Water Quality in the Ala Wai Canal

Summary by Solomon Chen
From August 2018, the SMART Ala Wai project has launched its water sampling effort in Ala Wai Canal. The goal of sampling is to conduct a water quality survey in the canal. Sampling is conducted on a monthly basis at selected stations along the canal using a Boston Whaler. So far, the team has surveyed the canal three times; the first sampling on 8/9, the second sampling on 10/21, and the third sampling on 11/29. The first sampling on 8/9 was scheduled to survey the canal at low tide and high tide respectively on the same day. The second sampling on 10/21 was set to survey the canal once during high tide, started around the time of highest tide on that day. The third sampling was set to survey the canal and the outer buoy located offshore Ala Moana. Due to sea state, the third sampling was the only sapling with water column profiling and sampling outside of the canal. The canal water quality survey consists of two parts — sensor profiling and water sampling.

In sensor profiling, surface transect along the length of the canal and vertical profile cast were conducted. Sensor bundle was attached to one side of the boat. The surface transect was only conducted during the first sampling, but discreet vertical profiles were done in both sampling. The vertical profiles were cast at four selected stations for the first sampling (site 1, 5, 9, 13 in fig.1). Vertical profiles were cast at 13 selected station for the second sampling (fig.1). The third sampling extended further toward offshore with 18 profiling and sampling station in total. Data of depth (absolute pressure), temperature, salinity (conductivity), oxygen concentration, chlorophyll, and turbidity were obtained as a time series. After sensor profiling, data analysis and visualization were done in Python and QGIS to construct a chemical and a physical understanding of the water column.

According to the result, the canal generally has two layers of the water body with distinct salinity (hence density) properties. A halocline is observed in most of the vertical profiles at depth around 0.5. Notably, the canal generally has an anoxic layer of water at the bottom (, most profound at station 2 and station 9). However, the oxygen level differentiation is not appreciable after the turning of the canal (in which section that is close to the open ocean). The canal water is generally warmer at the bottom. The temperature difference varies between 0 to 2-degree Celsius. Chlorophyll value varies between depth at different sites, however, its concentration difference along depth is less appreciable after the turning of the canal, too. Turbidity value remains relatively the same at all stations along the depth.

In water sampling, samples were collected with a peristaltic pump (Cole-Parmer Masterflex). One end of the tubing was connected to the sensor bundle. For the first water sampling, water samples were collected at all four stations at surface level and the bottom. Water samples were collected at surface level (~0.25m), pycnocline (various depth), and the bottom (various depth depends on canal bathymetry). A duplicate was collected with each sample for DNA analysis. One set of the samples were filtered at CMORE and then sent to the S-Lab for nutrient analysis. The other set of samples were filtered at CMORE as well; the filters of this set of samples were preserved for DNA extraction and analysis.

Overall, the canal sampling is continuing to construct a time series of nutrient content and physical/chemical properties inside the canal. Sampling strategy and instrumentation are being improved every sampling. Most of all, the sampling effort is working toward data collection of much higher resolution than what has ever been done in Ala Wai Canal.
Figure 1. Station map and vertical profiles