How long do you purge a well if one of the parameters will not stabilize
How long do you purge a well if one of the parameters will not stabilize (even if it is one of the more important ones)?
First, it's important to identify which parameters will tell you when a well is properly purged. Common purging indicator parameters are pH, electrical conductivity (or specific conductance), dissolved oxygen, redox/ORP, temperature and turbidity. From both my own experience and the published literature on purging, the most reliable and adequately conservative indicators are EC and DO. pH tends to stabilize before EC and DO and often changes less than 0.5 pH units, sometimes much less.
Redox/ORP are measures of the net of all oxidizing and reducing reactions occurring within the groundwater and can show wide natural variation, and therefore may not appear stable. Temperature, if measured down within the well screen zone, might be a useful indicator but, when measured at the well head, it is susceptible to change due to ambient temperature at the well head, sunlight, flow rate changes, and even some sampling devices (such as electric submersible pumps). Turbidity is the most problematic – it is not a measure of water chemistry but water quality and, therefore, changes in turbidity don't reflect changes in the subsurface geochemistry. Turbidity should not be used to determine purging completeness, but it should be MEASURED at the point of sampling whenever your target analytes may be affected by turbidity (e.g., metal ions, PAHs, etc.) to support the analytical data and any significant changes observed.
My recommendation is to purge until EC and DO are stable and turbidity is below 10-20 NTU (the lower, the better). When a well is initially pumped, DO will tend to be higher than the actual groundwater concentration and, conversely, EC will tend to be lower. As water from within the well casing and screen is replaced with water from the sand pack and formation, the DO typically sloped downward in the range of 1-4 mg/L or less, while the EC will climb as DO falls (since metal ions are held in solution at the lower DO and higher CO2).
In that example, EC and DO tell the whole story. It's interesting to note that pH and turbidity in this example would give a false indication of stabilization – the pH stabilizes long before EC and DO, while the turbidity starts out low and remains low, providing no indication of changing geochemistry.
The answer to the question is this: a well should be purged until the key parameters of DO, EC and pH are stable. “Stable” meaning: three consecutive sets of readings that fall within your stabilization criteria. If the parameter that isn't stabilizing is temperature, redox/ORP or turbidity, I wouldn't be concerned. If the parameter that won't stabilize is EC, DO or pH, I would try to identify the reason behind this. It could be that the sensors are unstable (requiring service or replacement), or an air leak in the flow cell tubing or fittings.
First, it's important to identify which parameters will tell you when a well is properly purged. Common purging indicator parameters are pH, electrical conductivity (or specific conductance), dissolved oxygen, redox/ORP, temperature and turbidity. From both my own experience and the published literature on purging, the most reliable and adequately conservative indicators are EC and DO. pH tends to stabilize before EC and DO and often changes less than 0.5 pH units, sometimes much less.
Redox/ORP are measures of the net of all oxidizing and reducing reactions occurring within the groundwater and can show wide natural variation, and therefore may not appear stable. Temperature, if measured down within the well screen zone, might be a useful indicator but, when measured at the well head, it is susceptible to change due to ambient temperature at the well head, sunlight, flow rate changes, and even some sampling devices (such as electric submersible pumps). Turbidity is the most problematic – it is not a measure of water chemistry but water quality and, therefore, changes in turbidity don't reflect changes in the subsurface geochemistry. Turbidity should not be used to determine purging completeness, but it should be MEASURED at the point of sampling whenever your target analytes may be affected by turbidity (e.g., metal ions, PAHs, etc.) to support the analytical data and any significant changes observed.
My recommendation is to purge until EC and DO are stable and turbidity is below 10-20 NTU (the lower, the better). When a well is initially pumped, DO will tend to be higher than the actual groundwater concentration and, conversely, EC will tend to be lower. As water from within the well casing and screen is replaced with water from the sand pack and formation, the DO typically sloped downward in the range of 1-4 mg/L or less, while the EC will climb as DO falls (since metal ions are held in solution at the lower DO and higher CO2).
In that example, EC and DO tell the whole story. It's interesting to note that pH and turbidity in this example would give a false indication of stabilization – the pH stabilizes long before EC and DO, while the turbidity starts out low and remains low, providing no indication of changing geochemistry.
The answer to the question is this: a well should be purged until the key parameters of DO, EC and pH are stable. “Stable” meaning: three consecutive sets of readings that fall within your stabilization criteria. If the parameter that isn't stabilizing is temperature, redox/ORP or turbidity, I wouldn't be concerned. If the parameter that won't stabilize is EC, DO or pH, I would try to identify the reason behind this. It could be that the sensors are unstable (requiring service or replacement), or an air leak in the flow cell tubing or fittings.