Prepared in cooperation with the Mercer School District

Water Quality, Hydrology, and Simulated Response to 

Changes in Phosphorus Loading of Mercer Lake, Iron County, 

Wisconsin, with Special Emphasis on the Effects 

of Wastewater Discharges

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2012–5134
Cover photos. (upper left) historical photo of J.A. Tederstrom boat landing, Mercer Lake, Mercer, Wis., 

(lower right), public boat landing, Mercer Lake, Mercer, Wis., by Dale Robertson, USGS, 

(background) 2010 aerial photo of Mercer Lake and its watershed, Iron County, Wis. 
Water Quality, Hydrology, and Simulated Response 

to Changes in Phosphorus Loading of Mercer Lake, 

Iron County, Wisconsin, with Special Emphasis on 

the Effects of Wastewater Discharges

By Dale M. Robertson, Herbert S. Garn, William J. Rose, Paul F. Juckem, and Paul C. Reneau

Prepared in cooperation with the Mercer School District

Scientific Investigations Report 2012–5134

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2012

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as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested citation:

Robertson, D.M., Garn, H.S., Rose, W.J., Juckem, P.F., and Reneau, P.C., 2012, Hydrology, water quality, and response 

to simulated changes in phosphorus loading of Mercer Lake, Iron County, Wisconsin, with emphasis on effects of 

wastewater discharges on water quality: U.S. Geological Survey Scientific Investigations Report 2012-5134, 58 p 
iii

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................2

History of Activities near Mercer Lake .............................................................................................2

Shallow Lakes and Internal Phosphorus Loading...........................................................................5

Lake-Sediment Quality ........................................................................................................................6

Purpose and Scope ..............................................................................................................................6

Mercer Lake and Its Watershed .................................................................................................................6

Land Use and Population.....................................................................................................................8

Data-Collection Methods and Sites............................................................................................................9

Lake-Stage and Water-Quality Monitoring.......................................................................................9

Lake Classification..............................................................................................................................10

Stream Monitoring..............................................................................................................................10

Estimating Input from the Ungaged Near-Lake Drainage Areas ................................................11

Groundwater Monitoring...................................................................................................................11

Lake-Sediment Analyses...................................................................................................................11

Phosphorus Release Rates from Mercer Lake Sediments.................................................11

Composition of Mercer Lake Sediments................................................................................13

Paleoecological Analysis .........................................................................................................13

Stratification, Water Chemistry, and Trophic Conditions ......................................................................14

Stratification and Dissolved Oxygen ...............................................................................................14

Water Chemistry .................................................................................................................................14

Inferred Lake Water Quality from Analyses of Lake Sediment............................................................22

Quality of the Sediments of Mercer Lake ................................................................................................23

Organic Content, Phosphorus, and Trace Elements .....................................................................23

Synthetic Organic Compounds.........................................................................................................23

Hydrology and Water Budget ....................................................................................................................27

Change in Storage ..............................................................................................................................27

Precipitation.........................................................................................................................................28

Evaporation..........................................................................................................................................28

Gaged Surface-Water Inflows..........................................................................................................28

Surface-Water Outflow......................................................................................................................29

Groundwater Flow near and into Mercer Lake .............................................................................29

Ungaged Near-Lake Drainage Area ................................................................................................36

Water-Budget Summary....................................................................................................................36

Sources of Phosphorus...............................................................................................................................38

Precipitation.........................................................................................................................................38

Surface-Water Inflow .......................................................................................................................38

Gaged Inlets................................................................................................................................38

Ungaged Near-Lake Drainage Areas .....................................................................................38

Groundwater........................................................................................................................................40

Septic Systems....................................................................................................................................40

Release of Phosphorus from Lake Sediments ...............................................................................42

Surface-Water Outflow .....................................................................................................................42
iv

Phosphorus-Budget Summary .........................................................................................................42

MY 2008–09 .................................................................................................................................42

Typical Hydrology ......................................................................................................................44

Prior to 1965 with Typical Hydrology.......................................................................................44

1965 to 1995 with Typical Hydrology .......................................................................................45

Future with Typical Hydrology .................................................................................................45

Simulated Changes in Water Quality in Response to Changes in Phosphorus Loading .................46

Modeling Approach............................................................................................................................46

Verification and Adjustments for Model Biases............................................................................49

Response in Water Quality to Basinwide Changes in Phosphorus Loading.............................49

Response in Water Quality during Typical Hydrologic Conditions .............................................49

Simulated Effects of Internal Phosphorus Loading.......................................................................50

Response in Water Quality to Past and Future Anthropogenic Changes 

in the Watershed ...................................................................................................................50

Importance of Internal Phosphorus Loading to Lake Productivity......................................................50

Relative Importance of Natural and Anthropogenic Changes in the Watershed 

to Lake Productivity .......................................................................................................................51

Summary and Conclusions.........................................................................................................................51

References....................................................................................................................................................53

Appendix 1. Wastewater indicators in sediment (Schedule 5433 compounds), analyzed 

at the U.S. Geological Survey National Water Quality Laboratory in Denver, Colo............57

Figures

1. 2010 Aerial photo of Mercer Lake with its watershed delineated .......................................3

2. 2010 Aerial photo of Mercer Lake with the near-lake drainage area 

adjacent to Mercer Lake and surface-water and groundwater 

data-collection sites identified...................................................................................................7

3. Graph of the population of Mercer, Iron County, Wis., 1910–2010 .......................................9

4. Photographs illustrating sediment-sample collection for A, phosphorus 

release analyses and B, composition analyses ....................................................................12

 5. Continuous data diagrams of temperature and dissolved oxygen distribution 

during open water periods of monitoring years 2008–09, collected at the 

Deep Hole site in the East Basin of Mercer Lake..................................................................15

6. Graph of dissolved oxygen concentrations measured throughout 

Mercer Lake, on March 26, 2008..............................................................................................16

7. Scatter plots of near-surface A, total phosphorus concentrations, 

B, chlorophyll a concentrations, and C, Secchi depths in the 

East and West Basins of Mercer Lake during monitoring years 2008–09.........................16

8. Graph of near-surface and near-bottom total phosphorus concentrations in 

the East and West Basins of Mercer Lake during monitoring years 2008–09..................19

9. Near-surface A, total phosphorus concentrations, B, chlorophyll a concentrations, 

and C, Secchi depths in the East Basin of Mercer Lake from 1973 to 2011 ......................20

10. Trophic state index values based on water quality measured in 

the East Basin of Mercer Lake from 1973 to 2011 .................................................................21

11. Graph of estimated changes in water clarity (Secchi depth) inferred from 

changes in diatom assemblages in the sediment core collected in Mercer Lake..........22
v

12. Bar graph of monthly precipitation at Mercer Lake during 

monitoring years 2008–09 relative to long-term average precipitation 

for the period 1971–2000............................................................................................................29

13. Graphs of daily discharge, phosphorus concentrations, and phosphorus 

loads at A, Little Turtle Inlet, B, storm drain outlet near the boat landing, 

and C, Mercer Lake Outlet, during monitoring years 2008–09 ............................................30

14. Map of hydrologic features simulated with the GFLOW analytic 

groundwater-flow model near Mercer Lake..........................................................................32

15. Scatter plot of measured and simulated water levels for the GFLOW 

model of the area near Mercer Lake.......................................................................................33

16. Scatter plot of measured and simulated streamflow and groundwater 

inflow to Mercer Lake for the GFLOW model of the area near Mercer Lake...................34

17. Map of simulated groundwater-flow directions and area contributing 

groundwater to Mercer Lake....................................................................................................35

18. Pie diagrams of water budgets for Mercer Lake for monitoring 

years 2008–9 and for typical hydrologic conditions..............................................................36

19. Bar graph of the monthly accuracy of the water budget on the basis of 

comparisons of total water inputs and outputs plus storage change for 

Mercer Lake during monitoring years 2008–09 .....................................................................37

20. Bar graph of concentrations of dissolved phosphorus in groundwater 

from piezometers near Mercer Lake.......................................................................................40

21. Pie diagrams of phosphorus budgets for Mercer Lake for monitoring 

years 2008–09 and for typical hydrologic conditions............................................................44

22. Diagrams of measured and simulated summer-average near-surface 

concentrationsof A, total phosphorus, B, chlorophyll a, and C, Secchi depths 

in Mercer Lake in response to various phosphorus-loading scenarios............................48

Tables

1. Chronological history of major activities potentially affecting the water 

quality of Mercer Lake.................................................................................................................4

2. Population of the Town of Mercer from 1910 to 2010 .............................................................4

3. Morphometric characteristics of Mercer Lake.......................................................................8

4. Land use/land cover in the contributing areas to Mercer Lake ...........................................8

5. Near-surface, summer-average (May–September) total phosphorus, 

chlorophyll a, and Secchi depths for the East and West Basins, and 

trophic state index values based on data from the East Basin of 

Mercer Lake from 1973 to 2011.................................................................................................18

6. Phosphorus and trace element concentrations in the bottom sediment 

of Mercer and Grand Portage Lakes.......................................................................................24

7. Concentrations of wastewater compounds detected in the bottom 

sediment of Mercer and Grand Portage Lakes .....................................................................25

8. Summary of water-budget components for Mercer Lake during 

monitoring years 2008–09 and during typical hydrologic conditions.................................28

9. Summary of the phosphorus-budget components for Mercer Lake, 

during monitoring years 2008–09, during typical hydrologic conditions, 

and with future development in the watershed.....................................................................39

10. Phosphorus input from septic systems, sewage treatment plants, and 

untreated areas into Mercer Lake...........................................................................................41
vi

11. Estimated areas of aerobic and anaerobic sediment below 6.5 feet in 

Mercer Lake and estimated sediment phosphorus release from 

May 1 to September 30 ..............................................................................................................43

12. Summary of estimated rates of phosphorus release from the sediments 

of Mercer Lake............................................................................................................................43

13. Estimated water quality in Mercer Lake in response to various 

phosphorus-loading scenarios based on the Canfield and Bachman 

natural-lake model and Carlson trophic-state-index equations.........................................47

Conversion Factors

Multiply By To obtain

Length

inch (in) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 0.004047 square kilometer (km2

)

Volume

cubic foot (ft3

) 0.02832 cubic meter (m3) 

acre-foot (acre-ft) 1,233 cubic meter (m3

)

Mass

pound, avoirdupois (lb) 0.4536 kilogram (kg) 

Miscellaneous rates and yields

inch per year (in/yr) 25.4 millimeter per year (mm/yr)

foot per day (ft/d) 0.3048 meter per day (m/d)

cubic feet per second (ft3/s) 0.02832 cubic meter per second (m3/s)

gallons per day (gal/d) 0.003785 cubic meters per day (m3

/d)

gram per square centimeter (g/cm2

) 89,217.0 pounds per acre (lb/acre)

pound per year (lb/yr) 0.4536 kilogram per year (kg/yr) 

pound per square foot (lb/ft2

) 4.8824 kilogram per square meter (kg/m2)

pounds per acre (lb/acre) 112.07 milligrams per square meter (mg/m2

)

pounds per square mile (lb/mi2

) 0.1751 kilograms per square kilometer (kg/km2)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F=(1.8×°C)+32

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 

25 °C).

Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 

(NGVD 29). Sea level refers to 0 feet above the vertical datum.

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) 

or micrograms per liter (μg/L). All dissolved chemical constituents were collected and filtered 

through a 0.45-μm filter. Chlorophyll a samples were filtered through a 5-μm filter.

Concentrations of chemicals in sediment are given in micrograms per kilogram of sediment (μg/

kg) or milligrams per kilogram of sediment (mg/kg).
vii

Acknowledgments

Technical Reviewers

Richard L. Kiesling, Hydrologist, U.S. Geological Survey, Minnesota Water Science Center, 

Mounds View, Minn. 

Lori M. Fuller, Geographer, U.S. Geological Survey, Michigan Water Science Center, 

Lansing, Mich.

Local Project Coordinators

Douglas Chidley, Mercer Lake Association, Mercer, Wis.

James Kreitlow, Water Resources Management Specialist, Wisconsin Department of Natural 

Resources, Rhinelander, Wis.

Field Support

Jerry Huffmaster, Mercer Lake Association, Mercer, Wis.

Publishing Support

Elizabeth A. Ciganovich, Technical Writer-Editor, U.S. Geological Survey Science Publishing 

Network, Madison, Wis.

Marta Anderson, Visual Information Specialist, U.S. Geological Survey Science Publishing 

Network, Madison, Wis. 

Michelle M. Greenwood, Reports Specialist, U.S. Geological Survey, Middleton, Wis.

Jennifer L. Bruce, Geographer, U.S. Geological Survey, Middleton, Wis.

James L. Kennedy, Geographer, U.S. Geological Survey, Middleton, Wis.

Laura L. Nelson, Office Automation Assistant, Middleton, Wis.

Approving Official

Kevin J. Breen, Bureau Approving Official, Hydrologist, U.S. Geological Survey, 

New Cumberland, Penn. 

Water Quality, Hydrology, and Simulated Response 

to Changes in Phosphorus Loading of Mercer Lake, 

Iron County, Wisconsin, with Special Emphasis on the 

Effects of Wastewater Discharges

By Dale M. Robertson, Herbert S. Garn, William J. Rose, Paul F. Juckem, and Paul C. Reneau 

Abstract

Mercer Lake is a relatively shallow drainage lake in 

north-central Wisconsin. The area near the lake has gone 

through many changes over the past century, including 

urbanization and industrial development. To try to improve 

the water quality of the lake, actions have been taken, such as 

removal of the lumber mill and diversion of all effluent from 

the sewage treatment plant away from the lake; however, it is 

uncertain how these actions have affected water quality. 

Mercer Lake area residents and authorities would like to 

continue to try to improve the water quality of the lake; 

however, they would like to place their efforts in the actions 

that will have the most beneficial effects. To provide a better 

understanding of the factors affecting the water quality of 

Mercer Lake, a detailed study of the lake and its watershed 

was conducted by the U.S. Geological Survey in collaboration 

with the Mercer Lake Association. The purposes of the study 

were to describe the water quality of the lake and the composition 

of its sediments; quantify the sources of water and phosphorus 

loading to the lake, including sources associated with wastewater 

discharges; and evaluate the effects of past and future changes in 

phosphorus inputs on the water quality of the lake using eutrophication models (models that simulate changes in phosphorus and 

algae concentrations and water clarity in the lake).

Based on analyses of sediment cores and monitoring 

data collected from the lake, the water quality of Mercer Lake 

appears to have degraded as a result of the activities in its 

watershed over the past 100 years. The water quality appears 

to have improved, however, since a sewage treatment plant 

was constructed in 1965 and its effluent was routed away from 

the lake in 1995. Since 2000, when a more consistent monitoring program began, the water quality of the lake appears 

to have changed very little. During the two monitoring years 

(MY) 2008–09, the average summer near-surface concentration of total phosphorus was 0.023 mg/L, indicating the lake 

is borderline mesotrophic-eutrophic, or has moderate to high 

concentrations of phosphorus, whereas the average summer 

chlorophyll a concentration was 3.3 mg/L and water clarity, 

as measured with a Secchi depth, was 10.4 ft, both indicating 

mesotrophic conditions or that the lake has a moderate amount 

of algae and water clarity.

Although actions have been taken to eliminate the 

wastewater discharges, the bottom sediment still has slightly 

elevated concentrations of several pollutants from wastewater 

discharges, lumber operations, and roadway drainage, and 

a few naturally occurring metals (such as iron). None of the 

concentrations, however, were high enough above the defined 

thresholds to be of concern.

Based on nitrogen to phosphorus ratios, the productivity 

(algal growth) in Mercer Lake should typically be limited by 

phosphorus; therefore, understanding the phosphorus input to 

the lake is important when management efforts to improve or 

prevent degradation of the lake water quality are considered. 

Total inputs of phosphorus to Mercer Lake were directly 

estimated for MY 2008–09 at about 340 lb/yr and for a recent 

year with more typical hydrology at about 475 lb/yr. During 

these years, the largest sources of phosphorus were from 

Little Turtle Inlet, which contributed about 45 percent, and the 

drainage area near the lake containing the adjacent urban and 

residential developments, which contributed about 24 percent. 

Prior to 1965, when there was no sewage treatment plant 

and septic systems and other untreated systems contributed 

nutrients to the watershed, phosphorus loadings were estimated to be about 71 percent higher than during around 2009. 

In 1965, a sewage treatment plant was built, but its effluent 

was released in the downstream end of the lake. Depending 

on various assumptions on how much effluent was retained in 

the lake, phosphorus inputs from wastewater may have ranged 

from 0 to 342 lb. Future highway and stormwater improvements have been identified in the Mercer Infrastructure 

Improvement Project, and if they are done with the proposed 

best management practices, then phosphorus inputs to the lake 

may decrease by about 40 lb.

 Eutrophication models [Canfield and Bachman model 

(1981) and Carlson Trophic State Index equations (1977)] 

were used to predict how the water quality of Mercer Lake 

should respond to changes in phosphorus loading. A relatively 
2 Water Quality, Hydrology, and Simulated Response to Changes in Phosphorus Loading of Mercer Lake, WI, and Effects of Wastewater Discharge

linear response was found between phosphorus loading and 

phosphorus and chlorophyll a concentrations in the lake, with 

changes in phosphorus concentrations being slightly less 

(about 80 percent) and changes in chlorophyll a concentrations 

being slightly more (about 120 percent) than the changes in 

phosphorus loadings to the lake. Water clarity, indicated by 

Secchi depths, responded more to decreases in phosphorus 

loading than to increases in loading.

Results from the eutrophication models indicated that 

the lake should have been negatively affected by the wastewater discharges. Prior to 1965, when there was no sewage 

treatment plant effluent and inputs from the septic systems 

and other untreated systems were thought to be high, the lake 

should have been eutrophic; near the surface, average phosphorus concentrations were almost 0.035 mg/L, chlorophyll a

concentrations were about 7 μg/L, and Secchi depths were 

about 6 ft, which agreed with the shallower Secchi depths during this time estimated from the sediment-core analysis. The 

models indicated that between 1965 and 1995, when the lake 

retained some of the effluent from the new sewage treatment 

plant, water quality should have been between the conditions 

estimated prior to 1965 and what was expected during typical 

hydrologic conditions around MY 2008–09. The models also 

indicated that if the future Mercer Infrastructure Improvement 

Project is conducted with the best management practices as 

proposed, the water quality in the lake could improve slightly 

from that measured during 2006–10.

Because of the small amount of phosphorus that is 

presently input into Mercer Lake any additional phosphorus 

added to the lake could degrade water quality; therefore, 

management actions can usefully focus on minimizing future 

phosphorus inputs. Phosphorus released from the sediments 

of a degraded lake often delays its response to decreases in 

external phosphorus loading, especially in shallow, frequently 

mixed systems. Mercer Lake, however, remains stratified 

throughout most of the summer, and phosphorus released from 

the sediments represents only about 6 percent, or a small fraction, of the total phosphorus load to the lake. Therefore, the 

phosphorus trapped in the sediments should minimally affect 

the long-term water quality of the lake and should not delay 

the response in its productivity to future changes in nutrient 

loading from its watershed.

Introduction

Mercer Lake is a relatively shallow lake with surfacewater inflows and outflows (a “drainage lake”) in the Northern 

Lakes and Forests Ecoregion (Omernik and others, 2000) in 

Iron County in north-central Wisconsin. The town of Mercer is 

located on the north side of the lake, between Mercer Lake and 

Grand Portage Lake (fig. 1). The area near Mercer Lake has 

gone through many changes since 1894, when the first train 

arrived in the area and was followed by early settlers, roads, 

mills, stores, and other businesses. Historically, iron mines 

and the lumber industry were very important for this area, but 

more recently, the area has become very popular for fishing 

and other outdoor activities. Because of the many uses for the 

lake and surrounding area, the lake is an important resource 

for Iron County. The popularity of the lake has resulted in 

additional development around Mercer Lake, and residents 

feel that this development and past activities in the Mercer 

Lake watershed, especially historical wastewater discharges 

to the lake, have affected the quality of the lake and its 

sediments. Despite actions to reduce the sewage and chemical input to the lake, Mercer Lake residents believe that the 

historical addition of nutrients in sewage may have and still 

does affect the water quality and plant and algal productivity 

in the lake and that the addition of chemicals, especially from 

lumber operations and street use, may have altered the quality 

of the sediments in the lake. Residents believe that water quality of the lake may have improved, but water quality continues 

to be poorer than it was historically. In 2006, residents felt that 

the amount of muck in the lake had increased significantly 

and feel it dominates the bottom substrate, and both residents 

and the Wisconsin Department of Natural Resources (WDNR) 

believe that aquatic weeds in the lake are a problem. Residents 

are also concerned about how future development in the 

watershed will affect the water quality of the lake. Mercer 

Lake area residents and authorities would like to improve 

the water quality of the lake; therefore, understanding the 

factors affecting water quality, especially the effects of past 

wastewater discharges, is very important. 

To obtain a better understanding of the factors affecting 

the water quality of Mercer Lake, the U.S. Geological Survey 

(USGS) conducted a detailed study from 2007 to 2011 in 

collaboration with the Mercer Lake Association. This study 

was partially funded through the Cooperative Water Program 

of the USGS in cooperation with the Mercer School District, 

and the Lake Protection Grant Program of the WDNR. 

The results of this study are described in this report.

History of Activities near Mercer Lake

In the 1700s and 1800s, the area near Mercer Lake was 

relatively pristine, except for a relatively large Chippewa 

Indian village located near the lake (Outdoor Network, Inc., 

2011). Local residents remember the Chippewa camps being 

occupied until the early 20th century. The area near the lake 

remained relatively unaffected by development until the 

1880s, when iron mining became an important industry in 

Iron County and the lumber industry moved into northern 

Wisconsin. To support these activities, railroad lines were 

extended into northern Wisconsin, making the area truly 

accessible for the first time. The mining boom created a 

demand for timber to build mines, and villages sprang up to 

support them. Most of the growth around Mercer Lake, other 

than the Chippewa village, began in 1894 when the first train 

arrived, and with it brought early settlers who needed roads, 

mills, stores and other businesses (table 1). The first Mercer 
90°6' 90°

46°12' 46°9'

Grand

Portage

Lake

MERCER

Martha

Lake

Lake

Tahoe

Little Turtle River

Little Turtle River

Mercer

Lake

Little

Martha

Lake

Grand

Portage

Lake

MERCER

Martha

Lake

Mercer

Lake

Lake

Tahoe

Little Turtle River

Little Turtle River

Little

Martha

Lake

WWTP A

WWTP B

0 0.5 1 MILE

0 0.5 1 KILOMETER

Wastewater treatment plant

Mercer Lake drainage basin boundary

Near-lake drainage area boundary

EXPLANATION

Study site in

 Iron County

Base map from the World Imagery Basemap from ESRI, 1:40,000,

Web Mercator Auxiliary Sphere.

Introduction 3

Figure 1. 2010 Aerial photo of Mercer Lake, Iron County, Wis., with its watershed delineated. The original (WWTP A) and 2012 

(WWTP B) locations of the wastewater treatment plants are identified on the figure.

school was erected in 1894. Mercer was established as a town 

in 1909, along the north shore of Mercer Lake. The town was 

named after John Mercer, a timberman. In 1910, the population of Mercer was 311 (table 2).

In the late 1800s, lumbering flourished and Mercer 

became a well-established community in northern Wisconsin. 

Initially, pine logs from the Mercer-Manitowish area were 

floated down the Manitowish and Turtle Rivers, into the 

Flambeau River, to the downstream sawmills (Outdoor 

Network, Inc., 2011). Unmanaged logging took its toll on the 

area, bringing forest fires in 1911 that burned many acres and 

threatened lives. Following clear cutting of the forest, the vast 

barren areas were burned, rather than replanted, in the hope 

of providing good farm land, but farming never became a 

major industry in Iron County. Soon most of the desirable pine 

trees had been harvested from Iron County, leaving behind 

considerable hemlock and hardwoods. These trees were too 

heavy to log and transport by available methods, resulting 

in the establishment of many smaller mills closer to areas of 

hardwood growth and the available railroad lines. In the late 

1930s, a moderate scale sawmill was established between 

Mercer and Grand Portage Lakes. 

In the 1930s, Mercer became a popular vacation area 

for fishing and hunting (Outdoor Network, Inc., 2011). In 

1930, the population of Mercer was 666 (table 2). Cash crop 

agriculture never reached significant levels in Iron County; 

at best, the logging camps and mines in the area created a 

demand for small-scale truck farming, and hay was grown for 

livestock. By the 1940s and 1950s, most of the small subsistence farms were either abandoned or absorbed by larger operations. Currently, limited dairy farming is the main agricultural 

activity in the more fertile part of northern Iron County. 
4 Water Quality, Hydrology, and Simulated Response to Changes in Phosphorus Loading of Mercer Lake, WI, and Effects of Wastewater Discharge

Table 1. Chronological history of major activities potentially affecting the water quality of Mercer Lake, Iron County, Wis.

[WDNR, Wisconsin Department of Natural Resources]

Date Activity Reference

1894 Early settlers and first Mercer school established with the arrival of the 

railroad

Outdoor Network, Inc., 2011.

1909 Town of Mercer established Town of Mercer, 2011.

1890s–1920s Peak lumbering operations and cutting of pines and hardwoods Outdoor Network, Inc., 2011.

1911 Major forest fires in surrounding cutover areas Outdoor Network, Inc., 2011.

Late 1930s Establishment of a sawmill at the Midwest Forest Products industrial site D. Keefe, Mercer Lake Association, written 

commun., 2010.

1965 Wastewater treatment plant constructed and sewage effluent pipe installed to 

discharge near the outlet of Mercer Lake

Mercer Lake Association, 2007.

1967–68 Lumber debarking and shipping company started at the Midwest Forest 

Products site

D. Keefe, Mercer Lake Association, written 

commun., 2010.

1973, 1976 Dye tracer and water-quality studies done by WDNR to evaluate the effect 

of sewage effluent discharge near the lake outlet in response to continued 

complaints about water quality

L. Sridharan and B. Zellmer, Wisconsin 

Department of Natural Resources, written 

commun., 1980.

1987–1990 Mercer treatment plant disinfection violations of discharge permit; continuing complaints about water qualityWisconsin Department of Natural Resources, 

written commun., various dates.

1994–2001 Underground storage tank at Midwest Forest Products site removed in 1994 

that was found to be leaking diesel fuel with some groundwater contamination; monitoring of the site completed in 2000, and the monitoring site was 

discontinued because it was thought to be of no risk in 2001 by WDNR

J. Kreitlow, Wisconsin Department of Natural 

Resources, written commun., 2010.

1995 New treatment plant constructed south of Mercer to divert sewage out of the 

Mercer Lake watershed

Mercer Lake Association, 2007.

1998 Old treatment plant removed and the site restored C. Olson, Wisconsin Department of Natural 

Resources, written commun., 1998.

2005 Lumber debarking and shipping company discontinued completely D. Keefe, Mercer Lake Association, written 

commun., 2010.

The saw mill has been the only significant industrial site 

near Mercer Lake. The saw mill burned down several times 

and was rebuilt by different owners. In 1967–68, a subsidiary 

of a paper company modified this site for debarking, treating, 

and shipping logs (table 1). This operation continued until 

about 2005, but at that time it was operating only minimally 

(D. Keefe, Mercer Lake Association, written commun., 2010). 

The property contained an underground diesel fuel storage 

tank and was the subject of investigation and monitored 

for leaks (J. Kreitlow, Wisconsin Department of Natural 

Resources, written commun., 2010). The diesel fuel storage 

tank was removed in 1994 and was found to have contaminated local soil and groundwater, but no private water supplies 

were found to have been affected. Monitoring wells indicated 

that the site was no longer an environmental risk, and the site 

was allowed to close in 2001. 

The first municipal wastewater treatment plant for the 

town of Mercer (Sanitary District #1) was constructed in 

1965 on the northwest shore of Mercer Lake (WWTP A in 

fig. 1), and it was designed to treat sewage from most of the 

town of Mercer, which had a population of 1,048 in 1960 

(table 2). This secondary treatment facility employed an 

Table 2. Population of the Town 

of Mercer, Iron County, Wis., from 

1910 to 2010.

[Data from U.S. Census Bureau, 2011]

Year Population

1910 311

1920 567

1930 666

1940 936

1950 974

1960 1,048

1970 1,003

1980 1,425

1990 1,325

2000 1,732

2010 1,407
Introduction 5

activated sludge process and aerobic digestion and had an 

average capacity of 85,000 gal/d (P. Budde, Strand Assoc., 

written commun., 1980). The effluent from the plant passed 

through a 3,000-ft long, 6-in. diameter asbestos pipe laid 

along the bottom of the lake and discharged near the Mercer 

Lake Outlet to the Little Turtle River. At that time, emergent 

and submergent aquatic vegetation was moderate to dense 

and the lake was considered to have an aquatic weed problem (Wisconsin Department of Natural Resources, 1970). It 

was believed that much of the effluent from the plant was not 

leaving the lake and may have been affecting the water quality 

of the lake. Therefore, two dye studies were conducted. Based 

on the first dye tracer study, conducted in 1973, the WDNR 

concluded that effluent was circulating back into the lake and 

was adversely affecting the water quality of the lake. It was 

recommended that the outfall line be extended farther into the 

outlet. The tracer study was repeated in 1976, and the WDNR 

then concluded that the effluent was being drawn out of the 

lake into the Little Turtle River. The reason for the different 

conclusions of the two studies was thought to be the result of 

the dye plume not being observed long enough in the 1973 

survey (L. Sridharan and B. Zellmer, Wisconsin Department 

of Natural Resources, written commun., 1980). In addition to 

the possibility of the effluent not completely leaving the lake, 

the plant had frequent operating problems and often had difficulty meeting discharge permit limits. Operation difficulties 

and complaints about the treatment plant continued through 

the 1980s. The residents also believed that the pipe installed 

along the lake bottom may have ruptured soon after construction and spilled effluent directly into the main body of the lake 

until 1995 (D. Chidley, local resident, oral commun., 2006). 

A new treatment plant was constructed south of Mercer in 

1995 (WWTP B in fig. 1), and it released its effluent outside 

of the watershed of the lake (Mercer Lake Association, 2007). 

The new facility is a sequencing batch reactor type of plant 

and discharges into three cells that seep into groundwater 

believed to flow away from the lake. The design flow for 

this plant is 82,700 gal/d, adequate for 1,100 people. The old 

treatment plant on the lake was completely removed and the 

site was restored (C. Olson, Wisconsin Department of Natural 

Resources, written commun., 1998). 

Since the late 1890s, many factors have affected the 

quality of the water and sediments in Mercer Lake, such as 

nutrient inputs from septic and municipal effluent, chemical 

inputs from lumber mills and roads, and actions have been 

taken to ameliorate the effects of these factors (table 1). 

Shallow Lakes and Internal Phosphorus Loading

Mercer Lake is relatively shallow lake. Shallow lakes, 

defined as having a maximum depth less than about 20 ft 

(Osgood and others, 2002), typically experience only short 

periods of stratification, when the cooler, more dense, bottom 

water (hypolimnion) does not mix with the warmer, less dense, 

surface water (epilimnion), rather than a single extended 

period of stratification from early summer through early fall. 

Lakes in which water mixes frequently throughout the openwater period are called “polymictic,” whereas lakes in which 

water mixes only during spring and fall and have extended 

periods of stratification in summer and under the ice are called 

“dimictic.” Deeper lakes with relatively large surface areas 

may also be prone to frequent deep mixing. Osgood (1988) 

described the functional aspects of mixing based on the mean 

depth and surface area of lakes in terms of the Osgood Index.

The Osgood Index is defined as the mean depth (z, in meters) 

of a waterbody divided by the square root of the surface area 

(A, in square kilometers), or z/A0.5. The Osgood Index is used 

to describe the degree to which a lake or reservoir is likely 

to mix because of the forces of wind. Lakes with Osgood 

Index values greater than 9 generally are dimictic, and lakes 

with Index values less than 4 tend to be polymictic. Mercer 

Lake has an Osgood Index of 3.6; therefore, the lake should 

be polymictic. However, given that the lake has two basins 

(fig. 1), only part of the surface area is exposed to the wind 

at any time. The Osgood Index of the East Basin of the lake 

is 6.1 and the West Basin is 4.2; therefore, this indicates that 

both basins may be between these two categories and may mix 

intermittently. 

When deep water of a productive lake (a lake with 

abundant plants or algae) becomes isolated from surface mixing because of thermal stratification, the bottom water often 

becomes anaerobic or devoid of dissolved oxygen because 

of the loss of oxygen associated with the decomposition of 

organic substances. When oxygen is completely depleted 

(anoxia) near the bottom sediment, the rate of phosphorus 

release from the deep sediments (termed “internal loading”) 

often increases dramatically (Mortimer, 1941). Typically, in 

deeper, dimictic lakes, phosphorus released from the deep 

sediments is contained in the hypolimnion of the lake and is 

released to the epilimnion primarily at fall turnover. Therefore, 

phosphorus concentrations near the surface (termed “nearsurface concentrations”) in dimictic lakes usually remain 

stable or decrease as summer progresses (Welch and Cooke, 

1995). In polymictic lakes, however, when the deeper water 

becomes anaerobic during the short periods of stratification 

and phosphorus is released from the sediments, the frequent 

mixing events deliver the phosphorus to the surface of the 

lake throughout summer. The frequent mixing results in nearsurface concentrations of phosphorus increasing throughout 

the summer and the lake being very productive in late summer (Welch and Cooke, 1995). Polymictic lakes are extremely 

vulnerable to phosphorus contributed from areas outside of 

the lake (termed “external loading”) and difficult to rehabilitate because the high rate of internal loading usually does not 

quickly respond to decreases in external phosphorus inputs 

(Newton and Jarrell, 1999; Nurnberg, 1998; Welch and Cooke, 

1995). 

Shallow lakes typically have two alternative stable 

states—phytoplankton (algae) dominated or macrophyte 

(plant) dominated (Newton and Jarrell, 1999). In moderate 

densities, macrophytes are beneficial in these lakes because 
6 Water Quality, Hydrology, and Simulated Response to Changes in Phosphorus Loading of Mercer Lake, WI, and Effects of Wastewater Discharge

they keep sediment from being resuspended by the wind 

and, therefore, help improve water clarity. Macrophytes 

also provide a place for attached algae to grow and remove 

phosphorus from the water column. If the macrophytes are 

removed or if external phosphorus inputs increase, the lake 

can shift from a macrophyte-dominated state to an algal-dominated state. Once a lake is in the algal-dominated state, macrophytes have a difficult time reestablishing themselves, because 

algae reduce the penetration of light through the water. Of 

these two conditions, it is commonly believed that the macrophyte-dominated state, as in Mercer Lake, is more desirable 

for human and biological use than the algal-dominated state 

(Newton and Jarrell, 1999). It is believed that Mercer Lake 

has more macrophytes than it once had, but macrophytes may 

have always been common in the lake. 

Although the phosphorus load to Mercer Lake is believed 

to have decreased as a result of remediation efforts in the 

watershed (table 1), the internal phosphorus load may limit 

the response of the lake to decreases in external phosphorus 

loading. The impact of this internal phosphorus load in 

delaying the response in a lake’s productivity to decreases in 

external nutrient loading has been documented in several lakes 

(Larsen and Mercier 1976; Larsen and others, 1979 and 1981; 

Robertson and Rose, 2008). Without knowing the importance 

of internal loading, it is not possible to predict with any degree 

of confidence how it could delay future improvements in the 

water quality of the lake.

Lake-Sediment Quality 

The bottom sediments of a lake can provide a historical 

record of the chemical inputs to the lake, because many 

substances entering the lake eventually are deposited in the 

sediments in the deeper areas. Many organic chemicals, 

or hydrophobic compounds, are insoluble and have strong 

tendencies to accumulate in the sediments. Because of the 

tendency for these substances to be deposited in bottom 

sediments, much information about the history of chemical 

and particulate inputs may be obtained by analyzing sediment 

samples. Some elements, such as phosphorus, can migrate 

through the lake sediments and be released back into the water 

column (resolubilized) when environmental conditions such 

as dissolved oxygen, pH, and temperature change; therefore, 

other indicators of past nutrient conditions are needed to 

examine historical changes in lake productivity. Changes in 

the assemblages of diatoms (a type of phytoplankton) in the 

lake sediments are often used to infer historical changes in the 

nutrient concentrations in the water column of lakes (Garrison, 

2005).

Anthropogenic (those resulting from human activity) contaminants are deposited into lakes from the atmosphere, from 

point sources such as sanitary and storm sewers, industrial 

facilities, lumbering operations, and from nonpoint sources 

such as from urban and agricultural areas. The concentration 

and bioavailability (the degree to which a substance is available to living creatures) of contaminants in sediments can be 

major issues of concern in some areas because of their ability 

to persist and accumulate in sediment and aquatic organisms. 

Since the late 1890s, it is believed that the main anthropogenic 

contaminants that have entered Mercer Lake were from point 

sources such as sanitary and storm sewers, lumbering operations, and general urban areas (table 1). For many Midwest 

lakes, agricultural activities have strongly affected water quality; however for Mercer Lake, contaminants from agricultural 

activities are not believed to have had significant effects.

Purpose and Scope

The primary purposes of this report are to describe the

conditions in Mercer Lake as observed in 2008–09, discuss 

factors that may delay expected improvements in water quality, and provide information useful for consideration of further 

measures that may improve the water quality of this lake in the 

face of increases in property development. Specifically, this 

report (1) describes the present and historical water quality of 

Mercer Lake, (2) quantifies water and phosphorus budgets, 

including contributions from internal loading, on the basis of 

data collected from November 2007 through October 2009; 

(3) presents the results of model simulations that demonstrate 

the potential effects of changes in phosphorus inputs on lakewater quality, including effects from wastewater discharges; 

and (4) describes the sediment quality of the surficial sediments of Mercer Lake. Information from this study improves 

our knowledge of how lakes, in general, should respond to 

changes in wastewater discharges.

Mercer Lake and Its Watershed

Mercer Lake is a drainage lake in Iron County just south 

of Mercer, Wis. Mercer Lake is in the Little Turtle River 

Basin, the main stream flowing into and out of the lake (fig. 

1). Grand Portage Lake is immediately upstream of Mercer 

Lake. An additional small tributary, the Tahoe Inlet (outlet 

stream from Lake Tahoe), flows into the lake on the west side 

near the outlet. After leaving the lake, the Little Turtle River 

flows into the Little Turtle Flowage, the Turtle-Flambeau 

Flowage, and then into the Flambeau River. In this study, the 

watershed was divided into three areas: the area upstream of 

the major inlet to the lake (the “upstream gaged area”), the 

Tahoe watershed, and the drainage area adjacent to the lake 

(the “near-lake area”) that includes drainage from the storm 

drain immediately east of the boat launch (figs. 1 and 2). 

The area and volume of Mercer Lake are 184 acres and 

1,949 acre-ft as given on the 1968 WDNR lake survey maps. 

In this study, the morphometry of the lake was reevaluated on 

the basis of an aerial image obtained from the 2005 National 

Agricultural Imagery Program (U.S. Department of Agriculture, 2006). By using geographic information system (GIS) 

techniques, the resulting surface area of Mercer Lake was 

determined to be 179 acres and the volume to be 1,793 acre-ft 
s4

s3

Tahoe Inlet

Site G

Site F

Site E

Site H

Site D

Site C

Site B

Site A

Little Turtle Inlet

Outlet

Deep Hole

East Basin

West Basin

s1

s6

s2

s5

P7

P6

P5

P4

P3

P2

P1

Stage

WWTP A

Boat launch

s4

s3

Tahoe Inlet Stage

WWTP A

Site G

Site H

Site F

Site E

Site D

Site C

Site B

Site A

Storm drain

Boat launch

Little Turtle Inlet

Outlet

Deep Hole

East Basin

West Basin

s1

s6

s2

s5

P7

P6

P5

P4

P3

P2

P1 5 ft

10 ft

15 ft

15 ft

20 ft

20 ft

24 ft

17 ft

21 ft

0 ft

3 ft

5 ft

10 ft

15 ft

15 ft

20 ft

20 ft

24 ft

17 ft

21 ft

0 ft

3 ft

90°5' 90°3'

46°10' 46°9'30"

5 ft

Gage site

Lake stage gage site

Lake site

Main lake site

Piezometer

Sediment site

Wastewater treatment plant

Near-lake drainage area boundary

Lake-depth contour, in feet (ft) as labelled

EXPLANATION

Base map from the World Imagery Basemap from ESRI, 1:40,000,

Web Mercator Auxiliary Sphere. Bathymetry from Wisconsin

Department of Natural Resources, Mercer Lake - 

Iron County lake map, June 1968.

0 0.125 0.25 MILE

0 0.125 0.25 KILOMETER

Mercer Lake and Its Watershed 7

Figure 2. 2010 Aerial photo of Mercer Lake, Iron County, Wis. with the near-lake drainage area and surface-water and groundwater 

data-collection sites identified. The location of the original wastewater treatment plant (WWTP A), and the contours of similar depth 

are identified on the figure.
8 Water Quality, Hydrology, and Simulated Response to Changes in Phosphorus Loading of Mercer Lake, WI, and Effects of Wastewater Discharge

Table 3. Morphometric characteristics of Mercer Lake, 

Iron County, Wis.

[NA, not available]

Characteristic (units) Entire Lake East Basin West Basin

Area (acres) 179 84 95

Volume (acre-feet) 1,793 976 817

Maximum depth (feet) 24.0 24.0 21.0

Mean depth (feet) 10.0 11.7 8.6

Residence time, study 

period (years)

0.46 NA NA

Residence time–typical 

hydrology (years)

0.33 NA NA

Osgood Index1 3.6 6.1 4.2

1

 Osgood and others, 2002.

drainage). The depth of the surficial materials may be up 

to 100 ft over igneous and metamorphic bedrock (Young 

and Hindall, 1972; Mudrey and others, 1982). Soils of the 

rolling-to-undulating uplands are primarily sandy loam, loamy 

sand, and sand, with permeabilities of 2.5 to 5 in/hr or more, 

and muck or peat soils in the wetlands (Hole and others, 

1968; Oakes and Cotter, 1975). This glacial drift is the only 

significant source of groundwater, yielding moderate to large 

quantities of water. Scattered throughout the area are many 

small closed depressions, or internally drained areas 

commonly with ponds or wetlands but without outlets, that do 

not contribute surface runoff to the lakes. 

Land Use and Population

Land use in the watershed is primarily a mixture of 

forest (74.9 percent), wetlands (6.6 percent), open water 

(8.5 percent), and low-density residential (5.9 percent), with 

smaller areas of urban (2.2 percent), grassland/shrubland 

(1.1 percent), agriculture (0.4 percent), and golf courses 

(0.4 percent) (table 4; Multi-Resolution Land Characteristics 

Consortium, 2001). Therefore, agriculture inputs of nutrients 

should not be a factor in this area, but inputs from urban and 

low-density residential uses may be significant, especially 

along the north shore of the Lake.

The town of Mercer generally has not experienced the 

recent rapid growth that other areas in the northwoods have 

had (Garn and others, 2010), and the town retains much of 

its “northwoods character.” Decennial censuses since 1910 

exhibit variable population growth, with larger increases in the 

1930s, 1970s, and 1990s (fig. 3). The population has increased 

from 311 in 1910 to 1,732 in 2000; however, in 2010, the 

population decreased to 1,407, a loss of 19 percent from 2000. 

(table 3). The area and volume determined in this study were 

used for all of the computations in this report. Based on this 

information, the mean depth of the lake is 10 ft. Mercer Lake 

is comprised of two basins: the East Basin with a maximum 

depth of 24 ft and the West Basin with a maximum depth of 

21 ft. The two basins are separated by a sill with a maximum 

depth of about 10–15 ft.

The watershed of Mercer Lake consists of surficial 

glacial deposits, including pitted outwash and other ice contact 

deposits, ground moraine and end moraines composed of sand 

and gravel. The basin has low to moderate relief and flat to 

steep slopes with many swamps and several upstream lakes 

(Grand Portage Lake immediately upstream, and Martha, 

Little Martha and other small lakes farther upstream in the 

Table 4. Land use/land cover in the contributing areas to Mercer Lake, Iron County, Wis.

[Data from the Multi-Resolution Land Characteristics Consortium, 2001]

Basin/drainage Area 

(acres)

Forest 

(percent)

Wetlands 

(percent)

Low-density 

residential 

(percent)

Moderateand highdensity 

residential 

(percent)

Agriculture 

(percent)

Grassland/

shrubland 

(percent)

Golf 

course 

(percent)

Open 

water 

(percent)

Upstream gaged 

area

6,564 75.0 7.7 5.3 1.3 0.5 1.0 0.0 9.2 

Tahoe watershed 769 77.2 0.1 8.0 3.2 0.0 1.9 3.8 5.9 

Near-lake area 292 66.5 0.0 15.2 18.3 0.0 0.0 0.0 0.0 

Entire watershed 7,625 74.9 6.6 5.9 2.2 0.4 1.1 0.4 8.5 
0

200

400

600

800

1,000

1,200

1,400

1,600

1,800

2,000

1900 1920 1940 1960 1980 2000 2020

Population of Mercer, Wisconsin

Year

Data-Collection Methods and Sites 9

Figure 3. Population of Mercer, Iron County, Wis., 1910–2010.

Data-Collection Methods and Sites

As part of this study, USGS personnel collected stream 

and lake data from November 2007 through October 2009. In 

addition, water-quality data for the lake were collected by the 

WDNR and volunteers from the Lakes Association as part of 

the WDNR’s Wisconsin Citizen Lake Monitoring Program 

(Wisconsin Department of Natural Resources, 2008). Data 

collected by the WDNR were supplied by J. Vennie (Wisconsin Department of Natural Resources, written commun., 

2009). Citizen Lake Monitoring data were obtained from 

the WDNR’s Surface Water Integrated Monitoring System 

(SWIMS) database (http://dnr.wi.gov/lakes/clmn/Stations.

aspx?location=26, accessed December 12, 2011). All available data, but primarily near-surface concentrations of total 

phosphorus and chlorophyll a, plus Secchi depths, were used 

to characterize long-term changes in the water quality of the 

lake; however, only data collected from November 1, 2007, 

to October 31, 2009, were used to describe the hydrology and 

phosphorus inputs to the lake. This latter period was divided 

into two monitoring years (MY): November 2007 through 

October 2008 (hereafter referred to as “MY 2008”) and 

November 2008 through October 2009 (hereafter referred to 

as “MY 2009”).

Lake-Stage and Water-Quality Monitoring

A continuously recording (at 15-minute intervals) lakestage gage was installed and operated on the southwest side 

of Mercer Lake (fig. 2). The gage was operated for the 2-year 

monitoring period.

Water-quality data for the lake were collected by the 

WDNR from 1973 to 1974 as part of a statewide assessment 

and during 1975 to 1976 and 2000–02 as part of other general 

assessments. In 2001, volunteers from the local community and the Mercer Lake Association (Citizen Monitoring) 

began monitoring the lake. From 2001 to 2002, only Secchi 

depths were measured at the Deep Hole Site in the East Basin 

(fig. 2). During 2004, 2005, and 2007, only Secchi depths 

were measured near the deepest location in the West Basin 

(also referred to as the home base site). From 2006 to 2011, 

Secchi depth, near-surface total phosphorus, and chlorophyll 

a were measured at the Deep Hole site in the East Basin; 

all three parameters were also measured in the West Basin 

during 2008 and 2009, and Secchi depth was measured in 

2011. All Citizen Lake Monitoring was done in accordance 

with protocols described by Wickman and Herman (2005). In 

2008 and 2009, USGS sampling was done at the Deep Hole 

site in the East Basin and near the deepest location in the West 

Basin (fig. 2). USGS protocols involved collecting profiles of 

water temperature, dissolved oxygen, specific conductance, 

and pH with a multiparameter meter, and water clarity (Secchi 

depth) with a standard 8-in.-diameter black-and-white Secchi 

disk during each visit. Near-surface samples were collected 

with a Van Dorn sampler and were analyzed for total phosphorus and chlorophyll a concentrations and, during midsummer, for various nitrogen species (nitrite plus nitrate and 

Kjeldahl nitrogen). Samples collected near the lake bottom 

(near-bottom samples) were collected about 3 ft above the 

sediment-water interface and were analyzed for total phosphorus concentration. In addition, water samples were collected at 

the Deep Hole site in the East Basin during spring turnover in 

2008 and 2009 and were analyzed for common ions and other 
10 Water Quality, Hydrology, and Simulated Response to Changes in Phosphorus Loading of Mercer Lake, WI, and Effects of Wastewater Discharge

characteristics such as color, turbidity, alkalinity, total dissolved 

solids, and silica. Almost all of the monitoring was conducted during the open water season, except during 1974–75 and 2008–09, 

when sampling was also done in March through the ice.

All of the lake-stage, water-quality, and quality-assurance 

data collected by the USGS were published in the USGS 

online Annual Water Data Report series (U.S. Geological 

Survey, 2009–10), which is available on the World Wide Web 

at http://wdr.water.usgs.gov/index.html.

In addition to the sampling described above, the USGS 

conducted a synoptic survey on March 26, 2008, to determine 

the extent of low dissolved oxygen concentrations (hypoxia) 

under the ice. During this survey, profiles of water temperature, dissolved oxygen, specific conductance, and pH were 

collected with a multiparameter meter at 10 locations throughout the lake (Sites A–H and the main lake sites in fig. 2).

Lake Classification

One method of classifying the water quality of a lake is 

with trophic state index (TSI) values based on near-surface 

concentrations of total phosphorus, chlorophyll a, and Secchi 

depths, as developed by Carlson (1977). The indices were 

developed to place these three characteristics on similar scales 

to allow comparison of different lakes. TSI values based on 

total phosphorus concentrations (TSIP), chlorophyll a concentrations (TSIC), and Secchi depths (TSISD) were computed 

for each sampling by use of equations 1–3. The individual 

index values were averaged monthly, and the monthly average values were then used to compute summer (May through 

September) average TSI values:

TSIP = 4.15 + 14.42 (ln total phosphorus) (1)

[total phosphous in milligrams per liter *1,000]

TSIC = 30.6 + 9.81 [ln chlorophyll a (in micrograms per liter)] (2)

TSISD = 77.12 – 14.41 [ln Secchi depth (in feet)] (3)

Oligotrophic lakes have TSI values less than 40; a limited 

supply of nutrients; typically have low phosphorus concentrations, low algal populations, and high water clarity; and contain oxygen throughout the year in their deepest zones (Wisconsin Department of Natural Resources, 1992). Mesotrophic 

lakes have TSI values between 40 and 50; a moderate supply 

of nutrients; a tendency to produce moderate algal blooms and 

moderate clarity; and occasionally have oxygen depletions in 

the deepest zones of the lake. Eutrophic lakes have TSI values 

greater than 50; a large supply of nutrients; severe waterquality problems, such as frequent seasonal algal blooms and 

poor clarity; and oxygen depletion is common throughout 

the deeper zones of the lake. Eutrophic lakes with TSI values 

greater than 60 are often further classified as hypereutrophic 

lakes, and they typically have even more severe water-quality 

problems, including frequent extensive algal blooms. 

Stream Monitoring

Three stream sites (two tributary sites and one outlet 

site; fig. 2) were equipped with instrumentation to continuously monitor flow at 15-minute intervals. At all three 

sites, water level was measured to determine flow by use of 

standard stage-discharge relations (Rantz and others, 1982). 

Two tributary sites were monitored: the main inlet site, Little 

Turtle River at Garnet Street, station number 0535739503; and 

the storm drain outlet near the boat landing, station number 

0535739504. At the storm drain, a 7-inch weir was installed to 

provide accurate flow estimates. A gaging station was installed 

at the bridge crossing the Little Turtle River at the Mercer 

Lake Outlet, station number 053573952. The data are stored 

and maintained in the USGS National Water Information 

System (NWIS) database. 

Water samples were collected at the three monitoring 

sites by USGS staff or a local observer approximately monthly 

and during flow events by use of the Equal-Width-Increment 

(EWI) method (Edwards and Glysson, 1999) or by grab 

methods. Water samples were analyzed for concentrations of 

total phosphorus. Phosphorus loads for each site were then 

computed for the entire monitoring period using the Graphical 

Constituent Loading Analysis System (GCLAS; Koltun and 

others, 2006). GCLAS is a program developed by the USGS 

to estimate loads of water-quality constituents from instantaneous measurements of streamflow and constituent concentration, which collectively can be used to compute instantaneous 

loads. Generally, concentrations are linearly extrapolated 

between measurements except at the beginning and end of 

each event. Prior to the extrapolations, additional concentrations are often added to the time series to better describe concentrations just prior to an event and just following an event or 

to describe events without measured concentrations. Concentrations at the beginning of an event were estimated from 

concentrations measured during previous base-flow periods. 

Concentrations at the end of the events were estimated from 

concentrations measured shortly after the end of an event. 

To estimate flow and phosphorus loading from the Tahoe 

watershed, intermittent discharge measurements and water 

samples were collected from a small tributary, the outlet 

stream from Lake Tahoe that enters Mercer Lake on the west 

side (Tahoe Inlet in fig. 2). All water samples were analyzed 

for total phosphorus. The measured flows were then correlated 

with the continuously recorded flow at the main inlet site on 

Little Turtle River to develop a regression relation between 

the two sites [Tahoe Flow (cfs) = Little Turtle Flow (cfs) * 

0.239 – 0.802]. The regression relation and the daily flows at 

Little Turtle Creek were used to estimate daily flows at the 

Tahoe Inlet. Temporally varying offsets were then applied 

to all estimated flow data to force estimated flows to equal 

those measured at the outlet. Daily phosphorus loads from the 

Tahoe Inlet were determined by multiplying the volumetrically 

weighted concentration from all of the samples measured at 

the outlet by the estimated daily flow.
Data-Collection Methods and Sites 11

All water-quality data and daily flows and loads were 

published in the USGS Water Data Report Series (U.S. Geological Survey, 2009–10), which is available on the World 

Wide Web at http://wdr.water.usgs.gov/. 

Estimating Input from the Ungaged Near-Lake 

Drainage Areas

Smaller subwatersheds making up the area immediately 

around the lake, not including the storm drain near the boat 

landing (referred to herein as “ungaged near-lake area”; fig. 2) 

also contribute runoff to the lake. Drainage from ungaged 

near-lake areas includes runoff from areas drained by unchannelized direct runoff to the lake, runoff conveyed in numerous 

small channels, and additional runoff from the storm sewered 

parts of the town that flows overland to the lake when the 

capacity of the storm sewer is exceeded. Because of the complex nature of the sources of this runoff, the use of runoff coefficients was not practical. This near-lake runoff was estimated 

from the residuals in the mass-balance equation after all other 

inputs and outputs were determined, including inputs from 

groundwater. To temporally distribute the near-lake runoff, the 

runoff required to make the calculated change in the lake stage 

equal the measured (actual) change in the lake stage was calculated on a daily basis. Because of the inability to perfectly 

measure or estimate other inputs, outputs, and changes in lake 

stage, the calculations yielded negative runoff for some days, 

which is not possible in reality. To deal with this, runoff for the 

calculated negative days was assumed to equal zero; therefore, 

on those days when calculated runoff was positive, the calculated values needed to be adjusted (reduced) to offset the sum 

of the negative values. To make this adjustment, the positive 

daily flow values were multiplied by the ratio of the sum of the 

positive values minus the sum of the negative values divided 

by the sum of the positive values.

Groundwater Monitoring

Seven shallow, small-diameter piezometers (1/2-in.-diameter observation wells) were installed around the periphery of 

Mercer Lake (fig. 2) to help define areas contributing groundwater to the lake, determine the phosphorus concentration in 

the groundwater entering the lake, and quantify the phosphorus 

loading from groundwater. The piezometers were installed 

approximately 2–3 ft below the water table, to depths of 5–8 ft. 

Groundwater gradients, which are determined from the differences in water elevation in the piezometers and elevation of the 

lake surface, in all of the piezometers were measured six times 

during the 2-year monitoring period: three times in the September–October early fall period, two times in the May–June early 

summer period, and once in March. In addition, water elevations in two of the piezometers were measured in November 

2008. Dissolved phosphorus concentrations were measured in 

each piezometer for each of the early summer and early fall 

samplings if the gradient indicated flow toward the lake.

To provide information for the calibration of a local 

groundwater model (described in later sections), water-surface 

elevations of selected nearby lakes and streams were surveyed 

on September 2008 by use of a Real-Time Kinematic Global 

Positioning System (RTK-GPS). The elevations surveyed have 

an expected accuracy of about 0.1 ft. Concurrent base-flow 

discharge measurements were made at stream sites during the 

survey.

 All lake, stream, and groundwater samples were analyzed by the Wisconsin State Laboratory of Hygiene (WSLH) 

in accordance with standard analytical procedures described in 

its “Manual of Analytical Methods, Inorganic Chemistry Unit” 

(Wisconsin State Laboratory of Hygiene, 1993).

Lake-Sediment Analyses

Phosphorus Release Rates from Mercer Lake 

Sediments

Release rates of phosphorus from the sediments during 

aerobic and anaerobic conditions (used to estimate internal 

loading) were determined from analysis of sediment cores 

collected at four locations in the lake (lake-monitoring sites 

and two auxiliary sites, sites s1–s4 in fig. 2) by using the 

sediment-core-incubation techniques described by James and 

Barko (1991). A Wildco sediment-core sampler equipped with 

acrylic core liners was used to collect the cores (fig. 4A). At 

the two deep lake-monitoring sites (s1 and s2), four cores were 

collected: two cores for aerobic analysis and two cores for 

anaerobic analysis. At the two shallow sites in the West Bay 

(s3 and s4), two cores were collected for aerobic analysis only. 

After retrieval, the core liners, containing both sediment and 

overlying water, were immediately sealed with rubber stoppers 

and stored in a protective container for shipping to the WSLH. 

Additional lake water was collected for incubation with the 

collected sediment. 

At the WSLH, the sediment cores were carefully drained 

of overlying water. Then additional water, collected from the 

lake, was filtered and siphoned onto the sediment contained 

in each of the core liners. All of the liners containing the 

sediment were placed in the dark and incubated at a constant 

temperature (approximately 22o

C) for 1– 2 weeks. The incubation temperature was a constant temperature close to that 

above the sediments in Mercer Lake during summer. 

The aerobic/anaerobic environment of each core was 

controlled in the laboratory by gently bubbling either air 

(aerobic environment) or nitrogen (anaerobic environment) 

through an air stone placed just above the sediment surface. 

The limited bubbling action insured complete mixing of the 

water column but did not disrupt the sediment. Eight of the 

cores (two from each location) were subjected to aerobic 

conditions and four of the cores (two each from the two deep 

lake-monitoring sites, s1 and s2) were subjected to anaerobic 

conditions. Therefore, all incubations were run in duplicate.
A

B

12 Water Quality, Hydrology, and Simulated Response to Changes in Phosphorus Loading of Mercer Lake, WI, and Effects of Wastewater Discharge

Figure 4. Photographs illustrating sediment-sample collection for A, phosphorus release analyses 

and B, composition analyses.
Data-Collection Methods and Sites 13

Samples for soluble reactive phosphorus analysis were 

collected daily from water above the sediment for each core 

and filtered through a 0.45-mm filter. The volume of water 

that was removed was replaced with an equivalent volume of 

lake water with a known concentration. Rates of phosphorus 

release from the sediments (in lb/ft2/d) were then calculated 

as a linear change in mass in the overlying water divided by 

the time and the cross-sectional area of the incubation core 

liner. All results were corrected for the water (and phosphorus) 

that was removed from above the core and replaced with lake 

water. All of the aerobic and anaerobic sediment release rates 

were then averaged to obtain an average aerobic release rate 

and an average anaerobic release rate. 

The amount of phosphorus released from the sediment 

(internal loading) was obtained by summing the amount 

released from the aerobic and anaerobic areas. The area of 

anaerobic sediment was estimated from the depths at which 

a dissolved oxygen concentration of less than or equal to 

approximately 0.5 mg/L was measured in the profiles collected 

in the deepest areas of the lake, assuming that the dissolved 

oxygen concentrations were horizontally uniform throughout 

the lake. The area of aerobic sediment was computed by 

subtracting the area of anaerobic sediment and the area of 

sediment at depths less than 6.5 ft from the total area. It was 

assumed that no phosphorus was released from areas less than 

6.5-ft deep, because these areas typically have little accumulated organic sediment. From the 2 years of profile data, the 

areas of aerobic and anaerobic sediment were estimated for 

specific dates throughout the year. The daily sediment release 

was then computed as the product of the release rates and the 

respective aerobic and anaerobic areas.

Composition of Mercer Lake Sediments

Surficial sediment samples from the lake bottom were 

collected from four locations in Mercer Lake: monitoring 

sites s1 and s2 in the East and West Basins, a shallow site in 

the near the original sewage treatment plant location, site s5, 

and a shallow site near the outlet of the lake where the pipe 

discharged its effluent, site S6 (fig. 2). In addition, a sediment 

sample was collected at a deep location in the northeast bay 

of Grand Portage Lake. Sediment samples for physical and 

nutrient analysis, trace elements, and organic compounds were 

collected on September 8 and 9, 2008, and for polychlorinated 

biphenyls (PCBs) analysis on October 8, 2008. The four sites 

in Mercer Lake were chosen to represent the variability in the 

lake and to ascertain the potential effects of the discharge from 

the sewage treatment plant. Sediments from Grand Portage 

Lake were examined to represent reference sediment conditions, 

because sediment from Grand Portage Lake should have had 

minimal affects from sewage input, urbanization, and industry.

Surficial sediment samples from approximately the top 

2-in. layer of sediments were collected with an Ekman dredge 

(fig. 4B) for analyses of physical characteristics, nutrients, 

trace elements, PCBs and other organic compounds. Samples 

were deposited in pre-cleaned polypropylene or pre-combusted glass jars as appropriate. Samples to be analyzed for 

trace elements and organic compounds were chilled in ice and 

sent to the USGS National Water Quality Laboratory (NWQL) 

in Denver Colo., and those analyzed for physical characteristics, nutrients, and PCB constituents were chilled in ice and 

sent to the WSLH. Analytical methods at NWQL followed 

approved USGS protocols (Fishman and others, 1994; Fishman and Friedman, 1989; Garbarino and Struzeski, 1998), and 

samples sent to the WSLH followed their approved protocols 

(Wisconsin State Laboratory of Hygiene, 1993).

Sediment samples were analyzed for a wide variety of 

constituents, including metals and wastewater indicators, to 

characterize the sediment quality. Analyses for trace metals 

were performed according to methods described in NWQL 

Schedule 316. Analyses for wastewater indicators were done 

according to methods described in NWQL Schedule 5433, 

which was developed for the determination of wastewater 

indicator compounds typically found in domestic and industrial wastewater. The wastewater analytical list contains about 

60 individual compounds (Appendix 1), including surfactants, 

food additives, antioxidants, flame retardants, plasticizers, 

industrial solvents, disinfectants, pharmaceuticals, personalcare products, polycyclic aromatic hydrocarbons (PAHs), 

and high-use domestic pesticides (Burkhardt and others, 

2006). Some of these compounds are known or suspected of 

disrupting endocrine functions in aquatic vertebrate organisms 

(see Appendix 1). 

Paleoecological Analysis

A separate paleoecological study of the lake-bottom sediments of Mercer and Grand Portage Lakes was done by the 

WDNR, Bureau of Integrated Science Services, to determine 

the ages of the sediment from specific depths and to quantify 

historical changes in sedimentation rates in the lakes, changes 

in water clarity, and changes in the macrophyte community 

(Garrison, 2005). Sediments from the deep region of the West 

Basin of Mercer Lake (20 ft) and the northeast bay of Grand 

Portage Lake (32 ft) were extracted by use of a piston coring 

device on July 9, 2003, and then analyzed to reconstruct 

changes in the water clarity and macrophyte community in 

the lakes by examining changes in the diatom assemblages at 

different depths in the core. Results from Grand Portage Lake 

were used as reference conditions and were thought to have 

minimal effects of inputs from sewage treatment plant, and 

urban and residential development. 
14 Water Quality, Hydrology, and Simulated Response to Changes in Phosphorus Loading of Mercer Lake, WI, and Effects of Wastewater Discharge

Stratification, Water Chemistry, and 

Trophic Conditions

Stratification and Dissolved Oxygen

During MY 2008–09, Mercer Lake can be described 

as a dimictic lake, meaning that it typically undergoes two 

extended periods of mixing (often referred to as “turnover”) 

during spring and fall and thermally stratifies during summer 

and winter. During summer, a weak thermocline (the depth 

interval where the temperature decreases rather abruptly) 

develops in late May to early June and remains established 

through September (fig. 5). The thermocline developed from 

about 13 to 16 ft below the surface. Water temperatures in the 

deepest areas of the lake were from about 12 to 14°C in early 

summer (early June) and increased to about 16 to 18°C by late 

summer (early September). During summer, surface mixing 

penetrated down into the thermocline several times, but mixing 

was insufficient to cause complete mixing down to the bottom 

of the lake (fig. 5). The incomplete mixing was sufficient to 

cause bottom temperatures to increase throughout the summer. 

Temperature profiles collected in both basins demonstrated 

little horizontal variation in thermal stratification throughout 

the lake during summer.

Dissolved oxygen concentrations were near saturation 

throughout the lake from just after the ice melted until the lake 

stratified; however, once a thermocline developed, concentrations in the deep areas quickly decreased, while concentrations 

in shallow waters remained near saturation (fig. 5). By midJune, dissolved oxygen concentrations near the bottom of the 

deepest areas of the lake were almost zero (less than 0.5 mg/L) 

and unable to support fish. This anaerobic condition in the 

bottom waters is common in eutrophic lakes (Wetzel, 1983). 

During late fall, the lake completely mixed, and dissolved 

oxygen concentrations at all depths were near saturation again. 

From the 2 years of data, it appeared that anaerobic conditions 

near the bottom developed about June 15 and lasted to about 

September 10. 

Temperature and dissolved oxygen profiles were also collected under the ice in late winter. These profiles demonstrated 

that the lake was inversely stratified with cool water (near 0o

C) 

just below the ice and warming to about 5oC near the bottom. 

In the March profiles, very low dissolved oxygen concentrations were found: 2.4–2.6 mg/L under the ice and less than 

1 mg/L about 5 ft or more beneath the ice (about 6.5 ft beneath 

the lake surface). Therefore, a synoptic survey was done on 

March 26, 2008, to determine the extent of hypoxia in the lake 

(fig. 6). The very low oxygen concentrations in the middle of 

the lake also occurred in almost all other areas, except near to 

where the Little Turtle River enters the lake (Site A). By late 

winter, dissolved oxygen concentrations greater than 5 mg/L 

were only found just beneath the ice and near the inlet. Dissolved oxygen concentrations throughout the lake were well 

below the Wisconsin state standard of 5 mg/L for warm-water 

lakes (Shaw and others, 1993), and typically below the 2-mg/L 

critical concentration for most fish.

Water Chemistry

Water in Mercer Lake is a little harder than most lakes in 

north-central Wisconsin, reflecting the igneous bedrock and 

glacial deposits in the area. The hardness was about 88 to 97 mg/L 

as calcium carbonate (CaCO3), which classifies it as moderately 

hard water (Hem, 1985). Hardness is caused primarily by the 

presence of calcium and magnesium. According to Lillie and 

Mason (1983), relatively low concentrations of calcium, magnesium, and alkalinity characterize lakes in this region. They found 

that for 189 lakes sampled in this region, average concentrations 

of calcium, magnesium, and alkalinity were 10, 5, and 37 mg/L, 

respectively. During this study, concentrations of calcium, magnesium, and alkalinity in Mercer Lake were 26, 6.5, and 82 mg/L, 

respectively.

Specific conductance throughout the lake during spring and 

fall overturns and near the surface during other periods was about 

160–220 μS/cm, but it increased to greater than 300 μS/cm near 

the bottom when the deep areas were anaerobic. The pH in the 

upper part of the lake typically increased from about 7.5 standard 

units, in early summer, to about 8.0–8.5 in mid-to-late summer. 

The chloride concentration during spring overturn was 

12.6 mg/L. The median concentration of chloride in lakes in this 

area reported by Lillie and Mason (1983) was 2 mg/L. Concentrations of chloride are indicators of urban development in the 

watershed and road salting in the winter; however, concentrations 

of chloride in Mercer Lake are still very low when compared to 

lakes in southeastern Wisconsin lakes. According to U.S. Environmental Protection Agency (1988) water-quality criteria, freshwater 

aquatic organisms should not be affected if the 4-day average 

concentration of dissolved chloride remains less than 230 mg/L.

Phosphorus and nitrogen are essential nutrients for plant 

growth and are the nutrients that usually limit algal growth in 

most lakes. Elevated nutrient concentrations can cause high 

algal populations or “blooms”; therefore, high nutrient inputs 

can be the cause of accelerated eutrophication (that is, accelerated aging and greater lake productivity). Concentrations of 

near-surface total phosphorus greater than 0.024 mg/L indicate 

eutrophic conditions (Carlson, 1977). Near-surface phosphorus 

concentrations in Mercer Lake demonstrated little seasonality 

(fig. 7A). During MY 2008 and MY 2009, concentrations ranged 

from about 0.01 mg/L to 0.05 mg/L in midsummer. Concentrations demonstrated no consistent differences between the East 

and West Basins of the lake. The average near-surface summer 

(May–September) phosphorus concentration during MY 2008–09 

was about 0.023 mg/L (table 5). Therefore, based on total phosphorus concentrations, Mercer Lake would typically be classified 

as on the border between mesotrophic and eutrophic. 

In MY 2009, there appeared to possibly be a discrepancy 

between the phosphorus data collected by the USGS and the 

Citizen Monitoring Program. Therefore, a comparison study was 

conducted in August 2010 using side-by-side sampling. This comparison study demonstrated little difference (within 0.002 mg/L) 

between the two groups; therefore, the variability in MY 2009 

appears to be real changes in phosphorus concentrations in the 

lake. 
Mar. Apr. May June July Aug. Sept. Mar. Apr. May June July Aug. Sept. 

Water temperature (2008) Water temperature (2009)

Dissolved oxygen (2008) Dissolved oxygen (2009)

05

101520

Depth, in feet Depth, in feet

05

101520

Figure 5. Temperature and dissolved oxygen distribution during open water periods of monitoring years 2008–09, collected at the Deep Hole sitof Mercer Lake, Iron County, Wis.
0

5

10

15

20

0 2 4 6 8

Dissolved oxygen concentration, in milligrams per liter

Depth, in feet

A

B

Deep Hole in East Basin

C

Deepest location in West Basin

D

E

F

G

H

Sites monitored

EXPLANATION

16 Water Quality, Hydrology, and Simulated Response to Changes in Phosphorus Loading of Mercer Lake, WI, and Effects of Wastewater Discharge

Figure 6. Dissolved oxygen concentrations measured throughout Mercer Lake, Iron County, Wis., on March 26, 2008. 

Site locations are illustrated in figure 2.

Figure 7 (right). Near-surface A, total phosphorus concentrations, 

B, chlorophyll a concentrations, and C, Secchi depths in the East and 

West Basins of Mercer Lake, Iron County, Wis., during monitoring 

years 2008–09.
A

C

B

0

3

6

9

12

15

Secchi depth, in feet

USGS - East Basin

Citizen monitoring - East Basin

USGS - West Basin

Citizen monitoring - West Basin

Summer average

Eutrophic threshold

Mesotrophic threshold

0

0.01

0.02

0.03

0.04

0.05

Mar. 2008 May 2008 July 2008 Aug. 2008 Oct. 2008 Dec. 2008 Mar. 2009 May 2009 July 2009 Sept. 2009

Total phosphorus concentration, 

in milligrams per liter 

0

1

2

3

4

5

6

7

8

9

Chlorophyll a, in micrograms per liter

EXPLANATION

Eutrophic

Mesotrophic

Oligotrophic

Eutrophic

Eutrophic

Mesotrophic

Mesotrophic

Oligotrophic

Oligotrophic

Stratification, Water Chemistry, and Trophic Conditions 17
Table 5. Near-surface, summer-average (May–September) total phosphorus, chlorophyll a, and Secchi depths for the East and West Basins, and tro

based on data from the East Basin of Mercer Lake, Iron County, Wis. from 1973 to 2011. [mg/L, milligrams per liter; μg/L, micrograms per liter; ft, feet; —, not available or no data] Year East Basin West Basin Trophic state index 

Total phosphorus (mg/L) Chlorophyll a (μg/L) Secchi depth (ft) Total phosphorus (mg/L) Chlorophyll a (μg/L) Secchi depth (ft) Phosphorus Chlor

1973 0.040 — 7.5 — — — 57.3 

1974 0.030 — 5.7 — — — 52.3 

1975 0.020 — 6.5 — — — 47.3 

1976 0.030 — 8.0 — — — 53.2 

1979 0.027 5.5 10.0 — — — 51.7 4

2000 0.015 — — — — — 43.2 

2001 — — 20.0 — — — — 

2002 0.018 4.0 10.5 — — — 45.8 4

2004 — — 7.3 — — 7.3 — 

2005 — — 10.6 — — 10.6 — 

2006 0.017 3.9 10.9 — — — 45.0 4

2007 0.017 2.1 11.1 — — 11.4 44.4 4

2008 0.022 3.9 10.1 0.024 3.4 10.4 48.0 4

2009 0.024 3.1 10.0 0.024 3.0 10.5 48.4 42010 0.018 2.6 12.4 — — — 44.8 4

2011 0.019 3.2 — — — 10.1 46.7 4

Average 2006–10 0.019 3.1 10.7 — — — 46.1 4

Average 2008–09 0.023 3.5 10.1 0.024 3.2 10.4 48.2 4

1Trophic state index values for 2004, 2005, 2007, and 2011 include Secchi data from the West Basin.
0

0.03

0.06

0.09

0.12

0.15

Mar. 2008 May 2008 July 2008 Aug. 2008 Oct. 2008 Dec. 2008 Mar. 2009 May 2009 July 2009 Sept. 2009

Total phosphorus concentration, in milligrams per liter 

East Basin - near surface

West Basin - near surface

East Basin - near bottom

West Basin - near bottom

Date

EXPLANATION

Stratification, Water Chemistry, and Trophic Conditions 19

Figure 8. Near-surface and near-bottom total phosphorus concentrations in the East and West Basins of Mercer Lake, 

Iron County, Wis., during monitoring years 2008–09.

Near-bottom total phosphorus concentrations were similar to those near the surface during unstratified periods; however, phosphorus concentrations increased relatively quickly 

after the near-bottom water became anoxic in winter in the 

West Basin and in summer in the deeper East Basin (fig. 8). 

Maximum concentrations in the two basins reached approximately 0.14 mg/L. The higher concentrations in summer in 

the East Basin than in the West Basin may have resulted from 

this basin being deeper, which resulted in longer and more 

extensive stratification and anoxia. During summer in the 

West Basin, phosphorus concentrations began to occasionally 

increase, but then decreased, presumably with mixing in this 

shallower basin. It is unclear why phosphorus concentrations 

did not increase in the East Basin during winter stratification; 

both basins had similar hypoxic conditions below about 13 ft. 

Since 1973, near-surface total phosphorus concentrations 

have ranged from 0.011 mg/L to 0.048 mg/L, and the low and 

high concentrations were both measured in 2009 (fig. 9A). 

From the 1970s to the 2011, summer-average concentrations 

have ranged from 0.015 mg/L in 2000 to 0.040 mg/L in 1973. 

Although summer-average concentrations have been variable, 

there appears to be a decrease in total phosphorus concentrations, which is consistent with that expected with the discontinuation of discharge of sewage effluent from the town of 

Mercer. 

During spring of 2008 and 2009, near-surface total 

nitrogen concentrations (computed as the sum of Kjeldahl 

nitrogen and dissolved nitrite plus nitrate) ranged from 

0.55 to 0.61 mg/L; Kjeldahl nitrogen represented more than 

95 percent of the total nitrogen. These values are similar to those 

measured in the late 1970s and 2000s (0.43 to 0.96 mg/L); the 

highest and lowest concentrations were both measured in 1973. 

The ratio of the near-surface concentrations of total nitrogen to total phosphorus (N:P ratio) is often used to determine the 

potential limiting nutrient for productivity in a lake. The specific 

value of this ratio that determines which nutrient potentially 

is limiting differs under different ambient conditions, such as 

water temperature, light intensity, and nutrient deficiencies (Correll, 1998); however, a ratio greater than about 16:1 by weight 

usually indicates that phosphorus should be the potentially limiting nutrient. The N:P ratios for 1970s data ranged from 7:1 to 

28:1, with an average ratio of 18:1. This indicates that phosphorus should have been the potentially limiting nutrient, but nitrogen does appear to occasionally limit productivity. Since 2002, 

the N:P ratios have ranged from 20:1 to 24:1, with an average of 

22:1. These ratios indicate that phosphorus typically should be 

the nutrient typically limiting productivity in the lake. 

Further decreases in phosphorus concentrations should not 

only increase the N:P ratio, but also favor the growth of green 

algae over the growth of blue-green algae. Blue-green algae 

usually are not limited by nitrogen, because they can fix 

nitrogen from the atmosphere. Blue-green algae are the least 

desirable type of algae, because they commonly form extensive 

blooms or scums on the water surface, are potentially toxic, 

and are usually the least preferred food by grazing zooplankton. 

Therefore, phosphorus should be the nutrient of concern when 

considering management efforts to improve or prevent degradation of the water quality of Mercer Lake.
A

B

C

0

0.01

0.02

0.03

0.04

0.05

1972 1976 1980 1984 1988 1992 1996 2000 2004 2008 2012

Total phosphorus concentration,

 in milligrams per liter 

U.S. Geological Survey

Citizen monitoring

Wisconsin Department of Natural Resources

Summer-average concentration

0

5

10

15

20

25

1972 1976 1980 1984 1988 1992 1996 2000 2004 2008 2012

Secchi depth, in feet

U.S. Geological Survey

Citizen monitoring

Citizen monitoring–West Basin

Wisconsin Department of Natural Resources

Summer-average Secchi depth

0

2

4

6

8

10

12

1972 1976 1980 1984 1988 1992 1996 2000 2004 2008 2012

Chlorophyll a, in micrograms per liter 

U.S. Geological Survey

Citizen monitoring

Wisconsin Department of Natural Resources

Summer-average concentration

Date

20 Water Quality, Hydrology, and Simulated Response to Changes in Phosphorus Loading of Mercer Lake, WI, and Effects of Wastewater Discharge

Figure 9. Near-surface A, total phosphorus concentrations, B, chlorophyll a concentrations, and C, Secchi depths in 

the East Basin of Mercer Lake, Iron County, Wis., from 1973 to 2011. 
30

40

50

60

1972 1976 1980 1984 1988 1992 1996 2000 2004 2008 2012

Trophic state index

Total phosphorus

Chlorophyll a

Secchi depth

Date

Eutrophic

Mesotrophic

Oligotrophic

Stratification, Water Chemistry, and Trophic Conditions 21

Figure 10. Trophic state index values based on water quality measured in the East Basin of Mercer Lake, Wis., from 1973 to 2011. 

Chlorophyll a is a photosynthetic pigment found in 

algae and other green plants. Its concentration is commonly 

used as a measure of the density of the algal population in 

a lake. Average concentrations between about 2 and 7 μg/L 

indicate mesotrophic conditions, and average concentrations 

greater than 7 μg/L indicate eutrophic conditions (Carlson, 

1977). During MY 2008–09, concentrations ranged from 

0.9 to 9 μg/L (fig. 7B). No consistent differences were 

observed between the East and West Basins of the lake. 

Concentrations were generally highest in spring and early 

summer, lowest in midsummer, and then increased slightly in 

late summer. The average near-surface summer chlorophyll a

concentrations during MY 2008–09 was about 3.4 μg/L 

(table 5). Therefore, based on chlorophyll a concentrations, 

Mercer Lake would typically be classified as mesotrophic.

Since 1979, chlorophyll a concentrations ranged from 

1.39 μg/L to 10 μg/L (fig. 9B). The only chlorophyll a

measurement prior to 2001 was 5.5 μg/L, measured on July 

9, 1979. Since 2001, summer-average concentrations have 

ranged from 2.1 to 4.0 μg/L. Based on these data, no consistent long-term change was apparent. 

During MY 2008–09, Secchi depths in Mercer Lake 

ranged from 5.9 to 9 ft under the ice and during early spring to 

about 12–14 ft in mid-to-late summer (fig. 7C). Clarity consistently increased as summer progressed. During MY 2008–09, 

the average Secchi depth during summer was about 10 ft 

(table 5). Since 1973, Secchi depths have ranged from 3 ft in 

1974 to 20 ft in 2001 (fig. 9C). Summer-average Secchi depths 

have ranged from 5.7 ft in 1974 to 20 ft in 2001 (Secchi 

depths were only measured in late summer in 2001) (table 5). 

Secchi depths between 6.6 and 13.1 ft indicate mesotrophic 

conditions and less than 6.6 ft indicate eutrophic conditions 

(Carlson, 1977). Therefore, based on Secchi depths, Mercer 

Lake would typically be classified as mesotrophic. Since 1973, 

it appears that the water clarity has improved (fig. 9C).

TSI values based on near-surface concentrations of total 

phosphorus, chlorophyll a, and Secchi depth indicate that in 

MY 2008 and MY 2009 Mercer Lake was typically mesotrophic (fig. 10 and table 5). During almost every sampling, the 

lake was classified as mesotrophic based on chlorophyll a concentrations and Secchi depths; however, on many occasions 

the lake was classified as eutrophic based on total phosphorus 

concentrations greater than 0.024 mg/L. Since 1973, all three 

TSIs decreased, which indicates that the water quality of the 

lake has improved. Based on summer-average TSI values for 

Secchi depth, it would appear the changes were quite variable; 

however, the large variations in 2001 and 2004 were probably caused by an unusual sampling frequency rather than 

changes in the lake. In both of these years, Secchi depths were 

measured only in late August and September. These unusual 

summer averages demonstrate the importance of a consistent 

monitoring program when trying to detect changes in water 

quality. The improvements in water quality from 1980 to after 

2000 are consistent with those expected with the discontinuation of discharge of sewage effluent from the town of Mercer. 
Inferred Lake Water Quality from 

Analyses of Lake Sediment

Results of analyses of sediment cores extracted from the 

deep areas of the West Basin of Mercer Lake and the northeast bay of Grand Portage Lake on July 9, 2003, were used 

to quantify historical sedimentation rates and reconstruct 

changes in the water quality and the amount of macrophytes 

in the lakes (Garrison, 2005). The average annual sedimentation rates for Mercer and Grand Portage Lakes were about 

0.011 g/cm2, which are below average for the lakes that have 

been studied in Wisconsin. The annual sedimentation rate 

in Mercer Lake was 0.003 g/cm2 in the early 1800s, but it 

steadily increased to about 0.022 g/cm2 in recent years. Sedimentation rates in Grand Portage Lake also increased since 

the early 1800s to about 0.02 g/cm2 in the 1990s, but they may 

have decreased slightly since 1995. 

Based on Garrison’s analysis of historic diatom assemblages, macrophytes have always been abundant in Mercer 

Lake (since the 1800s), but low in Grand Portage Lake. The 

amount of macrophytes in Mercer Lake, however, may have 

fluctuated through time, whereas those in Grand Portage 

Lake have remained low. The changes in diatom assemblages 

were also used to estimate changes in the water clarity (Secchi depth) in Mercer Lake (fig. 11; Garrison, 1995). It was 

estimated that average Secchi depths in Mercer Lake were 

about 11.5 ft in the 1800s, but decreased relatively rapidly 

around 1900 in response to increases in erosion and nutrient 

input from development and lumbering in the area. Secchi 

depths remained relatively steady until the early 1960s, when 

average Secchi depths rapidly decreased to about 8 ft, likely 

in response to phosphorus input from the sewage plant. Garrison inferred that during this time, the phosphorus concentrations were highest and macrophyte density was lowest in the 

entire record for Mercer Lake. Since the 1960s, Secchi depths 

improved but have been quite variable, which suggests that 

phosphorus concentrations have decreased and the macrophyte 

density has increased. The increase in macrophyte density 

could have been caused by improved water clarity (Garrison, 

2005) and, possibly, a longer growing season resulting from 

increases in air temperatures in the region. Anderson and 

others (1996) demonstrated that lakes in northern Wisconsin 

had a trend toward earlier ice out dates from 1968 to 1988, 

which infers that the growing season has increased since 1968. 

Since around 1900, nutrient concentrations in Grand Portage 

Lake increased slightly, but they have not changed much since 

around 1930. Based on changes in the diatom assemblages, 

water quality in Mercer Lake appears to have been adversely 

affected by watershed activities, primarily runoff from the 

town of Mercer and especially inputs from the sewage treatment plant (Garrison, 1995). Improvements inferred in the 

lake since the 1960s agree with changes measured in lake 

water quality since 1973 (fig. 9).

Figure 11. Estimated changes in water clarity (Secchi 

depth) inferred from changes in diatom assemblages in the 

sediment core collected in Mercer Lake, Iron County, Wis. 

(after Garrison, 2005).

1820

1840

1860

1880

1900

1920

1940

1960

1980

2000

8 9 10 11 12

Secchi depth, in feet

Year

22 Water Quality, Hydrology, and Simulated Response to Changes in Phosphorus Loading of Mercer Lake, WI, and Effects of Wastewater Discharge
Quality of the Sediments of Mercer Lake 23

Quality of the Sediments of Mercer Lake

Many factors may have affected the quality of the sediments in Mercer Lake, especially wastewater discharges from 

septic systems, the municipal sewage treatment plant, lumber 

mill and processing operations, and streets and urban areas. 

Urbanization in the basin increases the likelihood that synthetic organic compounds and trace metals will be transported 

to the lake. The bottom sediments of a lake provide a historical 

record of these chemical inputs, because many substances 

entering the lake eventually are deposited in the sediments 

in deeper areas. Some of these substances may be released 

back into the water column with changing environmental 

conditions.

Many of the priority pollutants identified by the U.S. 

Environmental Protection Agency (EPA) were among the elements and compounds examined in the sediment analysis for 

Mercer Lake. Many of these substances have been identified 

as suspected carcinogens or potentially toxic in other ways to 

wildlife or humans. Their occurrence in both the water and 

sediment of aquatic systems depends largely on the characteristics of the watershed and land use within it. Organic content, 

nutrients, PAHs, disinfectants, detergents, fuel components, 

combustion products, and some metals (cadmium, chromium, 

copper, lead, and zinc) could indicate the water was affected 

by municipal waste input and urban runoff. PAHs, PCBs, and 

chlorinated hydrocarbons and many metals are byproducts of 

industrial activities, and their presence in elevated concentrations in natural waters could indicate the lake was affected 

by industrial discharges. Other chlorinated hydrocarbons 

and some metals are derived from pesticides and herbicides, 

and they would most likely occur in aquatic systems that 

receive agricultural drainage, which should not be much of a 

factor in the Mercer Lake watershed. In lakes that are remote 

from point-source discharges, the primary source of many 

of these substances is atmospheric deposition. In still other 

systems, weathering or leaching of certain elements from 

rocks and soils in the watershed can produce unexpectedly 

high concentrations of certain constituents in receiving waters, 

even if inputs from industry and agriculture are minimal or 

nonexistent.

Over the past several years, different entities including several States, EPA, Canadian Provinces, and various 

researchers have each developed sets of effects-based sediment-quality guidelines (Wisconsin Department of Natural 

Resources, 2003). The guidelines were developed using 

empirical approaches that established databases describing a 

range of effects (such as reduced survival, growth, or reproduction of benthic macroinvertebrate organisms) to a range 

of concentrations of individual sediment-associated contaminants. The guidelines established two concentration levels 

based on biological effects— a lower effect concentration at 

which no or minimal effects are predicted (the threshold effect 

concentration or TEC) and an upper effect concentration at 

which adverse effects are highly probable or will frequently be 

seen (the probable effect concentration or PEC). The TECs 

are conservative guidelines of possible problems related to 

sediment chemistry; they represent a lower level of concern 

than the PECs, and exceedances do not necessarily indicate 

toxicity. A few environmental factors may increase or decrease 

the toxicity of some of the metals and organic compounds. 

Anoxic sediments that occur in the bottom of many lakes 

may cause some metals to be present in their reduced form 

that may be more toxic than the TECs and PECs indicate (for 

example aluminum), whereas, high percentages of organic 

matter and/or small grains in bottom sediments can reduce 

the bioavailability and toxicity of some metals (Ankley et al., 

1996) and organic compounds.

Organic Content, Phosphorus, and Trace 

Elements

The composition of the sediment from the four areas of 

Mercer Lake and the sediment of Grand Portage Lake (the 

lake that was thought to be relatively unaffected by human 

activities and was used as a reference site to represent background conditions) are summarized in tables 6 and 7. In 

general, concentrations in the deep areas of Mercer Lake were 

similar to those in Grand Portage Lake for phosphorus and all 

trace elements (table 6), but concentrations of phosphorus and 

a few trace elements (aluminum, iron, and manganese) were 

higher in the deep basins of both lakes than in the shallow 

bays. Aluminum, iron and manganese are naturally occurring elements abundant in soil and rocks (Hem, 1985). High 

concentrations of these elements are likely a reflection of the 

natural iron ore mineralization found in Iron County. Concentrations of lead and zinc were highest in the West Basin 

of Mercer Lake, likely as a result of effluent from the nearby 

storm sewer outlet, indicating urban storm water runoff and 

municipal sources. Concentrations of iron, manganese, cadmium, copper, lead and zinc exceeded their respective TECs, 

but only concentrations of lead in the West Basin of Mercer 

Lake were at the PEC. 

Synthetic Organic Compounds

Only a subset of the wastewater compounds for which 

samples were analyzed as part of this study (Appendix 1) were 

detected in the sediments of Mercer and Grand Portage Lakes 

(table 7); however, for most wastewater compounds, concentrations were less than the detection limit and therefore are not 

listed in table 7. In addition, many of the detected compounds 

were reported by the laboratory as estimated concentrations 

because of low concentrations and recovery, precision, blank 

contamination, or reference standard issues commonly associated with analyzing these compounds. The minimum reporting 

level (less-than values) for some of the compounds was also 

relatively high, greater than the TECs, because the sample size 

was smaller than optimum or had large organic content (for 
Table 6. Phosphorus and trace element concentrations in the bottom sediment of Mercer and Grand Portage Lakes, Iron County, Wis. [TEC, threshold effect concentration; PEC, probable effects concentration; NA, not available; <, less than] Constituent Reporting level Station name, site number on figure 2, and station identification number 

Mercer Lake, near Outlet, s6 460930090043200 Mercer Lake, Northwest Bay, s5 460950090041300 Mercer Lake, West Basin, deepest location, s2 460945090040600 Mercer Lake, East Basin, Deep Hole, s1 460937090033100 Grand Portage Lake, Northeast Bay Deep Hole 461030090023800 TE

Volatile solids (percent) and phosphorus (milligram per kilogram of sediment)2 Volatile solids 0.1 45.2 58.1 48.4 35.0 44.3 NA 

Total phosphorus 9.9 679 720 1320 885 1210 NA 

Phosphate, as phosphorus, iron and aluminum bound 5 83 83 399 186 514 NA 

Trace elements (milligram per kilogram of sediment)3 Aluminum, recoverable 2.8 5400 4300 8700 8800 10000 NA 

Boron, recoverable 1.8 < 3.6 < 3.6 < 9.0 < 9.0 < 5.4 NA 

Cadmium, recoverable 0.001 1.3 0.83 1.6 1.6 1.7 Chromium, recoverable 0.4 26 19 32 42 36 4

Cobalt, recoverable 0.001 3.5 2.2 4.9 4.9 6 NA 

Copper, recoverable 2 33 25 42 32 44 3Iron, total digestable 2.1 16,000 12,000 53,000 42,000 35,000 20,00

Lead, recoverable 0.001 64 28 130 85 84 3

Manganese, recoverable 0.3 960 550 640 580 610 46

Mercury, recoverable 0.0072 0.097 0.051 0.094 0.105 0.102 Zinc, recoverable 0.1 160 74 200 170 160 12

1Wisconsin Department of Natural Resources, 2003. 2Analyses conducted by the Wisconsin State Laboratory of Hygiene in Madison, Wis. 3Analyses conducted by the U.S. Geological Survey National Water Quality Laboratory in Denver, Colo.
Table 7. Concentrations of wastewater compounds detected in the bottom sediment of Mercer and Grand Portage Lakes, Iron County, Wis.—Contin[Concentrations are reported in microgram per kilogram of sediment except where noted. All analyses, except polychlorinated biphenyls (PCBs), were conducted at the U.S. GeQuality Laboratory in Denver, Colo. PCBs were analyzed by the Wisconsin State Laboratory of Hygiene in Madison, Wis. TEC, threshold effect concentration; PEC, probable ethan; E, estimated concentration reported; NA, not available; ND, no data; PCB, polychlorinated biphenyls; PAH, polycyclic aromatic hydrocarbon; CP, combustion product] Constituent Station name, site number on figure 2, and station identification number Standard Pos

Mercer Lake, near Outlet, s6 460930090043200 Mercer Lake, Northwest Bay, s5 460950090041300 Mercer Lake, West Basin, deepest location, s2 460945090040600 Mercer Lake, East Basin, Deep Hole, s1 460937090033100 Grand Portage Lake, NE Bay Deep Hole 461030090023800 TEC1 PEC1 Indicators of sewage contamination 3-beta-Coprostanol, solids < 1,700 < 2,800 E 740 E 660 < 1,800 NA NA Carn

3-Methyl-1H-indole, solids 270 E 210 E 220 250 250 NA NA Frag

co

beta-Sitosterol, solids E 4,500 E 4,800 E 5,600 E 5,000 E 5,500 NA NA Sewa

ste

beta-Stigmastanol, solids E 3,500 E 3,800 E 5,600 E 4,400 E 5,200 NA NA Sewa

ste

4-Nonylphenol diethoxylate,sd E 1,500 E 1,800 E 2,300 E 2,900 < 3,600 NA NA Sewa

m

4-t-Octylphenol diethoxylate < 170 < 280 < 230 < 220 < 180 NA NA Sewa

tab

Benzo[a]pyrene, solids E 30 < 280 E 70 E 60 E 30 150 1,450 Regu

re

Cholesterol, solids E 1,200 E 1,600 E 2,700 E 1,900 E 2,000 NA NA Feca

Acetophenone, solids < 520 E 700 < 680 < 650 < 540 NA NA Frag

ba

Indole, solids 2,800 2,000 4,100 E 2,200 3,000 NA NA Pesti

gr

Naphthalene, solids E 40 < 280 E 110 E 90 E 50 176 561 PAH

m

Indicators of street and fuel contamination 2,6-Dimethylnaphthalene, solids 200 E 150 510 390 370 NA NA Dies

ga

2-Methylnaphthalene, solids < 170 < 280 E 40 < 220 < 180 20.2 201 Gaso

(2

Fluoranthene, solids E 100 E 50 300 E 190 E 120 423 2,230 PAH

ag

Isophorone, solids E 40 E 190 E 60 E 50 E 40 NA NA Solv

s
Table 7. Concentrations of wastewater compounds detected in the bottom sediment of Mercer and Grand Portage Lakes, Iron County, Wis.—Contin[Concentrations are reported in microgram per kilogram of sediment except where noted. All analyses, except polychlorinated biphenyls (PCBs), were conducted at the U.S. GeQuality Laboratory in Denver, Colo. PCBs were analyzed by the Wisconsin State Laboratory of Hygiene in Madison, Wis. TEC, threshold effect concentration; PEC, probable ethan; E, estimated concentration reported; NA, not available; ND, no data; PCB, polychlorinated biphenyls; PAH, polycyclic aromatic hydrocarbon; CP, combustion product] Constituent Station name, site number on figure 2, and station identification number Standard Pos

Mercer Lake, near Outlet, s6 460930090043200 Mercer Lake, Northwest Bay, s5 460950090041300 Mercer Lake, West Basin, deepest location, s2 460945090040600 Mercer Lake, East Basin, Deep Hole, s1 460937090033100 Grand Portage Lake, NE Bay Deep Hole 461030090023800 TEC1 PEC1 Indicators of street and fuel contamination—Continued Phenanthrene, solids E 90 < 280 E 210 E 130 E 90 204 1,170 PAH

sd

Prometon, solids < 170 < 80 < 230 < 220 < 180 NA NA Her

p

Pyrene, solids E 80 E 40 230 E 160 E 90 195 1,520 PAH

ag

Indicators of lumber processing contamination Anthracene, solids < 170 < 280 E 30 E 30 E 10 57 845 PAH

po

p-Cresol, solids E 620 E 570 E 530 3,300 1,100 ND ND Wo

Indicators of general industrial contamination Phenol, solids E 3,300 E 7,600 < 440 E 730 E 680 4,200 12,000 Dis

sfu

Total polychlorinated biphe- nyls (PCBs in milligram per gram of sediment)3 0.024 ND ND 0.024 ND 60 676 Man

1 Wisconsin Department of Natural Resources, 2003. 2 Uses obtained from J.R. Ward, U.S. Geological Survey,. written commun., February 28, 2006. 3 Reporting limit is 0.024 milligram per kilogram of sediment.
Hydrology and Water Budget 27

example, anthracene) that made comparisons uncertain. Most 

guidelines (TECs and PECs) are usually expressed on a dry 

weight basis normalized at 1 percent total organic carbon to 

put the results on a common basis; however, this normalization process could not be performed because the information 

was not available. The high organic content (35 to 58 percent 

volatile solids) of the sediments in both lakes has a significant effect on decreasing the potential toxicity of some of the 

organic compounds to values below the TEC or PEC. 

Therefore, the toxicity for several compounds (based on the 

comparisons with the TECs and PECs in table 7) appears 

worse than what it actually would be if the data were normalized to the percent total organic carbon in the sample. 

The sediments of both lakes contained many constituents 

that are indicative of the past activities in their watersheds. 

In table 7, these constituents are divided into indicators of 

sewage, street and fuel, lumber industry, and general industrial 

contamination. Concentrations of most of the indicators of 

sewage contamination were as high in Grand Portage Lake as 

they were in Mercer Lake, except acetophenone, which was 

relatively high in the northwest bay of Mercer Lake. Acetophenone is used in detergents and as a flavor in beverages. 

Concentrations of many of the indicators of street runoff and 

fuels were higher in Mercer Lake than in Grand Portage Lake. 

Concentrations of the PAHs phenanthrene and pyrene, components of coal tar and oil, and 2-methylnaphthalene, a fuel component, were highest in sediment of the West Basin of Mercer 

Lake. Concentrations of 2-methylnaphthalene, phenanthrene, 

and pyrene were near or slightly exceeded the TECs at one or 

more sites in Mercer Lake. Concentrations of two indicators 

of lumber industry contamination were found in both lakes: 

anthracene and p-cresol. The concentration of p-cresol, a wood 

preservative without recommended guidelines, was highest at 

Deep Hole in the East Basin of Mercer Lake. Two indicators 

of general industrial contamination, phenol and PCBs, were 

found in the sediments of both lakes. Phenol, which is used 

in many kinds of manufacturing, was found in relatively high 

concentrations (near or just above the TEC) in the northwest 

bay and near the Mercer Lake Outlet. Total PCBs were found 

at the detection limit of 0.024 mg/kg in the Deep Hole of East 

Basin of Mercer Lake and near the lake outlet and were not 

of concern as were concentrations of all individual alochlors, 

which were below the detection limit. None of these compounds that were detected in the sediments of the lakes are 

known or suspected to have endocrine-disrupting potential. 

The results of the analysis of the bottom sediments of 

Mercer and Grand Portage Lakes indicate that industrial 

and municipal effluents and runoff from the watershed have 

produced slightly elevated concentrations of a few elements 

and compounds in the sediments. Concentrations of metals 

and organic compounds in sediments of the northwest bay and 

near the lake outlet were not markedly different from those 

in other areas of the lake, as would be expected if they had 

been affected by discharges from the old wastewater treatment 

plant.The findings for the synthetic organic chemicals are consistent with the land use in the Mercer Lake watershed. 

The low concentrations of PAHs and PCBs correspond to the 

relative absence of industries that would release these compounds in the watershed, unlike findings from more urban 

areas like Milwaukee (Thomas and others, 2007). Similar 

to that found in this study, Fitzpatrick and Peppler (2003) 

found elevated concentrations of pentachlorophenol, p-cresol, 

and other organic compounds in sediments of an abandoned 

sawmill pond in northern Wisconsin. Although the concentrations of several potentially toxic elements and compounds 

were found to slightly exceed their published TEC guidelines, 

they were generally well below PECs and of low concern. The 

actual effects of these concentrations on organisms are complicated by many interacting environmental factors and the actual 

effects are uncertain. 

Hydrology and Water Budget

Because the productivity in Mercer Lake is limited by 

phosphorus or possibly co-limited by phosphorus and nitrogen 

(based on N:P ratios) and because decreases in phosphorus 

concentrations should favor production of green algae over 

blue-green algae, decreases in the input of phosphorus should 

reduce phosphorus concentrations in the lake and thus improve 

water quality. Therefore, it is important to understand the 

origins of the phosphorus reaching the lake. Almost all of the 

phosphorus entering the lake is transported by water inputs; 

therefore, to quantify phosphorus inputs, it is necessary to first 

quantify the water inputs. Water budgets for the lake were 

quantified for each of the two monitoring years, MY 2008 and 

MY 2009.

The hydrology of the lake can be described in terms of 

components of its water budget. The water budget for a period 

of interest may be represented as follows:

	 DS = (P + SWIn + GWIn) – (E + SWOut + GWOut) (4)

where DS is change in the volume of water stored in the lake 

and is equal to the sum of the volumes of water entering the 

lake minus the sum of the volumes leaving the lake. Water 

enters the lake as precipitation (P), surface-water inflow 

(SWIn), and groundwater inflow (GWIn). Water leaves the lake 

through evaporation (E), surface-water outflow (SWOut), and 

groundwater outflow (GWOut). Each term of the water budget 

was estimated for MY 2008–09 on a daily basis.

Change in Storage

Changes in the volume of the lake were determined from 

water elevations measured continuously at the lake-stage gage 

on the south shore of the West Basin of the lake (fig. 2). In 

MY 2008, water levels fluctuated from 1584.4 ft in May to 

1583.6 ft in October, and in MY 2009, water levels fluctuated 

from 1584.1 ft in November to 1583.5 ft in July (U.S. Geological Survey, 2009–10). For MY 2008, the net change in lake 
28 Water Quality, Hydrology, and Simulated Response to Changes in Phosphorus Loading of Mercer Lake, WI, and Effects of Wastewater Discharge

level from the beginning to end of the period was −0.14 ft, 

representing a loss in storage of 23 acre-ft (table 8). For MY 

2009, there was a slight gain in stage of 0.04 ft, representing a 

gain in storage of 6 acre-ft.

Precipitation

During the 2-year study period, daily precipitation was 

measured with a recording gage at the main inlet site on the 

Little Turtle River during nonfreezing periods, and it was 

estimated from the average of that measured in the communities of Butternut (24 mi southwest of the lake), Upson (22 mi 

northwest), and Manitowish Waters (8 mi east) during freezing 

periods. Data from the nearby weather stations were obtained 

from the National Weather Service National Climatic Data 

Center (2007–09). Precipitation on the lake surface during 

MY 2008 was 25.93 in. (399 acre-ft), and during MY 2009 

was 21.12 in. (325 acre-ft). Precipitation was 81.5 percent of 

the long-term average (1971–2000) in 2008 and 66.4 percent 

in 2009, reflecting an extended dry period in the region. 

Monthly precipitation during each study year is compared to 

the 1971–2000 average monthly precipitation in figure 12. 

In MY 2008, precipitation totals for May through September 

were below average. In MY 2009, only December and October 

totals were above the long-term average, and all other months 

were well below average. 

Evaporation

The rate of evaporation from the lake surface was 

assumed to be similar to the average monthly rates estimated 

for Minocqua Lake (26.5 mi southeast of Mercer Lake) during 

2006–07 by Garn and others (2010). During 2006–07, Minocqua Lake also experienced very dry conditions and received 

about 85 percent of the long-term average precipitation. It was 

assumed that evaporation from the lake surface was zero during periods of ice cover and that the lake was ice covered from 

December through March. Annual evaporation was estimated 

to be 23.93 in. (368 acre-ft; table 8).

Gaged Surface-Water Inflows

Daily inflow from the Little Turtle River, the main tributary to the lake, was measured at the gaging station installed 

at Garnet Street (fig. 2). The annual average flow at this 

site was 5.20 ft3

/s (3,770 acre-ft) in MY 2008 and 4.35 ft3/s 

(3,150 acre-ft) in MY 2009 (table 8, fig. 13A). Annual flows in 

the Little Turtle River and other rivers in the region reflected 

the very dry conditions in north-central Wisconsin during 

both monitoring years. In 2009, the extremely dry summer prompted the Governor to declare counties in northern 

Wisconsin in a state of emergency because of drought conditions. In MY 2008, runoff at nearby long-term USGS gaging 

Table 8. Summary of water-budget components for Mercer Lake, Iron County, Wis., during monitoring years 2008–09 and during 

typical hydrologic conditions.

Typical conditions

MY 2008 MY 2009 2-year average Typical conditions

Volume 

(acre-ft) Percent Volume 

(acre-ft) Percent Volume 

(acre-ft) Percent Volume 

(acre-ft) Percent

Inputs to lake

Precipitation 399 7.0 325 7.0 362 7.0 489 6.8

Little Turtle Inlet 3,770 66.4 3,150 67.3 3,460 66.8 4,680 65.4

Tahoe Inlet 248 4.4 232 5.0 240 4.6 324 4.5

Storm drain 22 0.4 21 0.5 22 0.4 29 0.4

Groundwater 878 15.5 876 18.7 877 16.9 1,190 16.6

Ungaged area 359 6.3 72 1.5 216 4.2 441 6.2

Total input 5,680 100.0 4,680 100.0 5,180 100.0 7,150 100.0

Outputs from lake

Evaporation 368 6.5 368 7.9 368 7.1 403 5.6

Little Turtle Outlet 5,330 93.5 4,300 92.1 4,820 92.9 6,750 94.4

Groundwater 0 0.0 0 0.0 0 0.0 0 0.0

Total output 5,700 100.0 4,670 100.0 5,190 100.0 7,150 100.0

Change in storage (acre-ft and percent of total input)

Change in storage –23 –0.4 6 0.1 –9 –0.2 0 0.0
0

1

2

3

4

5

Nov.Dec.

Jan.

Feb.

Mar.

Apr.

May

June

July

Aug.

Sept.

Oct.

Nov. 2007 – Oct. 2008

Nov. 2008 – Oct. 2009

1971 – 2000 average

Precipitation, in inches

Hydrology and Water Budget 29

Figure 12. Monthly precipitation at Mercer Lake, Iron County, Wis., during monitoring years 2008–09 relative to long-term 

average precipitation for the period 1971–2000.

stations was 64 percent of the long-term average annual runoff 

(1936–2008) at Wisconsin River at Rainbow Lake near Lake 

Tomahawk, Wis., 65 percent of the average annual runoff 

(1991–2008) at Bear River near Manitowish Waters, Wis., 

and 67 percent of the average annual runoff (1991–2008) at 

the Trout River at Trout Lake near Boulder Junction, Wis. 

(U.S. Geological Survey, 2009). In MY 2009, runoff was 

40 percent of the average annual runoff at the Bear River 

station, 54 percent at Wisconsin River station, and 57 percent 

at the Trout River station (U.S. Geological Survey, 2010).

To estimate flow from the Lake Tahoe watershed, 

intermittent discharge measurements were collected from 

the stream leaving Lake Tahoe that enters the west side of 

Mercer Lake. The measured flows at the Tahoe Inlet and the 

continuously recorded flow at the Little Turtle Inlet were used 

to develop a regression relation to predict flow at the Tahoe 

Inlet from the flow at Little Turtle Inlet. The annual average 

flow at this site was 0.341 ft3/s (248 acre-ft) in MY 2008 and 

0.321 ft3/s (232 acre-ft) in MY 2009 (table 8).

Daily inflow from the storm drain outlet, the other monitored tributary to the lake, was measured at the gaging station 

near the boat landing (fig. 2). The annual average flow at this 

site was 0.031 ft3/s (22 acre-ft) in MY 2008 and 0.029 ft3/s 

(21 acre-ft) in MY 2009 (table 8). This site is characterized by 

very sudden changes in flow (fig. 13B). 

Surface-Water Outflow

Daily outflow from the lake was measured at the gaging 

station installed at the bridge crossing the Little Turtle River at 

the Mercer Lake Outlet (fig. 2). Outflow ranged from about 

2 ft3/s in late summer to 20 ft3/s in spring (fig. 13C). The average annual surface-water outflow was 7.35 ft3

/s (5,330 acre-ft) 

in MY 2008 and 5.94 ft3/s (4,300 acre-ft) in MY 2009 (table 8). 

Groundwater Flow near and into Mercer Lake

The two-dimensional, analytic-element, steady-state, 

groundwater-flow model, GFLOW (Haitjema, 1995) was used 

to describe the groundwater flow near Mercer Lake, define 

the groundwater-source area around the lake, and describe 

the groundwater flow into the lake. A complete description of 

analytic-element modeling is beyond the scope of this report; 

however, a brief description is given below. A complete discussion of the methods for applying the model can be found in 

Strack (1989) and Haitjema (1995).

An infinite aquifer, the underground layer of water-bearing soil and rock, is assumed in analytic-element modeling. 

In GFLOW, the study area (domain) does not require a grid or 

involve interpolation between cells. To construct an analyticelement model, features that affect groundwater flow (such 
Figure 13. Daily discharge, phosphorus concentrations, and phosphorus loads at A, Little Turtle Inlet, B, storm 

drain outlet near the boat landing, and C, Mercer Lake Outlet, during monitoring years 2008–09.

A

B

C

0

5

10

15

20

25

30

35

40

45

Discharge, in cubic feet per second, and concentration, 

in milligrams per liter * 1,000

0.0

0.5

1.0

1.5

Phosphorus load, in pounds per day 

0.0

0.1

0.2

0.3

0.4

0.5

Discharge, in cubic feet per second

0.0

0.5

1.0

1.5

2.0

2.5

Phosphorus load, in pounds per day,

 and concentration in milligrams per liter 

0

10

20

30

40

Nov. 2007

Jan. 2008

Mar. 2008

May 2008

July 2008

Sept. 2008

Nov. 2008

Jan. 2009

Mar. 2009

May 2009

July 2009

Sept. 2009

Nov. 2009

Discharge, in cubic feet per second, and concentration, 

in milligrams per liter * 1,000

0.0

1.0

2.0

3.0

4.0

Phosphorus load, in pounds per day 

Date

EXPLANATION

Outlet discharge

Phosphorus concentration

Phosphorus load

Inlet discharge

Storm sewer discharge

30 Water Quality, Hydrology, and Simulated Response to Changes in Phosphorus Loading of Mercer Lake, WI, and Effects of Wastewater Discharge
Hydrology and Water Budget 31

as surface-water bodies, aquifer characteristics, and recharge) 

are entered as mathematical elements or strings of elements. 

The amount of detail specified for the features depends on the 

distance from the area of interest. Each element is represented 

by an analytic solution. The effects of these individual solutions are added together to arrive at a solution for the groundwater-flow system. In GFLOW, the analytic elements are two 

dimensional and are used to simulate steady-state conditions. 

Groundwater-flow systems are three dimensional; however, 

two-dimensional models can provide reasonable approximations of groundwater flow lines (paths of groundwater movement) when the lengths of the flow lines are long compared to 

the aquifer thickness (Haitjema, 1995). In the study area, most 

groundwater was assumed to move through unconsolidated 

deposits that have a maximum saturated thickness of 200 ft. 

The lengths of flow lines from recharge areas to discharge 

areas are typically several thousand feet or more in length. 

The GFLOW model constructed in this study covers an 

area extending approximately 5–10 mi around Mercer Lake 

(fig. 14). The geometry of the single-layer model includes a 

bottom elevation set at 1,500 ft and an average aquifer thickness of 100 ft, which were based on well logs for the study 

area and additional information from Trotta and Cotter (1973). 

Recharge was applied uniformly across the simulated area. 

Surface-water features were simulated using several types of 

linesink elements. Line sinks are mathematical representation 

of a stream or lake. They can also be thought of as an infinite 

number of shallow wells placed along the line, all pumping at 

a finite rate of discharge. The model includes “far-field” and 

“near-field” linesinks to represent surface-water features. The 

near field represents the area of interest and includes Mercer 

Lake and most adjacent lakes and streams. The far-field 

area surrounds the near-field area of interest. In the far field, 

streams and lakes are simulated as coarse linesinks having 

little or no resistance between the surface-water feature and 

the groundwater-flow system. Resistance is the force tending 

to retard groundwater movement. The purpose of simulating 

the far field is to have the model explicitly define the regional 

groundwater divide surrounding the area of interest. 

Most near-field lakes and streams, which are in good connection with the groundwater-flow system, were included as 

linesinks with a specified resistance. Some lakes were not simulated, but instead used as water-level targets during calibration. In analytic-element modeling, resistance is computed by 

dividing the bed-sediment thickness by the vertical hydraulic 

conductivity of the sediment. Hydraulic conductivity is a measure of the permeability of the aquifer to groundwater movement. Near-field streams were simulated using linesinks with 

resistance of 0.5 day and widths ranging from 5 to 50 ft. Nearfield lakes were simulated as linesinks with resistance values 

dependent upon the lake type. Resistance was set to 10 days 

for drainage lakes (those with surface-water inflows and outflows) and 50 days for seepage lakes (those without surfacewater inflows or outflows), which follows the approach used 

by Juckem and Hunt (2007). Resistance values used in this 

model are similar to values used in other GFLOW models of 

areas near Mercer Lake (Hunt and others, 1998; Graczyk and 

others, 2003, Robertson and others, 2005; Juckem and Hunt, 

2007; Garn and others, 2010). Lake widths and depths were 

determined from USGS topographic maps. Lake stages were 

measured in September 2008 using a RTK-GPS survey instrument with vertical accuracy of approximately 0.1 ft. 

The GFLOW model was calibrated using the parameter 

estimation code PEST (Doherty, 2010). Measured streamflows, lake stages, and water levels from 112 well logs (using 

measured depth to water and corresponding topographic 

quadrangle map elevations), along with estimated groundwater 

inflow to Mercer Lake, were used as groundwater level and 

flow rate targets during model calibration. The estimated 

groundwater inflow rate used in the GFLOW model was determined by selecting a value that allowed the calculated stage 

of the lake (using equation 4, with all of the water-budget 

components described above) to equal or parallel the actual 

lake stage for periods when there was little or no runoff from 

the ungaged near-lake drainage area around the lake. Streamflows and nearby lake levels represented the average flows 

and levels for the 2-year monitoring period. Water levels for 

distant lakes were estimated from USGS topographic maps. 

Because groundwater levels from the 112 well logs were 

measured over a period of years and well elevations were 

estimated from topographic maps, these targets were given 

less weight (weight=0.2) during calibration than those for 

lake stages and flows. Weights assigned to targets for calibration were multiplied by the target residual (measured value 

minus simulated value) to give higher significance to the most 

accurate and important measurements during the calibration 

process. Weights for lake stages ranged from 0.2 for distant 

lakes with water levels estimated from topographic maps to 5 

for nearby seepage lakes with measured water levels. Weights 

for streamflows ranged from 0.00001 to 0.0001. Although it 

appears water levels and well logs were given more weight 

than streamflows, streamflows were actually given relatively 

higher weights, because streamflows were in units of cubic 

feet per day, as opposed to water levels that were in units 

of feet. Weight for groundwater seepage into Mercer Lake 

was set at 0.005, a relatively high weight for the calibration. 

Weights were generally guided by the expected measurement accuracy of the targets, but also by the model objective, 

which was to simulate the groundwater-contributing area to 

the lake. Because the groundwater-contributing area is directly 

related to both recharge and groundwater inflow to the lake, 

the groundwater-inflow target was the most heavily weighted 

target. Recharge and horizontal hydraulic conductivity were 

automatically varied during model calibration until weighted 

differences between measured and simulated groundwater 

inflow to Mercer Lake, water levels, lake stages, and streamflows were minimized.

The calibrated model had a recharge rate of 4.7 in/yr and 

hydraulic conductivity of 36.1 ft/d. The hydraulic conductivity 

is similar to values used in previous models developed in other 

nearby studies (Hunt and others, 1998; Graczyk and others, 

2003; Robertson and others, 2005; Garn and others, 2010), 
32 Water Quality, Hydrology, and Simulated Response to Changes in Phosphorus Loading of Mercer Lake, WI, and Effects of Wastewater Discharge

IRON COUNTY

VILAS COUNTY

Mercer Lake

Iron County

Vilas County

0 2 4 MILES

0 2 4 KILOMETERS

Base map compiled and modified from digital data sources,

U.S. Geological Survey, 1:250,000

90°10' 90° 89°50'

46°15'

46°5'

EXPLANATION

Simulated area

Near-field element (linesink with resistance)

Far-field element (linesink without resistance)

Base-flow target

Lake-stage target

Groundwater-level target

County boundary

Road

Figure 14. Hydrologic features simulated with the GFLOW analytic groundwater-flow model near Mercer Lake, Iron County, Wis.
1,540

1,550

1,560

1,570

1,580

1,590

1,600

1,610

1,620

1,630

1,640

1,540 1,550 1,560 1,570 1,580 1,590 1,600 1,610 1,620 1,630 1,640

Groundwater level targets: 

MD = -7.3 ft, MAD = 11.3 ft

Lake water level targets:

MD = -0.7 ft, MAD = 3.0 ft

EXPLANATION

Measured water level, in feet above the National Geodetic Vertical Datum of 1929 (NGVD 29)

Simulated water level, in feet above the National Geodetic Vertical Datum of 1929 (NGVD 29)

1:1 line

Hydrology and Water Budget 33

Figure 15. Measured and simulated water levels for the GFLOW model of the area near Mercer Lake, Iron County, Wis. The mean 

difference (MD) and mean absolute difference (MAD) between the measured and simulated water levels are given on the graph.

but the recharge rate is low and reflects the influence of the 

prolonged drought in the region. The recharge rate is within 

the range estimated by Dripps and others (2006) for the period 

1996–2000, a period prior to the start of the drought, but it 

is not expected to represent “normal” hydrologic conditions. 

Simulated heads and flows were reasonably close to measured 

values. The mean and mean absolute difference between simulated and measured groundwater levels were -7.3 and 11.3 ft, 

respectively (fig. 15). For lake stages within the modeled area, 

mean and mean absolute differences were -0.7 and 3.0 ft, 

respectively (fig. 15). The mean and mean absolute differences 

for streamflow were 0.6 and 0.7 ft3

/s, respectively (fig. 16). 

Groundwater inflow to Mercer Lake was within 0.1 percent of 

the target value (fig. 16).

Based on the similarity between measured and simulated water levels and flows, the model appears to accurately simulate groundwater flow near Mercer Lake during 

MY 2008–09. The model represents one set of steady-state, 

baseflow conditions; however, in reality there are seasonal 

variations in groundwater levels, lake stage, and streamflow. 

It was assumed that seasonal variations in groundwater levels 

and natural lake stages would be relatively uniform throughout the model area, thus having minimal effect on the extent 

of the groundwater-contributing areas as well as the net gains 

and losses to the groundwater-flow system immediately 

adjacent to near-field lakes. In addition, it was assumed 

that human-induced seasonal changes in lake stages (dams) 

would cause only short-term effects on gains and losses 
0

100,000

200,000

300,000

400,000

500,000

600,000

700,000

800,000

900,000

0 100,000 200,000 300,000 400,000 500,000 600,000 700,000 800,000 900,000

Streamflow targets; 

MD = 51,800 ft3/d or 0.6 ft3/s,

MAD = 60,500 ft3/d or 0.7 ft3/s

Groundwater inflow target 

for Mercer Lake

EXPLANATION

Measured streamflow and groundwater inflow, in cubic feet per day

Simulated streamflow and groundwater inflow, in cubic feet per day

1:1 line

34 Water Quality, Hydrology, and Simulated Response to Changes in Phosphorus Loading of Mercer Lake, WI, and Effects of Wastewater Discharge

Figure 16. Measured and simulated streamflow and groundwater inflow to Mercer Lake for the GFLOW model of the area near 

Mercer Lake, Iron County, Wis. The mean difference (MD) and mean absolute difference (MAD) between the measured and 

simulated streamflows and inflows are given on the graph.

and that those effects would be minimal on an annual basis. 

Based on these assumptions, the model can be used to define 

groundwater-contributing areas around Mercer Lake. It was 

further assumed that groundwater-flow directions remained 

relatively constant over a range of dry and wet years, because 

the water level change in the surrounding drainage lakes 

is small. To evaluate the influence of recharge rates on the 

groundwater-contributing area, a range of recharge rates, from 

the calibrated rate of 4.7 in/yr to 10 in/yr (a more reasonable 

estimate for “typical” conditions), were simulated with the 

model, whereas linesink elevations remained unchanged in the 

simulation. As expected, this change in recharge rates had relatively little influence on the simulated groundwater-contributing area to Mercer Lake. Thus, lacking differential changes in 

surface-water elevations, higher recharge during more typical 

years will simply change the amount of water flowing through 

the system, but not flow directions. 

The groundwater-contributing area for Mercer Lake was 

delineated using particle-tracking techniques in GFLOW. The 

groundwater-contributing area covers 3.0 mi2

 (shaded area 
EXPLANATION

Groundwater flow path (arrow indicates direction of flow)

Simulated water-table altitude, 5-foot contour interval

Road

River

Lake

Area contributing groundwater to Mercer Lake

Base map compiled and modified from digital data sources,

U.S. Geological Survey, 1:24,000

90°5' 90°

46°12'

46°10'

0 0.5 1 MILE

0 0.5 1 KILOMETER

1580

1590

1600

1595

1605

1610

1585

1615

1580

1610

1605

1590

1615

Mercer Lake

Figure 17. Simulated groundwater-flow directions and area contributing groundwater to Mercer Lake, Iron County, Wis.

Hydrology and Water Budget 35

in fig. 17) and extends beyond the surface-water contributing 

area, particularly to the south and east of Mercer Lake. Simulated groundwater flow indicates that discharge (groundwater 

input) occurs along the entire shoreline surrounding Mercer 

Lake, which matches the results from the near-lake piezometers. Net groundwater flow into the lake by direct discharge 

along shoreline segments was simulated to be 1.21 ft3/s (about 

877 acre-ft per year) for MY 2008–09. The area of greatest 

groundwater discharge to Mercer Lake is along the east and 

northwest shorelines (fig. 17).

Although this model was considered reasonably calibrated and was sufficient for estimates of groundwater inflow 

to Mercer Lake during the monitoring period, local heterogeneities that were not simulated with the model may locally 

alter the contributing area to a small degree. Use of the 

model for evaluations beyond the stated purpose (to estimate 

groundwater-contributing areas to the lake and groundwater 

exchanges with the lake) may require reevaluation of the 

model and may require recalibration of the model to match 

new or existing data that correspond to the new purpose.
Water inputs: typical year (7,150 acre-feet)

Water inputs: 2-year average (5,180 acre-feet)

Groundwater

16.9%

Tahoe Inlet

4.6%

Storm drain

0.4%

Ungaged area

4.2%

Water outputs: 2-year average (5,190 acre-feet)

Water outputs: typical year (7,150 acre-feet)

Little Turtle Inlet

66.8%

Precipitation

7.0%

Groundwater

16.6%

Evaporation

7.1%

Evaporation

5.6%

Outlet

94.4%

Outlet

92.9%

Little Turtle Inlet

65.4%

Precipitation

6.8%

Tahoe Inlet

4.5%

Storm drain

0.4%

Ungaged area

6.2%

36 Water Quality, Hydrology, and Simulated Response to Changes in Phosphorus Loading of Mercer Lake, WI, and Effects of Wastewater Discharge

Figure 18. Water budgets for Mercer Lake, Iron County, Wis., for monitoring years 2008–09 and for typical hydrologic conditions. 

Ungaged Near-Lake Drainage Area

Flow from the near-lake drainage area (fig. 2) includes 

unchannelized runoff going directly into the lake, runoff 

conveyed in numerous small channels, and additional runoff 

from the parts of the town that are drained by storm sewers. 

Part of this runoff is from the gaged storm-drain outlet, but 

the approach used to estimate the ungaged near-lake runoff 

removes this monitored runoff. The ungaged near-lake runoff 

was estimated as the residual in the mass-balance equation 

(eq. 4) after all other inputs, including that from groundwater, 

and outputs were determined. The annual average flow from 

the remaining ungaged near-lake area was estimated to be 

0.49 ft3

/s (359 acre-ft) in MY 2008 and 0.10 ft3/s (72 acre-ft) 

in MY 2009 (table 8). 

Water-Budget Summary

The Little Turtle Inlet is the largest source of water to 

Mercer Lake, supplying about 67 percent of the total input 

during the 2-year study period (table 8; fig. 18). Other sources 

were groundwater (16.9 percent), precipitation (7.0 percent), 

Tahoe Inlet (4.6 percent) and the total near-lake drainage that 

includes the urbanized area and storm drain (4.6 percent). 

The total input of water in MY 2008 (5,680 acre-ft) was more 

than in MY 2009 (4,680 acre-ft) because of the prolonged dry 

period and below-normal precipitation. The residence time—

that is, the length of time required for water entering the lake 

to completely replace the volume of water in the lake—was 

0.46 years, computed on the basis of the average inflow (total 

runoff plus precipitation minus evaporation and not including 

groundwater inputs) for the 2 monitoring years. 
0

100

200

300

400

500

600

700

800

900

1,000

Nov. 2007

Jan. 2008

Mar. 2008

May 2008

July 2008

Sept. 2008

Nov. 2008

Jan. 2009

Mar. 2009

May 2009

July 2009

Sept. 2009

Inputs

Outputs plus change in storage

Volume, in acre-feet

Date

Hydrology and Water Budget 37

Figure 19. Monthly accuracy of the water budget on the basis of comparisons of total water inputs and outputs 

plus storage change for Mercer Lake, Iron County, Wis., during monitoring years 2008–09.

The total annual output of water from the lake was 

very similar to the total input in both years, resulting in little 

change in storage from the beginning to the end of each study 

year. On average, about 93 percent of the total water leaving 

the lake left through the Mercer Lake Outlet and 7 percent left 

through evaporation (table 8; fig. 18). Based on results from 

the piezometers installed around the lake and the groundwater 

model, little water left the lake to the groundwater system. 

The quality or accuracy of the water budget was evaluated by comparing the monthly sum of all inputs with the sum 

of all outputs plus the change in storage for the lake (fig. 19). 

The differences between the calculated inputs and outputs 

reflect the cumulative errors in the estimates of all of the components in the water budget. The water budget was relatively 

accurate on a monthly and annual time scale. The largest 

monthly difference were in June 2008 and May 2009, which 

when expressed as percentages of the total measured output 

plus change in storage were 9.5 in June 2008 and 11.9 percent 

in May 2009. On an annual basis, the percentage error was 

about 0.1 percent for both years.

The hydrology documented in MY 2008–09 represents 

conditions during a series of dry years. To determine how 

the hydrologic loading to the lake would change under more 

typical hydrologic conditions, inputs for each of the budget 

components (table 8) were estimated during long-term average hydrologic conditions. During MY 2008–09, precipitation was 74 percent of the long-term average; therefore, 

precipitation, runoff volumes in the Little Turtle Inlet, storm 

drain, and Tahoe Inlet, and groundwater input were increased 

by 26 percent. A 26-percent increase in the input from the 

ungaged near-lake area was less than that input during 

MY 2008; therefore, the input from the ungaged near-lake area 

in MY 2009 (the wetter year) was increased by 18.5 percent, 

because precipitation in 2009 was 18.5 percent below the 

long-term average. During typical hydrologic conditions, 

evaporation from the lake was assumed to be equal to the 

average total annual free-surface evaporation of 26.2 in. estimated for the area by Farnsworth and others (1982). Outflow 

from the lake was then estimated from the total inputs to the 

lake minus that leaving the lake through evaporation and 

assuming that there were no long-term changes in water level. 

The resulting hydrologic inputs during average hydrologic 

conditions are given in table 8 and figure 18. During typical 

hydrologic conditions, the total water input is estimated to 

be 7,150 acre-ft, which is an increase of 38 percent from that 

measured in MY 2008–09.
38 Water Quality, Hydrology, and Simulated Response to Changes in Phosphorus Loading of Mercer Lake, WI, and Effects of Wastewater Discharge

Sources of Phosphorus

To help define where the phosphorus in Mercer Lake 

originates and how much may be controllable, a detailed 

phosphorus budget was computed for each monitoring year, 

and then this information was used to estimate the phosphorus 

budget for a year with typical hydrologic conditions. External sources of phosphorus to the lake include precipitation, 

surface-water and groundwater inflow, and contributions from 

septic systems. In addition to these external sources, phosphorus can be released from the bottom sediments of the lake, 

which is considered an internal source of phosphorus (internal loading). Phosphorus is removed from the lake through 

surface-water outflow and deposition to the lake sediments. 

Precipitation

Atmospheric deposition of phosphorus on Mercer Lake 

was determined from phosphorus concentrations measured 

in wetfall (rain and snow) and phosphorus-deposition rates 

measured in dryfall (dry particles from the atmosphere) at 

a weather station previously operated at Whitefish Lake in 

Douglas County, Wis. (Robertson, Rose, and Juckem, 2009). 

Total monthly wetfall deposition was computed by multiplying 

the average estimated phosphorus concentration of 0.016 mg/L 

by the monthly precipitation on the lake. Phosphorus in dryfall 

was estimated from deposition rates measured at Whitefish 

Lake, which was adjusted for lakes with few conifers in the 

surrounding area and was approximately 26 lb/mi2

/yr, equally 

distributed from May through October. Total phosphorus 

inputs to the lake from precipitation were 24.9 lb in MY 2008 

and 21.7 lb in MY 2009 (table 9). 

Surface-Water Inflow 

Phosphorus inputs (loads) were computed for the continuously monitored sites at the Little Turtle Inlet, the storm 

drain near the boat landing, and at the Mercer Lake Outlet. 

Phosphorus loads were also estimated for the Tahoe Inlet and 

the ungaged near-lake drainage area.

Gaged Inlets

Phosphorus in Little Turtle Inlet at Garnet Street (fig. 2) 

originates from the upper watershed, including Grand Portage 

Lake and other upstream lakes. Total phosphorus concentrations from 34 samples collected during the 2-year study 

period ranged from 0.009 to 0.040 mg/L (fig. 13A). The 

median and volumetrically weighted concentrations were 

0.016 and 0.018 mg/L, respectively. About 50 percent of the 

phosphorus was in the dissolved form, but this was based on 

only two samples. Daily phosphorus loads from the inlet were 

computed with GCLAS and summed to obtain annual totals. 

The total quantity of phosphorus delivered from Little Turtle 

Inlet was 152 lb in MY 2008 and 169 lb in MY 2009 

(table 9). Annual yields from Little Turtle Inlet watershed were 

14.8 lb/mi2

 in MY 2008 and 16.5 lb/mi2 in MY 2009. These 

yields are slightly less than those measured for the Bear River 

watershed near Manitowish Waters in the Northern Lakes and 

Forests ecoregion (24 lb/mi2; Corsi and others, 1997). The 

yields measured in this study are likely lower than those of the 

other studies because the two study years (MY 2008–09) were 

characterized by below-average precipitation and runoff and 

because of the trapping effect of the upstream lakes.

Phosphorus in Tahoe Inlet originates from the Tahoe 

Lake watershed, Tahoe Lake, and the forest and wetland areas 

downstream of Tahoe Lake. Total phosphorus concentrations 

from 10 samples collected during the 2-year study ranged 

from 0.017 to 0.082 mg/L. The median and volumetrically 

weighted concentrations of total phosphorus were both 

0.036 mg/L. Daily phosphorus loads were determined by multiplying the volumetrically weighted concentration from all of 

the samples measured at this site by the estimated daily flows. 

The total quantity of phosphorus estimated from this inlet was 

24.3 lb in MY 2008 and 22.8 lb in MY 2009.

Phosphorus in the storm drain near the boat landing originates mostly from urban sources, including runoff 

from streets in Mercer. Total phosphorus concentrations 

from 36 samples collected during the 2-year study ranged 

from 0.019 to 1.96 mg/L (fig. 13B). The median and volumetrically weighted concentrations of total phosphorus were 

0.360 and 0.833 mg/L, respectively. About 50 percent of the 

phosphorus was in the dissolved form, but this was based on 

only two samples. Daily phosphorus loads were computed 

with GCLAS and summed to obtain annual totals. The total 

quantity of phosphorus measured at this storm drain was 

15.0 lb in MY 2008 and 12.7 lb in MY 2009.

Ungaged Near-Lake Drainage Areas

The phosphorus concentration in the inflow from the 

ungaged near-lake drainage areas was estimated from phosphorus concentrations measured in the storm drain and at 

Tahoe Inlet. It was assumed that the flow from the ungaged 

near-lake area consisted of water from a combination of 

sources similar to that from the storm drain (20 percent) and 

the Tahoe Lake watershed (80 percent). Therefore, the phosphorus concentration for flow from these areas was set equal 

to 0.2*volumetrically weighted concentration from the storm 

drain + 0.8*volumetrically weighted concentration from Tahoe 

Inlet, or set equal to 0.076 mg/L. Daily phosphorus loads were 

then determined by multiplying the volumetrically weighted 

concentration (0.076 mg/L) by the estimated daily flows for 

the ungaged area. The total quantity of phosphorus measured 

from the ungaged area was 74.0 lb in MY 2008 and 14.9 lb in 

MY 2009 (table 9). Loading during MY 2009 was much less 

than that in MY 2008 because the very dry conditions resulted 

in a lack of runoff from this area. 
Table 9. Summary of the phosphorus-budget components for Mercer Lake, Iron County, Wis., during monitoring years 2008–09, during typical hydrolfuture development in the watershed. [Monitoring year, November–October; MY, monitoring year; all data in pounds per year; —, not estimated; bold type indicates total inputs to the lake] Budget component MY 2008 MY 2009 2-year average Percentage of 2-year total Typical hydrology Pre-1965 1965–95 0-percent 

retention 10-percent retention 25

r

Inputs to lake Precipitation 24.9 21.7 23.3 6.9 28.8 28.8 28.8 28.8 

Little Turtle Inlet 152 169 160 47.2 217 217 217 217 

Tahoe Inlet 24.3 22.8 23.5 6.9 31.8 31.8 31.8 31.8 

Storm drain 15.0 12.7 13.8 4.1 18.7 18.7 18.7 18.7 

Ungaged area 74.0 14.9 44.4 13.1 90.7 90.7 90.7 90.7 

Groundwater 39.5 39.3 39.4 11.6 53.3 53.3 53.3 53.3 

Septic systems 8.6 8.6 8.6 2.5 8.6 349 31.7 167 

Total external input 338 289 313 92.3 449 789 472 609 

Internal (sediment) loading 26.3 26.3 26.3 7.7 26.3 26.3 26.3 26.3 

Total external and internal input 364 315 340 100 475 816 498 635 

Outputs from lake Little Turtle Outlet 271 214 243 71.4 — — — — 
0

0.02

0.04

0.06

0.08

0.10

0.12

1 2 3 4 5 6 7

Piezometer identification number

Concentration, in milligrams per liter 

October 24, 2007

June 16, 2008

September 16, 2008

May 20, 2009

September 2, 2009

EXPLANATION

40 Water Quality, Hydrology, and Simulated Response to Changes in Phosphorus Loading of Mercer Lake, WI, and Effects of Wastewater Discharge

Figure 20. Concentrations of dissolved phosphorus in groundwater from piezometers near Mercer Lake, Iron County, Wis.

Groundwater

Loading of phosphorus from natural groundwater inflow 

and from contributions from septic systems in the groundwater 

were estimated separately. To determine the natural groundwater 

contributions, piezometers were installed around the lakeshore 

(fig. 2) and sampled to determine the concentrations of dissolved phosphorus in groundwater entering the lake (fig. 20). 

Based on water-elevation gradients and the groundwater 

model, groundwater enters the lake from the entire shoreline, 

with the possibility of a slight flow reversal near the boat landing. Average dissolved phosphorus concentrations in each of 

piezometers ranged from 0.012 mg/L at P5, near the outlet, to 

0.047 mg/L at P2, on the northwest side of the lake. The average concentration of all samples was 0.022 mg/L, whereas the 

median concentration was 0.017 mg/L. Phosphorus concentrations in some piezometers, such as piezometer P2, may be 

elevated above background concentrations and likely affected 

by septic-tank effluent or urban sources. Therefore, to estimate 

the phosphorus input to the lake from natural groundwater 

inflow, it was assumed that the concentration of groundwater 

entering the lake was equal to the median concentration measured in the piezometers (0.017 mg/L). During MY 2008–09, 

the annual phosphorus load contributed by natural groundwater inflow was estimated to be about 40 lb (table 9).

Septic Systems

Phosphorus loading from near-lake septic systems was 

estimated separately from background groundwater inflow by 

applying export coefficients to the near-lake septic systems. 

The latest population and occupancy information, site-specific 

soil information, and septic-system characteristics were evaluated and used to estimate the site-specific coefficients.

The input of phosphorus from septic systems (M) was 

estimated by use of equation 5 (Reckhow and others, 1980):

M = Es * (number of capita years) * (1-SR), (5)

where M is a function of a phosphorus export coefficient, 

Es, a soil retention coefficient, SR, and the number of people 

using the septic systems annually (capita years). This method 

of estimating phosphorus loading from septic systems is 

described in more detail by Garn and others (1996). In applying equation 5, it was assumed that the most likely value for 

Es was 1.5 lb of phosphorus per capita per year (table 10). 

Typical export coefficients range from about 1.0 lb per capita 

per year (Reckhow and others, 1980; Panuska and Kreider, 

2003) to over 2 lb per capita per year (Garn and others, 1996). 

Soils of the area around Mercer Lake consist predominantly 

of the Vilas-Lindquist Complex, Keweenaw-Pence Complex, 

Manitowish sandy loam, Annalake fine sandy loam, Padus and 

Pence sandy loam, and Seelyeville and Markey soils (Natural 

Resources Conservation Service, 2010). Many of these soils 

are underlain by sand and gravel, are well drained, have high 

subsoil permeabilities, and are classified as a poor filter with 

very limited application for septic-tank absorption fields. More 

than 75 percent of soils in the area are very limited for disposal of wastewater by infiltration. Therefore, an SR coefficient 

of 0.5 was used for these soils, plus an SR of 0.2 was added 

for the fraction of the phosphorus that is removed by storage 

of solids in the septic tank. Therefore, a most likely SR value 

of 0.70 (70-percent retention) was used in septic system input 

calculations (table 10). 
Table 10. Phosphorus input from septic systems, sewage treatment plants, and untreated areas into Mercer Lake, Iron County, Wis. [Es, phosphorus export coefficient; SR, soil retention coefficient; lb, pounds; —, not applicable] Scenario / time period Occupancy Es SR Houses Resorts Total capita years Septic system input (lb) 

Number Residents per unit Fraction of year occupied Number Residents per unit Fraction of year occupied MY 2008–09 Permanent 1.5 0.7 7 2.1 1 0 0 0 14.7 6.6 

Seasonal 1.5 0.7 7 2.1 0.3 0 0 0 4.4 2.0 

Total — — — — — — — — 19.1 8.6 

Pre-1965 Permanent 1.5 0.5 16 2.1 1 0 0 0 33.6 25.2 

Seasonal 1.5 0.5 49 2.1 0.3 6 12 0.3 52.5 39.4 

Total — — — — — — — — 86.1 64.6 

1965–95 0 percent retention in lake Permanent 1.5 0.7 13 2.1 1 0 0 0 27.3 12.3 

Seasonal 1.5 0.7 40 2.1 0.3 0 0 0 43.2 19.4 

Total — — — — — — — — 70.5 31.7 

10 percent retention in lake Permanent 1.5 0.7 13 2.1 1 0 0 0 27.3 12.3 

Seasonal 1.5 0.7 40 2.1 0.3 5 12 0.3 43.2 19.4 

Total — — — — — — — — 70.5 31.7 

25 percent retention in lake Permanent 1.5 0.7 13 2.1 1 0 0 0 27.3 12.3 

Seasonal 1.5 0.7 40 2.1 0.3 5 12 0.3 43.2 19.4 

Total — — — — — — — — 70.5 31.7 

Future Permanent 1.5 0.7 7 2.1 1 0 0 0 14.7 6.6 

Seasonal 1.5 0.7 7 2.1 0.3 0 0 0 4.4 2.0 

Total — — — — — — — — 19.1 8.6 
42 Water Quality, Hydrology, and Simulated Response to Changes in Phosphorus Loading of Mercer Lake, WI, and Effects of Wastewater Discharge

In MY 2008–09, not all households and facilities around 

the lake had septic discharge to the lake, because a large 

part of the shoreline was served by the sanitary sewer and 

many residences were a long distance from the lake. During MY 2008–09, it was estimated that 15 residences on 

the southwest shore were not served by the sanitary sewer 

and, therefore, potentially were not affecting the lake. Based 

on the 2000 census, the average household size for Mercer 

was 2.1 persons (ePodunk, Inc., 2007), and, therefore, this 

occupancy was used for these residences. It was assumed that 

about half of the residents living around the lake are permanent residents and half are seasonal or spend about 30 percent 

of the year in their lakeshore residences. The number of 

capita years was then calculated by multiplying the number 

of residences by the number of persons per household by the 

fraction of the year occupied; a total of 19 capita years during 

MY 2008–09 (table 10). During MY 2008–09, the annual 

phosphorus load contributed by septic inputs was estimated to 

be about 9 lb (tables 9 and 10).

Release of Phosphorus from Lake Sediments

Three types of information are needed to estimate how 

much phosphorus may be released from the sediments of the 

lake: the area from which phosphorus is being released, the 

aerobic or anaerobic characteristics of the sediment surface, 

and the rate of phosphorus release. It was assumed that phosphorus was released only from areas greater than 6.5-ft deep, 

because these areas typically accumulate organic sediment. It 

is important to determine where and how long the sediment 

interface is anaerobic, because phosphorus release is typically 

much higher during anaerobic conditions than during aerobic 

conditions. The dates when the sediment interface was anaerobic and the area of anaerobic sediment were estimated from 

the dates and depths at which a dissolved oxygen concentration of 0.5 mg/L or less was found in the profiles collected in 

the deepest areas of the lake. The sediment interface was typically anaerobic from about June 15 to about September 10, 

and typically below about 20 ft (fig. 5). The areas of aerobic 

and anaerobic conditions are given in table 11.

Rates of phosphorus release from the aerobic and 

anaerobic sediments in Mercer Lake were estimated from 

12 sediment cores collected from the lake (figs. 2 and 4A) and 

analyzed by the WSLH. Daily aerobic-release rates ranged 

from -1.93*10-8 lb/ft2

 (removal of phosphorus from the water 

column) to 1.58*10-8 lb/ft2, with an average aerobic release 

rate of 4.94*10-9 lb/ft2 (table 12). Daily anaerobic-release 

rates ranged from 1.38*10-7 lb/ft2 to 4.33*10-7 lb/ft2, with an 

average anaerobic release rate of 2.77*10-7 lb/ft2. The anaerobic release rate was about 100 times greater than the aerobic 

release rate, and the aerobic release rate was not much different than the release rate from a tube analyzed with no sediment in it (blank). 

The estimated amount of phosphorus released from the 

sediments of Mercer Lake was 26.3 lb from May 1 through 

September (table 11; 3.5 lb from aerobic sediments and 22.7 lb 

from anaerobic sediments). Given the very low phosphorus 

release rate from the aerobic sediments, the assumption that 

phosphorus was not released from sediment shallower than 

6.5 ft was not very important. The amount of phosphorus 

released before May 1 and after September 30 is expected to 

be very low, because the water immediately above the sediments is typically aerobic when the lake is well mixed and 

release rates decrease when the water temperature is cool during fall, winter, and spring (James and Barko, 2004); therefore, 

the annual input of phosphorus from the sediments of the lake 

is estimated to be 26.3 lb. 

Surface-Water Outflow 

Phosphorus is discharged from Mercer Lake into the 

Little Turtle River through the Mercer Lake Outlet. Total 

phosphorus concentrations in the 34 samples collected 

at the outlet during the 2-year study period ranged from 

0.012 to 0.034 mg/L (fig. 13C), and the flow-weighted-average 

concentration was 0.019 mg/L. Daily phosphorus loads were 

computed with GCLAS and summed to obtain annual totals. 

The total phosphorus output through the Mercer Lake Outlet 

was 271 lb in MY 2008 and 214 lb in MY 2009 (table 9).

Phosphorus-Budget Summary

MY 2008–09

During MY 2008–09, the average annual input of 

phosphorus to Mercer Lake was 340 lb (table 9), of which 

313 lb came from external sources (92.3 percent) and 26.3 lb 

came from the sediments in the lake (7.7 percent). The largest 

external source of phosphorus to the lake was from the Little 

Turtle Inlet, which delivered about 47 percent of the total load 

(fig. 21) compared with about 67 percent of the water input 

(fig. 18). The next largest contribution was from the near-lake 

area including much of the town and monitored storm drain, 

which delivered about 17 percent of the total phosphorus load, 

compared to only 4.6 percent of the water input. Contributions from precipitation and groundwater accounted for about 

7 and 12 percent of the total load, respectively, while septic 

systems accounted for 2.5 percent of the total load. Phosphorus loading in MY 2009 was about 15 percent less than that in 

MY 2008 because the continuing drought reduced most of the 

inputs except that from Grand Portage Lake.

The average annual export of phosphorus to the Little 

Turtle River was 243 lb: 271 lb in MY 2008 and 214 lb in 

MY 2009. Therefore, on average, about 70 percent of the 

external phosphorus load to the lake is exported out of the 
Sources of Phosphorus 43

Table 11. Estimated areas of aerobic and anaerobic sediment below 6.5 feet in Mercer 

Lake, Iron County, Wis., and estimated sediment phosphorus release from May 1 to 

September 30.

[ft2, square feet; lb, pounds; NA, not applicable]

Date Aerobic area 

(ft2

)

Anaerobic 

area 

(ft2

)

Aerobic 

release

(lb)

Anaerobic 

release

(lb)

Total release

(lb)

May 1–June 14 5,210,000 0 1.1 0.0 1.1

June 15–Sept. 10 4,290,000 922,000 1.9 22.7 24.6

Sept. 11–Sept. 30 5,210,000 0 0.5 0.0 0.5

Total NA NA 3.5 22.7 26.3

Table 12. Summary of estimated rates of phosphorus release from the sediments of Mercer 

Lake, Iron County, Wis.

[Locations of all sites are shown in figure 2; lb/ft2/d, pounds per square foot per day; R2, coefficient of variation; 

NA, not applicable]

Site number Replicate

identifier

Aerobic or 

anaerobic

Release rate

(lb/ft2

/d)

Regression fit 

from laboratory 

results

(R2

)

s1 A Aerobic 1.19E-08 0.94

s1 B Aerobic 1.06E-08 0.99

s2 A Aerobic –1.93E-08 0.96

s2 B Aerobic 1.52E-08 0.94

s3 A Aerobic –6.76E-09 0.99

s3 B Aerobic 1.58E-08 0.96

s4 A Aerobic 6.96E-09 0.52

s4 B Aerobic 5.12E-09 0.73

s1 C Anaerobic 2.76E-07 0.99

s1 D Anaerobic 1.38E-07 0.98

s2 C Anaerobic 4.33E-07 0.97

s2 D Anaerobic 2.61E-07 0.96

Average NA Aerobic 4.94E-09 NA

Average NA Anaerobic 2.77E-07 NA

Blank—No sediment NA Aerobic –6.76E-09 0.22
Phosphorus loading: typical year (475 pounds)

Precipitation

6.1%

Little Turtle Inlet

45.6%

Tahoe Inlet

6.7%

Storm drain

3.9%

Groundwater

11.2%

Septic

1.8%

Internal 

(sediments)

5.5%

Phosphorus loading: 2-year average (340 pounds)

Precipitation

6.9%

Groundwater

11.6%

Little Turtle Inlet

47.2%

Tahoe Inlet

6.9%

Storm drain

4.1%

Septic

2.5%

Internal 

(sediments)

7.7%

area

Ungaged

near-lake

19.1%

area

13.1%

Ungaged

near-lake

44 Water Quality, Hydrology, and Simulated Response to Changes in Phosphorus Loading of Mercer Lake, WI, and Effects of Wastewater Discharge

Figure 21. Phosphorus budgets for Mercer Lake, Iron County, Wis., for monitoring years 2008–09 and for typical 

hydrologic conditions.

lake. About 30 percent of the external phosphorus entering 

the lake is retained in the lake sediments, part of which may 

be released to the water column in the future. Lakes normally 

accumulate phosphorus in their bottom sediment; therefore, 

phosphorus inputs exceed phosphorus losses in most lakes 

(Graneli, 1999). A prolonged period when phosphorus exports 

exceed inputs would indicate that phosphorus inputs from the 

watershed have decreased and that phosphorus concentrations 

in the lake should decrease.

Typical Hydrology 

The phosphorus loading in MY 2008–09 represents 

what occurred during a series of dry years. To determine 

phosphorus loading to the lake during more typical or average hydrologic conditions, loads were reestimated for each 

component of the phosphorus budget (table 9) on the basis of 

long-term average precipitation and flow values (described 

earlier) using the same phosphorus concentrations as estimated 

for MY 2008–09. No adjustments were made to inputs from 

septic systems and internal loading. With these assumptions, 

the total annual phosphorus load to Mercer Lake during typical hydrologic conditions is estimated to increase to about 

475 lb (table 9), which is an increase of about 40 percent from 

the average annual loading estimated for MY 2008–09, compared to an increase in water inputs of about 38 percent.

Prior to 1965 with Typical Hydrology

Phosphorus loading to Mercer Lake in MY 2008–09, 

even after compensating for the dry conditions, is expected 

to be less than it was prior to when there were more septic 

systems around the lake (prior to 1965) and when some sewage effluent may have been retained in the lake (1965–95). 

Prior to 1965, it is expected that phosphorus contributions 

from septic systems and the town were much higher. Around 

1965, 65 homes, 6 resorts, and a church were located on the 

shoreline of Mercer Lake, all with septic systems (Wisconsin 

Department of Natural Resources, 1970). If it is assumed that: 

75 percent of the homes were nonpermanent (living on the 

lake 30 percent of the year), the average household size was 

2.1 persons (ePodunk, Inc., 2007), each resort was occupied by 12 people for 30 percent of the year, and that septic 

systems were less efficient than they were in MY 2008–09 

(SR coefficient of 0.5), then the total estimated load from 

septic systems was 65 lb (table 10). In addition, prior to 

1965, effluent from many of the residences farther from the 

lake probably reached the lake through pipes and overland 

runoff. The amount of phosphorus from these residences is 

very difficult to estimate. If it is assumed that their export is 

equivalent to 25 percent of the phosphorus that was discharged 

from the sewage treatment plant when it first opened in 1965 

(37,400 gal/d at a concentration of 10 mg/L; R. Gothblad, 
Sources of Phosphorus 45

Wisconsin Department of Natural Resources, written commun. 1975, “Mercer Sewage Treatment Plant-24-hour Survey 

Report”), then it is estimated that there was an additional 

source of 285 lb of phosphorus (table 10). Therefore, prior to 

1965, septic systems and sewage effluent may have contributed about 350 lb of phosphorus to the lake (table 10). If it is 

assumed that loadings from all of the other sources were similar to those estimated for MY 2008–09, then during typical 

years prior to 1965, the total annual load of phosphorus would 

have been 816 lb (table 9).

1965 to 1995 with Typical Hydrology

From 1965 to 1995, most of the sewage from the residences around Mercer Lake was treated by the newly constructed sewage treatment plant; however, households west of 

the Little Turtle River were not included in this system. During 

this time, about 53 homes and 5 resorts were in the untreated 

area immediately around the lake (Wisconsin Department of 

Natural Resources, 1970). If it is assumed that: 75 percent of 

these homes were nonpermanent (living on the lake 30 percent 

of the year), the average household size was 2.1 persons, 

each resort was occupied by 12 people for 30 percent of the 

year, and that septic systems had a similar efficiency to that 

estimated for MY 2008–09 (SR coefficient of 0.7), then the 

total estimated load from septic systems is estimated to have 

been about 32 lb (table 10). During this time, effluent from 

the sewage treatment plant passed through a 3,000-ft long 

pipe laid along the bottom of the lake and discharged into 

Mercer Lake near its Outlet. It was believed that some of the 

effluent from the plant was not leaving the lake and may have 

been affecting water quality. The amount of phosphorus discharged into the lake is very difficult to estimate. To estimate 

the potential input from this discharge, it was assumed that 

0, 10, and 25 percent of the total effluent discharged from 

the sewage treatment plant was retained in the lake. During 

1965–95, about 37,400–52,500 gal/d (average of about 

45,000 gal/d) of effluent with a phosphorus concentration of 

about 10 mg/L was released from the sewage treatment plant 

(Wisconsin Department of Natural Resources, written commun., 1995). This equates to an annual average discharge 

of phosphorus of about 1,370 lb. Therefore, about 32 lb of 

phosphorus would be added to the lake from septic systems 

and sewage treatment discharge with 0 percent of the effluent retained in the lake, 169 lb of phosphorus added with 

10 percent retained in the lake, and 374 lb of phosphorus 

added with 25 percent retained in the lake (table 10). If it 

is assumed that loadings from all of the other sources were 

similar to those estimated for typical hydrologic years around 

MY 2008–09, then during typical hydrologic years during 

1965–95, the total annual load of phosphorus would have been 

between 498 and 840 lb, depending on how much sewage 

treatment plant effluent was retained in the lake (table 9).

Future with Typical Hydrology

As of 2012, there are no immediate plans to add sewer 

lines to the residences on the southwest side of the lake; 

therefore, the amount of phosphorus from septic systems 

should remain similar to that estimated for MY 2008–09 

(8.6 lb; table 10). There are, however, plans to try to reduce 

the amount of runoff from stormwater reaching the lake 

as part of the Mercer Infrastructure Improvement Project, 

which includes highway and stormwater improvements, and 

other activities in the town of Mercer (Ayres and Associates, 

2010). Ayers and Associates applied the Windows Source 

Loading and Management Model (WinSLAMM) to estimate 

current inputs of phosphorus and sediment to Mercer Lake 

from the area near the town of Mercer and to estimate how 

these inputs could be modified with various best management 

practice alternatives. WinSLAMM is a watershed model that 

is commonly used to estimate runoff volumes and loads from 

urban areas (Pitt and Voorhees, 1993). Ayers and Associates 

(2010) estimated that with existing (2010) land use, 50.4 lb of 

phosphorus should be discharged to surface waters near from 

the area near the town of Mercer, and if anticipated changes 

in land use associated with Mercer Infrastructure Improvement Project are included, this discharge of phosphorus should 

increase to about 52.6 lb. Ayers and Associates (2010) also 

estimated that, with the installation of various best management practices, this discharge of phosphorus could be reduced 

by 36 percent. For this same area near the town, based on the 

information in table 9, it was estimated in this study that this 

area should contribute about 74.8 lb of phosphorus (input from 

the storm drain and 80 percent of the contributions from the 

ungaged near-lake area) during typical hydrologic conditions. 

Therefore, if the inputs from both of these areas were reduced 

by 36 percent, then the inputs from the storm drain would be 

11.9 lb and the total from the ungaged near-lake area would be 

57.7 lb (table 9). If it is assumed that loading from all of the 

other sources was similar to that estimated for typical hydrologic years around MY 2008–09, then during typical years 

after the Mercer Infrastructure Improvement Project is completed and the various best management practices are installed, 

the total annual load of phosphorus would decrease to about 

435 lb (table 9). In addition to highway improvements, additional development is expected around Mercer Lake. Much 

of this development is expected in areas that do not drain 

directly to the lake; therefore, the development should not 

affect loading to the lake. In addition, if areas draining to the 

lake are developed and development is performed as required 

by Wisconsin State law, then runoff and nutrient loading to the 

lake should not increase.
46 Water Quality, Hydrology, and Simulated Response to Changes in Phosphorus Loading of Mercer Lake, WI, and Effects of Wastewater Discharge

Simulated Changes in Water Quality in 

Response to Changes in Phosphorus 

Loading 

Empirical eutrophication models that relate phosphorus 

loading to specific water-quality characteristics can be used 

to determine how natural (climate driven) and anthropogenic 

changes in phosphorus loading could modify the water quality 

of Mercer Lake. These models were developed on the basis 

of comparisons between hydrologic and phosphorus loading determined for many different lake systems and specific 

measures describing lake water quality, such as near-surface 

phosphorus and chlorophyll a concentrations, and Secchi 

depth (Vollenweider, 1975; Canfield and Bachmann, 1981; 

Reckhow and Chapra, 1983; Cooke and others, 1993; Panuska 

and Kreider, 2003). One empirical model that has been shown 

to be good at predicting phosphorus concentrations of lakes in 

Wisconsin is the Canfield and Bachman (1981) natural-lake 

model (Robertson and others, 2002, 2005; Robertson, Rose, 

and Juckem, 2009; Robertson, Rose, and Fitzpatrick, 2009; 

Robertson and Rose, 2008). Therefore, the Canfield and Bachman natural-lake model (1981; equation 6) was applied to 

Mercer Lake to determine how changes in phosphorus loading 

should affect phosphorus concentrations in the lake:

(6) Total Phosphorus 

Concentration (mg/L) 

where L is the annual phosphorus-loading rate (in mg/m2 = 

lb/acre x 112.07), Z is the mean depth of the lake (in feet), 

and τ is the residence time of the water in the lake (in years). 

In this application, outputs from the model when the average annual hydrology and phosphorus loadings measured in 

MY 2008–09 were used as inputs were used as a base case 

(Base) for comparisons with other basinwide changes in 

loadings (tables 8 and 9). Outputs from the model when the 

hydrology and phosphorus loadings for typical hydrologic 

conditions were used as inputs were used as a base case 

(Reference) for all other comparisons.

The total phosphorus concentrations predicted with the 

Canfield and Bachman model were then used to predict chlorophyll a concentrations and Secchi depths through the use 

of Carlson’s (1977) TSI equations (equations 1–3). In other 

words, the chlorophyll a concentrations and Secchi depths that 

yielded a similar TSI value as that computed from the predicted total phosphorus concentration were computed. There 

are no true calibration factors when Carlson’s TSI equations 

are used to estimate chlorophyll a concentrations and Secchi 

depths in this manner; therefore, the output was adjusted to 

account for model biases by adjusting the results by the percentage of bias in the results for the base-case (Base) scenario.

= L/1000

0.305 * Z (0.162 x (L/Z)0.458 + 1/τ)

Modeling Approach

To estimate how the water quality of Mercer Lake should 

respond to changes in phosphorus loading, 17 scenarios were 

simulated with the Canfield and Bachman model and Carlson 

TSI equations (table 13 and fig. 22). Scenario 1 simulated the 

average conditions for MY 2008–09. Eight simulations (2–9) 

were used to determine the general response of the lake to 

basinwide changes in external phosphorus loading: simulated decreases in external phosphorus loading by 75, 50, and 

25 percent and increases in external phosphorus loading by 

25, 50, 75, 100, and 200 percent. External loading excludes 

phosphorus loading from the sediment. Internal phosphorus 

loading from the sediments is not expected to change quickly 

in response to changes in external loading and, therefore, 

was not modified in scenarios 2–9. Scenario 1 established a 

base condition (concentration) to which results from the other 

basinwide changes can be compared. Although the models 

can simulate the effects of dramatic reductions in phosphorus 

loading, such as a 75 percent reduction, this type of reduction 

may not be possible given that much of the phosphorus loading is coming from a relatively pristine upstream watershed, 

groundwater, and precipitation.

Eight scenarios (10–17) were then used to determine 

how changes in phosphorus loading caused by natural (hydrologic) and anthropogenic changes could have modified in 

the past or could modify the water quality of the lake in the 

future. Scenario 10 which had the hydrology and phosphorus loadings for typical hydrologic conditions is labeled as 

the “Reference” in figure 22. Three scenarios (10–12) were 

used to simulate the effects of changes in hydrology and the 

effects of internal phosphorus loading from the sediments. 

Four scenarios (13–16) were used to simulate the response 

of the lake to historical changes in phosphorus loading. 

In scenario 13, the response of the lake to inputs for typical hydrologic conditions prior to 1965 was simulated. In 

scenarios 14–16, responses of the lake to inputs for 1965–95 

with various levels of sewage treatment plant effluent retained 

in the lake were simulated. In scenario 17, the models were 

used to predict the response of the lake to the inputs estimated 

during typical years after the Mercer Infrastructure Improvement Project is completed and the proposed best management 

practices are installed. 

Four types of data are required as input into the Canfield 

and Bachman model (table 13): water-quality data (from 

table 5), morphometric data (mean depth), hydrologic data 

(residence time), and nutrient-loading data (from table 9). 

Although the loading data used as input to these models 

are summarized for the entire year, these models simulate 

water quality only for the growing season (May–September); 

therefore, the water-quality data in table 5 were used for 

comparisons. 
Table 13. Estimated water quality in Mercer Lake, Iron County, Wis., in response to various phosphorus-loading scenarios based on the Canfield and model (1981) and Carlson (1977) trophic-state-index equations. [lb, pounds; ft, feet; mg/L, milligrams per liter; μg/L, micrograms per liter] Simulations Scenario number Annual phosphorus load (lb) Water elevation above an arbitrary datum (ft) Surface area (acres) Mean depth (ft) Residence time (years) Predicted total phosphorus concentration (mg/L) 

c

c

General response Percent change in external phosphorus load – 75 percent 2 105 86 179 10.0 0.46 0.008 

– 50 percent 3 183 86 179 10.0 0.46 0.013 – 25 percent 4 261 86 179 10.0 0.46 0.018 0 percent (Base) 1 340 86 179 10.0 0.46 0.023 + 25 percent 5 418 86 179 10.0 0.46 0.027 + 50 percent 6 496 86 179 10.0 0.46 0.031 + 75 percent 7 575 86 179 10.0 0.46 0.035 + 100 percent 8 653 86 179 10.0 0.46 0.039 + 200 percent 9 967 86 179 10.0 0.46 0.053 

Specific natural and anthropogenic scenarios Typical hydrology (Reference) 10 475 86 179 10.0 0.32 0.024 

MY 2008–09 (no internal loading) 11 313 86 179 10.0 0.46 0.021 Typical hydrology (no internal loading) 12 449 86 179 10.0 0.32 0.023 Pre-1965 13 816 86 179 10.0 0.32 0.038 1965–95 (0 percent retention) 14 498 86 179 10.0 0.32 0.025 1965–95 (10 percent retention) 15 635 86 179 10.0 0.32 0.031 1965–95 (25 percent retention) 16 840 86 179 10.0 0.32 0.039 Future scenario with use of best-manage- ment practices 17 435 86 179 10.0 0.32 0.022 
Eutrophic

Mesotrophic

Oligotrophic

Eutrophic

Mesotrophic

Oligotrophic

Eutrophic

Mesotrophic

Oligotrophic

Phosphorus response

0.000

0.010

0.020

0.030

0.040

0.050

0.060

-100 -50 0 50 100 150 200

11 10

11

11

12 13 14 15 16 17

Percent change in external phosphorus loading

Total phosphorus, in milligrams per liter 

-100

-50

0

50

100

150

Percent change

Scenarios 

Reference

10 12 13 14 15 16 17

Chlorophyll a response 

0

2

4

6

8

10

12

14

-100 -50 0 50 100 150 200

Percent change in external phosphorus loading

Chlorophyll a concentration, in micrograms per liter 

-100

0

100

200

300

Percent change

Reference

Scenarios 

10 12 13 14 15 16 17

Secchi depth response

0

5

10

15

20

25

-100 -50 0 50 100 150 200

Percent change in external phosphorus loading

Secchi depth, in feet

-100

-50

0

50

100

150

Percent change

Reference

Scenarios 

Measured (2008–09) Directly simulated using eutrophication models

2–9 General response

Measured typical hydrology (2006–10) 1 Adjusted base simulation (2008–09) 

10 Typical hydrology

11 Monitoring year 2008–09 (no internal loading)

12 Typical hydrology (no internal loading)

13 Pre 1965

14 1965–95 (0 percent retention)

15 1965–95 (10 percent retention)

16 1965–95 (25 percent retention)

17 Future scenario

EXPLANATION

Measured water quality Simulations (with scenario numbers for 10−17)

A

B

C

48 Water Quality, Hydrology, and Simulated Response to Changes in Phosphorus Loading of Mercer Lake, WI, and Effects of Wastewater Discharge

Figure 22. Measured and simulated summer-average near-surface concentrations of A, total phosphorus, B, chlorophyll a, 

and C, Secchi depths in Mercer Lake, Iron County, Wis., in response to various phosphorus-loading scenarios.
Simulated Changes in Water Quality in Response to Changes in Phosphorus Loading 49

Verification and Adjustments for Model Biases

The ability of the Canfield and Bachman (1981) model to 

simulate the changes in phosphorus concentrations in the lake 

(termed “in-lake concentrations”) was evaluated by inputting 

the morphometry of the lake and the hydrologic and phosphorus loadings for MY 2008–09 into the model (equation 6) 

and simulating the average near-surface total phosphorus 

concentration. The simulated total phosphorus concentration for MY 2008–09 (0.021 mg/L; fig. 22A) is similar to 

that measured in the lake (0.023 mg/L; table 5). This similarity indicates that the model should accurately simulate the 

summer-average phosphorus concentrations in Mercer Lake 

in response to changes in phosphorus loading. Because of 

the small discrepancy in measured and simulated phosphorus 

concentrations, all of the simulated phosphorus concentrations 

were increased by 8 percent.

Determining how well the Carlson (1977) TSI equations 

simulate chlorophyll a concentrations and Secchi depths in 

Mercer Lake was accomplished by comparing the measured 

values with those estimated with equations 2 and 3 using the 

TSI value based on the adjusted simulated total phosphorus 

concentration. Given a TSI value of 49.2 (from equation 1 

with the adjusted simulated phosphorus concentration of 

0.023 mg/L), the unadjusted predicted chlorophyll a concentration was 6.7 mg/L and the unadjusted predicted Secchi 

depth was 6.9 ft (fig. 22B and C). The measured chlorophyll a

concentration in MY 2008–09 was 3.5 mg/L, which is less 

than that predicted with equation 2. The measured Secchi 

depth in MY 2008–09 was 9.7 ft, which is greater than that 

predicted with equation 3. The discrepancy in chlorophyll a

concentrations and Secchi depths indicates that measured 

chlorophyll a concentrations are lower than expected, and 

the clarity measured in Mercer Lake is better than would be 

expected given the measured phosphorus concentrations. 

These discrepancies may have been caused by specific characteristics in Mercer Lake that differ from other lakes with similar phosphorus concentrations, such as the extensive macrophytes in the lake, algal growth on the macrophytes, structure 

of the food web, or the binding of phosphorus to iron to make 

the phosphorus less available for algae growth. Because of the 

large discrepancies in measured and simulated chlorophyll a

concentrations and Secchi depths, all of the simulated chlorophyll a concentrations from equation 2 were decreased by 

47 percent and the Secchi-depth values from equation 3 were 

increased by 40 percent.

Response in Water Quality to Basinwide 

Changes in Phosphorus Loading

On the basis of model simulations for basinwide changes 

in external phosphorus loading (scenarios 2–9; table 13), inlake phosphorus concentrations should have a relatively linear 

response to a linear change in loading (fig. 22A); however, 

on a percentage basis, the changes in in-lake phosphorus 

concentrations are about 20 percent smaller than the changes 

in external phosphorus loadings. For example, a 50-percent 

change in external phosphorus loading should cause about 

a 40-percent change in in-lake phosphorus concentrations. 

Given that much of the phosphorus loading to Mercer Lake is 

coming from a relatively pristine upstream watershed, reductions in loading of more than about 50 percent may not be 

possible.

Simulated summer-average chlorophyll a concentrations 

also had a relatively linear response to changes in external 

phosphorus loading (fig. 22B); however, on a percentage 

basis, changes in chlorophyll a concentrations are about 

10–20 percent larger than the changes in external phosphorus 

loadings. For example, a 50-percent change in external phosphorus loading should cause about a 55- to 60-percent change 

in chlorophyll a concentrations.

Simulated summer-average Secchi depths had a nonlinear 

response to changes in external phosphorus loading, with a 

much larger response to decreases in loading than to increases 

in loading (fig. 22C). For example, a 50-percent decrease in 

external phosphorus loading should cause about a 70-percent 

increase in Secchi depths, whereas a 50-percent increase 

in external phosphorus loading should cause only about a 

27-percent decrease in Secchi depths.

Response in Water Quality during Typical 

Hydrologic Conditions

The region near Mercer Lake experienced very dry 

conditions during MY 2008–09, which resulted in hydrologic and nutrient loading to the lake being less than normal 

(tables 8 and 9); therefore, water-quality conditions during 

years with more typical hydrology may be different than those 

measured in MY 2008–09. Scenario 10 was used to determine 

how natural (climatic) changes such as phosphorus loading associated with more typical hydrology—rather than the 

atypical conditions during MY 2008–09—should affect water 

quality (labeled as “Reference” in fig. 22; table 13). With 

typical hydrologic conditions, the hydrologic and phosphorus 

inputs to the lake are expected to increase by about 40 percent 

(tables 9 and 13). On the basis of model simulations, this 

increase in water and phosphorus loading is expected to 

have very little effect on water quality: total phosphorus 

and chlorophyll a concentrations may increase slightly and 

water clarity may decrease slightly (table 13 and fig. 22). The 

small change in water quality results from the small expected 

changes in phosphorus concentrations in the inflowing water. 

The simulated conditions were then compared with conditions measured in the lake from 2006 to 2010, during which 

a range in hydrologic conditions occurred. During 2006–10, 

the average phosphorus concentration measured in the lake 

was 0.019 mg/L, the average chlorophyll a concentration was 

3.1 mg/L, and the average Secchi depth was 10.7 ft (fig. 22). 

All of the conditions measured during 2006–10 were similar 

to those measured in 2008–09, also indicating that changes 

in hydrology have little effect on the trophic conditions in the 

lake. 
50 Water Quality, Hydrology, and Simulated Response to Changes in Phosphorus Loading of Mercer Lake, WI, and Effects of Wastewater Discharge

Simulated Effects of Internal Phosphorus Loading

Two scenarios (11 and 12, in comparison with scenarios 

1 and 10, respectively) were used to determine the effects 

of internal phosphorus loading from the sediments on the 

water quality of the lake. Complete removal of the phosphorus loading from the sediments from scenarios 1 and 10 

(26.3 lb decrease; about 6–8 percent of the total phosphorus 

budget, table 9) resulted in simulated in-lake near-surface 

phosphorus concentrations decreasing by only 0.001 mg/L 

in scenarios 11 and 12. These small decreases in phosphorus 

concentrations resulted in modest decreases in chlorophyll 

a concentrations (0.3 mg/L) and modest increases in Secchi 

depths (0.5 to 0.7 ft) (table 13). Therefore, internal phosphorus 

loading in Mercer Lake has only a small effect on the present 

water quality of Mercer Lake and should have a small effect in 

the future if phosphorus loading to the lake is reduced. 

Response in Water Quality to Past and Future 

Anthropogenic Changes in the Watershed

Responses in water quality to anthropogenic changes near 

Mercer Lake were simulated with typical hydrologic conditions, and the results of the models were compared with those 

of scenario 10 (labeled as “Reference” in fig. 22). Four scenarios (13–16) were used to simulate the response of the lake 

to historical phosphorus loading. In scenario 13, the response 

of the lake to the inputs estimated for typical hydrologic conditions prior to 1965 were simulated. Prior to 1965, phosphorus inputs from septic systems and other untreated systems 

were estimated to be 349 lb, or about 340 lb more than in 

MY 2008–09 and represent about 43 percent of the total 

phosphorus budget (tables 9 and 10). This increase in loading resulted in simulated near-surface phosphorus concentrations increasing from 0.024 mg/L to 0.038 mg/L, an increase 

of 0.014 mg/L. This increase in phosphorus concentrations 

resulted in simulated chlorophyll a concentrations increasing 

to 7.4 mg/L (an increase of 3.6 mg/L) and simulated Secchi 

depths decreasing to 5.8 ft (a decrease of 3.4 ft) (table 13). 

Therefore, prior to 1965, water quality should have been much 

worse than in recent years, and the lake would have been classified as eutrophic. 

In scenarios 14–16, the response of the lake to the 

inputs estimated for years with typical hydrologic conditions between 1965 and 1995 were simulated with various 

levels of sewage treatment plant effluent being retained in the 

lake. In these simulations, phosphorus inputs from the septic 

systems (31.7 lb) were estimated to be higher than during the 

study period by 23.1 lb and inputs from sewage treatment 

plant effluent varied from 0 to 342 lb (tables 9 and 10). This 

increase in loading resulted in simulated near-surface phosphorus concentrations increasing from 0.024 mg/L to between 

0.025 and 0.039 mg/L, depending on how much effluent was 

retained in the lake. These increases in phosphorus concentrations resulted in simulated chlorophyll a concentrations 

increasing to between 4.0 and 7.7 mg/L and simulated Secchi 

depths decreasing to between 8.9 and 5.7 ft (table 13). Therefore, between 1965 and 1995, water quality in the lake could 

have been anywhere between the conditions estimated prior 

to 1965, if 25 percent of the sewage treatment plant effluent 

was retained in the lake, and what was expected during typical 

hydrologic conditions around MY 2008–09, if no sewagetreatment-plant effluent was retained in the lake.

In scenario 17, the response of the lake to the inputs 

estimated during typical years after the Mercer Infrastructure 

Improvement Project is completed and the proposed best 

management practices are installed was simulated. If the 

future construction is done with the proposed best management practices, the total annual load of phosphorus should 

decrease by about 40 lb to about 435 lb (tables 9 and 13). This 

40-lb decrease in loading resulted in simulated near-surface 

phosphorus concentrations decreasing from 0.024 mg/L to 

0.022 mg/L. This decrease in the average phosphorus concentration resulted in simulated chlorophyll a concentrations 

decreasing to 3.4 mg/L and simulated Secchi depths increasing to 9.9 ft (table 13). Therefore, if Mercer Infrastructure 

Improvement Project and the proposed best management 

practices are completed as proposed, the water quality could 

improve slightly. 

TSI values based on near-surface concentrations of total 

phosphorus and chlorophyll a, and on Secchi depths, indicate 

that Mercer Lake is usually classified as borderline mesotrophic-eutrophic with respect to phosphorus concentrations 

and mesotrophic with respect to chlorophyll a concentrations 

and water clarity (fig. 10). On the basis of the model simulations (fig. 22), the lake was probably more productive prior to 

1995, especially prior to 1965, and may have been classified 

as eutrophic based on all three trophic state variables. Model 

results also indicate that if construction around the lake in the 

future is done as proposed, the water quality of the lake should 

not change very much and may improve slightly if the best 

management practices are installed as they are being proposed; 

therefore, the lake should remain being classified as borderline mesotrophic-eutrophic with respect to total phosphorus 

concentrations and mesotrophic with respect to chlorophyll a

concentrations and water clarity. 

Importance of Internal Phosphorus 

Loading to Lake Productivity

When the deep water of productive lakes becomes 

anaerobic, the rate of phosphorus release from the sediments 

in these areas is often greatly accelerated. Typically, in deep 

dimictic lakes that remain stratified throughout summer, such 

as Minocqua Lake in north-central Wisconsin (Garn and others, 2010), phosphorus from internal loading is trapped in the 

hypolimnion during most of the summer and released into the 

shallow water primarily at fall overturn, which results in nearsurface phosphorus concentrations decreasing or changing 
Summary and Conclusions 51

very little as summer progresses . But in polymictic lakes, 

such as many shallow lakes like Butternut Lake also in northcentral Wisconsin (Robertson and others, 2008), phosphorus 

released from the sediments is not trapped in the bottom of 

the lake but is intermittently released to the shallow water 

throughout summer, which results in near-surface phosphorus 

concentrations increasing as summer progresses. Mercer Lake 

is a relatively shallow lake with an overall Osgood Index of 

3.6 (table 3); therefore, the lake might be expected to be polymictic. Given that Mercer Lake has two basins, only part of its 

surface area is exposed to the wind at any time. The Osgood 

Index of the East Basin of the lake is 6.1 and the West Basin is 

4.2, which indicate both basins may be between these two categories and may mix intermittently. Prior to this study, it was 

uncertain whether Mercer Lake acted like a dimictic or polymictic lake. Based on results of this study, Mercer Lake acts 

as a deeper, dimictic lake in which the water remains stratified 

throughout most of the summer (fig. 5), which results in most 

of the phosphorus released from the sediments being trapped 

in the hypolimnion of the lake and not causing near-surface 

phosphorus concentrations to increase as summer progresses. 

Although the phosphorus load to Mercer Lake has 

decreased as a result of mitigation efforts in the watershed 

(table 1), it was believed that the internal phosphorus load may 

delay the response of the lake to decreases in external phosphorus loading. Because Mercer Lake acts like a deep dimictic 

lake, contributions of phosphorus from internal loading during 

any given summer are relatively small (fig. 21) and, therefore, 

have minimal effect on that summer’s productivity. Although 

nutrient loading to the lake may have been quite high because 

of septic and sewage treatment plant inputs (table 10) and 

may have increased the phosphorus content of the bottom 

sediments (table 6), the overall contribution from internal 

phosphorus loading is quite small (table 9), as demonstrated 

by the relatively low near-bottom phosphorus concentrations in late summer (fig. 8). Internal loading represents only 

approximately 6 percent of the total phosphorus load to the 

lake. Therefore, the phosphorus trapped in the sediments of 

the lake should have minimal effect on the changes in future 

phosphorus concentrations and should not delay the recovery 

or response in the productivity of the lake to decreases in 

nutrients from its watershed. 

Relative Importance of Natural 

and Anthropogenic Changes in the 

Watershed to Lake Productivity 

One purpose of this study was to understand how changes 

in water quality resulting from natural changes in hydrology 

and nutrient loading caused by changes in climate compare 

with changes in water quality caused by anthropogenic modifications to the watershed, such as historical changes caused 

by septic and sewage input and future changes that may result 

from modifications to the watershed. Based on the comparison 

of model results for MY 2008–09 (dry years) to results for 

years with typical hydrology and nutrient loading, it appears 

that the water quality of Mercer Lake changes very little in 

response to natural changes in hydrology. This predicted 

lack of response in water quality was also observed in the 

measured data from 2006 to 2010 when relatively extensive 

monitoring was conducted. Results from the eutrophication 

models indicate that changes in the water quality of Mercer 

Lake in response to changes in anthropogenic modifications 

to the watershed should be much larger than the changes 

caused by natural hydrologic variability. Results of the models 

indicate that the lake should have been eutrophic prior to 1965 

with average near-surface phosphorus concentrations near 

0.038 mg/L, chlorophyll a concentrations about 7 mg/L, and 

Secchi depths about 6 ft. The predicted poorer water quality 

around 1965 agrees with the shallower Secchi depths estimated from Garrison’s (2005) sediment core analysis (fig. 11); 

however, the magnitude of the change is more than estimated 

from that analysis. The difference in response based on the 

eutrophication models and the sediment core analyses may 

have resulted from a possible overestimation of the additional 

contributions of phosphorus from the other untreated systems 

(285 lb in table 10). Input from the other untreated systems 

could only be roughly estimated. Since 1965, the results of 

both the eutrophication models and the sediment core analyses 

indicate that water quality should have gradually improved.

If the Mercer Infrastructure Improvement Project is 

done with the proposed best management practices and future 

development occurs with little increase in runoff and nutrient 

loading, then phosphorus loading to the lake should remain 

similar to that estimated in this study and the water quality of 

the lake should remain similar to that measured from 2006 to 

2010. Future nutrient loading estimates from Ayers and Assoc. 

(2010) indicate that minor decreases in nutrient loading should 

occur as part of the Mercer Infrastructure Improvement Project, which included highway and stormwater improvements, 

as long as the construction is done with the proposed best 

management practices; therefore, little change in water quality 

in Mercer Lake is expected in 2010–20.

Summary and Conclusions

Mercer Lake is relatively shallow 179-acre drainage lake 

in north-central Wisconsin. Mercer Lake’s watershed has gone 

through many changes since European settlers first arrived, 

including urban and industrial development and population 

growth. Residents felt that this development and past activities in the Mercer Lake watershed, such as historical wastewater discharges from septic systems, the municipal sewage 

treatment plant, lumber mill, and urban development, have 

affected the water quality of the lake. Over the past few years, 

many efforts were taken to improve the water quality of the 

lake, including removal of the lumber mill and diversion of 
52 Water Quality, Hydrology, and Simulated Response to Changes in Phosphorus Loading of Mercer Lake, WI, and Effects of Wastewater Discharge

all effluent from the sewage treatment plant away from the 

lake; however, it was uncertain what affects these actions had 

on water quality. Mercer Lake area residents and authorities 

would like to continue to try to improve the water quality 

of the lake; therefore, understanding the past, present, and 

future factors affecting water quality, especially the effects of 

wastewater discharges to the lake, is very important and was 

addressed in this report.

To provide a better understanding of the factors that 

affect the water quality of Mercer Lake, a detailed study of 

the lake and its watershed was done in monitoring years (MY) 

2008–09 by the USGS in cooperation with the Mercer School 

District. The purposes of the study were to describe the water 

quality of the lake and the composition of its sediments; 

quantify the sources of water and phosphorus loading to the 

lake, including those associated with septic and municipal 

wastewater discharges; and evaluate the effects of changes 

in phosphorus inputs on the water quality of the lake. Waterquality data were collected in the lake during MY 2008–09 

and compared with assembled historical data. Bottom sediments were used to describe historical water clarity and were 

analyzed for wastewater, industrial, and urban pollutants. 

Major water and phosphorus sources were measured directly, 

and minor sources were estimated to construct detailed water 

and phosphorus budgets for the lake. Lake eutrophication 

models were then used to simulate how the water quality of 

the lake should respond to past, present, and future changes in 

nutrient loading. 

During MY 2008–09, the average summer near-surface 

concentration of total phosphorus was 0.023 mg/L, indicating 

the lake is borderline meso-eutrophic, whereas the average 

summer chlorophyll a concentration was 3.3 mg/L and Secchi 

depth was 10.4 ft, both indicating mesotrophic conditions. 

Based on sediment core analysis and limited in-lake monitoring data, water quality appears to have improved since a sewage treatment plant was constructed in 1965 and its discharge 

completely routed away from the lake in 1995. Based on 

sediment core analyses, the poorest water clarity and highest 

algal productivity in the lake occurred around 1965; however, 

the density of macrophytes was higher prior to and after 1965. 

Since 2000, when a more consistent monitoring program 

began, the water quality of the lake appears to have changed 

very little. 

The bottom sediments had slightly elevated concentrations of several constituents associated with wastewater 

discharges from septic systems, the municipal sewage treatment plant, lumber mill, and urban runoff, and a few natural 

metals (such as iron). These elevated concentrations reflect 

modifications that have taken place in the watershed over the 

past 100 years. Although concentrations of several pollutants 

were elevated, none of the concentrations were much above 

the defined thresholds of concern. 

During the 2-year detailed monitoring period, the 

Little Turtle Inlet contributed about 67 percent of the total 

water inflow, groundwater contributed about 17 percent, 

and near-lake inflow and precipitation contributed about 

10 and 7 percent, respectively. The average residence time 

of water in the lakes was 0.46 years. Water leaves the lake 

primarily through the lake outlet (93 percent) or by evaporation (7 percent). MY 2008–09 were very dry years, and during 

years with typical hydrology, the total water input to the lake 

should increase by about 38 percent; however, the relative 

importance of each source should remain relatively similar. 

Productivity in Mercer Lake should typically be limited 

by phosphorus; therefore, understanding the phosphorus input 

is important when management efforts to improve or prevent 

degradation in the lake water quality are considered. During 

MY 2008 and 2009, the total annual input of phosphorus 

was 364 and 315 lb, respectively. During these years, the 

largest sources of phosphorus were from Little Turtle Inlet 

(about 45 percent), near-lake drainage area containing the 

adjacent urban and residential developments (17 percent), 

groundwater (12 percent), Tahoe Inlet (7 percent), and precipitation (7 percent). During years with typical hydrology, 

the phosphorus input should increase by about 40 percent, 

to about 475 lb, and the relative importance of the near-lake 

drainage area should increase. Phosphorus inputs were also 

estimated for three periods: 

1. Prior to 1965, when there was no sewage treatment 

plant and inputs from septic systems and other untreated 

systems were thought to be about 340 lb greater than 

in MY 2008–09 Phosphorus loading during years with 

typical hydrology was estimated to be about 815 lb. 

2. Between 1965 to 1995, when some of the effluent from 

the new sewage treatment plant was retained in the lake 

(estimated to be between 0 to 342 lb). Phosphorus loading 

during years with typical hydrology was estimated to be 

between about 500 and 840 lb.

3. The future, when the Mercer Infrastructure Improvement 

Project is completed. Inputs should decrease by about 

40 lb if the best management practices are installed as 

proposed. Phosphorus loading during years with typical 

hydrology should be about 435 lb.

The Canfield and Bachman eutrophication model and 

Carlson TSI equations were used to predict changes in water 

quality of Mercer Lake that should occur with changes in 

phosphorus loading. A relatively linear response was found 

between external phosphorus loading and phosphorus and 

chlorophyll a concentrations in the lake. On a percentage 

basis, the changes in phosphorus concentrations in the lake 

were about 20 percent smaller than the changes in external 

phosphorus loadings, whereas changes in chlorophyll a concentrations were about 10–20 percent larger than the changes 

in phosphorus loadings. Secchi depths, which indicate water 

clarity, had a nonlinear response to a linear change in external 

phosphorus loading, with a larger response to decreases in 

loading than to increases in loading.

The eutrophication models were then used to predict 

what historical and future water quality in the lake was and 

would be like when phosphorus loading was different. Results 
References 53

indicated that the lake was negatively affected by the wastewater discharges. Prior to 1965, when there was no sewage 

treatment plant and inputs from the septic systems and other 

untreated systems were thought to be higher than in recent 

years, the lake should have been eutrophic, with average 

near-surface phosphorus concentrations near 0.038 mg/L, 

chlorophyll a concentrations about 7 mg/L, and Secchi depths 

about 6 ft. The simulated degraded water quality agreed with 

the shallower Secchi depths estimated from the sediment core 

analysis; however, the magnitude of the change was more than 

that estimated from the sediment analysis. The models indicated that between 1965 and 1995, when some of the effluent 

from the new sewage treatment plant was retained in the lake, 

water quality could have been anywhere between the conditions estimated prior to 1965 and what was expected during 

typical hydrologic conditions around MY 2008–09, depending 

on the amount of sewage treatment plant effluent retained in 

the lake. The models also indicated that if the Mercer Infrastructure Improvement Project is conducted as proposed, with 

the best management practices, water quality could improve 

slightly from that measured during 2006–10. 

Because of the limited phosphorus presently input into 

Mercer Lake, increases in phosphorus loading could have a 

large effect on water quality. Therefore, management actions 

to minimize future phosphorus input into Mercer Lake are 

likely to greatly benefit water quality. Although the phosphorus load has decreased as a result of mitigation efforts in the 

watershed, especially a decrease in the input from septic and 

municipal wastewater, it was believed that the internal phosphorus load could delay the response of the lake to decreases 

in external phosphorus loading. Internal loading represents 

only approximately 6 percent of the total phosphorus load of 

the lake. Therefore, the phosphorus trapped in the sediments 

should have minimal effect on long-term phosphorus concentrations and should not delay the response in the productivity 

of the lake to future decreases in nutrient loading from its 

watershed. 

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Appendix 1 57

Appendix 1. Wastewater indicators in sediment (Schedule 5433 compounds), analyzed at the U.S. Geological Survey 

National Water Quality Laboratory in Denver Colo.—Continued

[EDP, endocrine-disrupting potential; S, suspected; K, known; μg/kg, microgram per kilogram; >, greater than; CP, combustion product; PAH, polycyclic aromatic hydrocarbon; --, no data. Table adapted from J. R. Ward, U. S. Geological Survey, written commun., February 28, 2006]

 Compound name Reporting level, 

μg/kg EDP Possible compound uses or sources

1,4-Dichlorobenzene 50 S Moth repellant, fumigant, deodorant. 

1-Methylnaphthalene 50 — 2-5 percent of gasoline, diesel fuel, or crude oil. 

2,6-Dimethylnaphthalene 50 — Present in diesel/kerosene (trace in gasoline). 

2-Methylnaphthalene 50 — 2-5 percent of gasoline, diesel fuel, or crude oil. 

3-beta-Coprostanol 500 — Carnivore fecal indicator. 

3-Methyl-1H-indole (skatol) 50 — Fragrance, stench in feces and coal tar. 

3-tert-Butyl-4-hydroxy anisole (BHA) 150 K Antioxidant, general preservative. 

4-Cumylphenol 50 K Nonionic detergent metabolite. 

4-n-Octylphenol 50 K Nonionic detergent metabolite. 

4-tert-Octylphenol 50 K Nonionic detergent metabolite. 

Acetophenone 150 — Fragrance in detergent and tobacco, flavor in beverages. 

Acetyl-hexamethyl-tetrahydro- naphthalene 

(AHTN) 

50 — Musk fragrance (widespread use) persistent in groundwater. 

Anthracene 50 — Wood preservative, component of tar, diesel, or crudeoil, CP. 

Anthraquinone 50 — Manufacturing dye/textiles, seed treatment, bird repellant. 

Atrazine 100 — Selective triazine herbicide. 

Benzo[a]pyrene 50 K Regulated PAH, used in cancer research, CP. 

Benzophenone 50 S Fixative for perfumes and soaps. 

beta-Sitosterol 500 — Plant sterol. 

beta-Stigmastanol 500 — Plant sterol. 

Bisphenol A 50 K Manufacturing polycarbonate resins, antioxidant, flame 

retardant. 

Bromacil 1700 K Herbicide, >80 percent noncrop usage on grass/brush.

Camphor 50 — Flavor, odorant, ointments. 

Carbazole 50 — Insecticide, Manufacturing dyes, explosives, and lubricants. 

Chlorpyrifos 170 K Insecticide, domestic pest and termite control.

Cholesterol 250 — Often a fecal indicator. 

Diazinon 170 K Insecticide, > 40 percent nonagricultural usage, ants, flies.

Diethyl phthalate 100 — Plasticizer for polymers and resins. 

Bis (2-Ethylhexyl) phthalate 250 — Plasticizer for polymers and resins, pesticides. 

d-Limonene 50 — Fungicide, antimicrobial, antiviral, fragrance in aerosols. 

Fluoranthene 50 — Component of coal tar and asphalt (only traces in gasoline or 

diesel fuel), CP. 

Hexahydrohexamethyl-cyclopentabenzopyran 

(HHCB) 

50 — Musk fragrance (widespread use) persistent in groundwater. 

Indole 100 — Pesticide inert ingredient, fragrance in coffee. 

Isoborneol 50 — Fragrance in perfumery, in disinfectants. 

Isophorone 50 — Solvent for lacquer, plastic, oil, silicon, resin. 

Isopropylbenzene 100 — Manuf phenol/acetone, fuels and paint thinner. 

Isoquinoline 100 — Flavors and fragrances. 

Menthol 50 — Cigarettes, cough drops, liniment, mouthwash. 

Metolachlor 170 — Herbicide, indicator of agricultural drainage.
58 Water Quality, Hydrology, and Simulated Response to Changes in Phosphorus Loading of Mercer Lake, WI, and Effects of Wastewater Discharge

Appendix 1. Wastewater indicators in sediment (Schedule 5433 compounds), analyzed at the U.S. Geological Survey 

National Water Quality Laboratory in Denver Colo.—Continued

[EDP, endocrine-disrupting potential; S, suspected; K, known; μg/kg, microgram per kilogram; >, greater than; CP, combustion product; PAH, polycyclic aromatic hydrocarbon; --, no data. Table adapted from J. R. Ward, U. S. Geological Survey, written commun., February 28, 2006]

 Compound name Reporting level, 

μg/kg EDP Possible compound uses or sources

N,N-Diethyl-meta-toluamide (Deet) 100 — Insecticide, urban uses, mosquito repellent. 

Naphthalene 50 — Fumigant, moth repellent, major component 

(about 10 percent) of gasoline. 

Nonylphenol diethoxylates 1000 K Nonionic detergent metabolite. 

4-Nonylphenol 750 — Nonionic detergent metabolite. 

Octylphenol diethoxylates 50 K Nonionic detergent metabolite. 

Octylphenol monoethoxylates 250 K Nonionic detergent metabolite. 

p-Cresol 250 S Wood preservative.

para-Nonylphenol (total) 500 K Nonionic detergent metabolite. 

Phenanthrene 50 — Manufacturing explosives, component of tar, diesel fuel, or 

crude oil, CP.

Phenol 50 — Disinfectant, manuf several products, leachate.

Prometon 170 — Herbicide (noncrop only).

Pyrene 50 — Component of coal tar and asphalt (only traces ingasoline or 

diesel fuel), CP.

2,2,4,4 -Tetrabromodiphenyl ether 50 — Fire retardant.

Tri(2-butoxyethyl) phosphate 150 — Flame retardant.

Tri(2-chloroethyl) phosphate 100 S Plasticizer, flame retardant.

Tri(dichloroisopropyl) phosphate 100 S Flame retardant.

Tributyl phosphate 50 — Antifoaming agent, flame retardant.

Triclosan 50 S Disinfectant, antimicrobial (concern for acquired microbial 

resistance).

Triphenyl phosphate 50 — Plasticizer, resin, wax, finish, roofing paper, flame retardant.
Publishing support provided by the U.S. Geological Survey

Science Publishing Network, Columbus Publishing Service Center

For more information concerning the research in this report, contact the

Director, Wisconsin Water Science Center

U.S. Geological Survey

8505 Research Way

Middleton, Wisconsin 53562

http://wi.water.usgs.gov/
Robertson, and others—Water Quality, Hydrology, and Simulated Response to Changes in Phosphorus Loading of Mercer Lake, WI, and Effects of Wastewater Discharge—Scientific Investigations Report 2012-5134