The Lake Olathe watershed, located in northeast Kansas, was investigated using bathymetric survey data and reservoir bottom-sediment cores to determine sediment deposition, water-quality trends, and transport of nutrients (phosphorus and nitrogen species), selected trace elements, selected pesticides, and diatoms as indicators of eutrophic (organic-enriched and depleted oxygen supply) conditions. To determine sediment deposition and loads, bathymetric data from Cedar Lake and Lake Olathe, both located in the Lake Olathe watershed, were collected in 2000 and compared to historical topographic data collected when the lakes were built.

Approximately 338 acre-feet of sediment deposition has occurred in Cedar Lake since dam closure in 1938, and 317 acre-feet has occurred at Lake Olathe since 1956. Mean annual sediment deposition was 5.45 acre-feet per year (0.89 acre-feet per year per square mile) for Cedar Lake and 7.0 acre-feet per year (0.42 acre-feet per year per square mile) for Lake Olathe. Mean annual sediment loads for the two reservoirs were 9.6 million pounds per year for Cedar Lake and 12.6 million pounds per year for Lake Olathe.

Mean concentrations of total phosphorus in bottom-sediment samples from Cedar Lake ranged from 1,370 to 1,810 milligrams per kilogram, and concentrations in bottom-sediment samples from Lake Olathe ranged from 588 to 1,030 milligrams per kilogram. The implication of large total phosphorus concentrations in the bottom sediment of Cedar Lake is that inflow into Cedar Lake is rich in phosphorus and that adverse water-quality conditions could affect water quality in downstream Lake Olathe through discharge of water from Cedar Lake to Lake Olathe via Cedar Creek.

Mean annual phosphorus loads transported from the Lake Olathe watershed were estimated to be 14,700 pounds per year for Cedar Lake and 9,720 pounds per year for Lake Olathe. The mean annual phosphorus yields were estimated to be 3.74 pounds per acre per year for Cedar Lake and 0.91 pound per acre per year for Lake Olathe. Phosphorus yields in the Cedar Lake watershed were largest of the six Kansas impoundment watersheds recently studied.

Concentrations of total ammonia plus organic nitrogen as nitrogen in bottom sediment increased from upstream to downstream in both Cedar Lake and Lake Olathe. Mean concentrations of total ammonia plus organic nitrogen as nitrogen (N) ranged from 2,000 to 2,700 milligrams per kilogram in bottom-sediment samples from Cedar Lake and from 1,300 to 2,700 milligrams per kilogram in samples from Lake Olathe. There was no statistical significance between total ammonia plus organic nitrogen as nitrogen and depth of bottom sediment.

Concentrations of six trace elements in bottom sediment from Cedar Lake and Lake Olathe (arsenic, chromium, copper, lead, nickel, and zinc) exceeded the U.S. Environmental Protection Agency Threshold Effects Levels (TELs) sediment-quality guidelines for aquatic organisms in sediment except for one lead concentration. Probable Effects Levels (PELs) for trace elements, however, were not exceeded at either lake.

Organochlorine and organophosphate insecticides were not detected in bottom-sediment samples from either Cedar Lake or Lake Olathe, but the acetanilide herbicides alachlor and metolachlor were detected in sediment from both lakes. The U.S. Environmental Protection Agency has not proposed TEL or PEL guideline concentrations for bottom sediment for any of the organophosphate, acetanilide, or triazine pesticides.

The diatoms (microscopic, single-celled organisms) Cyclotella bodanica, an indicator of low organic-enriched water, and Cyclotella meneghiniana, an indicator of organic-enriched water, were both present in bottom sediment from Lake Olathe. The presence of both of these diatoms suggests varying periods of low and high eutrophication in Lake Olathe from 1956 to 2000. The concentrations of two species in bottom sediment from Cedar Lake, Aulacoseira cf alpigena and Cyclotella meneghiniana, as well as two species in sediment from Lake Olathe, Aulacoseira cf alpigena and Stephanodiscus nigare, increased in sediment cores from the older bottom material to the more recent deposition near the top of the sediment cores. These diatom species indicate eutrophic conditions, and the increased concentration of these diatom species from the bottom of the cores to the sediment/water interface suggests that historically these lakes have been and continue to be eutrophic at times.

Comparison of constituent trends between Cedar Lake and Lake Olathe using reservoir bottom sediment was not possible because sediment from Cedar Lake was suspected of having been disturbed. However, trends that may be reflective of historical changes in water quality were not detected in sediment from either Cedar Lake or Lake Olathe for total phosphorus, trace elements (except lead), and organochlorine or organophosphate pesticides. A slight increasing trend in the concentration of total ammonia plus organic nitrogen as nitrogen was seen in the sediment profile from Lake Olathe but not in the profile from Cedar Lake. The acetanilide herbicides alachlor and metolachlor were more prevalent in more recently deposited sediment in Cedar Lake and Lake Olathe, as was the triazine herbicide atrazine in Lake Olathe bottom sediment, suggesting a possible increasing trend in lake-inflow water concentrations.

Trends in water-quality characteristics can be used by the Lake Olathe watershed managers to document historical changes in the watershed such as changes in land use, the suspension of the use of chlorinated insecticides, such as DDT and chlordane, and the use of hydrophobic fertilizers. The investigation described in this report provides a baseline of water-quality information to compare future changes in water quality or other watershed activities. With the addition of bathymetric surveys and the inclusion of additional reservoirs, reservoir sediment investigations can be used to estimate historical loads of phosphorus and other constituents in future water-quality assessments throughout Kansas.