Saldanha in the Western Cape of South Africa is a small port town on the northern shore of Saldanha Bay. The bay is a naturally sheltered harbour, making it ideal for exporting iron ore on massive ships. It serves the region’s base metal mines and a heavy minerals smelter, and is also the location of a large crude oil storage facility. In addition, the town is home to over 20,000 residents who follow their trade based on the economic drivers in the area, including shipping companies, industrial production plants and the railway line. As such, it is important that the water resources in the area, including groundwater, be monitored and protected from contamination.
Groundwater consulting company GEOSS, has taken the lead in an ongoing project to monitor the potential impacts of the aboveground activities on the region’s groundwater resources. GEOSS needed equipment that could handle sampling for potential VOCs (volatile organic compounds) VPHs (volatile petroleum hydrocarbons) and general chemistry at potentially contaminated locations. They also required a sampling solution that could be used in narrow diameter boreholes.
Based on their criteria, GEOSS selected a Solinst 1″ diameter Model 407 Bladder Pump. Solinst Bladder Pumps allow low flow purging and sampling – ideal when monitoring for VOCs. Low flow sampling involves extracting groundwater at rates comparable to ambient groundwater flow so that the drawdown of the water level is minimized, and the mixing of stagnant water with water from the screened intake area in a well is reduced, overall, providing a representative sample.
When a Bladder Pump is installed down a well (or borehole), the water rises inside the bladder and sample tubing to static level. Compressed gas is applied to the pump via the drive tubing using a Control Unit. Pressure causes the bladder to compress and closes the bottom check valve, forcing water from the bladder into the sample tubing. Pressure is released from the Bladder Pump during a vent cycle. The bladder opens as water refills the pump, while the top check valve prevents sample water from falling back into the bladder. Cycling the drive and vent provides water flow. With Solinst Bladder Pumps there is the assurance that there is no air/water contact during sampling, meeting the most rigorous standards for VOC groundwater monitoring.
Currently, GEOSS use a 1″ diameter Solinst Bladder Pump that they transport from site to site to sample groundwater. Between each sampling event, the Bladder Pump is decontaminated to prevent cross-contamination between wells. There are often over 30 samples to be collected each week, so efficient and proper decontamination is a crucial element to all sampling campaigns. They are using a portable setup that includes the Solinst Model 464 Electronic Pump Control Unit to regulate the vent/drive cycles, and a nitrogen cylinder for the gas source.
A Solinst Model 122 Interface Meter is also used in the project to measure water level and to detect any free phase in the groundwater. Michael Holloway, of GEOSS, said, “The equipment is used for the sampling and low flow purging of narrow diameter sampling boreholes. As hydrogeologists, we purge the borehole first before collecting a representative sample of the groundwater and not particularly water that has been standing stagnant inside the borehole.”
The Bladder Pump is versatile and can be used across a few sites within the project where a vehicle can’t get to… it’s easy enough to carry the entire Bladder Pump setup and then continue the fieldwork without wasting too much time. Overall, it is also mentioned the following advantages to using a Solinst Bladder Pump:
When selecting a groundwater sampler, there are some key factors to consider. The physical attributes of the well: diameter, calculated purge volumes, depth to water and sampling intervals, and the accessibly to the well for transporting sampling equipment. The well hydraulics, for recharge and recovery, turbidity, etc. must also be taken into account. Bladder pumps are EPA approved for low flow sampling and groundwater collection for Volatile Organic Compound (VOC) analysis.
Although there may be a high initial capital investment, the bladder pump will pay for itself through time savings and defensible data. The controller is only purchased once and can be re-used on numerous sites and sampling applications. The air supply can be rented inexpensively. Dedication of customized, pre-assembled systems significantly reduces the time needed in the field for setup. If portable pumps are required, then pumps are easily disassembled for decontamination between sampling events. Using low flow methods, only the sampling zone is purged, thus reducing purge volumes, sampling time, and disposal costs. High-quality samples with no turbidity or de-gassing also cut down the need for sample filtering or repeated sampling.
Determining the amount of applied pumping pressure to retrieve a sample is not difficult. 1 psi of pressure can raise a 2.3 ft column of water, which is about half of the column height of water in feet, expressed as psi. e.g. if the Bladder Pump’s intake is at 100 ft below ground surface, you would require approximately 50 psi of pressure to bring a sample to ground surface, add an extra 10 psi to allow for line loss.
When selecting a pump controller, make sure your choice has easy-to-follow pre-set pumping options. This helps take the guesswork out of determining suitable drive and vent times. For example, when sampling within a well with a ‘fast’ recharge rate, select a ‘high’ cycle rate (~6 sec/cycle of drive and vent times), versus a ‘slow’ recharge application, where a cycle rate of 115 seconds may be more suitable.
Solinst Bladder Pumps give defensible data due to consistent, high-quality samples in all types of applications. They have great potential to provide both time and cost savings. The advantages far outweigh the negative assumptions that may be misplaced on these instruments. The time spent in customizing and operating these systems should be considered a wise investment. They provide many benefits time and time again, in the long term.