How to use this calculator
- Enter casing diameter, total depth and depth to water.
- Choose how many well volumes to purge.
- Optionally include water stored in the filter pack.
- Read the purge volume and time.
How do you calculate well purge volume?
Water column h = total depth − depth to water
Casing volume = π × (D ÷ 2)² × h
Filter pack = π ÷ 4 × (Db² − D²) × L × porosity
Purge = (casing + pack) × number of volumes
Worked example
With casing internal diameter 50 mm, total well depth 20 m, depth to water 8 m, well volumes to purge 3, include filter pack water?: No, casing only, pump rate 4 L/min:
Purge volume: 70.7 litres
The calculator also gives water column 12 m; casing volume 23.6 L; one well volume 23.6 L.
Assumptions and limits
- Uses internal casing diameter.
- Filter pack water is counted only over the saturated length.
Frequently asked questions
How many litres per metre in a 2 inch well?
A 2 inch (50.8 mm) casing holds about 2.03 litres per metre, or 0.163 US gallons per foot. A 4 inch casing holds about 8.1 litres per metre.
Why purge three well volumes?
Water standing in the casing reacts with air and the casing and does not represent the aquifer. Purging three to five volumes, or until field parameters stabilise, brings in fresh formation water.
What if the well goes dry while purging?
Stop, let it recover to about 80% of the original level, and sample as soon as enough water returns. Note it on the field sheet.
References
- US EPA (1996). Low-Flow (Minimal Drawdown) Ground-Water Sampling Procedures, EPA/540/S-95/504
- ASTM D4448, Standard Guide for Sampling Ground-Water Monitoring Wells
- BIS IS 17614 (Part 11), Water quality sampling guidance for groundwater