SC By SCWS Team | February 2, 2026 | 10 min read
Reading Your Well Drilling Log
Your well drilling log is one of the most valuable documents you have as a well owner. This official record—filed with the state when your well was drilled—contains a wealth of information about what's underground and how your well was built. Whether you're troubleshooting problems, planning maintenance, or just curious about your property, knowing how to read this document empowers you to make better decisions.
📄 What Your Well Log Tells You
- • Exact depth of your well and where water was found
- • Type and depth of casing installed
- • Geological formations beneath your property
- • Static water level when drilling was completed
- • Initial yield estimate (how much water the well produced)
- • Seal and sanitary protection details
What Is a Well Drilling Log?
A well drilling log (officially called a Well Completion Report in California) is a standardized document that well drillers are legally required to file with the state. In California, this must be submitted to the Department of Water Resources (DWR) within 60 days of completing the well.
Think of it as your well's birth certificate combined with its medical record—it documents everything about how the well was constructed and what conditions were encountered during drilling. This information remains valuable for the life of the well.
How to Get Your Well Log
If you don't have a copy, here's where to find it:
🔍 Where to Find Your Well Log
Alternative
County Health Department
Local environmental health may have permit records
Should Have Copy
Property Records
Previous owner or seller should have provided it
For Recent Wells
Original Driller
Drilling company keeps records and can provide copies
Sections of a Well Log Explained
California's well completion report form (DWR 188) contains several sections. Let's walk through each one:
Section 1: Owner and Location Information
This section identifies:
- Owner name: Who owned the property when drilled (may be different now)
- Address/Location: Property location and legal description
- APN (Assessor's Parcel Number): County tax parcel identifier
- Township/Range/Section: Survey coordinates for rural properties
- Latitude/Longitude: GPS coordinates (newer logs)
Section 2: Well Specifications
Critical construction details:
| Field | What It Means | Why It Matters |
|---|---|---|
| Total Depth | How deep the well was drilled | Defines maximum pump setting depth |
| Borehole Diameter | Size of the hole drilled | Affects what casing/pump sizes fit |
| Drilling Method | Air rotary, mud rotary, cable tool, etc. | Indicates formation types encountered |
| Date Completed | When drilling finished | Well age affects maintenance needs |
| Intended Use | Domestic, irrigation, stock, etc. | Design was based on this use |
Section 3: Casing Information
This section details the pipe installed in the well:
- Casing diameter: Typically 4", 5", 6", or 8" for residential wells
- Casing material: Steel, PVC, or combination
- Casing depths: Where each casing section starts and ends
- Wall thickness: Usually expressed in gauge or schedule
- Perforations/Screen: Where water enters the casing (depth and length)
💡 Reading Casing Depths
A typical entry might read: "6" steel casing from 0 to 280 feet; perforated from 240-280 feet." This means solid steel casing lines the well from surface to 240 feet, then a perforated section allows water to enter from 240-280 feet.
Section 4: Annular Seal (Grout)
The annular seal is the material placed between the casing and the borehole wall:
- Seal material: Neat cement, bentonite, or concrete
- Seal depth: How deep the seal extends (minimum 20 feet in California)
- Purpose: Prevents surface contamination from reaching the aquifer
A proper sanitary seal is crucial for water quality protection. Wells with inadequate seals are vulnerable to contamination.
Section 5: Geological Log (Formation Data)
This is often the most interesting section—a layer-by-layer description of what's underground:
Sample Geological Log Entry
0-5 ft: Topsoil, brown, dry
5-25 ft: Clay, tan, sticky
25-60 ft: Decomposed granite, brown, moist
60-120 ft: Granite, gray, fractured, first water at 85 ft
120-200 ft: Granite, gray, hard, minor fractures
200-280 ft: Granite, gray, fractured, good water 240-280 ft
Understanding your geological log helps explain your well's behavior and potential. Key things to look for:
- Water-bearing zones: Noted as "water at X feet" or "good water zone"
- Formation changes: Where rock type changes often indicates aquifer boundaries
- Fracture notations: "Fractured" rock yields water; "solid" doesn't
- Total formations: Multiple water zones = more reliable supply
Section 6: Water Level Data
This critical section includes:
- Static water level: Water depth when not pumping (e.g., "65 feet below surface")
- Date measured: When static level was recorded
- First water encountered: The shallowest depth where water was found
⚠️ Static Level Changes Over Time
The static water level on your log was measured when the well was new. This level can change—seasonally, and over years due to drought or aquifer changes. Current levels may be different than what's recorded.
Section 7: Well Yield/Test Data
If a yield test was performed, you'll see:
- Yield: GPM (gallons per minute) produced
- Test method: Bail test, air lift, or pump test
- Drawdown: How far water dropped during testing
- Test duration: How long the test ran
Note that drilling log yields are often estimates or short-term tests. A proper well yield test after completion provides more accurate long-term data.
Common Formation Types in San Diego County
Understanding local geology helps interpret your log:
Decomposed Granite (DG)
Weathered granite, sandy texture. Can store water but variable yield. Common in foothills.
Granite (Hard Rock)
Water only in fractures. Yield depends entirely on hitting fracture zones. Mountains and East County.
Sandstone
Permeable sedimentary rock. Often produces good water. Found in coastal and valley areas.
Clay
Generally doesn't produce water. May act as confining layer creating artesian conditions below.
Alluvium/Gravel
Excellent water producer when saturated. Found in valleys and streambeds.
Shale
Low permeability. Poor water producer unless fractured. Can affect water quality.
How to Use Your Well Log
For Troubleshooting
When you have water problems, your log provides essential context:
- Running out of water? Check total depth vs. water zones—pump may be set too shallow
- Sediment in water? Review perforated section—screen may be in sandy zone
- Low yield? Compare current vs. original test data to see if decline occurred
- Considering deepening? Log shows what formations lie deeper
For Pump Replacement
Critical information for sizing a new pump:
- Casing diameter: Determines pump size that will fit
- Total depth: Maximum possible pump setting
- Water zones: Optimal pump placement for best yield
- Static level: Baseline for pump capacity requirements
For Property Sale/Purchase
The well log is essential for real estate transactions:
- Documents that well was properly constructed and permitted
- Shows original production capability
- Helps buyers understand what they're purchasing
- Required for some lenders and insurers
For Future Drilling Nearby
If you're considering a second well or neighbors are drilling:
- Indicates what depths are likely to be productive
- Shows which formations produced water
- Helps drillers estimate costs and success probability
Reading Between the Lines
Some things to notice that aren't always obvious:
Multiple Water Zones = More Reliable
If your log shows water encountered at several depths, your well has multiple sources. If one zone declines, others may sustain you.
Deep Static Level = More Storage
A well that's 300 feet deep with water standing at 50 feet has 250 feet of water column. That's significant storage to draw from.
Thick DG Layer = Recharge Zone
Decomposed granite often serves as a recharge zone, letting rain infiltrate to the aquifer. Wells in DG areas may recover faster after pumping.
Single Fracture Source = Vulnerable
If your log shows water only from one fracture zone, you're dependent on that single feature. Production could be variable.
When Your Log Is Missing or Incomplete
Sometimes well logs can't be found or contain limited information:
- Old wells (pre-1950s): May predate logging requirements
- Database gaps: Some reports weren't properly filed or digitized
- Hand-written logs: May be difficult to locate or read
- Multiple wells on property: Make sure you have the right well's log
If you can't find your log, a well video inspection can document current conditions including depth, casing condition, and water level.
Frequently Asked Questions
What is a well drilling log?
A well drilling log (also called a well completion report or driller's log) is an official document filed with the state that records all details of how a well was constructed. It includes total depth, casing specifications, geological formations encountered, water-bearing zones, static water level, and initial yield.
How do I get a copy of my well log?
In California, well logs are available through the Department of Water Resources (DWR) online well completion report database. You can search by address, parcel number, or geographic coordinates. The driller should have also provided a copy to the property owner. Your county environmental health department may also have records.
What does static water level mean on a well log?
Static water level is the depth to water when the well is not being pumped—it represents the natural water table elevation. It's measured from ground surface. For example, "static water level: 65 feet" means water stands 65 feet below ground when the pump is off. This level fluctuates seasonally.
What are the formations listed on a well log?
Formations describe the rock and soil types the driller encountered at different depths. Common entries include: topsoil, clay, sand, gravel, decomposed granite, granite, sandstone, and shale. Water-producing zones are often noted where fractured rock or permeable material was found.
Why does the well log show different casing depths?
Wells often have multiple casing sections: surface casing seals off shallow groundwater and unstable soil (typically 20-50 feet), while the main casing extends deeper. The annular seal (grout) between casing and borehole is noted, as is any perforated or screened section that allows water entry.
What does yield mean on a well drilling log?
Yield on a drilling log is typically a preliminary estimate based on drilling observations or a short pump test. It represents how much water the well produced initially, measured in GPM. A formal yield test after completion may show different results.
Need Help Interpreting Your Well Log?
Understanding your well log helps you make informed decisions about maintenance, repairs, and improvements. Our technicians can review your well log, explain what it means for your specific situation, and recommend appropriate service. We serve well owners throughout San Diego County.
