Can you explain the basic criteria for low-flow sampling?

Generally flow rates for the low-flow sampling will be in the range, of 200-1,000 ML/Min with a practical lower limit of about 100 mL/min. The reason for the practical limit is extended purging and sampling time unless only small sample containers will be taken.

• Drawdown is based on well performance not 0.1 m

• Fill larger bottles first then reduce flow to fill VOAs - once the well is stabilized, you don't want to INCREASE the flow rate during sampling, otherwise the stabilized conditions will be upset. Therefore, establish the highest practical purge rate for that well, then begin sampling at or below the purge rate, starting with the largest sample containers. The flow rate can be further reduced for filling small bottles (VOAs), and then paused momentarily to install an in-line sample filter if any filtered samples are required. Where permits or regulatory guidance dictates the "most to least volatile" bottle filling order, I encourage talking to the RPM to explain why this isn't necessary with a dedicated low-flow pump (it was intended for bailers).

• DO and SCT are the most reliable indictors

• ORP doesn’t always stabilize

• NTU is used to support sample data not necessarily for stabilization- Turbidity readings are also useful for identifying excessive pumping rates, but they won't tell you anything about when the water chemistry in the well screen zone is stable.

• Pump placement isn’t too critical.  If placed in the middle of 20’ screen it will pull water from most of screened area, see Varljen, et al, 2006 in Ground Water Monitoring and Remediation.

• Low flow is ideal in 20’ or less screened intervals

• Stabilization should be between the last three readings

• Spectra sample multi-port inlet pumps should be used in well screens <20’