Exam C/D is a 50-question, one-hour, closed-book specialized drilling exam with a 70 percent minimum score, weighted most heavily toward constructing the water well. It counts toward the CWD designation only alongside the general drilling exam, both passed within one 12-month period.
What this exam is and where it fits
Exam C/D: Air Rotary Drilling is one of NGWA's specialized drilling exams. It has 50 questions, a one-hour limit, and a 70 percent minimum passing score, and the exams are closed book.
NGWA operates a voluntary national certification program for groundwater contractors, started in 1970 and governed by contractors who set its standards. Many state agencies use these same exams within their own licensing programs, but NGWA certification itself is not a license; state licensing is separate and controlled by each state.
To earn the Certified Well Driller (CWD) designation, you must pass the general drilling exam plus at least one specialized drilling exam. Exam C/D is one of those specialized options, alongside cable tool, mud rotary, reverse circulation, jetting and driving, and augering and monitoring. You must pass both required exams within one 12-month period, scoring 70 percent or better on each.
On format, the PSI candidate bulletin is specific: Exam C/D has 50 questions, a minimum passing score of 70 percent, and one hour of testing time. All NGWA examinations are closed book, and no NGWA approval is needed before you sit an exam; certification application and eligibility review remain a separate step.
Official sources used: Groundwater | NGWA Certification exams — Certification exams page; Groundwater | NGWA Certification for contractors — Contractor certification page, eligibility section; PSI NGWA candidate information bulletin — Content pages 5–7, Exam C/D Air Rotary Drilling table
How the 50 questions are distributed
The bulletin's content table allocates the 50 questions across five duty areas, with constructing the water well carrying 34 of the 50 questions.
The PSI bulletin dated September 2022 gives the breakdown below. Well construction accounts for 34 of 50 questions. Rehabilitation is not listed as a separate C/D domain; that does not establish that every rehabilitation-related concept is excluded.
These counts describe how many questions fall in each area, not how difficult each area is. Use them to allocate study time proportionally, then link each area back to the NGWA study materials listed for drilling exams.
| Task area | Questions |
|---|---|
| Mobilize for the site | 6 |
| Construct water well | 34 |
| Develop wells | 7 |
| Decommission wells | 2 |
| Demobilize the site | 1 |
Official sources used: PSI NGWA candidate information bulletin — Content pages 5–7, Exam C/D Air Rotary Drilling table
Who can qualify for certification
Eligibility requires being at least 20 years old, having 24 consecutive months of full-time relevant experience, passing the two required exams within 12 months at 70 percent or better, and submitting references, an agreement form, and an affidavit.
The published eligibility rules are concrete. You must be at least 20 years old and have at least 24 consecutive months of full-time drilling or pump installation experience. Two professional contacts who are not affiliated with your current company must provide written references verifying that experience, and you submit an agreement form and an affidavit for the current year.
Application materials must be received within 12 months of the qualifying exam session, so treat that window as part of your plan rather than an afterthought. Nothing in the published material requires NGWA approval before you sit an exam; the bulletin states approval is not needed to test and that eligibility remains a separate process.
If your goal is state licensing rather than voluntary certification, confirm your state's own requirements directly with the licensing agency. Some states adopt NGWA's exams for their programs, but each state's rules govern.
- Age 20 or older
- At least 24 consecutive months of full-time drilling experience
- Both required exams passed within one 12-month period at 70 percent or better
- Written references from two professional contacts outside your current company
- Signed agreement form and current-year affidavit
- All application materials submitted within 12 months of the qualifying exam session
Official sources used: Groundwater | NGWA Certification for contractors — Contractor certification page, eligibility section; PSI NGWA candidate information bulletin — Content pages 5–7, Exam C/D Air Rotary Drilling table
The air rotary method in plain terms
Air rotary is a rotary drilling method that uses compressed air instead of drilling mud as the circulating fluid, and well development afterward removes fine material so the well produces sediment-free water.
USGS describes hydraulic rotary drilling as rotating a drill pipe fitted with a bit while circulating fluid to cool the bit and remove cuttings. Its air rotary description identifies air, rather than mud, as the circulating medium. This explains the basic distinction without prescribing an equipment configuration.
Once construction is complete, the well enters the development phase. Its objective is to remove clay, silt, and fine-grained sand from the area next to the screen or open hole so the well yields sediment-free water. The bulletin gives this area seven of the 50 questions.
Official sources used: USGS Basic Ground-Water Hydrology (Water-Supply Paper 2220) — Selected fundamentals, drilling methods and well development; PSI NGWA candidate information bulletin — Content pages 5–7, Exam C/D Air Rotary Drilling table
Worked exercise: annular area and velocity
For a centered 6.125-inch hole with 4.5-inch pipe, the annular area is about 0.0942 square feet and 400 actual cfm gives roughly 4,250 ft/min; in an 8-inch washed-out interval at the same flow, velocity drops to about 1,680 ft/min.
This is an original geometry exercise with the flow and dimensions supplied. It illustrates how the area of a ring affects the calculated mean velocity; it is not an NGWA operating target or a prediction of an exam question.
Practice exercise
Assume a circular borehole drilled with a 6.125-inch bit and drill pipe with a 4.5-inch outside diameter, the pipe centered in the hole, and air returning up the annulus at an actual flow of 400 cubic feet per minute at the section analyzed. (a) Calculate the annular area in square feet. (b) Calculate the mean annular velocity in feet per minute. (c) The same pipe string later passes through a washed-out interval measuring 8 inches across, with the same 400 actual cfm returning; recalculate the velocity there.
Show answer
(a) About 0.0942 square feet. (b) About 4,250 ft/min. (c) About 1,680 ft/min.
The annulus is the ring between two circles, so its area is pi/4 times the difference of the squared diameters, using the pipe outside diameter because the return air travels outside the pipe. For part (a): 6.125 squared is 37.5156, 4.5 squared is 20.25, the difference is 17.2656, and pi/4 times that is about 13.56 square inches, or 0.0942 square feet after dividing by 144. For part (b): velocity equals flow divided by area, so 400 cfm divided by 0.0942 square feet gives about 4,248 ft/min, roughly 4,250. For part (c): with an 8-inch hole, 64 minus 20.25 is 43.75, times pi/4 is about 34.36 square inches, or 0.2386 square feet, and 400 divided by 0.2386 gives about 1,676 ft/min, roughly 1,680. The classic slip is computing pi/4 times the square of the diameter difference, 1.625 inches here, which gives about 2.07 square inches and inflates velocity several times over. Note the assumptions behind the labels: the flow figure is the actual return at the section, not a compressor nameplate rating, and real returns vary with depth and ground conditions, so treat both velocities as labeled screening results under stated assumptions.
Official sources used: USGS Basic Ground-Water Hydrology (Water-Supply Paper 2220) — Selected fundamentals, drilling methods and well development
What NGWA recommends you study
NGWA recommends a short list of texts for drilling exams, with the Well Drillers Manual named specifically for the cable tool and air rotary exams, plus online training and task-analysis charts.
The exam page lists recommended study materials and states which exams each supports. Groundwater & Wells, third edition, and the Manual of Water Well Construction Practices, third edition, apply to all drilling exams, as do the Water Well Driller's Beginning Training Manual and the Procedures for Well Drilling Operations CD. The Well Drillers Manual is listed for the cable tool and air rotary exams specifically, which makes it the natural anchor for C/D.
Two other resources round out preparation. NGWA University offers self-paced online courses for the general drilling and water systems exams, built with NGWA-approved subject matter experts and including knowledge checks and study guides. Separately, the DACUM research charts analyze the major duties and tasks of drilling contractors and operators; treat them as a map of what the job involves, not as step-by-step procedures.
- Well Drillers Manual — listed for cable tool and air rotary exams
- Groundwater & Wells, third edition — all exams
- Manual of Water Well Construction Practices, third edition — all drilling exams
- Water Well Driller's Beginning Training Manual — all drilling exams
- Procedures for Well Drilling Operations (CD) — all drilling exams
- NGWA University online training — general drilling exam preparation
- DACUM chart for drilling contractors and operators — duty and task overview
Official sources used: Groundwater | NGWA Certification exams — Certification exams page; Drilling contractors and operators (DACUM research chart) — NGWA task chart, duties A–L
Scheduling, scores, and exam day
PSI administers the online exams year-round at more than 400 test centers, scores are issued immediately, and you should collect your score report before leaving the test center.
You schedule through PSI's test-taker website, and testing is available year-round at more than 400 centers nationwide. The program does not require special computer skills. If scheduling problems come up, NGWA asks you to email certification@ngwa.org so it can work with PSI.
Scores print immediately after you finish, so collect your score report at the center before leaving; NGWA can reissue scores for a fee but never over the phone. If you need accommodations, contact NGWA first, and PSI will then contact you to complete registration. All testing centers are ADA compliant.
Official sources used: Groundwater | NGWA Certification exams — Certification exams page
Common paper mistakes
The avoidable errors are geometric, interpretive, and strategic: using the wrong annular-area formula, treating question counts as difficulty ratings, and reading a paper calculation as an operating decision.
The geometric slip appears in the exercise above: the annulus is the difference of two circle areas, not the area of a circle whose diameter equals the gap between pipe and hole. Squaring the diameter difference shrinks the area several times and corrupts every velocity built on it.
The interpretive slip is reading the bulletin's question counts as a difficulty ranking. The table says 34 of 50 questions concern constructing the water well; it says nothing about how hard individual questions are. Weight your study time by the counts, but judge difficulty from your own practice results.
A calculation based on stated flow and diameter assumptions answers that paper problem. It does not establish a suitable compressor, operating rate or field procedure.
Official sources used: PSI NGWA candidate information bulletin — Content pages 5–7, Exam C/D Air Rotary Drilling table; USGS Basic Ground-Water Hydrology (Water-Supply Paper 2220) — Selected fundamentals, drilling methods and well development
A short pre-exam checklist
Before booking, confirm the current bulletin, your two-exam plan, the eligibility paperwork window, and your weakest task area from the official weights.
Work through this list once your study plan is running. Each item traces back to the verified facts in this guide rather than to hearsay.
- Download the current PSI NGWA candidate information bulletin and confirm the exam format and content tables still match this guide
- Confirm your plan covers the general drilling exam plus Exam C/D, both within one 12-month period at 70 percent or better
- Line up your two references outside your current company and the agreement form and affidavit
- Budget the 12-month application window after your qualifying exam session
- Schedule through PSI and plan to collect your score report at the test center
- If you want state licensure as well, verify that state's rules with the licensing agency directly
Official sources used: Groundwater | NGWA Certification exams — Certification exams page; Groundwater | NGWA Certification for contractors — Contractor certification page, eligibility section; PSI NGWA candidate information bulletin — Content pages 5–7, Exam C/D Air Rotary Drilling table
Official sources used
Facts checked against official sources: