Why do Alum Treatments “Fail”?

Generally, the most common or root cause of failures is Under-dosing.

“Failure” Mechanisms

An alum treatment can be considered to have failed when there is evidence that internal loading from lake sediments has not been controlled to a significant extent. This would be evidenced by hypolimnetic phosphorus concentrations elevated to levels seen before treatment. The loading source must be distinguished from the external load. Treatments can and do lose effectiveness over time, and this must be considered normal.

• Under-Dosing. Under-dosing is probably the most common cause of ineffective alum treatments. Several potential failure mechanisms would be corrected by higher dosing. Dosing is covered in more detail below.
• Sinking / Mixing of Alum Floc in Lake Sediment. In lakes with loose or “fluffy” sediments, alum concentrated at the surface of the sediments may sink or be mixed with the sediments diluting it and allowing free phosphorus to be released. Current dosing recommendations account for this.
• Clarity-Induced Changes to Lake Ecology. There is a potential for the water clarity caused by alum to induce changes in lake ecology. One such concept has curly-leaf pond weed (a non-native, invasive species) increasing in coverage due to deeper light penetration. When it dies and seneces around July 1, it releases its own phosphorus (taken from the sediment) and causes oxygen depletion in littoral areas to release additional phosphorus. Effective alum dosing can limit, but not eliminate, the effect.
• Non-Chemical Internal Load Mechanisms. Bottom-feeding rough fish and wind and wave action can cause internal loads not affected by alum. Lakes with extensive shallow areas may be susceptible. Alum treatment is generally limited to areas greater that five feet in depth, and wind may move alum to deeper areas. Untreated shallow areas can yield substantial internal loads due to intermittent stratification and oxygen depletion.
• Application Problems. Poor application control could cause diminished results, but documentation of such cases does not apparently exist. It is doubtful that poor application would cause failure of a correctly dosed treatment.
• External Loads Not Controlled. If substantial external loads are not controlled, lake response to alum treatment may be limited. Failure or success of an alum treatment should be considered on the basis of phosphorus export from the sediments, not surface water quality which reflects internal and external loading.

History of Alum Dosing

Alum dosing has trended toward higher doses over the past eight years. Data from MPCA (attached) indicate the trend in Minnesota applications. There has been a general tendency not to dose to the maximum extent possible. Current recommendations such as by Rydin and Welch (1999) are likely to yield the highest dosing rates.

Maximum Dosing Based on Aluminum Toxicity in Fish. Aluminum can become toxic when water column pH drops reaches 5.5. Calculations of buffering capacity yielded the maximum dose that is not toxic to fish. This dose has rarely been applied in Minnesota. Langdon Lake is one example that was dosed to that level, though pH was not depressed during the application. This method is at least 17 years old.

Reduced Dose Based on Internal Load Estimates. In Minnesota, most doses have been reduced from the maximum determined from toxicity limits. The basis became the amount needed to intercept and bind the internal load amount for a number of years, such as ten or twelve, along with a safety factor. This can often underestimate the needed long-term dose because aluminum is bound up with constituents other than phosphorus. Also, the internal load is a product of the total amount of phosphorus available in the sediment, so that the sediment phosphorus pool replaces that bound by aluminum.

Dosing as by Rydin and Welch (1999). Eugene Welch, an expert in lake alum treatments, says that most alum treatments have been under-dosed. Rydin and Welch have determined an alternative dosing method, based on the “free-phosphorus” in the upper layer of sediment. This requires a special analysis of sediment cores from the subject lake. The method still requires a large factor of safety, and will generally will result in the highest dosing rates of any method. This method is probably the most scientifically based because it is measured on the source of the loading.

MnDNR and MPCA Permitting. MnDNR permits alum treatments in Minnesota. Alum treatments have been permitted in the past with the understanding each lake would have only one opportunity to receive an alum treatment. Because previous dosing methods have yielded lower alum amounts, MnDNR is apparently prepared to permit re-treatments on the basis that the earlier doses only represented partial doses.

Recommendation

• The Board should consider alum treatment as an effective means for controlling internal phosphorus loading in lakes.
• The Rydin and Welch (1999) method should be employed in determining future alum dose rates for MCWD lakes.

Local Water Quality Alum Treatment Information

Water Body	County	Year of Treatment	Treatment Rate	Application Method	Total Dose
			(g Al/m2)		(lbs Al)
Long Lake	Hennepin	1996	30	surface	60,000
Cedar Lake	Hennepin	1996	27	surface	35,000
Lake of the Isles	Hennepin	1996	18	surface	10,000
Lake Susan	Carver	1998	30	surface	19,600
Centerville Lake	Anoka	1998	18	surface	57,600
Lake St. Clair	Becker	1998	37	surface	40,600
Calhoun	Hennepin	2001	42	At 10 ft depth	122,000
Harriet	Hennepin	2001	32	At 5 ft depth	34,000
Clifford	Douglas	2002	50	Surface	81,000
Faille	Douglas	2002	30	Surface	21,000
Langdon Lake	Hennepin	1998	70	surface	48,000