HKU-DMDSS
HKU Drive-in Multi-Depth Sampling System (HKU-DMDSS)
Introduction
HKU Drive-in Multi-Depth Sampling System, HKU-DMDSS is designed to meet the needs of high-resolution and periodical sampling of coastal groundwater. The metal free sampling system combines the advantages of traditional monitoring wells and the gas vapor sampling system. The system consists of a series of hyper-hard nylon water tubes installed in different depths. The deepest depth is about 4 m, which is the maximum sample depth of most peristaltic pumps and is suitable for most submarine groundwater discharge (SGD) and other biogeochemical process studies in CGMZs. The tube is first hosted inside a steel pipe that is driven by a portable electric hammer. After the tube reaches a desired depth, the steel pile is pulled out by a jack lift. The installation does not require major drilling and has minimum disturbance to the sediments. The system is innovative and costeffective and can be used for periodical sampling of coastal groundwater at both tidal and seasonal scales for years.The whole system includes two components: sampling tubes and drive-down system.
(1) Sampling tube. The sampling tubes are essential to the whole system because they will be left underground and water is sampled through the ports at the end of the water tube near the nylon tip. The configuration schematic of the sampling tube is shown in Figure 1. The nylon tip (70 mm, outer diameter [OD] 25 mm) has a cone ending and the detailed geometry of the tip is shown in Figure 1a. The upper end of the tip is hollowed inside (35 mm, OD 9 mm) to host the water tube (inner diameter or ID 5 mm). A lock hole (OD 3 mm) is made at 27.5 mm from the upper end to interlock the tip and water tube. Two barbs (OD 20 mm) at both sides of the tip are made so that the tip stays underground when the steel driving pipe is pulled out. A number of 18 to 24 water inlet ports (OD 3 mm) are made in the lower part of the tube. Those ports allow porewater to enter the tube in all directions. Filtering screens (75 × 50 mm) are used to cover all water inlet ports. The screen and the water tube are held in place with cable ties. The water tube is inserted into the hollowed space of the nylon tip, and then a hole at the tube is made to align with the lock hole of the tip. A lock pin (length 25 mm and OD 3 mm plastic rod) is inserted to mate the tube and the tip together. The open end of the water tube at the ground surface is capped with an end plug (length 25 mm and OD 3 mm nylon plug with silicone tube and fish line) as shown in Figure 1c. A mark is made on the plug surface to identify the water inlet depth.
(2) Drive-down system. The drive-down system consists of two types of steel driving pipes (length 39 & OD 1/2) and two types of drive-down adapters (driving down adaptor-A [DDA-A] and driving down adaptor B [DDA-B]).
Driving pipe-A is threaded at both ends for length extension. Driving pipe-B (Figure 2b) is threaded at one end and the other end is linked with a sharp jackhammer tip at another end. Two unique adaptors are used to couple the hammer bit end to the upper end of driving pipes as shown in Figure 2. DDA-A is used for water tube installation. The hammer bit slightly offsets from the water tube channel to protect the tube during the installation. DDA-B is used to connect the hammer bit to the driving pipe-A in the ground (Figure 2). Drive down adapter B and driving pipe A are used to make a probehole to the desired depth. Although the probehole may collapse, this setting makes the metal tip capable of penetrating the sediments easily to the desired depth. Using this setting, the nylon cone tip can stay in situ more easily after a slight rotation of the steel wall above the ground.
Advantages
This sampling system is of many advantages compared to previous in situ samplers/sampling systems.
1. The system is completely metal-free and will not be eroded, and can monitor the solutes in CGMZs over years.
2. The sampling system is cost effective, and most materials used in the system are available in the local hardware shops, but Drive down adapter-A and Drive down adapter-B are tailor-made by a technician.
3. The whole sampling system and driving down system can be handled by 1 to 2 persons, and no heavy vehicle system is involved, which is suitable for soft sediments along sea or lake shores or hyporheic zones.
4. The system induces negligible disturbance of the sediments, as the pipe of the sampling system is much smaller than other in situ samplers/sampling systems.
5. The sampling system is capable of taking samples both over diurnal cycle and seasonal cycle with high temporal sampling resolutions.
6. The sampling system is easily installed in a cross transect to investigate fluctuations of FW-SWI, and spatial heterogeneity of biogeochemical process within CGMZs.
Installing Procedure in the Field
Before the installation, the water tubes for the desired depths with paired end plugs were prepared.
The procedures of installation in the field were summarized as follows:
Step1. The electrical hammer was connected to a portable power generator; and all parts of the drive-down system were prepared in the field.
Step2. The Driving pipe B was assembled with the DDA-B and the hammer bit and driven down by the electrical hammer. Steel driving pipes were added one by one to reach the desire depth. Then the driving pipes were pulled back with a hi-lift jack, leaving a hole in the beach.
Step 3. A water tube with length as same as the depth of the hole was insert into driving pipe-A and connected to DDA-A. All the connected part was inserted into the hole made in step 2 and driven down by the hammer. The water tube system was then driven down together with the driving pipes with the electric hammer exactly. Again driving pipe-A was added one by one until the desired depth was reached.
Step 4.The driving pipes were pulled back with hi-lift jack and water tube remained at the desired depth. During the installation, water inlet ports near the nylon tip were protected by the steel wall of the driving pipes; the water tube length above ground was measured to calculate the exact depth of the nylon tip. The upper end of the tube was capped by the end plug unit to prevent sediments.
Step 5. After all the water tubes were installed at different desired depths following steps 1-5, the ground base (without cover) was installed. Figure 3 shows the details of the ground base and Figure 4 b shows the ground base as seen at the field site. Water was injected and pumped out repeatedly through the water tube to prevent the water inlet ports being blocked by sediments. Finally the ground base was covered and secured with cable ties.
References
Luo, X., K. L. Kwok, Y. Liu, and J. Jiao (2017), A Permanent Multilevel Monitoring and Sampling System in the Coastal Groundwater Mixing Zones, Groundwater, 55(4), 577-587.