re Carlson Trophic State index) Results The results of the August 5, 2013 sampling are shown in Figure 2. The analytical data was plotted to determine the TSI and is shown in Figure 3.
Figure 2 – August 5, 2013 Dudley Pond Water Quality Data
Figure 3 – Carlson Trophic State Index Results for August 5, 2013, Sample Points 24, 25 & 27
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 6.79 to 8.07. The low pH of 6.79 was found in the deep hole (bottom) at Sample Point 25, which is not unusual and is probably due to bacterial decomposition of biomass and the absence of photosynthesis. The high pH of 8.07 was found at the mid-point of Sample Point 27 and is most likely due to a lack of good mixing and carbon dioxide, a weak acid, being consumed due to photosynthesis. The balance of the pH readings were similar without much variation, which could be the result of mixing within the water column. DO and DO Percent Saturation – DO percent saturation measurements ranged from 0.8 % found at the bottom of Sample Point 25 to 110.7 % at 3 feet at Sample Point 27. DO Percent Saturation measurements increased with depth at Sample Points 24 and 27 indicating limited mixing, good light penetration and benthic photosynthesis. The DO percent saturation measurements at Sample Point 25 decreased with depth. The low DO percent saturation measurement at the bottom of Sample Point 25 indicates no photosynthesis, anoxic bacterial decomposition of biomass and a lack of mixing below the thermocline, which has been measured at about 12 feet in the past. Temperature – The temperature ranged from 14.4 C at the bottom of Sample Point 25 to 25.82 C at the surface of Sample Point 27. In all cases bottom temperatures were lower than surface temperatures. The bottom temperature at Sample Point 25 was significantly less than the surface temperature due to the thermocline. The slight temperature decrease from top to bottom at each sample point down to a depth of 12 feet indicates some vertical mixing in the water column above 12 feet. Oxidation Reduction Potential (ORP) – The ORP ranged from -20 mv at the bottom at Sample Point 25 to 193 mv at 3 feet at Sample Point 24. Above 12 feet the ORP was relatively uniform at each of the sample points, top to bottom. During past samplings negative ORPs have been measured at the bottom of Sample Point 25, which is indicative of active anoxic decomposition, no photosynthesis, low dissolved oxygen and minimal mixing (below the thermocline). Secchi Depths – Secchi depths (a measure of color, turbidity and suspended solids, such as algae) on August 5, 2013 was 10.75 feet (3.26 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, which was 10.75 feet (3.26 m). Phosphorus Concentrations – Total phosphorus was found at two locations on August 5 - 0.03 ppm at 12 feet at Sample Point 25 and 0.02 ppm at the bottom at Sample Point 27. The sample taken at the bottom at Sample Point 27 contained suspended solids including algae, which may be the reason that phosphorus was found in the sample. Nitrogen Compounds – Nitrogen compounds were not present in the samples taken except the sample at the bottom of Sample Point 27. The sample taken at the bottom at Sample Point 27 contained suspended solids including algae, which may be the reason that the total nitrogen concentration was found to be 0.88 ppm. Alpha Chlorophyll (Chlorophyll a) – On August 5, 2013 Chlorophyll a, an indirect measure of the algae concentration, for sample points above the thermocline ranged from 4.65 ug/l (ppb) at one foot at
Sample Point 27 to 13.03 ug/l (ppb) at the bottom of Sample Point 27. Chlorophyll a concentrations appear to marginally increase with depth at all sample points. This may be due to gravity settling of algae and/or the influence of benthic algae. 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, although the increased chlorophyll a concentration may be due to gravity settling of algae facilitated by the lack of mixing below the thermocline. The elevated chlorophyll a concentration found at the bottom of Sample Point 27 may be due to the algae that was noted in the sample. Carlson Trophic State Index – As Dudley Pond is primarily used for recreational purposes a TSI less than 50 is tolerable and less than 40 would be great. The TSI on August 5, 2013 for the three sample points ranged from 43 to 53 indicating that the Dudley Pond trophic index was in the mesotrophic range. Only Chlorophyll a concentrations in samples taken at a depths of 12 feet and less were used in the determination of the Carlson Trophic Index for August 5, 2013. The 0.03 ppm total phosphorus analysis found at 12 feet at Sample Point 25 resulted in a TSI of 53. If the high total phosphorus reading of 0.03 ppm at 12 feet at Sample Point 25 is disregarded the TSI range for August 5, 2013 would be 43 – 48, which is solidly mesotrophic and indicates relatively good recreational water quality.