There are a variety of reasons why groundwater is usually sampled. It can be gathered from a monitoring well or as a “grab” sample.
The collection of a “grab” groundwater sample is the easiest as it doesn’t require the use of an entirely installed groundwater monitoring well. Common methods of collecting the “grab” groundwater sample include the use of a jar or bailer from a trench or excavation that has been made with an excavator or backhoe. Use of a drilling rig is another method that’s also used in collecting the “grab” samples. In this case, the sampler is usually pushed into the target depth then pushed more into the aquifer. An aquifer is a porous rock that lets water pass through it.
There are two crucial important techniques needed when it comes to groundwater sampling: understanding of aquifers as well as being able to get accurate project results from the sample. These two are crucial regardless of whether the sampling is being done as a routine sampling for a potable water supply well or if it’s for environmental assessment. These are
Groundwater and Wells
Groundwater sampling has been made an interesting topic due to legislation changes, increased awareness on the effects of industrial activities and pressure on the aquifer supplies. These factors, therefore, influence in determining the quality and availability of groundwater for many government agencies and businesses. Hence, getting a sample from a monitoring borehole that represents the true quality of the groundwater in any given aquifer becomes an arduous task.
Two critical factors determine the quality of a groundwater sample gotten from a monitoring well or borehole:
- The screened interval length
- The variability in porosity of the adjoining strata
Inflows from a fissure or strata that is more permeable tend to control and bias the borehole sample producing a flow-weighted average sample that’s not a representation of the initial sample. Boreholes with longer screened intervals tend to have lots of bias with results being uncertain.
What then are the methodologies used in getting the groundwater sample?
Methodologies Used in Groundwater Sampling
Three main methodologies are mostly used when sampling borehole groundwater:
Low flow sampling
This methodology requires the pumping rates to be sustained at 0.5 liters per minute or less and is designed to primarily optimize sampling conditions for volatile organic compounds. For this method to work, the chemical parameters need to be monitored using flow cells that help determine the end of the purging before sampling.
Moreover, this method requires that there be no disturbance of the water column before the pumping and the minimal drawdown for the water column must also be maintained.
Passive sampling (Zero Purge)
The zero purge or passive sampling methodology makes use of passive sampling equipment to collect groundwater samples at distinct depths with minimal disturbance in the borehole column.
Fixed volume purging
Most countries use the fixed volume purging as the default guidance when it comes to groundwater sampling. In this method, high pumping rates are used to collect three or more times the typical water volume needed in the well lining by pumping it from the monitoring borehole. When pumping, chemical stability can be monitored by reducing the purge volumes.
With all methodologies, there are various equipment that can be used to achieve the desired result. Give us a call today for all your groundwater sampling needs.