How to Read Boring Logs and Well Construction Diagrams - A Field Guide for Environmental Consultants

A monitoring well installed at the wrong depth costs $3,000 to $8,000 to replace. Misreading lithology descriptions leads to incorrect conceptual site models. Poor well construction details invalidate years of groundwater data. Learning to read boring logs and well construction diagrams prevents these expensive mistakes.

Standard Boring Log Components

Every boring log contains the same basic information, though formats vary by drilling company and consultant. The header shows project details, drilling method, and completion date. The main body displays depth intervals, soil descriptions, and sampling information in columns.

- Depth column: Shows footage below ground surface, typically in 0.5-foot or 1-foot intervals - Lithology column: Visual representation using standard symbols and colors - Description column: Text description of soil type, color, moisture, and consistency - Sample column: Shows sample locations, types, and laboratory numbers - Groundwater column: Records water levels encountered during drilling

Most logs use the Unified Soil Classification System (USCS) for soil types. Clay appears as CL or CH, sand as SP or SW, and silt as ML or MH. The second letter indicates plasticity (L for low, H for high) or gradation (P for poorly graded, W for well graded).

Reading Lithology Symbols

Standard symbols represent different soil and rock types across the industry. These symbols appear in the lithology column and correspond to written descriptions.

- Dots: Fine sand or silt, with dot size indicating grain size - Circles: Gravel or coarse material, with circle size showing relative grain size
- Brick pattern: Clay or fine-grained cohesive soil - Horizontal lines: Bedrock or very dense material - Wavy lines: Organic material or peat - Triangle symbols: Fill material or anthropogenic deposits

Colors follow geological conventions. Brown indicates oxidized conditions, gray shows reduced conditions, and black suggests organic content or contamination. Red or orange colors typically indicate iron oxide staining.

Water level indicators use horizontal lines with “GW” notation and depth measurements. Static water levels appear as solid lines, while encountered water during drilling shows as dashed lines with timestamps.

Well Construction Details

The well construction diagram appears alongside or below the boring log. This diagram shows the physical components installed in the borehole and their exact depths.

- Casing: Solid PVC or steel pipe from surface to the screened interval - Screen: Slotted pipe that allows groundwater entry, typically 10 or 20 feet long - Filter pack: Clean sand placed around the screen, usually extending 2-5 feet above the screen top - Bentonite seal: Clay material placed above the filter pack to prevent vertical migration - Grout: Cement mixture filling the annular space from bentonite seal to surface

Screen slot sizes matter for sampling and development. Standard slots range from 0.010 inches for fine sand aquifers to 0.040 inches for coarse sand and gravel. The diagram should specify slot size and screen material.

Total depth, screen interval, and static water level must align logically. The screen should span the water table with adequate length below for seasonal fluctuations. A 2-inch diameter well requires different development procedures than a 4-inch well.

Key Information for Report Review

When reviewing boring logs for accuracy and completeness, focus on these critical elements that affect data quality and regulatory compliance.

Drilling Method and Date

The drilling method affects sample quality and contamination potential. Hollow stem auger drilling provides continuous samples but can cross-contaminate between intervals. Direct push methods like Geoprobe offer faster installation but limited sample recovery. The completion date indicates whether seasonal groundwater conditions apply.

Sample Intervals and Types

Continuous samples every 2-5 feet provide better lithologic control than grab samples every 10 feet. Split-spoon samples from standard penetration tests offer both lithology and geotechnical data. Check that sample intervals align with described lithology changes and contamination zones.

Groundwater Measurements

Static water levels should stabilize within 24-48 hours of completion. Immediate readings during drilling often reflect drilling fluid rather than formation water. Multiple measurements over time show whether the well is properly developed and representative of aquifer conditions.

Well Development Information

Properly developed wells produce clear water with stable pH and turbidity readings. Development volumes should equal 3-5 well volumes minimum. Turbidity below 50 NTUs indicates adequate development for most sampling programs.

Ohio note: Ohio VAP requires specific well construction standards under OAC 3745-300-08, including minimum 2-inch diameter wells and detailed construction documentation for all monitoring points.

Common Problems and Red Flags

Several recurring issues appear in boring logs that signal potential data quality problems or installation errors.

Missing or inconsistent depth measurements indicate poor quality control. Lithology descriptions that contradict visual symbols suggest rushed fieldwork. Groundwater levels that seem unrealistic for local conditions may reflect measurement errors or inadequate development time.

Well construction problems include screens set too high above the water table, inadequate filter pack thickness, or missing bentonite seals. These issues compromise long-term monitoring and may require well replacement.

Sample gaps in contaminated zones represent missed opportunities for delineation. Continuous sampling through suspected source areas provides better characterization than widely spaced grab samples.

Bottom Line

Boring logs and well construction diagrams contain the foundation data for every environmental site investigation. The depth measurements, lithology symbols, and construction details directly affect groundwater monitoring results and cleanup decisions. Careful review prevents costly field errors and ensures regulatory compliance. For sampling procedures once wells are installed, see our Low-Flow Groundwater Sampling guide and PFAS Sampling Best Practices guide.