Comparison of Water Quality in Low Population and High Population Lakes of Waukesha County

Prepared by:

Aquarius Lofton, Tammi Nagel and Jean Strick

November 24, 2003

Abstract

We measured phosphorus and total nitrogen in ten pre-selected lakes of Waukesha County to determine whether higher population densities in the communities surrounding a lake would result in reduced water quality as characterized by elevated levels of nutrients (phosphorus and total nitrogen).  Of the five lakes designated as low population, only one of the lakes exhibited detectable levels of total nitrogen and/or phosphates, compared to three of the five lakes designated as high population.  Despite a greater proportional occurrence of total nitrogen and phosphate detections in the high population lakes, the results of T-tests conducted for each group yielded P-values of greater than 0.05 for both phosphate and total nitrogen (0.098 and 0.466, respectively), indicating no statistically significant difference in water quality between the groups of lakes.  Additionally, phosphate and total nitrogen concentrations do not correlate with total lake area or depth.  In an effort to further investigate the effect of human activities on water quality, future studies should be designed to consider land use in drainage basins up-gradient of lakes, and to take into account seasonal nutrient variations.  

 

Keywords:   water quality, population, total nitrogen, phosphate.

 

Introduction

Land use surrounding lakes can affect the input of nutrients into the lake, and the resulting water quality.  In undisturbed settings, the banks of lakes are typically densely vegetated.  As a result, the input of nutrients such as phosphorus and nitrogen are low due to reduced levels of run-off from the land (Pitois et al, 2001).  With an increase in both urban and rural human populations, nutrients from sediment erosion, septic systems, and fertilizers are exported to lakes from the surrounding watershed at a rate that can increase algal growth, resulting in eutrophication, or aging, of a lake at rates greater than would naturally occur in the absence of human populations (Christopherson and Smith, 1995). 

According to the Great Lakes Water Quality Agreement, programs that are aimed towards controlling pollution to our Wisconsin Lakes are essential to preserve the ecosystem (Gilbertson, 1999).  It is the role of the scientist to assist engineers in construction of waste treatment plants that minimize excessive introduction of nutrients into the Great Lakes (Gilbertson, 1999).  This same type of preservation and restoration can be employed in smaller lakes throughout Wisconsin.  The smaller county lakes are also in danger of pollution from agricultural sources, insecticides, and industrial chemicals (Annin, 1999).  It is also the role of the scientist to educate the public about the effects of land use on water quality (Christopherson and Smith, 1995).  Lakes with higher population densities may have an increased use of fertilizers and pesticides to preserve lawns and landscaping, resulting in overflow to the shoreline waters.  Our research team hypothesized that water quality, as determined by reduced phosphorus and total nitrogen, is higher in lakes with lower population densities when compared to lakes with larger populations.

 

                                    Materials and Methods

We tested our hypothesis on October 8, 2003 and October 9, 2003.  Prior to water sample collection, ten lakes in Waukesha County were selected for sampling and designated as either low or high population.  To determine a lake’s designation, census information for the year 2000 was obtained from the Wisconsin Department of Administration for Waukesha County (Demographic Service Center, Wisconsin Department of Administration, 2003).  Next, the area of the towns, villages, and/or cities surrounding each of the ten lakes was estimated using a grid overlay with a known area.  The grid was placed over a map depicting Waukesha County communities, and the area of each community was estimated by addition of grid-squares covered by the community (Milwaukee Maps, Inc., 2003).   Population density for each lake was then calculated by dividing the total population of surrounding towns, villages, and/or cities, by the estimated area of each respective community.  The resulting population density, measured in people per square kilometer, was then used to designate each lake as either high or low population.  Lakes with surrounding communities averaging population densities of greater than 250 people per square kilometer were designated as high population.  Lakes with surrounding communities averaging population densities of less than or equal to 115 people per square kilometer were designated as low population.  Eagle Spring Lake, Pine Lake, Beaver Lake, North Lake, and Okauchee Lake were designated as low population lakes.  Lower Phantom Lake, Nagawicka Lake, Oconomowoc Lake, Lac La Belle, and Pewaukee Lake were designated at high population for the purpose of this study.  Population data and population density calculations are included as appendices to this report.  Average population densities calculated for each lake are included in Table 1.

Water samples were collected from each of the ten lakes and immediately analyzed in the field for total nitrogen and phosphate (as orthophosphate).  Water samples were collected from an average of 2-5 meters off the shoreline.  Water temperature was measured in conjunction with testing using a thermometer.  A LaMotte Nitrate-N and Phosphate in Water Test Kit was used to complete water quality analysis.  Prior to beginning each test, test tubes were rinsed with a sample of the lake water about to be analyzed.  The phosphate test was initiated first by filling a test tube with approximately 10 milliliters (mL) of lake water.  1.0 mL of reagent provided in the kit was added to the water sample and mixed.  Approximately 0.1 grams (g) of phosphate reducing reagent were added to the sample, and then mixed until the reagent was fully dissolved.  The water sample was then set aside for five minutes, in accordance with test kit instructions, during which time a second water sample was prepared for total nitrogen analysis.  The total nitrogen test was initiated by first filling a test tube with approximately 2.5 mL of lake water.  The 2.5 mL sample was diluted to 5.0 mL with acid reagent, and allowed to equilibrate for two minutes.  Following the two minute equilibration period, approximately 0.1 g of reducing reagent was added, and the water sample was inverted at a rate of approximately 40 times per minute for a total of one minute.  The water sample was then set-aside for ten minutes, in accordance with test kit instructions.  Following recommended equilibration times, the phosphate and total nitrogen samples were read using a LaMotte Axial Reader, and the results were noted.  Total nitrogen and phosphate readings were recorded in parts per million (ppm). 

Water clarity measurements with a Secchi disc were originally proposed for inclusion in this study, but were deemed inappropriate because sufficient lake depths were not accessible by the research team.  General clarity and water color observations are included on Table 1.  Table 1 also includes the results of total nitrogen, phosphate, and temperature measurements obtained for each lake.  In addition to measured parameters, the area of each lake and maximum depth, as obtained from the Wisconsin Department of Natural Resources (WDNR, 2003), are also tabulated.  General information pertaining to vegetation and bank types noted surrounding each lake is also included in Table 1.

 

 

Table 1. – Population Densities, Lake Area, Maximum Depth, Water Quality Parameters, and Visual Observations of Ten Waukesha County Lakes.  

 

Total nitrogen and phosphate concentrations were compiled into an ExcelÓ spreadsheet for graphing and statistical analysis.  A T-test was completed to evaluate whether there was a statistically significant difference between total nitrogen and phosphate in low population Waukesha County lakes, when compared to lakes designated as high population.   To evaluate whether total lake area (square kilometers) and maximum depth (meters) affect nutrient concentrations, area and depth were correlated individually with phosphate and total nitrogen concentrations. 

Results

Although total nitrogen and/or phosphate were detected in three of the five high population lakes, compared to only one of the five low population lakes, P-values generated for each set of data (0.098 and 0.466, for phosphate and total nitrogen, respectively) indicated no significant difference in water quality between the groups of lakes.  The average amount of phosphates in low population lakes  (Fig. 1) was lower than the average amount of phosphates in high population lakes. 

 

 

 

 

 

 

 

 

 

 

 

 

 

Fig 1. – Comparison of Average Phosphate Concentrations in Five Low Population Lakes and Five High Population Lakes of Waukesha County

 

The average amount of total nitrogen was higher in the low population lakes (Fig 2.), due to a single detection of total nitrogen (1ppm) in one of the five low population lakes (North Lake). 

 

 

 

 

 

 

 

 

 

 

 

 

Fig 2. – Comparison of Average Total Nitrogen Concentrations in Five Low Population Lakes and Five High Population Lakes of Waukesha County

 

When lake area and phosphate concentrations were correlated, a correlation coefficient of –0.33 was calculated, indicating a very weak negative correlation between lake area and phosphate concentrations.  Likewise, a correlation coefficient of –0.32 was calculated when lake area and total nitrogen concentrations were correlated, also indicating a very weak negative correlation.  Very weak correlation coefficients of –0.15 and 0.13 were calculated when maximum lake depth was correlated with phosphate and total nitrogen concentrations, respectively.  Based on calculated correlation coefficients, phosphate and total nitrogen concentrations do not correlate with lake area or depth.       

The water at one of the low population lakes, Pine Lake, was observed to have a green tint.  Water in the remaining four low population lakes was clear and colorless.  The water at four of the five high population lakes was observed to have a green or brown tint.  The water in high population Oconomowoc Lake was clear and colorless.  Water color observations were not statistically analyzed. 

 

Discussions

Based on the results of our research, there was no statistically significant difference in water quality between the groups of lakes included in this study; however, among the lakes designated as low population, only one of the five lakes (North Lake) had detectable levels of total nitrogen and phosphate.  North Lake was sampled at a location near a stream that entered on the northernmost tip of the lake.  It is possible that the stream was a source of nutrients entering the lake near the sample location.   Similarly to lakes, water quality in streams and rivers can deteriorate due to land practices along their banks (Bolda, 1997).  Land use in the drainage basin contributing to the stream entering North Lake was not taken into consideration prior to this study.  Further investigation of land-use in the greater drainage basin of North Lake could identify processes that contribute to addition of nutrients, such as a concentration of agricultural activities.  Research has shown that agricultural activities, including fertilization of cropland, have the potential to add nutrients to the local watershed (Pitois et al, 2001). 

Among lakes designated as high population lakes, only two of the five lakes had levels of total nitrogen and phosphates that were both below detection (Lac La Belle and Pewaukee Lake).  The waters of Lac La Belle and Pewaukee Lake were green in color due to a noted growth of algae.  Nitrogen and phosphorus are considered “limiting” nutrients for algal growth (Pitois et al, 2001).  Addition of these of nutrients to the watershed, as would be expected in highly populated areas, can contribute to algal growth (Pitois et al, 2001).  It is possible that nitrogen and phosphorus were previously present in the waters of Lac La Belle and Pewaukee Lake, but that they had been utilized during the summer months for production of algae and were therefore no longer present at detectable levels in the lake water.  Future studies of lake water quality should take into account possible seasonal fluctuations in nutrient levels. 

 

Literature Cited

 

Annin, P. 1999. Great Lake Effect. Newsweek, 134: 52-55.

 

Bolda, K.S., and W.J. Meyers. 1997. Conducting a long-term water quality monitoring project: as case study on the McCloud River, California. Journal of Soil and Water Conservation, 52: 49-54.

 

Christopherson, J., and E. Smith. 1995. The Tahoe landscape: a BMP education program. Journal of Soil and Water Conservation, 50: 272-274.

 

Demographic Services Center, Wisconsin Department of Administration. 2003. Population projections for Wisconsin communities: 2000-2010. Retrieved October 7, 2003 from http://www.doa.state.wi.us.

 

Gilbertson, M. 1999. Water Quality Objectives: Yardsticks of the Great Lakes Water Quality Agreement. Environmental Health Perspectives, 107: 239-242.

 

Milwaukee Map Service, Inc. 2003. Street map of Milwaukee County and Waukesha County.

 

Pitois, S., M.H. Jackson, and B.J.B. Wood. 2001. Sources of the eutrophication problems associated with toxic algae: an overview. Journal of Environmental Health, 64:25-32.

 

Wisconsin Department of Natural Resources. 2003. Wisconsin DNR lake maps: Waukesha County.  Retrieved October 7, 2003 from http://www.dnr.state.wi.us/org/water/fhp/lakes/lakemap/waukesha.htm.

Appendices