Draft Dudley Pond Alum Treatment Review Toni Moores, October 2013 On behalf of the Wayland Surface Water Quality Committee (SWQC) a review of the literature and communications with experts were undertaken regarding efficacy of phosphorus (P) treatment using alum in ponds/lakes. The objective of this review was to try to understand if Dudley Pond is a candidate for alum treatment to reduce the concentration phosphorus that is thought to be aggravating algae and weed growth. This report contains a summary, recommendations, methods, results and a discussion plus an appendix containing notes and links to relevant documents. Summary 1. Water quality data taken over time indicates that water quality in Dudley Pond appears to be slowly improving and P concentrations appear to be decreasing. Trophic State Indexes indicate the Pond has been eutrophic in the past and is at present typically mesotrophic (2013 TSI range 35 - 48), which is appropriate for recreational purposes. 2. Alum, iron and calcium have been used to precipitate (inactivate) P in lakes and ponds with varying degrees of success. These chemicals can reduce P concentrations in the water column, inactivate P in sediments, improve water clarity, reduce algae concentrations, but are not very effective in minimizing aquatic weeds. It is generally recommended that external sources (storm runoff & septic systems) of P be minimized before alum treatment is considered. 3. The contribution of P from septic systems adjacent Dudley Pond is difficult to estimate, so the SWQC does not understand the significance of P from septic systems relative to P contributions of surface water runoff and internal recycling of P from sediments. 4. With recent legislation regarding the elimination of P in detergents and its use as a landscaping fertilizer, it is expected that P entering Dudley Pond from surface runoff and groundwater (septic systems) will continue decrease over time; however, the aging of septic systems around the Pond may result in an increasing load of P from septic systems. 5. Historical data as well as recent data gathered by the SWQC indicates that there are significant quantities of P in the sediments, which recycle within the Pond during the spring and fall turnovers as indicated by SWQC spring water samples containing P. 6. In a study conducted in 1981 & 1982 by IEP, when the trophic state of the Pond was eutrophic, IEP recommended alum treatment after stormwater management alternatives were implemented, which they were in 1986, and the estimated cost was $18,000. The IEP study suggests that alum treatment would facilitate management of aquatic weeds as well as algae. One literature source indicated that milfoil is capable of taking up P via their roots and releasing it into the water column.
7. This review has facilitated a better understanding of the P water quality sampling results that the SWQC gathers in Dudley Pond during the spring, summer and fall of each year. Recommendations 1. Because alum treatment is not very effective in minimizing aquatic weeds and the Pond has acceptable water quality for recreational purposes, it does not appear that alum treatment to reduce the P content of the water column or the sediments would be cost effective. 2. If blue-green algae blooms become more frequent and severe, alum or preferably PhosLockŽ treatment should be considered. 3. It is recommended that data be gathered during 2014 to try to attempt to understand the P contribution of septic systems via groundwater. Methods Literature on the internet was gathered via Google. E mails were exchanged with Ken Wagner and Jim Sutherland, both experts in aquatic weed/algae management. Results Information collected which is thought to be relevant is contained in the appendix of this report. Discussion 1. As a general rule it is recommended that external sources (runoff, septic systems, etc.) of P be minimized before treatment of ponds/lakes is undertaken to minimize P concentrations. The primarily reason for this is to minimize the cost and frequency of treatment. 2. Various chemicals, including alum, sodium aluminate, iron, and calcium, have been used to precipitate soluble phosphorus in the water column and in pond sediments; alum is most commonly used. A relatively new phosphorus precipitation technique, developed in Australia and marketed by SePro in the US, is PhosLockŽ, a clay treated with lanthanum, appears to be more effective than alum. 3. Phosphorus precipitation with alum has had mixed result. Removal via chemical precipitation of soluble P from the water column with alum is almost instantaneous and very nearly complete. The added advantage of alum treatment is that much of the suspended solids, including algae, become incorporated into the alum floc and are removed from the water column via precipitation and gravity settling, thus improving clarity. Alum treatment is not very effective in minimizing aquatic weeds. The reason for the mixed results using alum is thought to be an inadequate understanding of P chemistry in the pond sediments. 4. “Under aerobic conditions, the P exchange equilibria are largely unidirectional toward the sediments; however, under anaerobic conditions, inorganic P exchange at the sediment-water interface is strongly influenced by low redox conditions. Sediment
phosphorus release (internal loading) can be an important source of phosphorus and can maintain high phosphorus concentrations in the water column, even in the absence of significant external loading” (Marsden 1989; Holz and Hoagland 1999). Anaerobic conditions and high pH (>9) maximize the release of P from pond sediments, including release of P precipitated by alum and iron. Blue-green algae (cyanobacteria) blooms can result in pH values in the 9 – 9.5 range and when the algae die they consume oxygen in the water as they decompose and the redox falls, resulting conditions for the release of P from sediments. This may be the mechanism by which blue-green algae increase the soluble P concentration in the water column. Minimizing anaerobic conditions and the associated release of P from sediments may have been the reason that pond circulators have been reported to be effective in the control of algae and milfoil. In Dudley Pond the SWQC has measured pH values in the 8 – 8.5 range resulting from bright sunshine and active photosynthesis, but have not measured pH values during a blue-green algae bloom. SWQC water quality measurements have seldom found anaerobic conditions at the Pond bottom at Sample Points 24 and 27, but anaerobic conditions and negative redox values are almost always found at the bottom of Sample Point 25 (“the deep hole”) which is confirmed in the literature reviewed.
5. So, based on the literature review, the Dudley Pond picture that emerges is: In order to understand the relative importance of sources of P in Dudley Pond crude calculations have been made. The calculations indicate that the amount of P contained in the Pond sediments is orders of magnitude greater that the P entering the Pond each year from runoff and septic systems
During summer, as long as the redox and DO concentrations are elevated in the shallow areas (above the 12 foot thermocline), it is believed that very little P is released from the sediments.
The soluble P from runoff and ground water (septic systems) plus the small amount released during the growing season from shallow sediments is mixed in the water column by thermals and wave action and, during the growing season, soluble P is essentially completely consumed by algae and weeds. SWQC does not usually find any measurable concentrations of P in our August samples.
During the summer, below the thermocline which has been measured at approximately 12 feet in the deep hole where the DO is depleted from decomposition of biomass and no photosynthesis, resulting in low redox values, P gets released over the summer, but is trapped in the deep hole below the thermocline because the thermocline prevents mixing of water that is deeper than 12 feet.
With the advent of autumn in the shallow areas (< 12 feet) the algae and weeds die and start decomposing and the fall turnover mixes the contents of the Pond, including the deep hole. We would expect to measure P in fall samples, but historically we have seen P in samples taken in November only occasionally. The reason for this is not clear.
During the winter, when there is very little photosynthesis due to low sunlight, lower levels of DO and low temperatures, some of the P in the sediments in shallow areas is released and P continues to be released in the deep hole.
When the spring turnover occurs the P in the deep hole as well as the shallow areas is mixed in the water column and is slow to be incorporated into biomass because weeds and algae are just beginning to grow. It is suspected that this is why the SWQC typically see P in our water quality samples during the March sampling.
During the spring and summer, the P in the deep hole is isolated by the thermocline and P enters the Pond from runoff and ground water and is almost immediately incorporated into biomass.
With the advent of fall the P cycle starts all over again.
In the past we have had blue-green algae blooms in Dudley Pond. It is expected that during these events P is released from the sediments due to elevated pH values, depressed DO concentrations and low redox values; however, SWQC has not made measurements or taken samples during blue-green algae blooms.
With the advent of regulations regarding the P content in detergents and the use of landscaping fertilizers, it is expected that the importance of P input into Dudley Pond from runoff and groundwater (septic systems) will decrease over time.
Since groundwater flows into the Pond from the east and southeast, groundwater may carry P into the Pond from septic systems, which would potentially complicate the precipitation of P using alum. To understand this source of P, samples of groundwater can be taken from the cold springs in the Pond and analyzed for P. The 1983 IEP study estimated the amount of groundwater that enters the Pond. So with P concentrations contained in groundwater entering the Pond a crude calculation could be made to understand the influence of P from groundwater (septic systems).
Appendix Aquatic Phosphorus Treatment Notes Reviewer Toni Moores October 2013
http://www.town.brewster.ma.us/files/longalum.pdf The treatment of Long Pond with aluminum compounds for the inactivation of phosphorus was successfully conducted in September and October of 2007. Approximately 370 acres were treated with aluminum sulfate (70,291 gallons) and sodium aluminate (37,856 gallons) on 17 days over a 28 day period. The ratio of alum to aluminate was 1.86:1, close to the targeted ratio. 2 2 2 Doses were approximately 10 g/m in the East Basin, 15 g/m in the West Basin, and 30 g/m in the Central Basin, as planned. The change in phytoplankton is very encouraging; cyanobacteria that were responsible for past blooms were abundant in September 2007, but have been minimal in biomass since then. Other algal groups more useful in the food web and less likely to cause problem blooms remain at moderate abundance. Beginning in November 2007, phosphorus levels began to rise and water clarity declined. Late winter and spring phosphorus levels were similar to pre-treatment levels and spring water clarity was within the historic range observed in the pond, suggesting no distinct benefit from the treatment. Multiple possible explanations exist, but none is completely consistent with all data, and there is a minimal track record for fall treatments and winter monitoring, so we have little context within which to evaluate this treatment for that time period. Several mechanisms may have combined to limit winter- spring control of phosphorus and improved clarity. However, beginning in May and progressing through August, water clarity increased dramatically, exceeding all measured values for the last decade. There was a decrease in clarity in September, consistent with historic trends, but an increase at the start of October, after which monitoring ceased. Water clarity during summer 2008 was the highest observed in over a decade. http://www.marinebiochemists.com/phosarticle.html Regardless of the treatment used, aquatic macrophytes will have a profound effect on the amount of P transported from the sediments to the water. Eurasian water milfoil is a major contributor of P to the water column. Phosphorus is taken up from the sediments and released through the leaves of the plant by diffusion. This diffusion occurs because of the high concentration gradient between the plant and the surrounding aquatic media. This fact must be taken into consideration when evaluating the effectiveness of a particular treatment. http://www.indianponds.org/wp-content/uploads/2011/02/Letter-with-White-Paper-9-252009.pdf . http://www.stormwater.ucf.edu/conferences/9thstormwatercd/documents/CurrentResearch.p df
http://www.sepro.com/phoslock/ Research and application of in-situ control technology for sediment rehabilitation in eutrophic water bodies B, Liu*, X.G, Liu*, J, Yang*, D. Garman** , K, Zhang*, H.G, Zhang* This paper was supplied by SePro re Phoslock. Excerpts appear below. Because of the prevalence of toxic algal blooms, high priority is given to control of phosphorus inputs to water bodies, but algal blooms may continue to occur long after phosphorus inputs have ceased due to remobilization of phosphorus bound to sediments (Paulo C.F.C. Gardolinskia, et al. 2004). Lots of factors can affect the release of phosphorus from sediment. Besides the physical & chemical components of sediment itself, disturbance, high temperature (>15°C), high pH(pH>8.0), low P concentration of the overlying water and anaerobic conditions may favor phosphorus release from the sediment. Fig. 2(a) indicated that dissolved oxygen was one of the main parameters for controlling the release of phosphorus from sediment, and anaerobic conditions can stimulate P release. The release in eutrophic waterbody was inhibited by aeration under normal conditions. Meanwhile, pH value was another important factor affecting the release of phosphorus from sediments, especially under alkaline condition, which promoted the NaOH-P release greatly (Jin, et.al, 2004). In fact, when cyanobacteria blooms occur, both DO and pH of water bodies may change a lot, on the one hand, the decay of algal matter may lead to the oxygen depletion in the water, which in turn can cause release of phosphates that were previously bound to oxidized sediment; on the other hand, pH value increases, some of which may even reach at 9~9.5, thus promote the NaOH-P release. Under the condition of both high pH value and aerobic, a great deal of phosphorus released from sediment, which reveals that aeration couldn’t restrain P release under high pH condition, not to mention the high pH coupling with anaerobic condition. When TP concentration in the overlying water was high, phosphorus in water column will not be absorbed by sediment under anaerobic conditions. The results of static and dynamic simulation experiments under different environmental conditions showed that with the application of modified clay, not only was the P concentration of the overlying water decreased immediately to a very low level, but the proportion of bioavailable phosphorus in the sediment was reduced and a majority of which were replaced by non- reactive species, even in the most severe conditions of high pH, anaerobic conditions and 2 disturbance: P release rate can be reduced to -19.25mg/m ∙d, while phosphate suppression 2 efficiencies of sediment can achieve 95% with the application rate at 0.5kg/m , and with the application of 3, 6 and 12 months, the transformation rate from bio-available P to stable P in the sediment can reach 40%, 53%, and 62% respectively. From the above data, we can see the effectiveness of Phoslock® on in-situ control of sediment restoration are as follows, original sediments, whose sorption capacities are dominated by Fe(OH)3, release P during the development of hypolimnetic anoxia that results in the reductive
dissolution of FeIII solid phases and release of Fe2+ into solution or reprecipitation as FeII sulfide. And an increase in the pH of a water body above pH 8 may result in re-release of the phosphorus from the aluminum flocs (Lewandowski et al., 2003). While the active ingredient in the clay, lanthanum, has a strong affinity for orthophosphate, forming the mineral rhabdophane with an extreme low solubility product Ksp =10-24.7 to 10-25.7 mol2 L-2 (Frankvan Oosterhout And Miguel Luring 2010), which can prevent P release during anoxia and high pH value conditions by adsorbing the P liberated from Fe(OH)3 and Al(OH)3 respectively. Although this binding is not influenced by redox state and a wide range of pH values, pH can influence the absorption capacity of the modified clay, the efficiency of Phoslock速 will be reduced when pH>9, which is mainly caused by the reasons that hydroxyl ions compete with phosphate for lanthanum binding sites. http://d.umn.edu/~skatsev/Publications/Katsev_ChemGeol_Phosphorus.pdf Our results provide explanations to the reports that lake restoration measures such as restricting phosphorus inputs to a lake or oxygenating the lake's hypolimnion (or both) in the long-term often fail to decrease sediment phosphorus effluxes. The re-deposition of sediment substances after their release into the water column (the feedback often overlooked in sediment RTMs) can critically affect the magnitude and dynamics of phosphorus efflux from sediments.
http://ceqg-rcqe.ccme.ca/download/en/205/ (good general overview by Canadians) Sedimentation of particulate phosphorus results in a slow but continuous loss from the water column. Phosphate is precipitated as insoluble iron, calcium, or aluminum phosphate and then released slowly. Exchange of phosphorus across the sediment/water interface is regulated by oxidation-reduction (redox) interactions, which are dependent on oxygen supply, mineral solubility and sorptive mechanisms (Stumm and Morgan 1996), the metabolic activities of bacteria and fungi, and turbulence from physical and biotic activities (Wetzel 2001). Lake sediments contain much higher concentrations of phosphorus than water. Under aerobic conditions, the exchange equilibria are largely unidirectional toward the sediments; however, under anaerobic conditions, inorganic exchange at the sediment-water interface is strongly influenced by redox conditions. Sediment phosphorus release (internal loading) can be an important source of phosphorus and can maintain high phosphorus concentrations in the water column, even in the absence of significant external loading (Marsden 1989; Holz and Hoagland 1999). http://dnr.wi.gov/lakes/grants/project.aspx?project=49233358 Good report w/ relevant info re sediment inactivation. In October just prior to stratification, East Alaska Lake is at its longest period of open-water stratification; and thus, phosphorus concentrations within the anoxic hypolimnion are near their maximum. During fall turnover, that phosphorus-rich water trapped all summer near the bottom of the lake is mixed throughout the water column where some of it can persist through the winter and be available to algae the following spring. In addition, phosphorus that builds up during winter stratification is mixed throughout the lake during spring turnover where it fuels algae throughout the summer. When the alum was applied in October of 2011,
temperature/dissolved oxygen data indicated that the lake was still stratified; meaning the lake had not yet turned over and the phosphorus-rich water was still near the bottom of the lake. When the alum was applied, it bound much of the phosphorus within the hypolimnion and prevented it from being mixed throughout the water column during fall turnover. Likewise, the alum prevented sediment phosphorus from being released during winter stratification which would have been mixed throughout the lake in spring. The prevention of hypolimnetic phosphorus from being released and mixed throughout the lake in the fall of 2011 and spring of 2012 is why near surface phosphorus concentrations in the summer of 2012 were lower than previously measured. During early fall 2012, a core was extracted from the deep hole where the 2 full dose rate of 132 g/m was applied, and a distinct layer was found near the surface of the sediment, indicating a substantial barrier to sediment phosphorus flux had been created (Photo 1). While a sediment layer appears to have built up over the top of the alum barrier, this is actually a result of the higher-density alum sinking below a less-dense layer of sediment following application. A phosphorus profile collected during the same October visit is also strong evidence of the success of the treatment, especially when compared to profiles collected the two previous years (Figure 2). As discussed earlier, phosphorus values within the hypolimnion in October are near their maximum. As illustrated in Figure 2, dissolved oxygen profiles (dashed lines) show that the lake was stratified during these sampling events in all three years. During the two years prior to the treatment, total phosphorus concentrations in the anoxic hypolimnion ranged from 0.1 to 1.24 mg/L, while the post treatment samples spanned from 0.040 to 0.088 mg/L. These data show that the alum treatment was successful in reducing the amount of phosphorus being released from bottom sediments during the summer of 2012. http://www.ecy.wa.gov/programs/wq/plants/algae/lakes/lakerestoration.html Aluminum, iron, or calcium salts can inactivate phosphorus in lake sediments. Lake projects typically use aluminum sulfate (alum) to inactivate phosphorus. Alum may also be applied in small doses for precipitation of water column phosphorus. When applied to water, alum forms a fluffy aluminum hydroxide precipitate called a floc. As the floc settles, it removes phosphorus and particulates (including algae) from the water column (precipitation). The floc settles on the sediment where it forms a layer that acts as barrier to phosphorus. Phosphorus, released from the sediments, combines with the alum and is not released into the water to fuel algae blooms (inactivation). Algal levels decline after alum treatment because phosphorus levels in the water are reduced. The length of treatment effectiveness varies with the amount of alum applied and the depth of the lake. Alum treatment in shallow lakes for phosphorus inactivation may last for eight or more years. In deeper lakes, alum treatment may last far longer. Long Lake in Kitsap County and Green Lake in Seattle provide examples of recent successful use. Click here to read an alum report from Green Lake. Some lake managers use alum to precipitate phosphorus from the water column by continuously injecting small amounts of alum during the summer months (micro-floc alum injection). The hypolimnetic aerator in Newman Lake, Washington, injects small amounts of alum into the water as it operates to provide additional management of phosphorus. Read more about beautiful Newman Lake (seen in the photograph) at this website.
http://www.mass.gov/eea/docs/dcr/watersupply/lakepond/downloads/practical-guide-nopics.pdf page 68 Phosphorus Inactivation
Tighe & Bond - Dudley Area Land Study, Draft Report, Jan2013 3.6.7 Surface Water Quality reports on Dudley Pond In 1981 the Town of Wayland contracted IEP, Inc. to perform a Diagnostic/Feasibility Study of Dudley Pond, which was finalized in 1983. The goal of the study was to identify the cause of pollutant/nutrient input to the pond and evaluate and recommend watershed and in-lake management strategies for reducing pollutant/nutrient inputs. The study found that groundwater travels from the southeast to the northwest of the pond, noting that the elevation of the groundwater divide in the northwest/downgradient portion of the watershed is not known and that additional information on groundwater levels is needed. The study notes that due to the high density of residential development surrounding the pond, non-point source nutrient contributions have a significant impact on the waterbody, specifically nutrient loading caused by stormwater runoff and leaching of subsurface sewage disposal systems. The greatest nutrient loading sources to Dudley Pond are septic systems (accounting for 32% of the phosphorus load) and stormwater runoff (58% of the phosphorus load). Editor’s note: Tighe & Bond were unwilling to share how these numbers were calculated. This excess phosphorus caused a “eutrophic” state in Dudley Pond, meaning that the excess phosphorus promoted the growth of plant and algae growth. The increase in aquatic vegetation and algae has interfered with recreational usage of the pond, impeding fishing, swimming, and boating. The study found that the soil types located close to the Pond have a low capacity to attenuate nitrogen and that the septic systems located close to the water eventually deplete the available attenuation capacity and leach into the pond. The study notes that as shoreline septic systems continue to age, more of them will contribute to phosphorus loading. At the time of the study the average age of the septic systems was 22 years and an estimated 51% of them consisted of cesspools (the majority of them on undersized lots).
IEP, DIAGNOSTIC/FEASIBILITY STUDY - DUDLEY POND WAYLAND, MASSACHUSETTS April 1983 [Editor’s note: text below was scanned and converted by OCR – efforts were made to correct recognition errors, but the text was difficult to convert, and some errors may remain.] 5.3 Internal Phosphorus Cycling pages 60 & 61 It is widely accepted (USEPA, 1980A) (Reckhow, 1979) that most lakes and ponds act as nutrient sinks (i.e. more nutrient sedimentation occurs than release from the sediments) on a net annual basis. Indeed this concept is implicit in most eutrophication models. However, it has been shown (Snow and DiGiano, 1976) that the pond bottom sediments may alternately act as a source or a sink at various times during a given year. The factors that determine which process occurs include nutrient concentrations in the hypolimnion, sediments, and interstitial water, dissolved oxygen and pH at the sediment/water interface, redox potential, the presence or absence of major cations (Ca, Fe, and Al), particulate settling velocity and flushing time. Whereas most of these factors may be highly variable throughout the year, monitoring them sufficiently to estimate nutrient release/sedimentation would be a very rigorous task. Hence,
the trophic state models (Dillon-Rigler, Vollenweider, Reckhow) applied in this study have, by necessity, simplified estimates of phosphorus retention by a pond as functions of basic morphological features such as retention time, mean depth, and/or normal levels of hypolimnetic dissolved oxygen. […] Thus the results of applying both formulae to Dudley Pond data indicate that between 52% and 82% of the annual total external phosphorus load is retained by the sediments. Phosphorus which may be recycled to the overlying waters of Dudley Pond, is likely to do so following the fall and spring turn-overs. Our water quality test results at Dudley Pond revealed comparatively low hypolimnetic concentrations of total phosphorus during July and August (0.05 and 0.04 mg/l respectively). Further, the small volume of the hypolimnion relative to the entire Pond volume, suggests that internal nutrient release is not an important factor in the overall phosphorus budget. In addition, our late winter (March 10, 1982) sampling results, revealed nearly equal phosphorus concentrations at the Pond surface (0.02 mg/l) and bottom (0.03 mg/l). Internal nutrient re- cycling following spring turn-over is likely to have a greater effect upon nuisance algae and macrophytes growth at Dudley Pond than would fall turn-over because of the increased availability of nutrients when water temperatures are warming, and active plant growth is beginning. The late winter water quality data for Dudley Pond points to external (watershed) nutrient sources as the prime contribution of phosphorus, resulting in the Pond’s current eutrophic status. If phosphorus is released from the sediments during certain periods of a year, these quantities are not considered as loadings and hence are not a part of the phosphorus budget. Trophic status is evaluated in the applied models by comparing predicted loadings to loadings which would yield eutrophic conditions. Whereas the latter, the pond's tolerance for phosphorus input, is computed in part by factors which define the pond’s ability to retain phosphorus, the predicted loadings must include only sources which are external to the Pond. Pages 92, 93, & 94 7.5 Nutrient Precipitation/Inactivation Nutrient precipitation is a lake restoration/management technique used to remove phosphorus from the water column, thereby limiting the growth of microscopic algae. Nutrient inactivation, on the other hand, is directed at reducing internal phosphorus recycling from the bottom sediment. Both techniques (precipitation and inactivation) involve treatment of the lake, usually with the chemicals aluminum sulfate or sodium aluminate. For precipitation treatments, the chemical is typically applied as a liquid slurry upon the water surfaces. During nutrient inactivation treatments the chemical is injected along the lake bottom or hypolimnion, by dispersing the chemical through weighted hoses attached to the pumps and surface vessel. Nutrient precipitation/inactivation treatments are most effective in ponds or lakes where external (watershed) nutrient loadings have been reduced. At Dudley Pond, the potential effectiveness and longevity of either a precipitation or inactivation treatment would be greatly enhanced by first curbing watershed nutrient contributions, primarily stormwater and septic. Precipitation/inactivation treatments seem to be equally successful in ponds or lakes, but more so in waterbodies that stratify and are characterized by a long hydraulic retention time (slow flushing). Dudley Pond clearly meets one of these criteria in that its hydraulic retention time is
1.54 years. Dudley also stratifies thermally during summer although the hypolimnion occupies only 4.6% of the total Pond volume. Phosphorus concentrations in the hypolimnion of Dudley Pond are elevated (approximately double) relative to epilimnetic concentrations during summer. Hypolimnetic total phosphorus was found to range between 0.04 mg/l - 0.07 mg/l in the three sampling rounds performed during the summer of 1981. Assuming an average hypolimnetic total phosphorus concentration of 0.05 mg/l, equals a phosphorus mass of 2.4kg of which a portion, could potentially be recycled throughout the Pond following fall turnover. In comparison to other eutrophic lakes however, neither the concentration nor mass of hypolimnetic phosphorus at Dudley is exceptionally high. A combined phosphorus precipitation/inactivation treatment of Dudley Pond is recommended once watershed nutrient contributions have been curbed, especially the stormwater loading component. Phosphorus precipitation and surface application of the chemical is suggested over the entire 92 acres of the Pond. Although epilimnetic phosphorus concentrations at Dudley are admittedly low (>0.01 - 0.03 mg/1) alum treatments performed elsewhere (i.e., Morse’s Pond, Wellesley) have demonstrated a remarkable ability to clear the water, through chemical coagulation/precipitation of microscopic algae and suspended organic complexes. Following a surface (precipitation) treatment, the alum floc which settles to the bottom will continue to adsorb some additional phosphorus that might otherwise be released to the water. Quantifying sediment phosphorus release rates and differentiating between aerobic and anaerobic bottom contributions is extremely difficult. Both sources are recognized, however, as potentially significant contributors. Throughout the deeper (>20 ft.) stratified portion of Dudley Pond, injection of the chemical at or below the hypolimnion is recommended to help reduce sediment phosphorus release. This area is approximately 19.2 acres or 21% of the Pond surface. The two chemicals, aluminum sulfate (alum) and sodium aluminate, have been most frequently used in previous phosphorus precipitation/inactivation lake restoration projects. The salts of these chemicals work in three ways: (1) by forming aluminum phosphate; (2) by entrapping phosphorus containing particles in the water column, and (3) by adsorbing phosphorus to the surface of the aluminum hydroxide (the main chemical product of the precipitation reaction [EPA, 1980Al). Alum dosage for precipitation treatment is determined through a series of jar tests whereby varying concentrations of the chemical are added to achieve the desired level of phosphorus reduction. In the case of Dudley Pond, dosage would also be based upon the removal of microscopic algae and color causing substances from the water. Dosage determinations of aluminum to bottom sediments (inactivation) are also determined by jar tests with a maximum dose established at that dose above which dissolved aluminum concentration exceeds 50 ug Al/l (Kennedy and Coke, 1982). Aluminum solubility is minimal between a pH range of 6 to 8, therefore, a dose of aluminum sulfate sufficient to reduce pH to 6.0 is considered as optimal. The range in pH at Dudley Pond (6.2-8.0) should allow for good floc formation and effective inhibition of phosphorus release in water depths greater than 20 feet. Calculations of dosage determined prior to treatment will assess the need for using sodium aluminate in combination with alum to mitigate a severe drop in pH at Dudley given its rather low buffering capacity. Long term studies of aluminum toxicity on aquatic organisms (fish, invertebrates, etc.) and lacking 1n the literature. Kennedy and Cooke (1982) review the work
which has been done and discuss the findings/conclusions of such researchers as Burrow (1977) Freeman and Everhart (1973) and Narf (1978). They conclude that aluminum toxicity does not appear to be a significant problem, as long as pH 1S controlled and/or RDA (residual dissolved aluminum) is not allowed to reach levels in the area of 50 ug Al/l. In that aluminum is one of the more abundant metals found in the earth's crust, is widely used in water treatment processes and is found in many foods, the potential adverse effects/risks of aluminum on human health, do not appear to be significant. The costs for a precipitation/inactivation treatment of Dudley Pond are difficult to define without first determining the dosage and possible requirements for adding sodium aluminate in combination with alum to prevent drastic changes in pH. Preliminary cost data may be drawn however, from two recent alum precipitation treatments performed in 1981 at Morse’s Pond in Wellesley and Spy Pond in-Arlington. Chemical c:ostsfor a total surface treatment of Morse’s Pond (surface area 102 acres, mean depth 7.7 ft.) were about $7,000, while chemical and application costs totaled approximately $10,800 at Spy Pond (Boschetti, personal communication, September 1982). Dudley is somewhat smaller in surface area than either Morse’s or Spy Pond, but its mean depth (9.23 ft.) lies between the other two pond depths. Therefore, the estimated total cost (1982) for a total surface treatment of Dudley Pond is approximately $12,000. The actual cost would be higher than $12,000 however, after allowing for an estimated $2,000 for deter- mining dosage and the additional $3,000 for the alum required to treat the hypolimnion of Dudley Pond. The total cost inclusive of final design and implementation is therefore estimated to be $18,000. In summary, a combined phosphorus precipitation/inactivation treatment is recommended at Dudley Pond after the implementation of the stormwater management alternatives. Such a treatment would serve to reduce sediment derived phosphorus from both the shallow aerobic and deeper bottom areas of Dudley Pond. In addition, an immediate short-term improvement in water clarity would be realized with a reasonable potential for attaining lasting benefit from more than one year. The comparative cost is low ($18,000)relative to the anticipated benefit. [Editor’s note: Stormwater management alternatives were implemented in 1986 through road paving and catch basins. Some of these catch basins are no longer effective.]