August 5, 2012 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, 2012 water quality data was gathered at three sample points and at three depths at each sample point. 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. The upper portion of the water column appeared to be well mixed at all sample points The trophic state of Dudley Pond was found to be high mesotrophic at the points sampled, based on the Carlson Trophic State Index (TSI), which is a function of Secchi depth, Chlorophyll a and Total Phosphorus (TP) concentration. The average TSI range for the three sample points was found to be 48 - 52. When water quality data for August 5, 2012 is compared to past data for mid-season samplings the trend is mixed; some better and some worse. At least the total phosphorus concentration was less than in years past. It is unclear whether the low TP concentration found on August 5, 2012 is an anomaly or if the ban on detergents containing phosphates is reducing the phosphorus contribution of septage entering the Pond.
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 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 one gallon 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
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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, 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, 2012 sampling are shown in Figure 2. The analytical data was plotted to determine the TSI and is shown in Figure 3.
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Date / Time:
8/5/2012
9:05 AM
Samplers: Andy Freedman, Toni Moores Weather: Partly cloudy, 78 F, light wind Barometric Pressure: 771.7 mm Hg
SP 24-top SP24-mid SP24-bot
SP 25-top SP25-mid SP25-bot
SP27-top SP27-mid SP27-bot
Depth (ft)
1'
3'
5' 6"
1'
12'
23' 6"
1'
3'
5' 6"
pH
8.64
8.72
8.6
8.82
8.08
6.9
8.48
8.63
8.68
ORP
128
106
93
168
152
-186
101.3
97.7
-13
DO
9.16
9.14
8.99
9.21
7.56
0.12
8.64
8.68
8.5
DO%
116.7
117
114.5
118.4
93
1.3
111.8
112
110.1
Temp
28.21
28.18
27.84
28.32
25.4
12.97
28.53
28.52
28.52
Secchi Depth
> 6'
Total Phosphorus (as P) OrthoPhosphate (as P) Ammonia (as N) Nitrate/Nitrite (as N) Total Nitrogen (as N) Chlorophyll É‘
ND ND ND ND 0.61 mg/l 6.07 ug/l
6' 9"
ND ND ND ND ND 5.74 ug/l
ND ND ND ND ND 7.75 ug/l
ND ND ND ND ND 2.99 ug/l
> 6'
ND ND ND ND ND 10.62 ug/l
ND ND ND ND ND 17.51 ug/l
ND ND ND ND 0.58 mg/l 5.73 ug/l
ND ND ND ND ND 5.37 ug/l
ND ND ND ND 1.2 mg/l 15.09 ug/l
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Figure 2 – August 5, 2012 Dudley Pond Water Quality Data
Figure 3 – Carlson Trophic State Index Results for August 5, 2012, Sample Points 24, 25 & 27
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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 8.82 to 6.9. Sample Point 24 had a pH that was similar at all three depths. The pH profile for Sample Point 25 decreased with depth starting at the surface with a pH of 8.82 and at the bottom a pH of 6.9. Because a thermocline usually exists in the summer at approximately 12 feet at Sample Point 25 no mixing occurs below the thermocline and sunlight does not penetrate to the bottom, it is not unusual to measure lower pH values at the bottom than at the top of the water column. The pH at the surface at Sample Point 27 was lower than at mid-point in the water column and at the bottom. The reason for this pH variation at Sample Point 27 is unknown. ORP – The ORP values ranged from +128 to -186, with the highest and lowest measured at the surface and bottom of Sample Point 25. The ORP usually decreases with depth; however, the decrease in the ORP with depth was unusual at Sample Point 27 and the reason is not known. DO and DO Percent Saturation – The DO usually decreases with depth, except when photosynthesis occurring with algae attached to the bottom (benthic algae). There were no surprises regarding DO and DO Percent Saturation at any of the Sample Points. Temperature – The temperature variations at Sample Points 24 and 27, top to bottom, were minimal indicating relatively good mixing in the top six feet of the Pond. For Sample Point 25 the variation in temperature from 28.32 C at the surface to 12.07 C at the bottom is fairly typical, when a thermocline exists during the summer. This thermal gradient is a feature of a thermocline, where the temperature below the thermocline does not change very much. Secchi Depths – Secchi depths (a measure of color, turbidity and suspended solids, such as algae) on August 5, 2012 was 6’ 9” (2.1 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 6’ 9” feet (2.1 m). Phosphorus Concentrations – On August 5, 2012 the total Phosphorus (TP) and orthophosphorus (soluble) OP concentration was not detectable at all sample points and all depths. A year ago on September 11, 2011 TP and OP were at non-detect levels, except for the bottom of Sample Point 25, where the TP concentrations was 70 ug/l. The lack of detectible TP at all locations on August 5, 2012 may indicate that the TP was totally consumed by the biomass in the Pond making TP the limiting nutrient for biomass production. Nitrogen Compounds – No nitrate or ammonia was detectable in samples taken on August 5, 2012. However detectable total nitrogen concentrations were found at the surface of Sample Points 24 and 27 and at the bottom of Sample Point 27 and ranged from 1.2 mg/l to 0.58 mg/l. Total Nitrogen is the sum of nitrate-nitrogen (NO3-N), nitrite-nitrogen (NO2-N), ammonia-nitrogen (NH3-N) and organically bonded nitrogen. As there was no nitrate, nitrite of ammonia present, the total nitrogen concentrations are most likely organic nitrogen from suspended organic materials such as algae or weed fragments. Alpha Chlorophyll (Chlorophyll a) – On August 5, 2012 Chlorophyll a, a measure of algae concentration, generally increased with depth at all Sample Points and ranged from 2.99 ug/l to 17.29 ug/l.
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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. 2012 for the three sample points ranged from 48 to 52 indicating that the Dudley Pond trophic index was high mesotrophic. Data Trends - The following comparisons are for data from Sample Point 25, which is the sample point with the most complete history of water quality data. Secchi Data - Comparing mid-season Secchi data from four past years to Secchi data from the August 5, 2012 data are shown below. Date Secchi (m)
August ‘78 1.14
August ‘81 2.1
August ‘07 1.6
Sept ‘11 2.98
August ‘12 2.05
Greater Secchi depths indicate better visibility and therefore better water quality. Total P Data – Higher Total P concentrations indicate poorer water quality because P is a nutrient that simulates the growth of algae and weeds. Total P concentrations appear to be seasonal and the highest concentrations have been found in the spring and fall. Date Total P ug/l
June ‘78 43
August ‘81 30
September ‘11 73*
August ‘12 ND
*On September 2011 the Total P concentration at Sample Point 25 at the bottom was 73 ug/l, which is thought to be an anomaly; however, higher concentrations of Total P are a possibility due to the lack of mixing below the thermocline. Chlorophyll a Data - Higher concentrations of Chlorophyll a indicate higher algae concentrations and poorer water quality.
Date Chlorophyll a ug/l
August ‘81 7.0
September ‘11 18.29*
August ‘12 10.37
*On September 2011 the Chlorophyll concentration at Sample Point 25 at the bottom was 44.09 ug/l, which is thought to be an anomaly; however, higher concentrations of chlorophyll are a possibility due to the lack of mixing below the thermocline. Chlorophyll is usually not present where the sun’s rays do not penetrate; however, it is possible that chlorophyll from the upper portions of the water column could settle to the bottom, assuming it is not degraded in the anaerobic environment at the bottom. TSI Data – The TSI range for August 5, 2012 was 48 – 52 compared to a TSI range of 44 – 50 for September 11, 2011.
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YSI Data – 2011 and 2012 are the only years that data from the YSI Multimeter exists and are shown below for comparison purposes. These are data are from the mid-season sampling.
Comparison Mid-Season YSI Multimeter Data
Depth (ft) pH ORP DO DO% Temp Secchi Depth Total Phosphorus (as P) OrthoPhosphate (as P) Ammonia (as N) Nitrate/Nitrite (as N) Total Nitrogen (as N) Chlorophyll ɑ
11-Sep-11
5-Aug-12
SP 25-top SP25-mid SP25-bot
SP 25-top SP25-mid SP25-bot
1' 7.8 150.2 8.6 101.1 23.45 9' 10" ND ND ND ND/ND 1.6 mg/l 5.3 ug/l
13' 7.72 147.6 8.51 97.1 22.05
ND ND ND ND/ND ND 5.47 ug/l
24' 6.89 -53.8 0.23 2.6 14.72
70 ug/l 0.02 mg/l 0.5 mg/l ND/ND 2 mg/l 44.09 ug/l
1' 8.82 168 9.21 118.4 28.32 6' 9" ND ND ND ND ND 2.99 ug/l
12' 8.08 152 7.56 93 25.4
23' 6" 6.9 -186 0.12 1.3 12.97
ND ND ND ND ND 10.62 ug/l
ND ND ND ND ND 17.51 ug/l
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