Assessment of the Hydrology, Water Quality, and Biology of Mercer Lake
Project Number: 9KB42
Project Chief: Dale Robertson
Project Topics: Iron County, Mercer, lake, nutrient, water, quality, shallow
Cooperators: Mercer School District
Period of Project: October 2007-current
Mercer Lake, Wisconsin. The USGS Lake Studies team provides scientifically sound information to help local communities determine effective management actions to protect or restore lakes.
Problem
Residents and authorities around Mercer Lake feel that the lake has degraded because of past activities in its watershed, such as effluent from the previous sewage treatment plant, residential development, and storm-water discharge. These inputs may have resulted in increased macrophyte growth and increased sedimentation. Recycling of phosphorus from the bottom sediments, and loading from upstream nutrient sources are now believed to be the major causes of current poor water quality.
Lake-area residents and lake users would like to implement management or restoration steps to return the lake to a more natural, improved condition. Before management decisions are made, a good understanding of the lake's hydrology, nutrient loading, and internal recycling of nutrients is needed.
Objectives
- Quantify the water quality of the lake;
- Determine chemical quality of the lake-bottom sediments near the old wastewater treatment plant and in the basins of the lake;
- Quantify the flows and phosphorus loads entering and leaving the lake;
- Determine water and phosphorus budgets;
- Evaluate and relate the measured water and phosphorus loads to observed and modeled water-quality responses using eutrophication models in WiLMS;
- Model the lake's response to future phosphorus-loading and management scenarios;
- Develop an Educational Outreach Program with the Mercer Lake School District.
A better understanding of the hydrologic and phosphorus inputs and potential response of the lake to various management scenarios will enhance the management of Mercer Lake.
Approach
The study will consist of data collection for two complete years, followed by data analysis, interpretation, and report preparation.
Water-quality and sediment chemistry:
Water-quality monitoring will be done at two locations in the lake- a main sampling site over the deepest location in the eastern basin and at an additional site in the center basin. The sites will be sampled 6 times per year. To augment this monitoring program, Mercer Lake’s Citizen Lake Monitor, Jerry Huffmaster, collects samples midway in time between USGS sampling dates. Data for TSI parameters will be compared with historical data and data from a paleoecological lake study by the WDNR.
Lake-bottom sediment will be collected at the three locations in the lake and analyzed for a wide variety of constituents including physical characteristics, wastewater indicators, and trace metals to characterize sediment quality.
Public boat landing on Mercer Lake, where storm runoff is being monitored.
Components of the water budget:
Changes in lake storage (volume) will be determined by a continuous-recording stage gage.
Precipitation will be monitored by a continuous-recording gage during non-freezing periods at the lake-stage or inflow gaging station. National Weather Service data will be used during winter.
Evaporation will be estimated from values determined from other lake studies.
Surface inflow will be determined by a continuous-recording gaging station on Little Turtle River at USH 51. Flow from Tahoe Creek will be measured intermittently near where it enters Mercer Lake. Inflow from the remaining unmonitored areas will be estimated through the use of runoff coefficients developed from the gaged watershed.
Surface outflow will be determined by a continuous-recording streamflow gage on Little Turtle River near the lake’s outlet.
Ground-water inflow and outflow will be quantified using the ground-water flow model GFLOW.
Phosphorus budget:
Phosphorus inputs and losses associated with each component in the water budget will be measured or estimated.
Phosphorus release rates from the bottom sediments will be determined from sediment cores from four sites in the lake. Phosphorus release from these cores under aerobic and anaerobic conditions will be determined in the laboratory by the State Lab of Hygiene.
Simulation of trophic response to changes in phosphorus loading:
Water and phosphorus budget information will be used in conjunction with lake water-quality information to assess Mercer Lake’s response to changes in nutrient loading and determine the lakes potential response to specific management scenarios with eutrophication models in WiLMS.
Selection of these specific scenarios will be done in consultation with the Lake Association.
Monitoring and Data
 Mercer Lake Monitoring - click the two red monitoring points to see Mercer Lake water levels, precipitation, and inlet streamflow.
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Publications and Reports
TBD
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