NWT RESEARCH DATABASE

Tags: physical sciences, water sampling, hydrology, sedimentology, water chemistry, bathymetry, organic carbon, erosion, water velocity

Objective(s): To correct for fossil particulate organic carbon (POC) input to the Mackenzie River, and to determine the loci of peatland organic carbon (OC) erosion and flux to the fluvial system.

Project Description: The objectives of this research project are to correct for fossil particulate organic carbon (POC) input to the Mackenzie River, and to determine the loci of peatland organic carbon (OC) erosion and flux to the fluvial system. WATER VELOCITY TRANSECT At each sampling location, an Acoustic Doppler Current Profiler (ADCP) will allow the research team to determine bathymetry, water velocity transect and water discharge. This device is to be mounted onto one side of a boat and immerged just below the river surface. It consists of four transductors oriented towards the river bottom. The transductors regularly send acoustic waves that interact with the suspended solid matter transported by the river. The frequency of the reflected wave, recorded by the device, is offset from the frequency of the incident wave, because of the relative velocity between the particle of solid matter and the ADCP, and thus the boat (Doppler effect). If the boat velocity is known (by a GPS coupled with the ADCP, for example), it is possible to calculate the velocity of the particle, which is the same as the velocity of the water flow surrounding the particle. The four transductors allow a 3-D reconstruction of the water velocity vector. By recording reflected waves with increasing elapsed time since the incident wave has been sent, water velocity is determined as a function of distance from the boat. Besides this water velocity vertical profile measurement, the ADCP has also an embedded sonar that determines river bottom. Finally, the ADCP also records the backscattered wave, which yields semi-quantitative information about the concentration of suspended matter in the river water. All the data is instantaneously recorded and treated with a laptop connected to the ADCP. Establishing the ADCP transect (water velocity over the river cross-section) needs the boat to cross the river from a bank to another. To check the reproducibility of the transect, four to six transects will be made. Altogether, and including the ADCP setup, this procedure will take approximately two hours at each sampling location. SAMPLING At each sampling location, the aim is to sample river water at various depths. This will be done thanks to a specific equipment: a point-sediment sampler. The research team have already successfully used this sampler in the Mackenzie River, along with in Bolivia and Peru to sample the Amazon River’s tributaries. It consists in a one meter-long plastic tube, that can contain approximately eight liters of water, and that has one cap at each of its tips. The sampler has to be attached via a steel cable to a winch mounted onto a boat. Once the boat is at the desired sampling position, the sampler is immerged, both sides open, down to the desired depth. Then, a small weight called "the messenger" is sent along the steel cable. The messenger rapidly goes down the cable, and when it strikes the top of the sampler, it triggers the closure of the caps, and thus the capture of river water at the desired depth. The sampler is then drawn up on the boat, the water it contains is poured into a bucket and weighed, its pH measured, and finally stored into clean hermetic plastic containers, waiting for the filtration. This procedure will be repeated for each sampling depth, constituting a sampling vertical. For some sampling locations, several vertical profiles will be made so as to assess the lateral variability of river material. To sample a river cross-section, several hours can be necessary. Finally, if possible, river bed material will be dredged using a simple home-made device. Coarse bank material will also be sampled if possible. These samples will be stored in hermetic plastic bags. FILTRATION Filtration will be achieved within a few hours after sampling, using under-pressure Teflon filtration units and PSE filter sheet, at 0.22 µm porosity. Filtering six to eight liters of water can be done in two or three hours. After filtration completion, filter sheets are brushed and rinsed so as to recover the solid material, which is poured with filtrated water into small glass bottles, to be brought back to the laboratory. Slightly less than a liter of filtrated water is also kept aside for chemical analysis. Results of this research will be communicated to communities via two methods. The first will be on the ground, during the research. This will allow a two-way-discussion of the research project and the communities’ interests in the findings. Second, the research team will write non-technical summaries for the Aurora Research Institute which will allow the findings to be disseminated over a wider group of individuals. The fieldwork for this study will be conducted from July 18, 2013 to July 29, 2013.