Utah Lake, part 4: Nutrient control

 

algae

Part 4 of a series on Utah Lake. The following observations and insights concerning Utah Lake have been developed during my nearly 50 years in Utah Valley as a professor, researcher, environmental engineer, consultant, and local citizen.


Concerns and issues related to nutrients as water pollutants have been increasing for some 60–70 years. The Clean Water Act of 1972 identified nutrients as polluting constituents to be addressed. Currently the Utah Division of Water Quality is moving ahead with plans to implement phosphorus and nitrogen removal regulations for wastewater treatment plants. However, many people are questioning and even contesting these plans based mainly on a belief that, in many cases, little, if any, benefits will be gained, coupled with concern over the huge costs of nutrient removal.

Primer on Plant Growth in Lakes

To grow well, photosynthetic plants need an adequate supply of nutrients and favorable environmental conditions. A common way to evaluate plant growth is to identify the main “factors” needed for growth and look for the one, or more, that is actually the limiting factor. To decrease growth, reduce the limiting factor even more; or to increase growth, supply more of it. For plant growth, a list of possible limiting factors includes light, temperature, carbon dioxide, phosphorus, nitrogen, other trace nutrients, inhibiting chemicals, and rate of loss (grazing, destruction, detritus, etc).

For water quality management purposes, aquatic plants might be grouped in two classes: macrophytes and algae. In lakes, the macrophytes most commonly significant are the rooted, vascular plants that grow from the bottom—in temperate climates growing from depths of up to some 15–20 feet. These macrophytes draw most of their nutrients from bottom sediments, where nutrients are normally abundant.

Lake algae are mostly tiny floating plants. They get their nutrients directly from the water. An abundant growth of algae provides a healthy primary base of the food chain in aquatic ecosystems. However, when excessive algae grows, a variety of water quality and aquatic habitat problems may occur, e.g., floating scums, windrows of algae, turbid water, increased pest and parasitic insects, bad odors, oxygen depletion as organic debris decomposes, and residual bad tastes and odors. In general, excessive algae growth causes a shift towards a more “swampy” condition.

In water quality management, the “fertilizers,” phosphorus and nitrogen, receive special attention since in most lakes they are the overall limiting factors that most determine the annual amount of algae produced. Even then, other factors limit growth some of the time, but these nutrients determine the amount of growth in most lakes. Of these two nutrients, phosphorus tends to be the more limiting in wet climates and nitrogen more limiting in arid climates. But the many and complex factors driving aquatic plant growth, with all of their seasonal and annual variations in each particular lake and for different parts of a lake, make evaluation and prediction of eutrophic level and water quality problems challenging and difficult.

The concept of trophic level is often used for classification of lake productivity:

  • oligotrophic—low productivity, usually having clear water;
  • mesotrophic—medium productivity, usually having slightly turbid water;
  • eutrophic—high productivity, usually having turbid water; and
  • hyper-eutrophic, very high productivity, usually having very turbid water.

Turbidity as used here is that resulting from the algal growth itself and the accompanying aquatic life and biological activity.

In trophic-level evaluation, two approaches are common. One is the trophic level as measured by the actual biological productivity, e.g., grams of biomass per square meter of surface area, or similar. The other uses the annual loading of phosphorus and nitrogen to predict the expected trophic level, if, in fact, either of these nutrients is the overall growth limiting factor. Of course, the measured productivity indicates what is really happening in the lake, but the predicted level is particularly valuable in estimating responses to changes in nutrient loading. However, a serious pitfall must be watched for and avoided—these predictions are erroneous if nutrients are not the limiting factors.

Nutrient Removal in Eutrophic Utah Lake?

Most experts would classify Utah Lake as eutrophic, edging toward hyper-eutrophic, in its actual productivity and ultra-hyper-eutrophic in its nutrient loadings—they are 10 to 15 times the amounts needed to support a eutrophic level of actual productivity. One might be initially alarmed by such high nutrient loadings—thinking they must certainly doom the lake to massive algae growth and widespread quality problems. However, scrutiny of the lake’s chemical characteristics and algae-growth dynamics indicates that the large nutrient loading is likely a moot point since it appears that algal growth is usually light-limited due to the lake’s natural turbidity. In addition, natural chemical reaction equilibria usually reduce available phosphorus to relatively low levels. This mineral co-precipitation of phosphorus with calcium, carbonate, silica, and other trace minerals removes huge amounts of phosphorus in Utah Lake. That is, most of the phosphorus is being naturally precipitated to the sediments where it is largely unavailable to algae.

Thus high loadings of phosphorus, nitrogen, and other trace nutrients to Utah Lake are of little pertinent concern since the overall loading doesn’t determine the amount of algal growth—rather growth is largely controlled by low light availability resulting from the turbid water—note again that this turbidity is largely from precipitated mineral sediments, with a smaller contribution from biological turbidity. If a “clear” lake had such high nutrient loadings, it would have a high probability of ongoing massive algal blooms—and it wouldn’t be clear any more. Often, conditions in Utah Lake are such (calm, warm conditions) that sufficient light penetration and adequate phosphorus and nitrogen are available to support rapid algae growth. These rapid growth episodes tend to be terminated, before they become a large bloom, by the return of wind and wave action that generates high turbidity that again limits the growth. Occasionally, before high mineral turbidity returns or phosphorus becomes limiting, nitrogen becomes limiting and undesirable, nitrogen-fixing, blue-green algae (actually cyanobacteria) dominate and produce blue-green blooms which sometimes result in transitory high levels of cyanotoxins in the water.

To summarize, Utah Lake is strongly eutrophic as to biological productivity, while experiencing ultra-hyper-eutrophic nutrient loadings. Another way of describing this condition is that, overall, phosphorus and nitrogen are not the limiting factors to algae growth in the lake.

For additional perspective, let’s assume that some still insist on trying to reduce nutrient loadings to Utah Lake to limiting levels with the postulated goal of lower actual productivity and cleaner/clearer water. To achieve limiting loadings—to make nutrients limiting at the existing eutrophic productivity level—would require removal of over 90 % of nutrients currently going into the lake. In addition to extensive, disruptive and costly agriculture and land use control programs in the drainage basin, advanced wastewater treatment systems for nutrient removal would cost hundreds of millions of dollars in facility costs and tens of millions per year in ongoing operating costs. Some proponents maintain that since we “don’t really know” whether wastewater nutrient removal might produce significant benefits, we should initially just use less-expensive treatment facility upgrades and modifications to remove perhaps 50%–80% of nutrients; and if that doesn’t reduce algae growth enough then, in a second effort, add more extensive, and much more expensive, facilities.

This seems double folly since only about 70% of the lake’s total nutrient loading comes from wastewater discharges. The science and math involved tell us that even 100% removal of wastewater nutrients would not reduce overall lake nutrient loadings enough to make them limiting. In fact, add in extensive watershed nutrient-control programs using best available technology and best management practices, at a staggering cost, and it is still essentially impossible that lake nutrient inputs could be reduced by the 90%–95% needed to become limiting at the lake’s natural eutrophic level. Restated: It is highly probable that the lake would be essentially the same quality as now even if every nutrient source were reduced to the highest degree possible—at a cost of many hundreds of millions of dollars. But even then, there remain major issues. Would phosphorus releases from the phosphorus-rich bottom sediments provide an abundance of phosphorus, and nitrogen removal cause a shift to increased toxic blue-green blooms?

Costs

Nutrient-control effectiveness and benefit–cost issues are major issues in this case. Advanced wastewater treatment is very expensive compared to conventional treatment. E.g., for Utah Valley, costs for advanced wastewater treatment to remove about 90% of sewage phosphorus would be, perhaps, $200 million for facility construction costs plus additional tens of millions each year in operating and maintenance costs—resulting in costs for wastewater treatment much higher than current levels. If comparable nitrogen removal were added, costs might nearly double again. Overall these costs are staggering, particularly in this case where benefits would likely be small or even nonexistent. Some people argue that we should do it because “it might help the lake,” in spite of the staggering costs and low probability of significant benefits. Is not proliferation of this kind of thinking a major contributor to current financial chaos across the nation and around the world? Rational and wise thinking would have us be rather certain that removal would be effective and that the benefits are at least somewhat in line with the costs, before undertaking nutrient removal projects.

Effect on Jordan River Nutrient Levels

With respect to nutrient levels in the Jordan River, high biological uptake and the chemically binding nature of the lake’s chemistry and sediments combine to make it an effective natural nutrient removal system that reduces nutrient concentrations to moderate levels before water flows into the Jordan River. It is doubtful that even extreme reductions in nutrient loadings to Utah Lake would result in significant changes in nutrient levels in the Jordan River, since residual concentrations are largely a function of lake biological activity and chemical equilibria and not the amount of nutrients coming into the lake.

In summary, the huge nutrient loadings to Utah Lake are likely a moot factor as to its overall algae growth and water quality, since growth-limiting natural turbidity is widespread and persistent. Also, chemically binding reactions and precipitation of phosphorus are in play to make phosphorus limiting at times, but even then concentrations of available phosphorus in the water are mainly dependent on equilibrium chemistry, and very little on the amount of phosphorus coming into the lake. The growth of nasty, sometimes toxic, blue-green algae is mostly associated with a transitory shortage of available nitrogen—since blue-green algae can get (fix) nitrogen from dissolved air, they out-grow other algae, sometimes causing a blue-green bloom that, upon decay, may cause toxic conditions for a short time. But this is a natural condition that occurs in this naturally eutrophic lake.

LaVere B. Merritt

LaVere B. Merritt is a professor emeritus of civil and environmental engineering at Brigham Young University. His research and public service have included many multidisciplinary studies on Utah Lake. He served as member and then chair of the Provo Metropolitan Water Board for many years and is a consultant to both public and private entities on Utah Lake matters.

2 Comments:

  1. Stephen Sowby, P.E.

    I agree totally with Dr. Merritt. Higher or more nutrient controls on WWTP’s discharging to Utah Lake are useless, costly, not needed, and would completely betray the science of water quality. The science and math does not support further nutrient controls in any way for Utah Lake. They may be warranted in other Utah streams and reservoirs where the water quality investigations, nutrient loads, science, and cost/benefit analysis supports these actions.

  2. Hello.This post was really remarkable, particularly because I was browsing for thoughts on this topic last Saturday.

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