Photo of drilling rig at Hollister test well

Key Issues

  • Replace an existing damaged well with a replacement well with sufficient yield and water quality to meet potable water supply needs of the City


  • Initially construct a less expensive test well to assess groundwater quality and yield
  • Design and construct a well to allow depth-discrete groundwater quality sampling to characterize vertical variability
  • Based on study findings, site the production well at a location likely to have better water quality

Installation of Test Well and Vertical Water Quality Profiling

Gilroy-Hollister Basin, San Benito County, California

A test well was installed and tested to determine the suitability of the site for a large municipal supply well.

The City of Hollister uses imported surface water and local groundwater for municipal supply. Historically the City has operated a number of wells for most of its potable water supply and fire protection. However, Well No. 3 (constructed in 1953) has been out of operation since its casing collapsed in 2010. The City retained Todd Groundwater to assist in the replacement of the well.

The City identified a location for a potential replacement well near Well No. 3. Based on previous hydrogeologic investigations, the selected location was anticipated to be easily able to meet the City’s production capacity requirements. However, the water quality was in question. As a result, Todd Groundwater recommended that a test well be constructed to evaluate depth-discrete water quality.

Photo of samples from Hollister test well being logged

Work included:

  • Geologic logging
  • Geophysical logging
  • Well design and preparation of well specifications
  • Well construction and development
  • Depth-discrete groundwater sampling
  • Constant rate pumping test

Diagram showing soil and water attributes with depth
Well Profile and Lithology

The test well was designed to include multiple discrete screened zones and an overall depth greater than the failed Well No. 3. The discrete screened zones were constructed in permeable aquifer units identified from formation samples and geophysical logs, and each discrete zone was sampled to evaluate depth-specific water quality.

The drilling, construction and testing of the Test Well indicated the presence of multiple coarse-grained aquifer units with the capacity to produce groundwater. However, the water quality in all of the individually screened aquifer units was poor. Specifically, the concentrations of nitrates in all screened zones were well above drinking water standards. Further evaluation of the other nitrogen compounds detected in the well indicated the likely source of the nitrates to be human waste from sewage. The source of this contamination is likely an existing waste disposal septage pit on a property adjoining the test well site. Accordingly, the test well was destroyed and the City has shifted the test well program to another site.