Hypolimnetic oxygenation and aeration in two Midwestern USA reservoirs

Figures & data

Table 1. Summary of reservoir management actions.

Action^a	Pleasant Lake^b	Vadnais Lake^c
Baseline study	1984–1986
Ferric chloride injection Mississippi River pump station^d	1987–present
Ferric chloride injection into watershed inflow^d	NA	1991–present
Hypolimnetic aeration^e, f	1995–2006^g	1987–2010^h
Hypolimnetic oxygenation	2014–2018	2012–2018^I

^aFor detailed site information and project background, see Walker et al. (1989), Walker (2000), Engstrom (2005), Cook (2009).

^bLocation: Ramsey County, MN 45^º05'41''N, 93^º05'58''W.

^cBathymetry available online:

^dhttps://www.dnr.state.mn.us/lakefind/showmap.html?mapid=C0500&downum=62004600.

^eLocation: Ramsey County, MN 45^º03'09''N, 93^º05'29''W.

^fBathymetry available online: https://www.dnr.state.mn.us/lakefind/showmap.html?mapid=B0486&downum=62003801.

^g.Iron injection using ferric chloride in all locations was based on a nominal dose concentration of 500 μg Fe/L (Walker et al. 1989).

^hFerric chloride injection into Vadnais Lake hypolimnion started in 1988.

ⁱPartial lift hypolimnetic aeration.

^jPleasant Lake hypolimnetic aeration failed in late 2006. No aeration 2007–2013.

^kHypolimnetic aeration removed and replaced in 2011.

^lFerric chloride injection continued.

Figure 1. Cut-away schematic of partial-lift hypolimnetic aerators. Air was injected into the bottom of the draft tube, which was open to the lake, creating an air-lift pump. Entrained water flowed downward and out through ports radially distributed at the bottom (12 total). Air is vented out of the top. Aerators were buoyant and anchored to the bottom. Draft tube length was approximately 9 m.

Figure 2. Hypolimnetic oxygenation diffuser placement montage in Vadnais. Diffusers run off the vaporization pressure of liquid oxygen. After manufacture at the lake side, diffusers were placed over deep holes and sunk to position by flooding the ballast pipe. (Photo credit: Jacobs. Schematic: Mobley Engineering, Inc.).

Table 2. Method summary.

Parameter	Method description	Note
Chlorophyll-a	Spectrophotometric: Standard Method 10200 H
Total Fe, total Mn	Microwave digestion: EPA 3052 followed by atomic absorption spectrometry by flame: Standard Method 3111	Past to 2016
Total Fe, total Mn	Microwave digestion: EPA 3052 followed by inductively coupled plasma mass spectrometry (KED mode): EPA 200.8	Starting in 2017
TP	Microwave digestion: EPA 3015A followed by flow injection analysis: EPA 365.1
DO probe	Hydrolab Quanta, saturated air method	Since 2010
ORP	Hydrolab Quanta combined pH/ORP, platinum electrode, KCl buffer solution combined pH/ORP calibration	Since 2010

Figure 3. Temperature isopleths for Vadnais (left) and Pleasant (right). Dots are data points. The bottom meter is truncated from plots to avoid interpolation errors from missing data points. Pleasant hypolimnetic mean temperature is warmer than for Vadnais because of Mississippi River inflows.

Figure 4. Pleasant partial destratification under first year gas flux rate.

Table 3. Summer mean daily oxygen delivery rates. Normal cubic meters per hour (Nm³/h) is converted directly from standard cubic feet per minute (scfm).

Year	Jun–Aug mean daily O2, kg/d		Jun–Aug mean daily O2, Nm^3/h
	Pleasant	Vadnais	Pleasant	Vadnais
2012	NA	1063	NA	31
2013	NA	1200	NA	35
2014	3327	1200	97	35
2015	1886	892	55	26
2016	1681	480	49	14
2017	1886	1097	55	32
2018	1646	857	48	25

Figure 5. Pleasant (left) and Vadnais (right) hypolimnetic DO (mg/L) at 10–15 m depth, Jun–Aug. Note that Vadnais had no HA nor HO in 2011.

Figure 6. Pleasant (left) and Vadnais (right) hypolimnetic ORP (mV) at 10–15 m depth, Jun–Aug. Note that aerators were removed from Vadnais in May 2011. HO was commissioned in October 2011.

Figure 7. Pleasant (left) and Vadnais (right) summer (Jun–Sep) hypolimnetic total Fe.

Figure 8. Pleasant (left) and Vadnais (right) summer (Jun–Sep) hypolimnetic total Mn.

Figure 9. Pleasant (left) and Vadnais (right) summer (Jun–Sep) hypolimnetic total phosphorus.

Figure 10. Summer (Jun–Sep) surface TP distribution in Pleasant (left) and Vadnais (right).

Figure 11. Summer (Jun–Sep) chlorophyll-a distribution in Pleasant (left) and Vadnais (right).

Figure 12. Distribution of trophic state indices 1984–2018 in Pleasant (left) and Vadnais (right), Jun–Sep values.

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