Please use this identifier to cite or link to this item: https://hdl.handle.net/11681/48297
Title: Water Quality Sampling Report : Results of Water Quality Monitoring During Dredging to Construct Shallow-Water Habitat on the Missouri River at the Little Sioux Bend Project Site
Authors: United States. Army. Corps of Engineers. Omaha District
Keywords: Water quality
Water--Sampling
Aquatic habitats
Pallid sturgeon
Sedimentation and deposition
Environmental management
Environmental protection
Publisher: United States. Army. Corps of Engineers. Omaha District.
Abstract: The Little Sioux Bend shallow-water habitat project (LSBSWHP) is one of several projects the U.S. Army Corps of Engineers (Corps) has constructed to enhance shallow-water habitat (SWH) along the lower Missouri River downstream of Gavins Point Dam. The LSBSWHP area is located in Harrison County, Iowa and Burt County, Nebraska along the Little Sioux Bend of the Missouri River between RM666 and RM669. The project area is on the Nebraska side of Missouri River and is centered on the old river channel that is the legal boundary between the States of Iowa and Nebraska. The LSBSWHP included the excavation of 464,063 cubic yards (354,802 cubic meters) of soil/sediment to create a 7,300 foot (2,225 meter) long flow-through chute that connected to the main channel of the Missouri River on the upstream and downstream ends. In constructing the chute, the top 18 inches (0.46 meters) of sediment/soil were removed and deposited in the designated spoil area and all the remaining material, an estimated 373,450 cubic yards, was excavated using a hydraulic dredge with the excavated material being discharged to the adjacent Missouri River. Dredging at the LSBSWHP commenced on 22-July-2015 and was completed on 11-September-2015 with 679 hours of active dredging being logged. Based on the dredging equipment used, the estimated average amount of material excavated was 550 cubic yards (420 cubic meters) per hour with the slurry estimated to be 11.1 percent solids. The average slurry discharge rate was 37.1 cubic feet (1.05 cubic meters) per second. Water quality monitoring was conducted at the LSBSWHP site during the construction of the chute. The objective of the monitoring was to assess the impact of the dredging discharge on water quality conditions in the Missouri River. Water quality was monitored at 15 sites at the LSBSWHP during periods of active dredging. Water quality monitoring sites were located on the dredging slurry discharge pipeline and the Missouri River upstream and downstream of the dredging slurry discharge. Water quality sampling at the LSBSWHP occurred on four dates during active dredging: 6-August-2015, 12-August-2015, 21-August-2015, and 1-September-2015. The sampled dredging slurry contained extremely high and variable levels of suspended sediment with sampled levels ranging from 5,970 to 235,000 mg/L. The variability of the sampled suspended sediment in the dredging slurry is attributed to the inconsistent nature of hydraulic dredging. The sampled levels of total suspended sediment solids in the Missouri River immediately downstream of the dredging discharge were significantly lower than the levels sampled in the dredging slurry. This indicated that the suspended material in the dredging slurry discharge likely settled out quickly and was incorporated into the bedload of the Missouri River and/or was deposited on the riverbed. In concert with the high levels of suspended sediment, the dredging slurry contained high and variable levels of sediment-bound constituents (i.e. total phosphorus and total metals). The estimated sediment discharged to the Missouri River from dredging ranged from a high of 373,450 cubic yards (dredging discharge characterization) to a low of 173,754 cubic yards (average of sampled dredging slurry conditions). The under estimation of the delivered sediment load based on the dredging slurry sampling is likely the result of under-sampling of the inconsistent and highly variable dredging slurry conditions. Total phosphorus levels sampled in the dredging slurry were less variable than the sampled total suspended sediment levels, and ranged from 3.38 to 27.90 mg/L. The total phosphorus flux rate of the dredging discharge was estimated from dredging slurry sampling to be from 0.0112 kg/sec (average sampled condition) to 0.0293 kg/sec (maximum sampled condition). This was from 6.6 to 17.3 percent of the average total phosphorus flux rate for the Missouri River at the LSBSWHP site during the time of dredging discharge. The estimated total phosphorus load delivered to the Missouri River from dredging, based on the sampling of the dredging slurry, ranged from a low of 27.3 metric tons (averaged sampled conditions) to a high of 71.6 metric tons (maximum sampled conditions). This compares to the 149.2 metric tons that was estimated from pre-construction sediment sampling and elutriate testing. It is noted that the sampling of sediment-bound total phosphorus in the dredging slurry is subject to the same under-sampling concern noted for the sampling of suspended sediment. Currently, the total phosphorus load to the Gulf of Mexico is estimated to be 154,300 metric tons per year, with the contribution of the Missouri River to this total load estimated to be between 16.8% (25,922 metric tons) and 20% (30,860 metric tons) (NRC, 2011). Levels of several metals, measured as total, sampled in the dredging slurry were quite high and exceeded total recoverable-based criteria identified in the State of Iowa’s water quality standards. Measured levels of total aluminum, copper, and zinc exceeded acute aquatic life criteria. Measured levels of total aluminum, arsenic, cadmium, copper, lead, selenium, and zinc exceeded chronic aquatic life criteria. Measured levels of total arsenic, copper, mercury, and thallium exceed human health criteria based on fish consumption. The sampled levels of total metals in the Missouri River immediately downstream of the dredging discharge were significantly lower than the sampled levels in the dredging slurry. The sampled low levels of total metals in the Missouri River immediately downstream of the dredging slurry discharge that were below applicable water quality standards criteria indicate that the applicable 404(b)(1) Guidelines water quality requirements for the dredging implemented at the LSBSWHP were met (i.e. no water quality standards’ violations and no significant degradation). The water quality sampling results at the LSBSWHP during dredging were compared to the pre-construction elutriate testing results conducted on sediment/soil samples collected at the project site. Overall, the elutriate testing of the sediment/soil samples collected at the LSBSWHP site prior to construction provided a reasonably good prediction of the water quality conditions that were sampled during dredging at the LSBSWHP site.
Description: Technical Report
Rights: Approved for Public Release; Distribution is Unlimited
URI: https://hdl.handle.net/11681/48297
Appears in Collections:Technical Reports