Hydrogeological Field Methods
Practical procedures for aquifer testing, well monitoring, and groundwater sampling in the field.
Driller's Log Interpretation
The driller's log is the first geological record from a new well — interpreting it correctly guides well design and aquifer assessment:
What to Record
- Formation descriptions: Color, grain size, consolidation, moisture content at each depth
- Water strikes: Depth and description of water inflow during drilling
- Drilling rate: Change in penetration rate indicates formation boundaries
- Drilling fluid loss: Lost circulation zones indicate fractures or high-permeability zones
- Cuttings description: Sample collected at regular intervals — grain size, mineral composition
Formation Identification
| Drilling Indicator | Formation Type | Aquifer Potential |
|---|---|---|
| Fast penetration, sand/gravel cuttings | Alluvium, outwash | High |
| Medium penetration, sandstone | Sandstone aquifer | Medium-High |
| Slow penetration, hard rock | Crystalline basement | Low (unless fractured) |
| Drilling fluid loss | Fractured zone, karst | High |
| Water strike at depth | Aquifer contact | Target zone |
| Clay cuttings, slow drilling | Shale, clay confining layer | Confining unit |
Slug Test Field Procedures
Quick, low-cost method for determining hydraulic conductivity:
Procedure
- Install piezometer: Screened interval in target aquifer, grouted to surface
- Measure static water level: Record to nearest mm — use electric water level meter
- Add or remove slug: Instantaneously add known volume (bailer) or solid cylinder
- Monitor recovery: Record water level at 1-sec intervals until 90% recovery
- Analyze data: Plot ln(head) vs time — slope gives K using Bouwer-Rice method
Bouwer-Rice Equation:
K = (r_c² × ln(R_e/r_w)) / (2L) × (1/t) × ln(h₀/h_t)
K = hydraulic conductivity (m/s), r_c = casing radius, L = screen length
R_e = effective radius, r_w = well radius, h₀/h_t = head at time 0 and t
K = (r_c² × ln(R_e/r_w)) / (2L) × (1/t) × ln(h₀/h_t)
K = hydraulic conductivity (m/s), r_c = casing radius, L = screen length
R_e = effective radius, r_w = well radius, h₀/h_t = head at time 0 and t
Equipment Needed
- Water level meter: Electric contact probe with depth tape — accuracy ±1 mm
- Slug: Solid steel or PVC cylinder of known volume — or bailer for removal
- Data logger: Pressure transducer with 1-sec sampling — records entire recovery
- Stopwatch: Backup timing if no data logger — manual readings at 5-sec intervals
Pumping Test Design
The gold standard for aquifer characterization — requires careful planning:
Test Components
- Pumping well: Production well or dedicated test well — screened in target aquifer
- Observation wells: 2-4 piezometers at different distances (10, 30, 100, 300 m)
- Pump: Submersible with flow meter — constant rate for 24-72 hours
- Water level monitoring: Continuous recording in all wells — pressure transducers
- Flow measurement: Electromagnetic or ultrasonic flow meter — accuracy ±2%
Test Duration Guidelines
| Aquifer Type | Minimum Duration | Recovery Time | Purpose |
|---|---|---|---|
| Unconfined (high-K) | 24 hours | 24 hours | Transmissivity, specific yield |
| Unconfined (low-K) | 48-72 hours | 48 hours | Transmissivity, storage |
| Confined | 24-48 hours | 24 hours | Transmissivity, storativity |
| Dual-porosity | 72-168 hours | 72 hours | Fracture and matrix properties |
Analysis Methods
- Cooper-Jacob (straight-line): Plot drawdown vs log(time) — slope gives T, intercept gives S
- Theis (curve matching): Match observed data to type curve — gives T and S simultaneously
- Recovery test: Analyze water level recovery after pumping stops — often more reliable than drawdown
Water Sampling Protocols
Low-Flow Sampling
Minimizes disturbance — provides representative groundwater samples:
- Setup: Pump at low flow (0.1-0.5 L/min) through tubing to surface
- Stabilization: Monitor pH, temperature, conductivity, dissolved oxygen until stable (±5% over 3 readings)
- Sample collection: Collect from end of tubing — minimize aeration
- Purpose: Dissolved metals, VOCs, nutrients — most accurate representation of aquifer conditions
Passive Diffusion Bag (PDB)
- Method: Deploy LDPE bag in well — equilibrates with groundwater over 2-4 weeks
- Purpose: VOC sampling — no pumping required, minimal disturbance
- Limitations: Only for dissolved gases and VOCs — not for metals or ions
Piezometer Installation
- Drilling: hollow-stem auger or mud rotary to target depth
- Screened interval: 1-3 m screen — narrow interval for precise water level
- Filter pack: Sand sized 2-3× screen slot — placed around screen
- Seal: Bentonite chips above filter pack — prevent vertical flow along borehole
- Grout: Bentonite-cement slurry to surface — isolate monitored zone
- Casing: PVC or steel — extends above ground with locking cap
- Survey: Measure top of casing elevation — convert water levels to elevation head
Field Equipment Checklist
Water level meter, data logger, flow meter, pump, sampling tubing, sample containers, cooler with ice, GPS, field notes, safety equipment (gas detector, PPE), decontamination supplies, calibration standards.