A New Approach to Water Quality Monitoring for Dredging-Getting Data Around-the-Clock

摘要

SAI Engineering,Inc.(SAI)and Littoral Ecological&Environmental Services(LEES)began work on the Los Angeles River Estuary(LARE)Pilot Capping Study for the Los Angeles District Army Corp of Engineers(COE)and MEC Analytical in the summer of 2001.The scope of SAI’s services included data collection and transmission to the COE in the form of a Geographical Information System(GIS)files and collecting real-time water quality data around the clock. rnDuring the initial planning phase,SAI determined that in situ monitoring would be ideal for the river dredging operation and placement at the contained aquatic disposal site.The relatively sheltered site in Los Angeles Harbor allowed deployment of the sondes in the vicinity of the disposal site.This allowed the COE to collect data 24 hours a day 7 days a week during placement of material at the site.The COE elected to monitor only at the disposal site.rnSAI and LEES provided design,assembly,deployment,and weekly servicing of two multi-variable water-quality monitoring instruments.SAI selected YSI 6920 Sondes capable of continuous water quality profiling or monitoring.These instruments provided time-marked data for the following water quality variables:turbidity,temperature, dissolved oxygen,salinity/conductivity,and depth.The sondess were chosen for reliability,cost,and service history in similar applications. rnSAI’s team designed a buoy system for the in-situ deployment of the self-contained data- logging sondes.Submerged taut-line moorings(buoy and anchor systems)incorporating protective cages for the sondes was used to moor the sondes up-and downcurrent of the dump site.The sondes were moored at the disposal site approximately five meters(17 feet)above the bottom.Anchorage on the seabed required a combination of 300-and 50- pound anchors connected by a 180-spreader line.rnDifferential GPS was used to record deployment locations and aid in instrument retrieval. To service the instruments,a grapnel hook was towed across the spreader line by a service boat,thus snagging the line and allowing retrieval of the anchors and the instruments.Instrument assemblies were also equipped with echolocation pingers to aid in emergency relocation.LEES deployed and monitored the sondess throughout the course of disposal.rnIn addition to the real-time sondes,MEC collected water samples and water quality measurements at specific times during dredging and disposal operation using a Seabird