DRAFT August 3, 2014 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 3, 2014 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 3, 2014 was much better than August 24, 1981, but one of the worst since 2011 when the SWQC started gathering water quality data. Water quality data found on August 3, 2014 including DO Percent Saturation, OxidationReduction Potential and the Total Nitrogen concentrations were significantly different than in the summer samplings of the past. The “abnormal” water quality parameters on August 3, 2014 may have been negatively influenced by a large amount of surface water runoff from a large rain event that occurred approximately 36 hours prior to taking data and gathering samples. Historical Dudley Pond water quality data appears below.
Date TSI * August 24, 1981 49 - 54 Sept. 11, 2011 44 - 48 August 5, 2012 48 - 52 August 5, 2013 43 - 48 August 3, 2014 47 - 53 * 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 7.48 – 10.83 ppb
Secchi Depth 2.1’ 9.83’ 6.75’ 10.75’ 8.24’
Total Phosphorus* 30 ppb ND ND 20 ppb ** ND
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) Thermocline – Temperature readings were taken at various 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 3, 2014 sampling are shown in Figure 2. The analytical data was plotted to determine the TSI and is shown in Figure 3 and Thermocline data is plotted in Figure 4.
Figure 2 – August 3, 2014 Dudley Pond Water Quality Data
Figure 3 – Carlson Trophic State Index Results for August 3, 2014, Sample Points 24, 25 & 27
Dudley Pond Sample Point 25 Thermocline 30 25
Temp C
20 15 10 5 0 0
5
10
15
20
25
Depth in Feet Figure 4 – Dudley Pond August 3, 2014 Thermocline Plot 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 6.65 to 7.78. The low pH of 6.65 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 7.78 was found near the surface of Sample Point 25 and 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 2 % found at the bottom of Sample Point 25 to 99.5 % at 1 foot depth at Sample Point 25. DO Percent Saturation measurements did not vary much within the top 15 feet of the water column indicating relatively good mixing and light penetration to about 8 feet, with benthic photosynthesis in depths of less than 8.25 feet. 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 measured at about 15 feet. All of the DO Percent Saturation measurements taken on August 3, 2014 were below the 100 – 150 % range found during the summer sampling events in the past. The reason for this is unknown. Temperature – The temperature ranged from 13.7 C at the bottom of Sample Point 25 to 25.66 C near the bottom (5.5 feet) Sample Point 27. 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 15 feet indicates some vertical mixing in the water column above 15 feet. Oxidation Reduction Potential (ORP) – The ORP ranged from 34.9 mv at a number of locations to 62.2 mv at 1 foot at Sample Point 24. ORP values were remarkably uniform even at the bottom of Sample Point 25, where ORPs are usually very low, if not negative when the water is warm. All of the ORP readings on August 3, 2014 were lower than the 100 – 150 mv range found in past summer samplings. The reason for these findings are not known. Secchi Depths – Secchi depths (a measure of color, turbidity and suspended solids, such as algae) on August 3, 2014 was 8.25 feet (2.5 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 8.25 feet (2.5 m).
Phosphorus Concentrations – There was no Total Phosphorus found except 0.04 mg/l at the bottom of Sample Point 25 and is thought to be the result of bacterial decomposition of biomass and essentially no vertical mixing. Because this value is below the thermocline it was not used to calculate the Tropic State Index. Nitrogen Compounds – Total Nitrogen was found at all sample points and all depths and ranged from 1.7 mg/l at the bottom of Sample Point 25 to 0.58 mg/l at 1 foot at Sample Point 25. Ubiquitous Total Nitrogen concentrations have not been seen during summer samplings in the past and may be due to large amounts of runoff from a storm approximately 36 hours prior to taking samples. 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 7.48 ug/l (ppb) at the bottom of Sample Point 24 to 10.83 ug/l (ppb) at 1 foot at Sample Point 24. Chlorophyll a concentrations appear to decrease marginally with depth at Sample Points 24 and 27. The Chlorophyll a concentration increased at Sample Point 25 from 8.09 ppb at a depth one foot, to 10.5 ppb at 12 feet and 45.11 ppb at the bottom (24 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, although the increased Chlorophyll a concentration may be due to gravity settling of algae facilitated by the lack of mixing below the thermocline. 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 3, 2014 for the three sample points ranged from 47 to 53 indicating that the Dudley Pond trophic index was in the mid to upper mesotrophic range. Only Chlorophyll a and Total Phosphorus concentrations in samples taken at depths of 15 feet and less were used in the determination of the Carlson Trophic Index for August 3, 2014. Thermocline – Figure 4 is a graphic representation of water temperature versus depth. 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 15 feet. This discontinuity indicates the depth at which the thermocline 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 portion of the water column. Historically the Dudley Pond summer thermocline has occurred at 12 – 14 feet.