By Sajjad Din
Low flow, low stress, or minimal drawdown sampling is the process of slowly purging a monitoring well while collecting data on the geochemistry of the water. Groundwater is pumped out of the subsurface via tubing placed at a depth roughly equal to the centre of the screen interval.
This is also to avoid disturbing sediments at the bottom of the well. Water is pumped at a rate of approximately 0.1 to 0.5 litres/minute. It passes through a flow-through cell with a probe attached to a multi-parameter water quality meter. Some coarser grained formations have been successfully sampled by this method with pumping rates up to 1 litres/minute. The flow-through cell is intended to reduce the effects of atmospheric conditions on the groundwater being purged and monitored.
Measurements are collected at three or five minute intervals. These measurements may include temperature, pH, dissolved oxygen, oxidation-reduction potential and conductivity. Water level monitoring is also carried out at the same time to ensure there is no significant drawdown during purging.
Once these parameters have stabilized for four distinct set of readings, the groundwater sample is collected. The purpose of such measurements is to confirm that water being drawn through the pump is actual “fresh” aquifer water. This assumes that the monitoring well is installed as per standard industry practices with a proper sand pack filter, seal and protective air tight cap and cover. During this type of purge the yield of the aquifer is not being investigated or evaluated. The main objective is to reduce stress to the environment being sampled and obtain accurate data and analytical results.
Low flow groundwater sampling
Low flow groundwater sampling has been around since the 1990s. It was developed to enhance the accuracy of pollutant assessment in groundwater and ensure that a representative groundwater sample had been collected. In recent years, this method has become more commonly used in Ontario. 2011 amendments to Ontario Regulation 153/04 seem to acknowledge that this method is now mainstream and that such water quality data would be collected in Phase II environmental site assessments.
- The main advantages for this method of sampling include:
- Less disturbance in the well, meaning greater test accuracy, particularly for volatile compounds.
- Lower wastewater amounts. This can add to project costs and is an issue for site occupants.
- Very useful data collection for future use and analysis.
Data collected can be used for various purposes beyond simply confirming connectivity to fresh aquifer groundwater. It can identify potential risks to potable well users in the area, analyse the effectiveness of in situ or ex situ water treatment methods, or the suitability of the groundwater.
- There are a few disadvantages to this type of sampling, including:
- Increased initial capital costs for pumps, water quality meters, etc.
- Increased costs due to higher operator training requirements.
- Increased sampling time due to more equipment being transferred from one site to another.
Maximizing the benefits
During a recent field demonstration and exercise, a group of students asked what this data means. My response was that the stabilization indicates that we have fresh aquifer groundwater. However, this answer did not satisfy the students. After all this effort to collect such data, was there not a system in which it could be input into a table, attached to a database and shared for public use?
Such a system could be like the Ontario Ministry of the Environment’s database for water well records, or the Hazardous Waste Information Network database. It could qualify a “type” of groundwater and its suitability for various purposes, taking into account the water quality standards for such parameters.