Collecting sediment free samples of low turbidity from monitoring wells installed in fine sand aquifers has been a difficult problem. Wells are often screened in aquifers containing thin lenses of silts and clays having a formation effective size of 0.1 mm (0.004“) or less. Fine sand silty formations are common to the California Bay area where wells, at depths of 6 to 15 meters (20 to 50 feet), are used to monitor landfills. Most of these wells use a conventional design consisting of a 5 cm (2 in.) or 10 cm (4 in.) diameter slotted or wire wrap screen with a filter pack externally placed around the screen to retain the fine sediment. Many of the wells are not useful because they do not provide clear, sediment free samples at reasonable sampling rates. Often, the wells are purged dry before the required purge volumes are removed.
In July, 1988, EMCON Associates installed a series of preassembled, dual walled screens filled with a fine filter pack around the Palo Alto, California landfill to monitor these fine sand aquifers at two different depths. Numerous tests were conducted to determine turbidity, pumping, and recovery rates for three wells that used 0.43 × 0.28 mm (40 × 60 # US Sieve) filter packs. The filter pack passed a #40 Sieve and was retained on a #60 Sieve.
This paper describes well development procedures, methods and measurements, and test results. Based on test results over a simulated four year sampling period, the following results have been achieved. 1. After the wells were stabilized, turbidity readings were consistently less than 20 Nephelometric Turbidity Units (NTU). Sediment was almost nil. 2. Using effective well development techniques, sampling and well recovery rates in fine sand formations can be improved so wells can be sampled in less than one hour. 3. There is no evidence of plugging of the dual walled screens during a simulated four year sampling program.