Lower Pilarcitos Creek Groundwater Basin Study - Coastside ...

Because drilling depths are shallow (about 65 to 85 feet deep), the production
wells should be drilled with cable tool methods. Cable tool methods have several
advantages in comparison to rotary drilling methods including the following:
• No drilling fluid (bentonite) is introduced to the borehole,
• Minimal borehole damage as the casing is advanced as drilling proceeds,
• Design of the well can accommodate either natural filter pack (preferred) or
filter pack designs,
• Aquifer soil samples can be properly classified resulting in custom-made well
design screens (aperture size and screen lengths) suited for each site, and
• Groundwater samples can be collected during drilling to determine if
groundwater quality is stratified with depth.
The pilot well and subsequent production wells should have a pump chamber of
at least 12-inch diameter and be constructed of steel. A surface seal should be installed
to about 20 feet; a County Health Department variance will be needed for this seal. The
wells should be drilled to bedrock. The well screen should be 12-inch telescope (natural
filter pack) or 6-inch diameter pipe size (filter pack) continuous wirewrap stainless steel
screen. The well screen should be placed at least three feet above the bedrock. The
aperture size of the screen should be designed from particle size analysis of
representative formation samples. After installation and construction, the well should be
developed with swab and bail methods for at least 30 hours. Mechanical well
development should be followed with development using the test pump. Detailed and
formal pumping tests (step-drawdown and constant discharge) should be conducted on
the well. A well construction report should be prepared to document the drilling,
construction, development, and testing of the production wells.
A phased approach to the wellfield would involve installation of wells on the north
side of the creek and construction of water conveyance and treatment facilities, followed
by extension of conveyance facilities to the south side of the creek.
Wellfield Operation and Life Expectancy
Yields from proposed production wells will be limited by seasonal water level
fluctuations and the relatively shallow depth of the aquifer. To assess the impact of
groundwater fluctuations at the test wells, water levels in nearby well 29F04 (see Figure
19) were assumed representative. Groundwater fluctuations at well 29F04 range from
22