Understanding Well Logs: How to Read Your Well Drilling Report

Somewhere in a California database sits a document that knows your well better than you do. It recorded every foot the drill bit traveled, every rock layer it punched through, and the exact moment it hit water. This document—your well log—is the Rosetta Stone of your water system. Yet most homeowners have never seen theirs, don't know how to read it, and couldn't find it if they tried. That ends today.

What Is a Well Log?

A well log is an official record created by the licensed well driller who constructed your water well. It documents everything about the drilling process and the completed well—from the geology encountered underground to the materials used in construction. Think of it as the birth certificate and blueprint of your well combined into one document.

In California, all licensed water well drillers are required by law to submit a Well Completion Report to the Department of Water Resources (DWR) within 60 days of finishing a well. This requirement has been in place since 1949, meaning most wells drilled in the past 75+ years have official records on file.

The well log serves multiple purposes: it creates a permanent public record of groundwater resources, helps future well technicians understand the well's construction, provides data for regional water management, and gives property owners essential information about their water supply.

Information Included in a Well Log

A comprehensive well completion report contains detailed information organized into several key sections. Understanding what each section tells you helps you make informed decisions about your well.

Well Location and Identification

The top section of the well log identifies the well's exact location using:

• Owner information: Name and address at time of drilling (may be previous owner)
• Property address: Physical location of the well
• Township, Range, Section: Legal land description for precise location
• APN (Assessor's Parcel Number): Links to county property records
• GPS coordinates: On newer reports, latitude and longitude
• Permit number: Local permit tracking number

Well Depth and Dimensions

This section describes the physical specifications of the well:

• Total depth: How deep the borehole was drilled (e.g., 480 feet)
• Completed depth: Final depth after installation (may differ from drilled depth)
• Borehole diameter: Width of the drilled hole (typically 6-12 inches)
• Casing diameter: Size of the installed pipe (commonly 5-8 inches)

Geological Formations (Driller's Log)

Perhaps the most valuable section, the driller's log describes the underground geology layer by layer:

This geological information tells you where water enters your well and helps technicians understand your aquifer when troubleshooting or planning rehabilitation.

Water-Bearing Zones

The log identifies depths where the driller encountered groundwater:

• First water: Shallowest depth where water was encountered
• Principal water-bearing zones: Layers that produce the most water
• Estimated yield per zone: How much each zone contributes (in GPM)

Understanding your water zones helps determine optimal pump placement and explains why water levels may vary seasonally. If you're experiencing well running dry issues, this information is crucial for finding solutions.

Casing and Construction Details

The casing section documents how the well was constructed:

• Casing material: Steel, PVC, or stainless steel
• Casing diameter and wall thickness: Structural specifications
• Casing depth: How deep the solid casing extends
• Perforations or screen: Where and how water enters the casing
• Annular seal/grout: Material sealing the space between casing and borehole
• Surface seal depth: How deep the sanitary seal extends (typically 20-50 feet)

Water Level and Yield Information

This critical data tells you about the well's water-producing capacity:

• Static water level: Depth to water when the pump isn't running
• Pumping water level: Depth to water during pumping
• Drawdown: Difference between static and pumping levels
• Estimated yield: Gallons per minute the well produces
• Test duration: How long the yield test ran

How to Read a Well Log

Reading a well log effectively requires understanding how to interpret the information and what it means for your well's performance.

Step 1: Verify the Well Location

Confirm the log matches your property. Check the APN, address, and legal description. If you have multiple wells or suspect the log may be for a neighboring property, verify the location details carefully.

Step 2: Note the Basic Specifications

Record the key numbers you'll need for maintenance and repairs:

• Total depth and completed depth
• Casing diameter (needed for pump sizing)
• Original static water level
• Original yield (GPM)

Step 3: Identify Water-Producing Zones

Look at the driller's log and note depths where water was encountered. The perforation or screen depths in the casing section should align with these water-bearing zones. This tells you where your water comes from.

Step 4: Understand the Geology

The geological description helps predict well behavior. For example:

• Fractured rock aquifers: Common in San Diego County, water flows through cracks
• Alluvial aquifers: Sand and gravel layers, typically higher yields
• Clay layers: Act as barriers, may separate aquifer zones

Step 5: Compare with Current Conditions

If you're troubleshooting, compare current measurements to the original log:

• Has the static water level dropped significantly?
• Is current yield lower than the original estimate?
• Are you pumping from the same zones, or has something changed?

Why Well Logs Matter

For Maintenance and Repairs

When something goes wrong with your well, the well log is the first document technicians need. It tells them:

• How deep to set the pump
• What size equipment fits the casing
• Where water enters the well
• What the original performance was (for comparison)
• What geological formations may be causing issues

Without a well log, troubleshooting becomes more expensive because technicians must investigate what should already be documented. Learn more about pump issues in our guide on types of well pumps.

For Property Sales

Well documentation significantly impacts real estate transactions:

• Buyer confidence: Complete records reassure buyers about water supply
• Lender requirements: Many lenders require well documentation for financing
• Inspection reference: Inspectors use logs to assess well condition
• Property value: Documented wells typically support higher valuations
• Disclosure compliance: Sellers should provide well information to buyers

For Troubleshooting Problems

Well logs are invaluable diagnostic tools. They help answer critical questions:

• Is the water level declining compared to when the well was drilled?
• Could deepening the well reach additional water-bearing zones?
• Are there formations that might be collapsing or causing sediment?
• Was the well properly sealed to prevent contamination?

Getting Your Well Log from the County

If you don't have a copy of your well log, here's how to obtain one in California:

California DWR Well Completion Reports

The California Department of Water Resources maintains a database of well completion reports. Access them through:

1. Online Map Application: Visit the DWR Well Completion Report Map at water.ca.gov. You can search by zooming to your property location or entering your address.
2. Direct Search: Use the DWR's online database to search by township/range/section or other criteria.
3. Request by Mail: Contact DWR with your property information if online search doesn't yield results.

County Environmental Health

Your county's environmental health department issues well permits and may have copies of well logs on file. In San Diego County, contact the Department of Environmental Health. They maintain records of permitted wells and can often provide documentation.

Original Drilling Company

If you know which company drilled your well, they may have retained records. Many established drilling companies keep files going back decades.

Previous Property Owners

Well documentation is sometimes passed along during property sales. If you purchased your property relatively recently, the previous owner or their real estate agent may have copies.

Using Well Logs for Troubleshooting

When your well isn't performing as expected, the well log provides a baseline for comparison:

Declining Water Levels

If your static water level is significantly lower than recorded on the original log, possible causes include:

• Regional aquifer depletion
• Increased demand from nearby wells
• Extended drought conditions
• Changes in local groundwater recharge

Reduced Flow Rate

Compare current yield to the original estimate. If it's significantly lower:

• Screen or perforations may be clogged with mineral deposits
• Aquifer may be producing less water
• Pump may need replacement or adjustment
• Well rehabilitation may restore flow

Water Quality Changes

The geological log helps explain water quality issues. For example, if your water has high iron, the log might show iron-bearing formations. Understanding the source helps select appropriate treatment.

Keeping Well Records

Good record-keeping protects your investment and simplifies future maintenance:

Documents to Maintain

• Well completion report: The original drilling log
• Well permit: From your county environmental health department
• Pump specifications: Make, model, horsepower, depth setting
• Service records: All maintenance and repairs performed
• Water test results: Historical water quality data
• Inspection reports: From professional well inspections

Creating a Well File

Keep all well-related documents in one location—a physical folder and digital backup. Include:

• Copies of all permits and official documents
• Service invoices and receipts
• Notes on any issues or observations
• Photos of equipment and wellhead
• Contact information for your well service company

Frequently Asked Questions

Your Well Log Is Your Well's Story

Your well log tells the complete story of your well—from the geological journey the drill bit took to the construction details that protect your water quality. Whether you're planning routine maintenance, troubleshooting an issue, or preparing to sell your property, this document is essential.

If you don't have a copy of your well log, take time to obtain one from the California DWR or your county. If you need help interpreting your well log or have questions about what the information means for your specific situation, our experienced technicians can review your documentation and provide guidance.