Effectiveness of alum in a hypereutrophic lake with substantial external loading

Figures & data

Table 1. Total phosphorus loading to Lake Ketchum, October 2010–October 2011.

Source	Total phosphorus inflows (kg)	% Phosphorus load
Internal (sediments)	455	73.0%
Inlet stream	146	23.4%
Surface runoff	13	2.1%
Groundwater (GW)	7	1.2%
Direct precipitation	2	0.3%
Total	623 (6414 mg/m^2 per year)	100%

Figure 1. Modeled vs. observed whole-lake volume weighted TP concentrations in Lake Ketchum, WA, Oct 2010–Oct 2011.

Figure 2. Observed phosphorus concentrations at 1 m, 5 m and in the main inlet of Lake Ketchum, WA, from Oct 2010 to Oct 2011.

Table 2. Summer (May–Oct) mean epilimnion TP concentrations (µg/L) as predicted by a mass balance model for various lake treatment alternatives in Lake Ketchum.

Treatment scenario	Pre	Year 1	Year 2	Year 3	Year 4
Observed	181	—	—	—	—
Modeled existing conditions	—	175	177	178	178
Alternative 1: Whole-lake sediment inactivation alum treatment	—	53	79	80	80
Alternative 2: Whole-lake sediment inactivation alum treatment repeated in year 3	—	53	79	38	64
Alternative 3: Whole-lake sediment inactivation alum treatment with alum injection on inlet stream	—	53	37	37	37
Alternative 4: Whole-lake sediment inactivation alum treatment with annual water column alum treatments	—	46	40	40	40

Table 3. Cost comparison of treatment alternatives for Lake Ketchum evaluated by the mass balance model (Burghdoff and Williams 2012).

	Cost in $1000s (2011 dollars)
Lake restoration alternative	Year 1	Year 2	Year 3	Year 4	4-Year Total	10-Year Total
Alternative 1: Whole-lake sediment inactivation alum treatment	$194	0	0	0	$194	$194^a
Alternative 2: Whole-lake sediment inactivation alum treatment repeated in year 3	$194	0	$174	0	$368	$890^b
Alternative 3: Whole-lake sediment inactivation alum treatment with alum injection on inlet stream	$428	$43	$65	$65	$601	$991^c
Alternative 4: Whole-lake sediment inactivation alum treatment with annual water column alum treatments	$230	$36	$36	$36	$338	$692^d

^a No repeated treatments in first 10 years.

^b Sediment treatments repeated every 2 years.

^c Sediment treatment will not need to be repeated until after 10 years.

^d Sediment treatment will need to be repeated in Year 6.

Table 4. Summer (May–Oct) mean nutrient and trophic state indicators before and after 2 successive alum treatments (µg/L) in Lake Ketchum. Inlet stream nutrient concentrations are annual means.

	Pre-treatment		Post treatment
Parameter	2011	2013	2014	2015	2016
TP – 1 m	181	289	34	15	16
TP – 5 m	2667	1427	186	17	23
SRP – 1 m	29	—	3	2	1
SRP – 5 m	2165	1235	83	1	2
TN – 1 m	1240^a	—	1166	663	552
TN – 5 m	4620^a	—	1639	698	752
TN:TP ratio	6.9	—	34.1	43.8	35.6
Chl – 1 m	81	56	45	17	4
Microcystin	0.94–551	0.54–539	16.1–675	n/a	n/a
Secchi (m)	1.0	1.7	2.1	4.0	3.5
TP - Inlet	557	474	458	456	385
SRP - Inlet	474	404	341	365	296

^a Total N calculated using the sum of TKN and NO₃ + NO₂.

Figure 3. Observed pre and post alum treatment 5 m phosphorus concentrations in Lake Ketchum, WA, Mar 2013–Oct 2016.

Figure 4. Observed pre and post alum treatment 1 m phosphorus concentrations in Lake Ketchum, WA, and observed main inlet phosphorus concentrations from Mar 2013–Oct 2016.

Burghdoff M, Williams G. 2012. Lake Ketchum algae control plan and technical appendix. Snohomish (WA): Public Works Department, Snohomish County Surface Water Management Division. Prepared with the assistance of Tetra Tech, Inc.

 Google Scholar