DRAFT Dudley Pond Water Quality Sampling Program April 2, 2014 Report 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 April 2, 2014 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. Based on somewhat uniform values top to bottom, the Pond was somewhat mixed and therefore it is assumed that the sampling occurred around the time of the spring turnover (water column mixing), even though there was a skim of ice at Sample Point 25. The Secchi depths found on April 2, 2014 were considerably poorer than in the past three years, which indicates that visibility in the water column was not as good as in recent years, possibly due to algae and/or suspended solids from recent rain storm. Based on the trophic state indexes shown below the water quality found on April 2, 2014 was worse than the water quality found during the previous three years. The poor Secchi reading and relatively high phosphorus and chlorophyll a concentrations at Sample Point 24 were responsible for increasing the TSI from 46 to 54 and had it not been for this poor results at Sample Point 24 the TSI range for Dudley Pond on April 2, 2014 would have been comparable to the previous three years
Date April 4, 2011 March 18, 2012 March 24, 2013 April 2, 2014
TSI 31 - 49 39 - 47 35 - 43 39 - 54
Chlorophyll a 2.72 – 3.99 ppb 1.98 – 4.62 ppb 1.39 – 4.46 ppb 2.24 – 5 ppb
Secchi Depth 12’ 1” 12’ 6” 14’ 4” 5’6” – 9’2”
Total Phosphorus 10 – 40 ppb 10 – 20 ppb Non-detect 10 – 20 ppb
This is the first spring sampling that total phosphorus and orthophosphorus were found at all sample points and all depths. As phosphorus is the rate limiting nutrient for weeds and algae, this finding along with poor Secchi depth measurements may indicate that the Pond will experience a larger crop of weeds and/or algae during 2014. The DO % Saturation and ORP data values were lower at Sample Point 25 than in the past three years, with DO % Saturation value a the bottom of Sample Point 25 being considerably lower (27%) than the spring of the previous three years. The reasons for this is unknown.
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-sight 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 March 24, 2013 sampling are shown in Figure 2. The analytical data was plotted to determine the TSI and is shown in Figure 3.
Figure 2 – April 2, 2014 Dudley Pond Water Quality Data
Figure 3 – Carlson Trophic State Index Results for April 2, 2014 at 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 5.92 at the mid-depth of Sample Point 27 to 6.61 at the mid-depth of Sample Point 25, without much variation from top to bottom, which indicates mixing within the water column, but the relatively low pH values indicates limited photosynthesis. DO and DO Percent Saturation – DO percent saturation measurements ranged from 121 % at the surface at Sample Point 25 to 27.5 % at the bottom of Sample Point 25. DO Percent Saturation values were relatively uniform top to bottom at Sample Points 24 and 27, which indicates some mixing was occurring. At Sample Point 25 DO Percent Saturation measurements were 121% at the surface, 95% at a depth of 12 feet and 27.5% at a depth of 24 feet. The lower percent saturation levels a twelve and twenty-four feet is most likely due to a lack of sunlight penetration (no photosynthesis), bacterial decomposition with some vertical mixing. Some vertical mixing may have been present because the thermocline, which eliminates vertical mixing below the thermocline, is not present until mid-late summer. Temperature – The temperature variation was 5.53 at the bottom of Sample Point 25 to 6.38 C at the bottom of Sample Point 24. The lack of significant temperature variation from top to bottom at Sample Points 24 and 27 indicates vertical mixing in the water column, which suggests that this sampling date was probably close to the spring turnover of the Pond. Oxidation Reduction Potential (ORP) – The ORP ranged from 137.5 mv at the mid-depth of Sample Point 25 to 167.6 mv at he mid-depth of Sample Point 27 and was relatively uniform at each of the sample points, top to bottom, but somewhat different from sample point to sample point. The following table is a comparison of ORP and DO % Saturation values at the bottom of Sample Point 25 (24 feet deep) for spring samplings for the past four years. Date ORP (mv) DO % Sat April 4, 2011 251 97.9 March 18, 2012 214.6 95 March 24, 2013 196.4 106.0 April 2, 2014 140.7 27.5 Clearly the ORP value on April 2, 2014 was marginally low compared to previous years, but the DO % Saturation value on April 2, 2014 was exceptionally low compared to values over the past three years. In most cases low DO % Saturation values are accompanied with low ORP values. It is not clear why the ORP and DO % Saturation values for April 2, 2014 do not correlate and why the DO % Saturation value found on April 2, 2014 is much lower than previous years
Secchi Depths – Secchi depth is a measure of color, turbidity and suspended solids, such as algae. At Sample Point 25 the Secchi depth on April 2, 2014 was 9.17’ (2.78 m), which was considerably poorer than the past three years. An accurate Secchi depth at Sample Point 27 could not be obtained because the bottom was clearly visible implying the Secchi depth was greater than the bottom depth. The Secchi depth at Sample Point 24 was 5.5’ (1.67 m), which is poorer than the past three years. Because the Chlorophyll A concentrations, an indirect measure of algae concentration, at Sample Point 24 were also elevated, the relatively low Secchi depth is thought to be due to a local algae bloom. Therefore, it is not clear which Secchi depth should be
used, so the average of the Secchi depths at Sample Points 24 and 25 was used [(2.78 + 1.67)/2 = 2.23 m]. Phosphorus Concentrations – Phosphorus is generally accepted as the growth limiting nutrient in fresh water systems. Total phosphorus is a measure of orthophosphate, condensed phosphates and organic phosphate and ranged from 0.01 to 0.02 mg/l (ppm) in the samples gathered on April 2. Condensed and organic phosphates are not readily usable as a nutrient by aquatic plants and algae. Orthophosphate is the form of phosphorus that is utilized to synthesize plants and algae and the concentration of orthophosphate in the April 2 samples was 0.01 mg/l at all sample points and all depths. This is the first year since 2010 that total phosphorus and orthophosphate has been detected at all three sample points and at all three depths. This finding may indicate that algae and/or weeds crops will be larger in 2014 than we have seen since 2010. Nitrogen Compounds – Nitrate concentrations ranged from 0.02 mg/l to 0.52 mg/l and were found at Sample Points 24 and 27. The presence of nitrate indicates an oxidizing environment and very limited bacterial decomposition going on in the water column. As the season progresses nitrate concentrations will disappear as a result of being incorporated into algae/weeds and/or via bacterial de-nitrification of nitrate to gaseous nitrogen, which will exit the Pond as a gas. Ammonia was only found in the sample from the bottom of Sample Point 25 at a concentration of 0.01 mg/l. TKN, total Kjeldahl nitrogen, includes ammonia and organic nitrogen compounds and was present only in the sample from the bottom of Sample Point 25 at a concentration of 0.55 mg/l. The presence of ammonia, TKN and lack of nitrate at the bottom of Sample Point 25 is most likely the result of bacterial decomposition of sludge and limited vertical mixing near the bottom of Sample Point 25. Alpha Chlorophyll (Chlorophyll a) – On April 2, 2014 Chlorophyll a, an indirect measure of the algae concentration, ranged from 2.24 ug/l (ppb) at the top of the water column at Sample Point 27 to 5 ug/l (ppb) at the bottom of Sample Point 25. 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 April 2, 2014 for the three sample points ranged from approximately 39 to 54 indicating that the Dudley Pond TSI was solidly in the mesotrophic range. Sample Points 25 and 27 had about the same TSI range of 39 - 46, which was in the low to midmesotrophic range. Because of relatively high phosphorus and chlorophyll a concentrations and a relatively poor Secchi depth Sample Point 24 had a TSI range of 45 – 54, which is in the mid to high mesotrophic range. It is difficult to attribute the relatively high TSI range found at Sample Point 24 to the presence of algae because the pH and DO % Saturation values were relatively low, which is an indication of low to moderate photosynthetic activity.