Category Archives: In-Lake Management

Use of Aluminum to Treat Lakes

Aluminum compounds are coagulants used in water and wastewater treatment to settle solids and pull dissolved solids out of solution. In water treatment these compounds convert impurities into particles that can be settled or filtered. In wastewater treatment aluminum does the same thing, but is also noted for its ability to find phosphorus and lower the fertility of effluents. Over 40 years ago it was hypothesized that aluminum could do in lakes what it did in treatment facilities and might be especially useful for inactivating phosphorus in sediment that was being recycled to create what we now call an “internal load”. While a lot has been learned in the intervening years that makes such treatments more effective, even the earliest treatments provided enough benefits to make continued use worthwhile. Although aluminum treatments will clear the water of most algae, it is not an algaecide, defined simply by the root words as something that kills algae.

Aluminum application to a lake.

While the majority of regulatory agencies in New England appear to understand why and how aluminum is used in lake management, there seems to be some confusion in a few places about aluminum use. The State of New York created a regulatory definition of algaecide that expands coverage to any additive that prevents algae from growing. This would include aluminum, which limits phosphorus availability, and is not on the federal list of registered algaecides, since the EPA does not consider it to be one. Consequently, aluminum treatment cannot be permitted in New York. Of course, by this logic algaecides would also include oxygen if added to keep phosphorus bound to iron and unavailable to algae, and to air used to circulate water, thereby disrupting the growth of many buoyant cyanobacteria. It would also include water used to dilute phosphorus concentrations or even flush a small lake. Arguments about other additives being “natural” simply do not hold water. New England states have generally not bought into this faulty logic, but apparently the Connecticut Department of Health has applied the New York definition of an algaecide in some cases and has not approved aluminum treatments in drinking water supplies, despite approval by CT DEEP for such treatments in recreational lakes.

There is no doubt that control of phosphorus before it enters a lake is preferable where feasible, but there are very real limits on our ability to do that, and where phosphorus has accumulated in a lake, it has to be inactivated to rehabilitate the lake. It is much like fixing a boat that has sprung a leak; the leak needs to be patched, but that won’t get rid of the water that has leaked in. Aluminum treatments offer control of internal loading, and can also be used to treat inflows where watershed management is not yet up to the task or to reduce phosphorus availability in the water column after significant loading events. We have learned over the years how to prevent toxicity, making it a relatively safe technique. Creating regulatory restrictions based on faulty logic or incomplete understanding of the technique hinders lake management in a time when we need every viable technique we can get.

Measuring Oxygen Demand

Knowing how much oxygen is consumed per unit volume of water or area of sediment in a lake is important to understanding lake metabolism and in planning for the provision of adequate oxygen. Systems that mix the lake or add air or oxygen must counteract the oxygen demand to be successful. Measuring demand is tricky, however, since the rate at which oxygen is removed is hard to isolate from factors that put oxygen into the water or allow it to move within water, and consumption of oxygen is not linear over the range from about 2 mg/L down to 0 mg/L.

Laboratory tests can be run in which oxygen loss over a set amount of time is measured in a closed bottle at a standard temperature, and this is useful, but this won’t include sediment oxygen demand, which is often the dominant source. For deep lakes, one can use temperature and dissolved oxygen profiles from a few weeks apart at a time when oxygen is >2 mg/L everywhere to get a reasonable estimate. Oxygen addition from the atmosphere and downward mixing necessitates using values from deeper water, usually below the depth of the thermocline, even if it has not strongly formed yet. Timing is critical. In New England stratification sets up from April into June and is highly weather dependent, but once it sets up, loss of oxygen near the bottom can be rapid and oxygen concentrations can become too low to use in demand calculations. When the oxygen concentration is less than about 2 mg/L, further removal slows down and calculations using low values will underestimate actual demand, something to be avoided when planning oxygen additions.

For proper data, subtraction of later oxygen from earlier oxygen at each depth increment can be summed to yield the mass of oxygen lost over a square meter of lake over time (Table 1). If the temperature has risen between measurements, the water will naturally hold less oxygen, so a correction for temperature-induced oxygen loss must be applied, based on the difference in saturation concentration at the earlier and later temperature measurements. In the example in Table 1, the thermocline forms at about 6 m but was very weak at the time of the April measurements. Yet it was apparent that little oxygen from above was reaching deeper. There was a slight increase in temperature, so the differences first obtained were adjusted down slightly. The difference in total oxygen demand is not large, but can be if the time between measurements is longer.

Table 1. Calculation of Oxygen Demand in a Deep Lake

In shallow lakes it is harder to find a time or place where factors other than oxygen demand are not significantly influential, but we have found one approach that often works.  Photosynthesis ceases overnight in the absence of light, and at least some nights there is calm that limits mixing. Measurements made after dusk and before dawn can be used if oxygen remains >2 mg/L throughout the water column, and the results can be quite striking. Oxygen demand from respiring algae or vascular plants will add to demand from decay, but that is all part of the demand and worth including. The same calculation approach used for the deeper lake is applied to the shallow lake (Table 2), but the whole water column is included.

Table 2. Calculation of Oxygen Demand in a Shallow Lake

For reference, oxygen demand higher than about 0.5 g/m2/d can eventually lead to oxygen depletion in stratified lakes. Values >1.0 g/m2/d will cause depletion in the bottom layer by mid-summer, and values >2.0 g/m2/d will cause oxygen depletion much earlier in summer. Making multiple measurements is recommended to characterize the range of oxygen demand and get a sense for possible error induced by not being able to control all oxygen inputs. With enough measures a pattern usually emerges that allow one to make a fairly accurate estimate of oxygen demand.

Temperature and Oxygen Profiles

If you polled people with a lot of lake assessment experience about what technique provides the most information for the money, chances are that Secchi disk transparency would be the choice, but temperature and oxygen profiles would get at least an honorable mention. The thermal structure of a lake is extremely important to the oxygen regime, which is critical to so many aspects of water quality and lake ecology. Temperature and oxygen profiles collected over the course of the year provide extremely valuable data for understanding your lake.

One needs an instrument that measures temperature and oxygen in water to collect these profiles, and a number of high quality instruments are on the market. YSI and Hach have the biggest market share, but there are others worth considering. Bear in mind that you get what you pay for. Temperature is based on differential movement of metals that respond to temperature change and instruments cannot be calibrated by users; one needs to check against a thermometer or known temperature once in a while, but temperature measures are generally reliable. Oxygen used to be done as a titration in lab or field, then moved to instruments with membranes that measured potential across that boundary, and now are done with luminescence, although all three approaches are still in use. Calibration is essential but not difficult for instruments.

A long enough cable is needed to reach the bottom from the boat or dock, and measurements are typically made every half meter (shallow lakes) to every 3 meters (very deep lakes), with 1 meter intervals most common. If the lake is deep enough to stratify, there will be a transition zone between upper and lower water layers and the thickness of that transition zone may be of interest, so more frequent measures (shorter vertical distance between readings) may be advisable near that boundary, called the metalimnion (with the point of inflection for temperature change called the thermocline). It is also important to get readings near the bottom, or even right in the sediment. Certainly the deepest reading should be no more than a few inches from the bottom, and you may find that oxygen declines sharply as the probe hits the sediment.

Graphing temperature and oxygen profiles provides a useful visual image of what is going on (Figure 1), and the pattern over time can be put on the same graph for comparison (Figure 2). Even shallow lakes may exhibit a decline in temperature, and a difference of only 3 Co can be enough to resist mixing. Even without a discernible change in temperature, there can be oxygen depression or even depletion near the bottom if the oxygen demand is high enough; oxygen doesn’t diffuse all that fast through water, and decay can outpace re-aeration.

Temperature and Oxygen Profile.

 

Oxygen Concentration over Depth and Time at a Deep Station.


So why are these profiles important? Higher temperatures increase metabolism and limit the types of organisms that can be present, perhaps most notably fish; trout do not do well at temperatures much above 21
oC. The temperature structure of the lake tells us about how much mixing is going on over space and time, and in the absence of adequate mixing, oxygen loss can occur. Loss of oxygen affects nearly all biological components of a lake; sensitive species cannot tolerate oxygen much less than 5 mg/L, few aerobic organisms do well at oxygen <2 mg/L, conversion of ammonium to nitrate ceases when oxygen runs out, and release of iron-bound phosphorus can occur at low oxygen concentrations. Much can be predicted about the status of a lake from temperature and oxygen profiles.

Odor in Lakes

When lakes smell bad, we notice. Some basic knowledge of sources of odor may help lake enthusiasts identify odors. Odors can be classified according to some complicated systems, but for our purposes we will stick to some basic descriptions.

Most people recognize “rotten egg” smell, which is hydrogen sulfide (H2S). It is formed when sulfate is metabolized in the absence of oxygen, usually in the bottom of a stratified lake. We sometimes smell H2S around salt marshes, as there are more sulfates in saltwater. In lakes, the presence of H2S means that oxygen has been depleted and the demand has become so great that sulfate (SO4) is being broken down. Other oxidized compounds are usually broken down first, such as nitrates (NO3), so oxygen demand is severe if H2S is being created. Most often the H2S is smelled in bottom water that has been sampled; the H2S tends to stay trapped in the bottom water layer until fall mixing. But with a mixing event, the smell can become detectable.

Certain cyanobacteria (blue-green algae) produce odorous compounds, specifically geosmin and methylisoborneol (MIB), which impart a musty to grassy odor. Often the offending cyanobacteria accumulate as a surface scum, making the odor obvious to those using the lake. If cyanobacteria are abundant enough to produce odor, there is legitimate concern about toxicity, but it is important to know that production of odor and production of toxins are not linked. Still, if enough cyanobacteria are present to create detectable odor, there are enough to create toxins above a safe threshold IF the cyanobacteria are toxin producing forms.

Decaying blue-green scum

Other algae besides cyanobacteria create odor. Actually, any algae will produce some odor if abundant enough, but certain algae can produce specific smells when abundant. Most notable are certain chrysophyta (golden algae), which produce odors such as cucumber, violet, spicy or fishy. No one is likely to confuse these options with a wine tasting event.

Green algae mats

Finally, dead algae tend to give off foul odors usually described as septic or decay. Dying filamentous green algae are particularly malodorous. If mats wash up on shore and start to decay, they are likely to be very noticeable to anyone with a nose.

A good reference to check if you want to know a lot more about odor in water is the American Water Works Association publication M57, called Algae: Source to Treatment.

Climate change impacts quantitatively assessed

Some impacts of climate change have known for years, but others are still surfacing. About a decade ago a representative of the USGS in Maine presented data at the NECNALMS conference for the date that ice cover broke up for multiple lakes in Maine. While there was variability typical of systems influenced by climate change, it was very clear that the date the ice was going out was getting earlier over decades. In about a 60 year period the average ice out date for the 2000s was about 2-3 weeks earlier than it had been in the 1950s. This should have been impressive enough in its own right, but apparently few other than ice fisherman really took notice.

Now we have another interesting measure that might be scarier. Oxygen consumption is an important feature in lakes, causing oxygen to become depleted (called anoxia) in many lakes deep enough to stratify, at which point oxygen can’t rapidly move downward from upper waters and decomposition gradually removes  oxygen from the bottom up to the boundary point, called the thermocline. Fish like trout that need cold water (<21 oC) but high oxygen (>5 mg/L) can get “squeezed”, faced with water too warm above and water with too little oxygen below. There may be no “trout water” during late summer, causing mortality. Further, loss of oxygen in deep water can allow phosphorus bound to iron to be released into the water column where it can support algae blooms, most often cyanobacteria that are favored by this type of release. In shallow water, high oxygen demand is indicative of elevated decomposition, and while complete loss of oxygen in water <15 ft deep is rare, that decomposition releases phosphorus that can fuel algae blooms. So oxygen consumption matters a lot to lake condition.

Data from Long Pond in Brewster and Harwich on Cape Cod, collected by the Natural Resources Department as part of a very useful water quality monitoring program, were plotted in an effort to understand the variation in oxygen consumption observed over time. What was found was that a relatively small difference in temperature, brought on by warmer spring air temperatures, resulted in a major increases in the rate of oxygen consumption, or oxygen demand (see figure). Oxygen demand below about 550 mg/m2/day is considered unlikely to cause severe anoxia, while values higher than 1000 mg/m2/day usually cause most of the bottom water layer to become anoxic in August and values greater than 2000 mg/m2/day will typically cause anoxia in July. Long Pond has an oxygen depletion problem, and it worsens appreciably with increasing temperatures in the deepest water.

Using a statistical technique called regression, the portion of the variability attributable to any tested factor can be evaluated. For Long Pond, and probably many other lakes, change in temperature explains much of the variation in oxygen demand (62% for Long Pond, a high percentage for a single factor). And this doesn’t require a large change either; the range of oxygen demand in Long Pond more than doubles for a temperature increase of only 3 Co (5.4 Fo)! The influence of the current direction of climate change is pushing our lakes toward a higher metabolism, almost like they have a fever, and the implications for all users, human or otherwise, are not good.

Oxygen demand as a function of temperature in Long Pond, MA.

 

Paper on aluminum treatments on Cape Cod approved for publication

A peer-reviewed article covering a dozen treatments of Cape Cod, MA lakes for control of internal phosphorus loading has been approved for publication in Lake and Reservoir Management. The paper is likely to appear in the June issue, but will be available online before then. Entitled “Aluminum Treatments to Control Internal Phosphorus Loading in Lakes on Cape Cod, Massachusetts”, it is authored by Kenneth J. Wagner, Dominic Meringolo, David F. Mitchell, Elizabeth Moran and Spence Smith and details projects from 1995 through 2016 where lakes and the underlying sediment were treated with aluminum sulfate and sodium aluminate to bind up surficial phosphorus bound to iron. That iron-bound phosphorus can be released when oxygen levels are low, as is usually the case in stratified lakes in New England. This internal source of phosphorus is often enough to support algae blooms, and comes with a low ratio of nitrogen to phosphorus, which tends to favor cyanobacteria. Consequently, inactivation of iron-bound phosphorus has been found to eliminate cyanobacterial blooms and increase water clarity in kettlehole lakes on Cape Cod.

Kettlehole lakes have limited surface inflow and depend on precipitation and groundwater as water sources. While the watershed is still important to lake condition over many years, it is that long-term accumulation of available phosphorus in the bottom sediments that drives conditions within any year. The review of treatments over the past two decades suggests some variability in results, but a positive impact on water quality overall for years after treatment. Two lakes have now been treated twice, one after about a decade and the other after 20 years. Surface phosphorus concentrations were reduced by 61% on average, while bottom phosphorus decreased by 84%. Chlorophyll-a, a pigment indicative of algae abundance, declined on average by 81%. These decreases translated into an increase in water clarity of 136% (more than a doubling from an average of 1.9 to 4.4 m) and a decrease in oxygen demand in deep water of 61%. Greater water clarity and more oxygen in deeper water were good for fish and other aquatic organisms as well as benefiting people with regard to water supply and recreation.

Aluminum application in process at Cliff Pond, Brewster, MA