Nitrate and pesticides in surficial aquifers and trophic state and phosphorus sources for selected lakes, eastern Otter Tail County, west-central Minnesota, 1993-96
Nitrate concentrations (as nitrogen) were analyzed in water from 73 wells completed in surficial aquifers. Water from about one-third of the wells had concentrations greater than 10 mg/L (milligrams per liter), the regulatory limit for drinking water established by the U.S. Environmental Protection Agency. Nitrate concentrations: (1) were greater in water from wells in agricultural settings than in nonagricultural settings; (2) were not greater in water from shallow wells (25 feet deep or less) in settings with rapid soil permeability than with moderate soil permeability, probably because the effects of permeability were offset by the effects of land use and well depth; and (3) were greater in water from shallow wells (25 feet deep or less) than from deep wells (greater than 25 feet deep).
Triazine herbicides were detected in water from 23 of the 73 sampled wells by immunoassay tests. Most of these wells are in agricultural settings. Ten pesticides, which included seven triazine herbicide compounds, were detected in water from 19 of 25 wells analyzed by gas chromatography/mass spectrometry. Atrazine and deethylatrazine, a degradation product of atrazine, were detected in water from 18 and 16 wells, respectively. None of the detected pesticides had concentrations that exceeded their respective regulatory limits for drinking water established by the U.S. Environmental Protection Agency.
Four lakes in the Otter Tail River Basin, which in downstream order are Little Pine, Big Pine, Rush, and Otter Tail Lakes, ranged in trophic state from upper oligotrophic to lower eutrophic. The Secchi disk transparencies were 4.0 to 7.4 feet, chlorophyll a concentrations (epilimnetic) were 4.4 to 28 micrograms per liter, and total phosphorus concentrations (epilimnetic) were less than 0.010 to 0.022 mg/L (except one concentration of 0.060 mg/L). The trophic state of these lakes may have become less eutrophic from upstream to downstream lakes.
Major external sources of phosphorus to Big Pine Lake were the Otter Tail and Toad Rivers. The phosphorus load from these two streams during March 16, 1995, to March 15, 1996 was 10,400 pounds. The phosphorus load from the Toad River (5,730 pounds) was greater than from the Otter Tail River (4,670 pounds) even though streamflow from the Toad River was about 70 percent less than the Otter Tail River. Phosphorus removal from Big Pine Lake through the Otter Tail River outlet during the 1-year period was 8,460 pounds. The total annual accumulation of phosphorus, which includes an estimated 700 pounds from ground-water discharge, was 2,640 pounds. The accumulated phosphorus probably was utilized by phytoplankton or was absorbed by nonliving particulate matter that eventually settled into bottom sediments.
Bottom sediments were an internal source of phosphorus to Little Pine and Big Pine Lakes. Increased total phosphorus concentrations (hypolimnetic) of 0.037 to 0.120 mg/L at depth during August 9-10, 1995, indicated phosphorus release from bottom sediments. The increased phosphorus probably was associated with anoxic conditions in the hypolimnion during summer stratification.
Phosphorus at depth in Little Pine and Big Pine Lakes was mostly orthophosphate. During the fall turnover of the lakes, this orthophosphate may have circulated to near the lake surface and became an available nutrient for phytoplankton during the following growing season. The internal phosphorus load to Little Pine Lake may have been important because about three-fourths of the lake probably became stratified and anoxic in the hypolimnion. The internal phosphorus load to Big Pine Lake may not have been important because only a small portion of the lake became stratified and anoxic at depth.
Additional publication details
|Publication Subtype||USGS Numbered Series|
|Title||Nitrate and pesticides in surficial aquifers and trophic state and phosphorus sources for selected lakes, eastern Otter Tail County, west-central Minnesota, 1993-96|
|Series title||Water-Resources Investigations Report|
|Publisher||U.S. Geological Survey|
|Publisher location||Mounds View, MN|
|Contributing office(s)||Minnesota Water Science Center|
|Description||vi, 43 p.|
|Online Only (Y/N)||N|
|Additional Online Files (Y/N)||N|