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

2016 Drought Highlights: Role of watersheds in lake condition

Looking down view tube at Secchi disk

Precipitation and flows were well below normal in spring and summer of 2016. For example, at Morses Pond in Wellesley, MA, there was no winter snow pack to speak of and precipitation in May through August was about half of the average for the previous decade (7.2 vs 14.2 inches). The situation was similar all over New England, and while evaporation exceeded precipitation during summer and caused low water levels, the reduction of nutrient inputs also resulted in high (sometimes record) water clarity. Blooms of algae were less common in lakes that are tightly linked to watershed inputs on a seasonal basis, which includes most impoundments on streams and river and other lakes with watersheds more than about 20 times the area of the lake. Phosphorus and chlorophyll-a concentrations were lower than average for a respective 72 and 80% of lakes surveyed by LEA in Maine, leading to Secchi transparency values higher than average for 72% of surveyed lakes. Unless internal recycling is the dominant source of phosphorus to a lake, reduced precipitation translates into less runoff, lower nutrient inputs, and higher water clarity.

The importance of a watershed to lake condition is clearly demonstrated, but that importance is mediated through two key processes: weather pattern and land management. In 2016 the weather did a lot of what we strive to do with land management, minimizing the transport of nutrients and other contaminants to lakes. We can’t control the weather, and having less water entering our lakes has its downsides (e.g., lower water levels, more impact from rooted plants), but the importance of watershed management to minimize nutrient inputs when the weather is not cooperating is underscored. If we can’t put a dome over our watershed and only open the roof when we want the water, we have to manage the watershed to limit inputs to the lake.

But what is the potential for watershed management to provide the benefits observed in 2016 as a result of low precipitation? The better than average conditions were associated with precipitation about 50% below normal. Nutrient loading is not necessarily proportional to water inputs, and we would expect disproportionately more loading with larger storms, but it seems reasonable to assume that we would need at least a 50% reduction of loading through watershed management to reap the same benefits provided by the 2016 weather pattern. Based on years of evaluation by the USEPA, phosphorus removal by best management practices rarely averages more than 50%, although well designed infiltration facilities can achieve 90% reductions. However, not all watershed soils are suitable for infiltration systems, so what all this means functionally is that we will be hard pressed to provide the level of watershed management necessary to maintain the conditions we observed in 2016.

We can view 2016 as setting the bar for potential lake condition with regard to nutrients, algae and water clarity. Low precipitation limited inputs, and while there were some negative effects of having less water, the water quality was about as high as could be expected in New England lakes. If your lake was not appreciably better than in other recent years, internal loading sources were most likely dominant or there is another source (e.g., direct discharge or extensive storm water piping that limits load reduction on the way to the lake) that requires attention. Yet for those lakes that did exhibit better than average conditions in 2016, maintaining those conditions by watershed management will require superior effort, as the practical limits to best management practices will necessitate application all over the watershed to achieve the level of loading reduction experienced in 2016.