Spatial distribution of nutrients, chloride, and suspended sediment concentrations and loads determined by using different sampling methods in a cross section of the Trenton Channel of the Detroit River, Michigan, November 2014–November 2015

Abstract

The Detroit River separates the United States and Canada as it flows from Lake St. Clair to Lake Erie. The Trenton Channel is a 13-kilometer-long branch of the Detroit River that flows to the west of Grosse Ile before rejoining the Detroit River near its mouth, just before the Detroit River flows into Lake Erie. The U.S. Environmental Protection Agency has listed both the Trenton Channel and Detroit River as Areas of Concern because of a list of Beneficial Use Impairments such as interrupted drinking-water services, loss of aquatic life, and reduced recreational use. Phosphorus loading from tributaries such as the Trenton Channel is one of the primary drivers of eutrophication in Lake Erie. The complex flow patterns and variable distribution of chemical constituents in the Trenton Channel make it difficult to accurately characterize the concentrations and loads of nutrients and other constituents conveyed through the channel to Lake Erie. In order to better understand the Trenton Channel’s contributions of nutrients (total phosphorus, orthophosphate, total nitrogen, and ammonia), chloride, and suspended sediment to Lake Erie and evaluate differences in results obtained by using different sample methodologies, the U.S. Geological Survey, in cooperation with the U.S. Environmental Protection Agency and Environment Canada, completed 12 sampling campaigns on the Trenton Channel in Detroit, Michigan, from November 2014 through November 2015. Acoustic Doppler current profiler (ADCP) techniques were used to characterize the distribution of velocity components within a cross section corresponding to a transect of the Trenton Channel at U.S. Geological Survey station 041686401 Trenton Channel of Detroit River at Grosse Ile, Mich. Three methods of collecting water-quality data at the same transect of the Trenton Channel were used: multiple-vertical depth-integrated (MVDI), fixed-point, and discrete samples. Horizontal and vertical variations in concentrations of nutrients, chloride, and suspended sediment were analyzed from discrete samples to better understand distributions of these constituents throughout the channel. Constituent loads were calculated by using individual sample concentrations and ADCP measurements for discharge made on the same day that the water-quality samples were collected. Constituent loads calculated from MVDI and fixed-point sampling methods were compared. The relation between MVDI and fixed-point samples helped quantify the differences between the sampling methods. Linear regression equations depicting the relation between concentrations measured by using MVDI and fixed-point samples were prepared. ADCP data indicates that velocities throughout the sampled transect remain uniform except for one location around 200 meters from the west bank of the channel. Secondary flow vectors suggest the presence of counter-rotating helical flow cells, and these helical flow cells could affect the mixing of constituents in transport by preventing cross-channel mixing. Flow discharges throughout the sampling campaign showed small variations, although lower flow rates were observed in the early winter months than in the summer months. Discrete sampling methods results displayed both heterogeneity throughout the channel horizontally, representing limited horizontal mixing in the channel, and displayed homogeneity throughout vertical transects, indicating mixing vertically. Comparisons between MVDI and fixed-point methods found consistently higher concentrations were measured in MVDI samples compared to concentrations measured in fixed-point samples. To correct for this bias between MVDI and fixed-point sample results, simple linear-regression equations were developed for all major constituents to help estimate constituent concentrations from fixed-point samples equivalent to those measured by using MVDI sampling techniques. Instantaneous constituent loads were developed by using velocity and discharge data obtained from ADCPs and constituent concentrations obtained from MVDI and fixed-point samples.