A Classification System for Water Well Boreholes

Published On: August 18, 2022By Categories: Drilling, Groundwater & Wells

Learning to use a uniform system for classifying water well boreholes.

By Thom Hanna, PG

One of the challenges facing the groundwater industry is providing consistent hydrogeologic data for water well boreholes.

Figure 1. Field guide.

Borehole logging is required for most all water well professionals, but few people have been instructed in what is important to record in a borehole log. This includes regulators that require geologic logs be provided as part of the documentation filed for well completion records.

Each state has different required levels of detail, most commonly asking for a “description.” It is important that groundwater professionals use clear and concise terms that have meaning to characterize water well boreholes so that other groundwater professionals can use the information.

There are several states and provinces that have adopted a classification system with minor modifications to reflect the local hydrogeology because it is concise and is readily adaptable for using in online drilling reports.

The Hydrogeologic Classification System for Water Well Boreholes (HCSWB) system was developed to establish a framework to create uniform borehole logs that can be correlated. The system consists of a methodology to record water well borehole data in a uniform approach for hydrologic interpretation.

It is discussed in the third chapter of Groundwater & Wells, Third Edition, which is titled “Subsurface Exploration and Hydrogeologic Logging.”

The HCSWB also provides the basis for storing and sorting data in digital format because the descriptions and identifiers are uniform and can be digitized so that records can be sorted by the descriptors.

The HCSWB incorporates important aspects of geologic description for distinguishing formation/aquifer characteristics and documents important hydrologic characteristics that control groundwater flow.

It is important to identify and properly describe the attributes of water-bearing strata that control the flow of water. The capacity of a formation to transmit water can vary greatly. Unconsolidated formations consisting of clay and silt, and bedrock formations such as granite, yield little water and form barriers to flow.

On the other hand, unconsolidated earth materials consisting of coarse sand and gravel and cavernous limestone can be prolific aquifers. The HCSWB was created to describe the water-transmitting capacity of the water-bearing formations.

The two main parts of the HCSWB are a classification system and a field guide (Figure 1) allowing fast identification of aquifer materials in the field. Figure 2 presents a flow chart with the logging procedures and categories of the classification system.

Sample Collection

The first step in creating a consistent hydrogeologic log is collection of a representative sample for description. The drilling method, the depth of the borehole, the penetration rate, and the type of drilling fluid will have significant influences on the sample.

  • Samples should be collected in a consistent fashion and care should be taken to capture a representative of the interval penetrated.
  • The samples should be washed and described when wet for more accurate color identifications. Care should be taken during the washing to not wash away any significant amount of fines.
  • It is recommended to log a series of samples together as opposed to individual samples because geologic transitions (e.g., fining upward depositional environments) can be noted and recorded.

Sample Description

Depth: This is the top and bottom of the depth interval being described. The length of the interval to be described will depend on the uniformity of the formation or aquifer and the level of detail that is appropriate for each borehole. Typically, samples are collected and described every 5 feet or 2 meters.

Aquifer/Formation: Formation names can be obtained from published data and are generally available from local, state, and federal geologic surveys. Important aquifers will also have formal names, quite often mimicking the formation name (e.g., Denver Formation or the Denver Basin Aquifer), and assist in better understanding the extent and regional characteristics of the aquifer/formation. If the aquifer or formation name is not available, this can be left blank.

Unconsolidated/Bedrock: This type of information will be the first descriptor that will give information concerning bedrock and alluvial contacts that are important for many regulatory agencies in managing surface and groundwater aquifers and are easy to identify on the log. In some areas this might not be important.

Unconsolidated Materials: When describing the cuttings from an unconsolidated material, a better understanding of the water-yielding characteristics is given by describing the smaller 20% fraction of the materials that control the flow. An example is formations described as “Sand and Gravel” indicates that at least 20% of the formation is sand. The field guide will help in providing consistent grain size descriptions.

  • Clay: Clays are classified when the majority of the formation is clay with less than 10% of the formation being other materials such as sands or silts. Clays are easy to distinguish from silts because they can be rolled into a rope 0.25 (¼) inch (6 mm) in diameter.
  • Silt: Silts are similar in appearance to clay but cannot be rolled into a rope 0.25 (¼) inch (6 mm) in diameter. For this designation there will be only 10% or less of the formation being sands or gravels.
  • Sand with Clay or Silt: For this designation 20% or more of the formation is clay or silt-sized material and 80% or less of the formation will be sands and gravels. This will be a mixture of materials that at first might seem to be permeable, but if it is a well-sorted formation, it will have a low permeability.
  • Sand, fine to medium: The smallest 20% or more of the formation ranges from 0.02 to 0.08 inch (0.5 to 2 mm). The remainder of the formation can be coarser sands but there will be more than 20% gravels in the materials with almost no silts and clays.
  • Sand, medium to coarse: The smallest 20% of the formation ranges from 0.08 to 0.2 inch (2 to 5 mm) in diameter. Less than 20% of the formation can be gravels with almost no silts and clays.
  • Sand with Gravel: The smallest 20% to 50% of the formation is sand but over half the formation (more than 50%) is gravel.
  • Gravel: The finest 20% or more of the formation is larger than 0.2 inch (5 mm) in diameter. These gravels will possibly be mixed with cobbles or boulders. There will be less than 5% sands in the sample.

Figure 2. Flow chart for logging water well boreholes.

Bedrock Type: Describing bedrock cuttings can be difficult in the field. Subtle differences such as siltstone/shale or igneous/
metamorphic rocks, especially granites, can be difficult to determine from borehole cuttings, but other descriptors such as hardness, noting of fractures, or formation/aquifer identification will provide additional detail.

Rock types and their metamorphic equivalents have been grouped into general categories that can be easily identified in the field from cuttings. Quite often flow will be a function of secondary permeability in bedrock materials, which is noted in the column for “Water Content.” Some bedrock types such as coals can be marker beds or be indicators of possible water quality issues.

  • Siltstone/shale: Siltstones and shales are very fine-grained sedimentary formations and slates and equivalents. The degree of induration is a function of depth of burial, where slates are the metamorphic equivalent of shales. Shales are generally considered aquitards and barriers to flow.
  • Sandstone: Sandstones are consolidated equivalents of sands that can have primary and secondary permeability. Sandstone aquifers are formed from deposits that were deposited as unconsolidated sands that have been buried, compacted, and cemented. The degree of compaction and type of cementation will control the primary permeability. Quartzites are the metamorphic equivalent; being very hard, they form good marker beds.
  • Limestone: Carbonate rock aquifers consist of limestone, dolomite, and marble. The yield of carbonate aquifers can vary greatly from confining layers to prolific water producers due to secondary permeability that is the result of dissolution of the limestone. Carbonate
    borehole cuttings can often be identified in the field by their reaction to weak acids (e.g., 10% hydrochloric acid) that make them effervesce.
  • Conglomerate: Conglomerates are consolidated equivalents of sands and gravels. These aquifers and formations are excellent marker beds for stratigraphic interpretation, and often represent the basal units of many formations.
  • Coal: Coals can be good aquifers, depending on the amount of fracturing, but in some areas they are important for noting areas of poor water quality or methane gas. Coals are also good marker beds.
  • Granite: Granites are igneous intrusive (granitoids) and metamorphic (e.g., gneiss, schists, and pegmatites) crystalline rocks. Color of granites are a function of the mineral content. Igneous and metamorphic granite aquifers are generally low-yielding formations that produce water through fractures. Very often these rocks act as aquitards.
  • Volcanic: Volcanic rocks are fine crystalline and extrusive formations (e.g., andesites, rhyolites) excluding basalts that are identified as a separate hydrogeologic classification. Groundwater flow is generally through fractures in volcanic rocks.
  • Basalt: Basalts can have significant primary and secondary permeability and are important aquifers in some areas. Basaltic aquifers form in layers of varying permeability determined by flow events.
  • Other: This category can be used to add any rock type that might aid in describing cuttings that is an important regional aquifer.

Hardness: Hardness is a relative term that needs to be standardized to provide consistency and meaning to the formation/aquifer description. Simple field tests can be performed to test hardness in the field:

  • Very Hard: Cuttings that are difficult to break with a hammer
  • Hard: Cuttings that are easy to break with a hammer or more than 30 blows/ft
  • Dense/Stiff: Cuttings that can be broken in your fingers or 10 to 30 blows/ft
  • Loose: Cuttings that are unconsolidated or less than 10 blows/ft

Color: Color provides information that can be used to correlate different formations. Reds and oranges can be indications of oxidation or flow of oxygenated water through the aquifer. The colors presented in the field guide are the most common colors that will be recognized in the field.

Water Content: Notations on water content will be influenced by the methods and drilling fluids used to drill the well. Water content data will help identify water production zones and the position of the water table. The following are the descriptors used for water content:

  • Dry: No moisture in cuttings
  • Moist: Cuttings are damp but there is no visible water
  • Wet: Cuttings have visible water
  • High Production: Locations of fractures or coarse-grained materials that yield significantly more water than adjacent aquifer materials
  • Lost Circulation: Zone of significant loss of drilling fluids or cuttings.

If the borehole is drilled with a heavy drilling fluid that does not allow for water content of the samples to be determined, this column should be left blank.

Other: The other area in the log provides the opportunity to add any additional comments that will enhance the formation/aquifer description. This will allow modifications of the borehole log to a particular application. One can include description of angularity or roundness, organic content, drilling conditions, or any other comments that will enhance the log.

Summary

The Hydrogeologic Classification System for Water Well Boreholes method allows groundwater professionals to make detailed, concise, and correlative logs based on the cuttings from water well boreholes. It also addresses a major industry challenge in the digital age of how to catalog, sort, and retrieve data to make borehole data readily available to all groundwater professionals. The HCSWB method is flexible,
detailed, and makes it easy to document the pertinent hydrogeologic data from water well boreholes.

By incorporating this methodology as a minimum standard, the logging of water well boreholes will be uniform, and the data generated will be interpretable by all groundwater professionals.

The field cards and a book describing how to describe water well cuttings using the HCSWB
is available in the NGWA bookstore. The card is also included in Groundwater & Wells, Third Edition, which can also be purchased by clicking here.
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Thomas M. Hanna, PG, is a technical director of water well products/hydrogeologist for Johnson Screens where he works in areas of well design, development, and well rehabilitation. He is a registered professional geologist in Arizona, Kentucky, and Wyoming and has worked for several groundwater consulting firms. Hanna can be reached at thom.hanna@johnsonscreens.com.

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