RESTORATION AND MANAGEMENT OF LAKES AND RESERVOIRS - CHAPTER 7 pps

7 Hypolimnetic Withdrawal7.1 INTRODUCTION The hypolimnetic withdrawal technique involves changing the depth at which water leaves the lake from the surface to near the maximum depth, so

7 Hypolimnetic Withdrawal

7.1 INTRODUCTION

The hypolimnetic withdrawal technique involves changing the depth at which water leaves the lake from the surface to near the maximum depth, so that nutrient-rich, rather than low-nutrient surface water is discharged Coincidentally, the hypolimnion detention time is shortened, the chance for anaerobic conditions to develop is decreased and the availability of nutrients to the epilimnion, through entrainment and diffusion, is reduced The technique is accomplished by installing a pipe along the lake bottom from near the deepest point to the outlet, and possibly beyond The outlet pipe is usually situated below lake level, so the device acts as a siphon It was named an “Olszewski tube” after its original user (Olszewski, 1961), but the technique itself is more commonly referred

to as hypolimnetic withdrawal This technique is applicable to stratified lakes and small reservoirs

in which anaerobic hypolimnia restrict the habitat for fish and promote the release of P, toxic metals, ammonia, and hydrogen sulfide from sediments

There are two important requirements for treatment success: (1) the lake level must remain relatively constant, and (2) thermal stability should not change While stratification may be weak-ened because epilimnetic water tends to be drawn downward, destratification will not occur provided the removal rate of hypolimnetic water is relatively slow Destratification should be avoided, because

it increases the transport of hypolimnetic nutrients and anoxic water to the epilimnion Polymictic lakes may not be good prospects for withdrawal To lessen the chances of destratification, directing inlet water to the metalimnion or hypolimnion may be possible This modification was installed in Lake Ballinger near Seattle (Figure 7.1) While the lake remained stratified with the inflow water directed to depth, the system was not tested without directed flow, for comparison Destratification has not generally been a problem with the technique Thermocline depth remained about the same

in seven of nine cases examined by Nürnberg (1987) Lowered lake level and thus head loss, will hamper recovery by reducing hypolimnetic water and P export (Livingstone and Schanz, 1994; Dunalska et al., 2001)

Preferentially removing hypolimnetic water, and therefore decreasing the residence time of the hypolimnion, should decrease the period of anoxia and increase the depth of the anoxic boundary resulting in a decrease in internal loading of P In a large fraction of cases, this has occurred (Nürnberg, 1987) Continued P export should ultimately reduce the sediment P pool Hypolimnetic withdrawal is an obvious and proven alternative, with relatively low cost, to accelerate recovery in stratified lakes where little improvement has followed wastewater diversion or wastewater P removal because of high internal loading There have been few new cases reported since the second edition

of this book, so most of the following remains relatively unchanged

The technique is employed inadvertently in reservoirs where hypolimnetic waters are normally discharged for power generation However, that procedure has not been evaluated for benefits to water quality in the reservoir itself Low DO content of discharged water has historically been a major problem with deep-discharge impoundments Multiple and shallower outlets have been incorporated into reservoir design to counteract low DO discharge Reducing discharge depth minimizes nutrient export Some combination of deep and shallow outlets may optimize the two goals of sufficient DO and high-nutrient export, but there is little mention of such a practice in the literature

FIGURE 7.1 Inlet and outlet structures designed for hypolimnetic withdrawal in Lake Ballinger (From KMC, 1981.)

Hall Creek

Lake Ballinger

Control weir

Pump Aerator McAleer Creek

Intake structure Epiliminion

Thermocline Metalimnion

Hypolimnion

7.2 TEST CASES

7.2.1 GENERAL TRENDS

Hypolimnetic withdrawal installation is documented in 21 lakes and 15 of those are in Europe (Björk, 1974; Nürnberg, 1987) Results are reported from 17 lakes (Nürnberg, 1987; Nürnberg et al., 1987) Four of the 21 lakes, two from the U.S and two from Canada, are either more recent

or not included by Nürnberg Morphometric and mixing characteristics of these lakes are shown

in Table 7.1 There are three other European lakes (Laacher and Lützel in Germany and Rudnickii Wielkie in Poland) with withdrawal systems cited by Dunalska et al., 2001) Ten additional cases were reported from Finland (Keto et al., 2004)

Internal loading from anoxic sediments during summer stratification occurred in all lakes prior

to withdrawal and in most cases, external loading was reduced Prior to withdrawal, Kleiner Montiggler See had been aerated with liquid oxygen and Reithersee was treated with iron chloride

to precipitate P followed by dredging (Nürnberg, 1987)

Withdrawal is initiated preferably after stratification, but before anoxic conditions occur The siphon pipe is located usually 1 to 2 m above the bottom at the greatest depth to maximize P transport (see Tables 7.1 and 7.2) In meromictic lakes however, it may be most effective to position the pipe above the monomolimnion so that it continues to be a sink for P If there are two basins, withdrawal from the shallowest basin may be more effective at reducing entrainment into the epilimnion (e.g., Lake Wononscopomuc, CT)

Hypolimnetic water withdrawal rates and consequent TP export and duration are shown in Table 7.2 These values varied among the lakes Years of withdrawal ranged from 1 to 10 for 20

of the listed lakes Sufficient data were available to estimate TP export and duration in only 11 of the 20 lakes The longest duration is for Kortowo, Poland, where the first withdrawal pipe was installed in 1956 Recent P budgets show 3.7 and 4.7 times more P exported from the lake than the inputs for 1999 and 2000 (Dunalaska et al., 2001)

Hypolimnetic and epilimnetic data were available on 12 lakes, but data were available for both

on only 10 lakes Maximum hypolimnetic TP concentration decreased in all 11 of 12 lakes and epilimnetic TP decreased in 8 of 12 where data were available The reduction in hypolimnetic TP

is a direct effect, but epilimnetic reduction in TP is an indirect effect demonstrating that entrainment

of P from hypolimnion to epilimnion was reduced The effect of withdrawal on epilimnetic TP was most significant as a function of grand total TP exported over the project life rather than annual export, whether expressed as total mass or per area (Figure 7.2; Nürnberg, 1987) The lakes involved

in this analysis were Burgaschi, Hecht, Kleiner Montiggler, Mauen, Meerfelder Maar, Piburger, Waramaug, and Wononscopomuc Lake Ballinger showed no change in epilimnetic TP due to increased external loading so it was not included

The longer withdrawal operated the greater was the proportional change in epilimnetic TP (Figure 7.3; Nürnberg, 1987) More data were available for this analysis The additional lakes besides those listed above for Figure 7.2 are Klopeiner, Kraiger and Wiler, although the latter was eliminated from the regression analysis due to high external P loading (Figure 7.3, open circle; Nürnberg, 1987) While substantial decrease in epilimnetic TP occurred in four lakes, on average, as long as

5 years may be necessary to see a significant decrease in epilimnetic TP (Figures 7.3) Recent data show a reduction of epilimnetic TP from 80 to 18 μg/L during 10 years of withdrawal in Lake Bled (Nürnberg and LaZerte, 2003)

The depth of hypolimnetic anoxia also decreased in 12 of 13 cases with adequate data, but that effect decreased as volume increased, and the days of anoxia decreased in 8 of 10 cases However, the reduction in anoxia could not be related to withdrawal rate or volume Thus, the case for lessened anoxia with withdrawal is not strong Thermocline position remained about the same in

8 of 10 cases and sank 2 to 3 m in 2 cases

TABLE 7.1

Morphometric Characteristics of Lakes Treated With Hypolimnetic Withdrawal

Lake

Watershed Area (10 3 m 2 )

Lake Area (10 3 m 2 )

Lake Volume (10 3 m 2 )

Water Res Time (yr)

Mean Depth (m)

Max.

Depth

Ballinger, Washington a 11,720 405 1,838 0.26 4.5 10.0 Monomictic Bled, Yugoslavia b NA 1438 25,690 3.6 17.9 30.2 Meromictic Burgäschi, Switzerland c 3,190 192 2,483 1.4 12.9 32.0 Meromictic Chain, British Columbia d — 460 2,760 0.5–3.0 6 9 Polymictic Devil’s, Wisconsin e 6,860 1,510 1,390 7.8* 9.2 14.3 Dimictic Germündener Maar,

W Germany f

430 75 1,330 8.0 17.7 39.0 Meromictic Hecht, Austria g 2,221 263 6,428 2.8 24.4 56.5 Meromictic Kleiner Montiggler, Italy h 1,252 52 518 NA 9.9 14.8 Meromictic Klopeiner, Austria i NA 1106 24,975 1.5 22.6 48.0 NA Kortowo, Poland j 1,020 901 5,293 NA 5.9 17.2 Dimictic Kraiger, Austria i NA 51 245 2.0 4.8 10.0 Dimictic Mauen, Switzerland k 4,300 510 1,989 0.6 3.9 6.8 Dimictic Meerfelder Maar,

W Germany l

1,270 248 2,270 4.5 9.2 18.0 Dimictic

de Paladru, France m 48,000 3900 97,000 4.0 25.0 35.0 Dimictic Piburger, Austria g,k 2640 134 1,835 1.9 13.7 24.6 Meromictic Pine, Alberta n 157,070 4,125 24,088 9.0 5.3 13.2 Dimictic Reither, Austria g,o NA 15 67 0.3 4.5 8.2 Dimictic Stubenberg, Austria i NA 450 NA NA NA 8.0 Polymictic Waramaug, Connecticut p 37,000 2,866 24,758 0.8 8.6 12.8 Dimictic Wiler, Switzerland i,q 257 31 325 1.0 10.0 20.5 NA Wononscopomuc,

Connecticut r

5994 1400 15,500 4.0 11.1 32.9 Dimictic

Source: From Nürnberg, G.K 1987 J Environ Eng 113, with additions With permission.

Data sources: a KCM 1981 Lake Ballinger Restoration Project Interim Monitoring Study Report; KCM 1986 Restoration

of Lake Ballinger: Phase III Final Report Kramer, Chin, and Mayo, Seattle, WA bVrhovsek, D et al 1985 Hydrobiologia 127; Nürnberg, G.K and B.D LaZerte 2003 Lake and Reservoir Manage 19 c Ambühl, H., personal communication.

dMcDonald, R.H et al 2004 Lake and Reservoir Manage 20 e Lathrop, R.C personal communication f Scharf, B.W 1983.

Beitrage Landespflege Reinland-Pfalz 9 gPechlaner, R 1978 Osterreichische Wasserwirtsch 30 h Thaler, B and D Tait.

1981 Tatigkeitsbericht des Biologischen Landeslabors autonome Provinz Bozen 2 i Hamm, A and V Kucklentz 1981.

Materialien der Bayrischen Landesanstalt fur Wasserforschung, Munchen, FDR, 15 jOlszewski, P 1961 Verh Int Verein.

Limnol 14; 1973 Verh Int Verein Limnol 18 kGächter, R 1976 Schweiz Z Hydrol 38 lScharf, B.W 1984 Natur und

Landschaft 59 mLascombe, C and J De Beneditis 1984 Verh Int Verein Limnol 22 nSosiak, A 2002 Initial Results of

the Pine Lake Restoration Program Alberta Environment, Edmonton, Alberta oPechlaner, R 1975 Verh Int Verein Limnol 19; 1979 Arch Hydrobiol Suppl 13 pNürnberg, G.K 1987 J Environ Eng 113 qEschmann, K.H 1969

Gesundheit-stechnik Zurich 3 rKortmann, R.W et al 1983 In: Lake Restoration, Protection and Management USEPA-440/5-83-001; Nürnberg, G.K et al 1987 Water Res 21; NA, not available.

TABLE 7.2

Specific Characteristics of Withdrawal Systems

Lake

Pipe Depth (m)

Withdrawal

Diameter b (cm)

Pipe Outflow c (m)

Annual TP Export (kg)

Duration (yr)

Volume (10 3 m 3 /yr)

Rate (m 3 /min)

Burgäschi 15.0 1000 3.0 33.0 0.5 147.1 5.0

Kleiner Montiggler d 13.0 16 NA NA –0.5 16.0 1.0

Meerfelder Maar 16.0 190 0.6 30.0 1.2 40.0 1.5

Waramaug a 8.5 1330 6.3 31.8 0.0 131.9 3.0

Wononscopomuc a 15.1 201 0.9 NA 0.0 21.0 5.0

a Active pumping.

b Inner diameter.

c Below lake level — or above for negative values.

d Operation only during spring.

Source: From Nürnberg, G.K 1987 J Environ Eng 113, with additions (with permission); NA, not available.

FIGURE 7.2 Changes in epilimnetic TP concentrations (after – before) vs grand total TP export via

hypolim-netic withdrawal (calculated as annual export multiplied by years of operation): Regression line is shown, y =

46 − 30 log x, n = 8, r2 = 0.75 (From Nürnberg, G.K 1987 J Environ Eng 113 With permission.)

1 )

0

− 20

− 40

− 60

Grand total TP export (kg)

7.2.1 SPECIFIC CASES

7.2.1.1 Mauen See

This is one of the most successful cases of hypolimnetic withdrawal (Gächter, 1976) An Olszewski tube was installed in 1968 in this Swiss lake at a depth of 6.5 m (Table 7.1) Prior to installation, external P loading was reduced from about 700 to 300 mg/m2 per yr (Nürnberg, 1987) The discharge

of 4 m3/min provided a hypolimnetic (> 4 m) water residence time of 0.2 years Marked improve-ment in lake quality followed installation Hypolimnetic DO and Secchi visibility increased and hypolimnetic TP decreased by 1,500 μg/L, the most of any lake examined (Nürnberg, 1987) Epilimnetic TP decreased by 60 μg/L Oscillatoria biomass decreased from a before-treatment

summer maximum of 152 g/m2 to 41 g/m2, 7 years after installation

Before installation, internal P loading from lake sediments during June and July was more than

200 times that of external loading After installation, internal loading progressively decreased to only four times external loading Sediment P release progressively declined for the 6 years of observation following installation During that time, P export exceeded external loading (360 kg/yr)

by a total of 3,700 kg, resulting in a decrease in P content of the surficial sediments

7.2.1.2 Austrian Lakes

Pechlaner (1978) reported on the response of three lakes following installation of Olszewski tubes; Piburger See, Reither See, and Hechtsee Characteristics of the three lakes are given in Tables 7.1 and 7.2 All three lakes are relatively small but important to local populations and tourists for recreation, especially swimming The Olszewski tubes were installed to accelerate the restoration process following sewage effluent divertion

The tube in Piburger See draws water from a depth of 23 m, which is nearly the maximum depth

of the lake (24.6 m) Total length of the tube is 639 m, with a diameter of 8.9 cm Hypolimnetic water is discharged at 0.6 m3/min at a point downstream that is 13.5 m below lake level

While oxygen content markedly increased, there was no recognizable oligotrophication In

1970, the same year as the tube was installed, the DO content at the time of ice cover increased

by 63% over pre-tube conditions in 1969 DO continued at the improved level or higher for the next 7 years However, the lake’s trophic state did not change because epilimnetic TP declined by

5 μg/L (Pechlaner, 1979)

Piburger See tends to be morphometrically meromictic The lake mixed completely only twice during the 9 years of observation, even though the monomolimnion was effectively replaced about

Eng 113 With permission.)

−.4

−.8 0

Withdrawal duration (yr)

three times per year by virtue of the tube discharge Increased circulation of deep water across the bottom sediments, as well as the removal of P-rich water overlying the sediments, resulted in increased internal loading that tended to compensate for the increased losses of P via the tube (see effect of dilution on internal loading, Chapter 6) Phosphorus losses through the tube over 3 years were 79–192% more with the tube than would have occurred without it (Pechlaner, 1979)

In contrast to Piburger See, Reither See improved markedly in quality following installation

of a tube (Pechlaner, 1978) The tube was placed near the maximum depth (8.2 m) of the dimictic lake in 1972 Tube diameter was 10 cm and water discharged at 0.24 m3/min from the hypolimnion

of the 1.5-ha lake

Epilimnetic TP decreased from annual means of 38 and 43 μg/L in 1974 and 1975 to 21 μg/L

in 1977 Transparency nearly doubled over the 4-year period following installation There was some uncertainty about change in phytoplankton biomass, due to interference with detritus However, there were less blue-green algae after installation

A larger tube (18 cm) was placed in Hechtsee in 1973 The depth of placement, however was not near the maximum depth as in the other two lakes Because of meromixis, odors from the monomolimnion were quite strong Therefore, the tube was placed at 25 m, considerably less than the 56.6 m maximum depth, in order to protect the recreational environment around the lake from nuisance odors Tube discharge from the 26.3 ha lake varied from 1.2 to 1.8 m3/min

Because monomolimnetic water was not withdrawn, DO remained at zero from 25 m to the bottom DO increased significantly above 25 m after installation of the tube and P transport from the lake increased markedly even though the tube was placed above the monomolimnion During the first four years following installation, P output (203 kg) exceeded input (93 kg) by 110 kg, which was the actual decrease in lake TP content TP above 25 m declined by 70–80% from

1973 to 1977, while, as expected, TP below 25 m changed little and actually showed some increase (Pechlaner, 1978)

7.2.1.3 U.S Lakes

Withdrawal systems were installed in the shallower of two basins of Lake Wononscopomuc, Connecticut, in 1980 Hypolimnetic water was discharged from the shallow basin’s maximum depth

of 15.1 m at 0.9 m3/min (Table 7.2), which was sufficient to replace the hypolimnetic volume in 5.6 months (Kortmann et al., 1983; Nürnberg et al., 1987)

Lake quality improved substantially Hypolimnetic TP decreased from about 400 μg/L before

to less than 100 to 50 μg/L over 5 years and epilimnetic TP decreased from 24–30 μg/L to 10–14 μg/L following the start of withdrawal The decreased TP was apparently due to reductions in internal loading, which was verified by a 79% decrease in measured sediment release in the shallow basin after 2 years of withdrawal (Nürnberg et al., 1987)

DO in the hypolimnion also increased and the anoxic factor (days of anoxia) decreased from 50–65 before to less than 30 after withdrawal Transparency remained high and unchanged (> 5

m), but metalimnetic blooms of O rubescens were eliminated by the treatment.

Two systems were installed in Lake Waramaug, Connecticut, in 1983 One withdrew water from 8.5 m in one end of the long, S-shaped lake (12.8 m maximum depth) and discharged it at 6.3 m3/min (Table 7.2) The other system withdrew water from the hypolimnion at the other end

of the lake, returning it aerated There were no significant trends in TP, either in the hypolimnion

or epilimnion, during the first 3 years following withdrawal However, the anoxic factor decreased from 76–89 to 75 days Reasons for no significant response in TP were: (1) insignificant magnitude

of TP removal or duration of removal, or (2) excessive external loading (Nürnberg et al., 1987) The other U.S lake treated with withdrawal is Lake Ballinger, north of Seattle The device was installed in 1982 and allows the lake inlet stream to be directed to the hypolimnion through a 276

m, 30.5 cm diameter pipe The option also exists to allow all or some fraction to enter the epilimnion

if inflow temperature exceeds 16°C and there is a tendency to destratify the water column A control

weir exists at the outlet to adjust the fraction of hypolimnetic and epilimnetic water discharged The mean flow through the 381 m, 30.5 cm outflow pipe was 3.4 m3/min, which resulted in a replacement time for the hypolimnion of about 3 months (KMC, 1986)

Anoxia occurred for only 2 weeks in 1983, the year after installation, and the hypolimnion remained oxic with at least 3 to 4 mg/L DO during the stratification period in 1984, which was thought to be due to reduced ammonia in the inflow Hypolimnetic DO remained above 2 mg/L during 1985 Maximum hypolimnetic TP decreased from about 450 to 900 μg/L during 1979–1981, before installation, to about 100 to 150 during 1982 to 1985, after installation TP at overturn in

1984 was 15 μg/L, the lowest level ever observed

More recent data are unavailable Operation of the system has been intermittent in recent years due to odors from the discharge stream that borders a golf course The lake was treated with alum

in 1993

Internal loading was reduced from a high pre-installation value of 227 kg in 1979 to only 17

kg in 1984 The overall decrease in internal loading was 70% Unfortunately, a substantial increase

in external loading during the late 1970s and early 1980s prevented much reduction in epilimnetic

TP and consequent improvement in lake quality (KCM, 1986)

A 1,677-m hypolimnetic withdrawal pipe was installed in Devil’s Lake, Wisconsin, in 2002

at the maximum depth, which varies from 13.5 to 15.7 m (Table 7.1,2) The outflow rate is controlled to vary from 6.8 to 10.3 m3/min, depending on lake level The outflow P concentration averaged 725 μg/L for 48 days of operation in 2002, discharging 446 kg of hypolimnetic TP (Lathrop, personal communication; Lathrop et al., 2004) Data were not yet available to assess lake quality improvement

The project was initiated because high internal P loading from deep-water sediments caused excessive amounts of planktonic and periphytic algae, even though external inputs from cultural sources were eliminated in prior decades Potential indirect benefits of reduced productivity included reduction in swimmer’s itch by decreasing parasite–host snail densities feeding on periphyton, and reduced fish mercury concentrations by shortening the extent and duration of hypolimnetic anoxia, which is necessary for sulfate-reducing bacteria to convert inorganic Hg to methyl Hg

Hypolimnetic withdrawal was chosen as the only suitable technique to restore the lake to its original pristine condition because field and laboratory results confirmed that internal P loading could be significantly reduced after multiple withdrawals Other techniques to reduce internal P loading were rejected due to: (1) high cost, e.g., aeration, hypolimnetic water treatment, (2) opposition to adding chemicals, e.g., alum, to one of the state’s high use, “outstanding resource waters,” and (3) long-term ineffectiveness of other techniques, e.g., aeration, alum, without continual

or periodic retreatment The siphon withdrawal system has the important advantage of no operation cost An additional benefit was alleviation of recently recurring flooding problems in the State Park from high lake levels (Lathrop, personal communication; Lathrop et al., 2004)

7.2.1.4 Canada

The restoration of Pine Lake, near Red Deer, Alberta, began in 1991 to improve water quality to

a mesotrophic state that existed prior to European settlement (Sosiak, 2002) Epilimnetic TP concentrations reached medians around 100 μg/L during the mid 1990s with chlorophyll (chl) a

medians of 20–50 μg/L Most (61%) of the lake TP originated from internal loading Controls on external loads from surface sources (36%) took place during 1996–1998 and a 1,400-m hypolimnetic withdrawal pipe was installed in 1998 to reduce internal loading (Table 7.1)

The withdrawal system produced high rates of P loss (Table 7.2), and along with external controls, has reduced lake TP and improved water quality (Sosiak, 2002) TP concentration

decreased by 44–47% and chl a by 76–81% during 1996–2000 Median TP concentrations of 53–61 μg/L equaled those expected from recovery, while chl a (7.5–11.1 μg/L) and transparency (2.7–3.4

m) exceeded expectations Since 2000, TP and chl concentrations have remained relatively low

(Sosiak, personal communication) However, there have been blooms of Gloeotrichia, which was

apparently absent before treatment

While external controls probably contributed to the recovery, most of the reduction in lake TP (29%) occurred during, and was attributed to, hypolimnetic withdrawal Some of the improvement, however, was probably due to lower year-to-year surface runoff, which was positively related to lake TP over the 15 year period of data Nevertheless, monitoring of other Alberta lakes did not show a widespread decline in lake TP that could be attributed to regional climatic conditions (Sosiak, 2002)

There have been no significant adverse water quality effects in the outlet stream, although temperature and DO were lower immediately downstream Odors have not been a problem Such

a complete, long-term data set for a project is unusual and is continuing Such thorough monitoring has allowed a definitive assessment of the recovery and project cost-effectiveness

Chain Lake is small, shallow and polymictic in British Columbia (Table 7.1) Raising the lake level 1.3 m in 1951 created eutrophic conditions (McDonald et al., 2004) Low nutrient dilution water was diverted to the lake in the 1960s and a small area to 9 m was dredged to enhance stability

Summer TP and chl a reached concentrations of 300 and 100 μg/L, respectively, with blue-green algal blooms Withdrawal has consistently exported water and TP over the 9 years of operation (Table 7.2) Transparency has increased significantly (∼ 1 m) over that time period Downstream adverse effects from degraded water quality were partially mitigated by a fountain aerator

7.3 COSTS

Installation costs (in 2002 U.S dollars) for the three systems in the U.S lakes were as follows: Lake Ballinger (41 ha, 3.4 m3/min flow) — $420,000; Lake Waramaug (287 ha, 6.3 m3/min) —

$62,000 (Davis, personal communication; KMC, 1981); Devil’s Lake (151 ha, 9.1 m3/min) —

$310,000 (Lathrop, personal communication); Pine Lake (412 ha, 5.3 m3/min) — $282,000, not including contributed labor and equipment Relatively low cost and low annual maintenance are definite advantages of hypolimnetic withdrawal

7.4 ADVERSE EFFECTS

Discharge of hypolimnetic water containing high concentrations of P, ammonia, hydrogen sulfide and reduced metals and no oxygen may cause a water quality problem downstream If the outflow stream contains an important fishery and is otherwise used for recreation or water supply, then special precautions are necessary to minimize adverse effects Withdrawal water from Lakes Wononscopomuc and Waramaug is aerated and mechanically cleaned before being discharged downstream and the intake pipe end in Lake Waramaug is elevated to avoid high concentrations and fertilization effects downstream (Nürnberg et al., 1987)

Discharge from Lake Ballinger must be interrupted at times due to odors (2 of the 6 months

of stratification) and high nutrient content is apparently responsible for extensive periphyton growth downstream from the outlet Odors are a nuisance to users of the adjacent golf course The discharges from Hect, Klopeiner, and Kraiger See contained high concentrations of toxic substances so they were stopped during the late summer Mixing of discharge hypolimnetic water with epilimnetic water would minimize adverse downstream effects

7.5 SUMMARY

The advantages of hypolimnetic withdrawal are threefold: (1) relatively low capital and operational costs, (2) evidence of effectiveness in a large fraction of cases, and (3) potentially long-term and even permanent effectiveness In most cases hypolimnetic DO increased, resulting in a decrease in

the anoxic volume and the days of anoxia Internal P loading usually decreased and if there was not an offsetting high external loading, epilimnetic TP also decreased

The effectiveness of withdrawal apparently depends on magnitude and duration of TP transport from the hypolimnion Thus, it is important to exchange the hypolimnion volume as frequently as possible A low rate of replacement may limit the effectiveness of this technique A desirable exchange rate is severalfold during the stratification period and the desired magnitude can be determined by comparing the oxygen deficit rate with the rate of oxygen transport For example, the flow directed to the hypolimnion in Lake Ballinger was established based on the desire to add oxygen at double the oxygen demand rate in the hypolimnion As a result, hypolimnetic water was exchanged about every 3 months In Mauensee, it was exchanged every 2.4 months Therefore, an exchange rate of at least once in 2 to 3 months is recommended to assure the effectiveness of withdrawal In addition, results indicate that at least a 3 and possibly a 5 year duration of TP export may be necessary to see improvement in lake (epilimnetic) quality

There is a possibility of negative effects on downstream water quality due to low DO, high nutrient content and reduced substances If the outflow stream contains an important fishery and

is otherwise used for recreation or water supply, then special precautions may be necessary to maintain water quality The extent to which DO in the outflow water will be reduced can be estimated by comparing the existing DO deficit in the lake with the input load of DO (Pechlaner, 1979) If low DO is expected in the outlet, then aeration equipment should be installed Whether the high P content will cause nuisance attached algal and secondary BOD problems downstream will depend upon the extent to which periphyton growth is limited by nutrients compared with other factors

REFERENCES

Björk, S 1974 European Lake Rehabilitation Activities Rep Inst Limnol University Lund, Sweden.

Davis, E.R 1983 Personal communication The Hotchkiss School, Lakeville, CT

Dunalska, J., G Wisniewski and C Mientki 2001 Water balance as a factor determining the Lake Kortowskie

restoration Limnol Rev 1: 65–72.

Eschmann, K.H 1969 Die sanierung des wiler Sees durch albeitung des Tiefenwassers Gesundheitstechnik

Zurich 3: 125–129.

Gächter, R 1976 Die Tiefenwasserableitung, ein Weg zur Sanierung von Seen Schweiz Z Hydrol 38: 1–28 Hamm, A., and V Kucklentz 1981 Moglichkeiten und Erfolgsaussichten der Seenrestaurierung Materialien

der Bayrischen Landesanstalt fur Wasserforschung, Munchen, FDR, 15: 1–221.

Keto, A., A Lehtinen, A Mäkelä and I Sammalkorpi 2004 Lake Restoration In: P Eloranta, Ed., Inland

and Coastal Waters of Finland, University of Helsinki and Palmina Centre for Continuing Education.

KMC 1981 Lake Ballinger Restoration Project Interim Monitoring Study Report Kramer, Chin and Mayo, Seattle, WA

KMC 1986 Restoration of Lake Ballinger: Phase III Final Report Kramer, Chin, and Mayo, Seattle, WA.

Kortmann, R.W., E.R Davis, C.R Frink and D.D Henry 1983 Hypolimnetic withdrawal: Restoration of Lake Wonoscopomuc, Connecticut In: Lake Restoration, Protection and Management USEPA-440/5-83-001 pp 46–55

Lascombe, C., and J De Beneditis 1984 Une expérience de soutirage des eaux hypolimniques au Lac de

Paladru (lsère-France): Bilan des cing premières années de fonctionnement Verh Int Verein Limnol.

22: 1035

Lathrop, R.C Personal communication Wisconsin Dept Nat Res., Madison

Lathrop, R.C., T.J Astfalk, J.C Panuska and D.W Marshall 2004 Restoring Devil’s Lake from the bottom

up Wisc Nat Resour 28: 4-9.

Livingstone, D.M and F Shanz 1994 The effects of deep-water siphoning on small, shallow lake Arch.

Hydrobiol 32: 15-44.

McDonald, R.H., G.A Lawrence and T.P Murphy 2004 Operation and evaluation of hypolimnetic withdrawal

in a shallow eutrophic lake Lake and Reservoir Manage 20: 39–53.