Research about Lake Mendota, Lake Monona

USGS2025

Seasonal stratification drives bioaccumulation of pelagic mercury sources in eutrophic lakes

Increased lake eutrophication, influenced by changing climate and land use, alters aquatic cycling and bioaccumulation of mercury (Hg). Additionally, seasonally dynamic lake circulation and plankton community composition can confound our ability to predict changes in biological Hg concentrations and sources. To assess temporal variation, we examined seasonal total Hg (THg) and methylmercury (MeHg) concentrations and stable isotope values in seston, waters, sediments, and fish from two adjacent urban eutrophic lakes in Madison, Wisconsin. In Lake Monona, surface sediment THg concentrations were elevated due to comparably higher urbanization and historical industrial inputs, whereas Lake Mendota sediments had lower concentrations corresponding with largely agricultural and suburban surrounding watershed. Surface water THg and MeHg were similar between lakes and seasonally dynamic, but water profiles exhibited elevated concentrations in the meta- and hypolimnion, highlighting water column MeHg production. Seston MeHg concentrations were often highest at shoulder seasons, possibly owing to metalimnetic MeHg delivery, but also differences in biomass and water clarity. The ∆199Hg and δ202Hg values in seston were similar between lakes, despite differing sediment THg concentrations and values, suggesting a shared bioaccumulated source of MeHg. Measurement of MeHg stable isotopes further elucidated that seston and fish predominantly bioaccumulated pelagic-sourced MeHg.

Wisconsin↗

USGS1999

Evaluation of the effectiveness of an urban stormwater treatment unit in Madison, Wisconsin, 1996-97

An urban stormwater treatment unit was tested as part of an ongoing program of urban nonpoint- pollution research in Madison, Wis. Flow measurements were made and water samples were collected at the inlet to, outlet from, and bypass around the treatment chamber of the device that was installed to collect the runoff from a city maintenance yard. About 90 percent of the runoff water from the 4.3-acre basin was treated by the unit. The remaining 10 percent bypassed the treatment chamber when the flow rate reached approximately 500 gallons per minute. A 24-percent difference between the estimated amount (405 kilograms) and the actual amount (536 kilograms) of retained material in the treatment chamber may be attributed to bedload material that the automatic samplers could not effectively collect. Assuming this, 8 percent of the total mass in the untreated runoff water was estimated as the unsampled bedload. On the basis of water-sample data collected over the course of the study, the suspended solids removal efficiency of treatment chamber was about 25 percent, and the efficiency of the unit as a whole was 21 percent. If the unsampled bedload material was accounted for, the treatment-chamber efficiency was 33 percent. About 19 percent of the total phosphorus was removed from the water that passed through the treatment chamber and 17 percent was removed by the unit as a whole. Total polycyclic aromatic hydrocarbon (PAH) loads were reduced about 39 percent by the treatment chamber and 34 percent by the unit as a whole; these were some of the most effectively removed constituents. Total metals were reduced about 20 to 30 percent by both the treatment chamber and the unit as a whole. In general, dissolved constituents were unaffected by the unit. The material retained in the treatment chamber had high concentrations of lead and PAH and may be subject to special disposal restrictions based on those concentrations and the presence of benzo(a)anthracene. The chemical makeup of the retained material in other similar stormwater treatment units will probably vary depending on the land use and activities in the drainage basin.

Wisconsin↗