Groundwater Samples and monitoring wells - Gerard van Dijk
Feb 09 2011
Author: Gerard van Dijk on behalf of Eijkelkamp Soil & Water
Are concentrations above certain pre-set limits one generally decides to drill deeper, install a monitoring well also in that layer and determine the situation in that layer. If an impermeable layer is drilled through the perforated layer must be repaired by using bentonite pellets or a bentonite-water grout. Since there is often very limited space available the bentonite often gets blocked on its way down and does not arrive at the required depth. This may cause serious and high risks of spreading of pollutants to deeper and up-till then unpolluted layers! A new way of installation of bentonite plugs can limit this risk.
When half-way the eighties the U.S. and some Northern European countries (mainly the Netherlands) developed techniques to sample groundwater it was clear from the beginning that this should be done from small diameter but classical type wells. Much attention should be paid to the cleanliness of the well material. Ordinary PVC (even for drinking water pipes) contains lead, zinc-calcium or tin in such a high concentration that it pollutes the water with that contaminant when the well is new. Monitoring well diameters may vary between 10 and 150 mm where 50 mm is normally the standard for wells where groundwater tables are deep. Wells that are not in regular use are purged (pumped) directly prior to sampling. Larger wells (100-150 mm) have the big disadvantage that large volumes of potentially toxic purging water must be pumped, collected and discarded off. A well of 50 mm allows for the use of impeller type sampling pumps. They can purge at a high speed and are constructed in stainless steel and teflon. If decontaminated correctly one can take good samples on any parameter.
Old wells have often been constructed using the wrong materials. Therefore they cannot be used for general environmental sampling. One of the reasons is that often glue is used to connect PVC pipes. The contaminants of the glue are found in the samples.
Nowadays PVC pipes can be used with an organic not traceable purely organic stabilizer. These pipes can also be used for trace metal sampling. PVC will weaken and collapse in petrol polluted soil. Therefore HDPE pipes are a better option. The material is cheap and has also the advantage of not being brittle in cold weather. Simple screw connections result in water tight connections.
The filter sand used to install around the filter section of these pipes must be extremely clean too. Otherwise polycyclic aromatic hydrocarbons (PAH) or trace metals can be found. Generally filter sand from Scandinavia or Northern Germany is used to avoid any problems.
A problem that often occurs is that a monitoring well already gets plugged by soil particles during installation. Plugged monitoring wells or wells in clay soils can be purged (and sampled) only with patience. It was worthwhile to see if plugging of well screens during installation could be avoided.
Another problem is plugging perforated impermeable layers with bentonite. Activated bentonite swells in water (not in hydrocarbons !) and is a non-toxic strongly water absorbing natural clay variety. During use, the pellets often get stuck in the small space between drill-casing and the blind section of the monitoring well. When preparing a grout by mixing bentonite powder with water an extremely sticky mud is pumped down the hole which in reverse will also “plug” all tubes and the pump used, if this is not cleaned directly after use. Of course this causes a lot of waste and work and a lot of side equipment at the drilling site.
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