Stability groundwater parameters

As discussed before, groundwater samples can be collected when a sufficient volume of water has been removed from the well (e.g., three to five well volumes) and groundwater parameters have stabilized. If parameters have not stabilized after five well volumes have been removed, then the well may be sampled (acceptance of sampling following the fifth purge volume is dependent on the study objectives). Table 1 summarizes the criteria used for establishing the stability of groundwater parameters. The time intervals between the parameter measurements depend on the well characteristics and the hydraulic properties of the aquifer and must be sufficiently spaced to provide results representative of aquifer properties. ... 

Table 1 Stabilization criteria for groundwater parameter measurements...

The persistence of the triazine herbicides in surface and groundwater and in soil is dependent to some extent on their susceptibility to chemical hydrolysis. The environmental stability of the triazine herbicides to hydrolysis is dependent upon environmental parameters such as temperature, pH of the water or soil solution, and the presence of dissolved constituents that may catalyze hydrolysis. 

We use water quality indicators, such as pH, temperature, conductivity (specific conductance), dissolved oxygen, oxidation-reduction potential (ORP), and turbidity, as groundwater well stabilization parameters. Stable values of three consecutive measurements of these parameters are considered an indication of a stabilized well. 

Table 3.4 lists the parameters in the order by which they would usually stabilize. Temperature and pH are usually the first ones to stabilize because they are not very sensitive to the influx of fresh water. That is why they are not as good stabilization indicators as the other parameters. Their measurements, however, are important for data interpretation and should always be made during groundwater sampling. Dissolved oxygen content and turbidity, which is the measure of the particulate matter content in water, are typically the last ones to stabilize. 

Table 3.4 Groundwater stabilization parameter performance criteria...

As shown in Table 3.4, acceptance criteria for stabilization parameters may have a range of recommended values. Because some are more stringent than others, they should be used with caution and be adjusted as necessary based on the knowledge of site-specific conditions. For example, the criterion for turbidity of + 10 percent may be too stringent for some hydrogeological conditions or for the conventional sampling method. On the other hand, this criterion may be easily achieved for wells with dedicated submersible pumps and non-turbid, clear groundwater. 

Field parameter measurements (pH, temperature, conductivity, etc.) may be part of the sampling procedure as indicators of surface water quality (not stabilization indicators as in groundwater sampling). 

Dissolved oxygen (DO) concentration is an important indicator of biological activity and chemical processes in groundwater and wastewater. It serves as a well stabilization indicator in groundwater sampling and is a key parameter in wastewater treatment process control. 

Turbidity is a drinking water quality parameter and a groundwater well stabilization indicator. The clarity of water defines a physical property of turbidity. Suspended matter, such as clay and silt particles, organic matter, microscopic organisms, and colloids, causes natural waters to be turbid. Turbidity is measured optically as a lightscattering property of water. 

There are two major approaches in the analysis of slope stability. The first one is the forward approach in the analysis of slope stability that requires data on shear strength properties and pore pressure conditions. The former are derived from a range of field and laboratory techniques, whereas the latter demand improved techniques capable of instrumenting rapid groundwater and soil suction responses to rainfall without damping the transient peak conditions. Probable worst-case parameter values are assumed, and a conservative value of the factor of safety is derived. 

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