Groundwater Elevation Measurement - Water Levels, Surveying, and Flow Direction

Overview

Groundwater elevation measurements are one of the most fundamental and frequently performed tasks in environmental fieldwork. They answer two basic questions: how deep is the water below the ground surface, and which direction is it flowing?

Every groundwater sampling event begins with water level measurements. Every site investigation report includes a groundwater flow map. The accuracy of these maps - and the cleanup decisions based on them - depends on getting the measurements right in the field.

Equipment

Water Level Meter (E-Tape)

An electronic water level indicator (commonly called an e-tape) is a flat tape marked in graduated increments with a sensor probe at the end. When the probe contacts water, it completes a circuit and the meter emits an audible tone and/or visual signal. You read the depth to water from the graduated tape at the reference point (top of casing).

Common manufacturers include Heron, Geotech, and Solinst. All work on the same principle and produce equivalent results. Choose based on tape length needed for your site’s well depths.

Maintenance: Rinse the probe and tape with deionized water between wells to prevent cross-contamination. Check the batteries before each field event. Carry a backup battery or a spare meter.

Survey Equipment

Measuring depth to water is only half the equation. To calculate groundwater elevations and determine flow direction, you need accurate surveyed elevations at each well’s measuring point (top of casing). This requires either professional surveying or GPS equipment.

Professional surveyor: Most accurate option. A licensed surveyor establishes horizontal coordinates and vertical elevations for all monitoring well locations relative to a known datum (typically NAVD88 for elevations and NAD83 for horizontal coordinates). Vertical accuracy to 0.01 feet is typical. Use a surveyor for any project where elevation accuracy matters for regulatory submittals.

GPS (for horizontal coordinates): Standard GPS or smartphone GPS is adequate for horizontal well locations (latitude/longitude) when used for general mapping purposes. However, standard GPS does not provide the vertical accuracy needed for groundwater elevation calculations. A well elevation error of even 0.5 feet can completely change the interpreted flow direction on a flat site.

RTK GPS: Real-time kinematic GPS systems can achieve survey-grade accuracy (0.05-0.1 feet vertical) and are increasingly used by consulting firms for well surveys. If your firm has RTK capability, this can replace a professional surveyor for many applications.

Self-survey (differential leveling): Using an optical level (auto-level) and stadia rod, a field technician can establish relative elevations between wells on a site. This is adequate for determining flow direction when absolute elevations are not required. The key is accuracy - careful leveling technique can achieve vertical accuracy within 0.01-0.02 feet between wells.

Measuring Water Levels

Procedure

1. Open the well and allow venting. Remove the well cap and allow the well to vent briefly. Wells sealed tightly can develop pressure or vacuum conditions that affect the initial water level reading.

2. Lower the e-tape slowly. Feed the tape into the well along the side of the casing. Listen and watch for the signal indicating water contact.

3. Note the initial contact depth. When the signal activates, stop and read the tape at the top of the well casing (the measuring point, or MP). Record this reading.

4. Verify the reading. Pull the tape up a few inches above the water surface, then lower it again slowly. The depth should match your first reading within 0.01 feet. If it does not, repeat until you get a consistent reading.

5. Measure total well depth. After recording the water level, lower the tape to the bottom of the well and record the total depth. This tells you the length of the water column and whether the well has accumulated sediment since the last measurement.

6. Record all measurements. At a minimum, document:

   • Well ID
   • Date and time of measurement
   • Depth to water below measuring point (to 0.01 feet)
   • Total depth of well
   • Measuring point description (e.g., “top of PVC casing, north side”)
   • Any observations (LNAPL sheen, odor, well condition, damage)

Timing

Measure all wells on a site during the same field visit, as close together in time as practical. Groundwater levels can change over hours due to tidal effects near coasts, barometric pressure changes, nearby pumping, or precipitation events. If water levels are measured days apart, the flow map may not represent a single snapshot of conditions.

For sites with many wells, measure them in a consistent order and note the time of each measurement.

LNAPL (Light Non-Aqueous Phase Liquid)

If a well contains floating product (gasoline, diesel, or other LNAPL), the e-tape will signal at the top of the product layer, not at the water surface. To distinguish product from water:

• Use an interface probe (oil-water interface meter), which produces different signals for product and water. It will give you both the depth to product and the depth to water beneath the product.
• If you do not have an interface probe, note that the e-tape reading represents the top of the product, not the water surface. Record it as “depth to product” and note the well for follow-up with an interface probe.

The presence of LNAPL affects the true groundwater elevation calculation. The product layer depresses the water surface in the well, and a correction factor is needed to estimate the actual potentiometric surface elevation.

Calculating Groundwater Elevations

Groundwater elevation at each well is calculated by subtracting the depth to water from the surveyed measuring point elevation:

Groundwater Elevation = Measuring Point Elevation - Depth to Water

For example: If the top of casing is surveyed at 842.56 feet (NAVD88) and the depth to water is 12.34 feet, the groundwater elevation is 842.56 - 12.34 = 830.22 feet.

This converts depth-to-water measurements (which vary with ground surface elevation) into comparable elevation values that can be mapped across the site.

Determining Groundwater Flow Direction

Three-Point Method

With groundwater elevations from at least three wells, you can determine the direction of groundwater flow. Water flows from higher elevation to lower elevation, perpendicular to the contour lines of equal elevation (equipotential lines).

The manual three-point method:

1. Plot the three well locations on a site map to scale
2. Label each with its groundwater elevation
3. On the line connecting the highest and lowest elevation wells, interpolate to find the point with the same elevation as the middle well
4. Draw a line through that interpolated point and the middle well - this is an equipotential line (a line of equal groundwater elevation)
5. Draw a line perpendicular to the equipotential line from the high side toward the low side - this is the groundwater flow direction

Potentiometric Surface Maps

For sites with more than three wells, groundwater elevations are contoured to create a potentiometric surface map (often called a groundwater contour map). Contour lines connect points of equal groundwater elevation, similar to topographic contour lines on a land surface map.

Creating a contour map:

The most common approach in practice is to sketch the contours by hand first based on the measured elevations, then digitize the map for the report. Hand contouring uses professional judgment - you consider the geology, known boundaries (streams, rivers, drainage features), and well spacing when drawing the contour lines.

Software tools (Surfer, ArcGIS, AutoCAD) can interpolate contours automatically, but automated contouring should always be reviewed and adjusted based on site knowledge. Software does not know where geologic boundaries, preferential flow paths, or pumping wells are located.

Contour interval: Choose a contour interval appropriate for the site’s hydraulic gradient. A flat site with a gradient of 0.001 might use 0.5-foot contour intervals. A steep site might use 2-foot or 5-foot intervals. The contour interval should be small enough to show the flow pattern but not so small that measurement uncertainty makes the contours unreliable.

Hydraulic Gradient

The hydraulic gradient is the slope of the groundwater surface - how much the elevation changes over a given distance. It is calculated as:

Hydraulic Gradient = (Elevation Difference) / (Distance Between Wells)

The gradient tells you how fast groundwater is likely moving (in combination with aquifer properties) and in what direction. A steeper gradient generally means faster flow. Typical hydraulic gradients in unconsolidated aquifers range from 0.001 to 0.05 feet per foot.

Common Mistakes

Inconsistent measuring point. Always measure from the same point on the well casing. Mark the measuring point (e.g., “north side, top of casing”) and note it in the well completion records. If someone measures from the south side one event and the north side the next, and the casing is not perfectly level, you introduce error into your trend data.

Not recording time of measurement. If wells are measured over several hours and a pump turns on nearby partway through, the later measurements may be affected. Recording the time lets you identify whether something changed during the measurement event.

Using uncalibrated or low-accuracy elevation data. A hand-held GPS elevation is not accurate enough for groundwater flow calculations on flat sites. Even a 1-foot error in one well’s elevation can reverse the apparent flow direction. Use surveyed elevations for any quantitative flow analysis.

Forgetting to decontaminate. The e-tape probe goes from well to well. If you do not rinse it between wells, you risk cross-contaminating wells. This matters most on sites with LNAPL or high dissolved-phase concentrations.

Measuring during or immediately after heavy rain. Precipitation can temporarily affect shallow water levels, especially in wells that are not properly sealed. Measurements taken during or immediately after a storm event may not represent equilibrium groundwater conditions. Note weather conditions in your field records.

Source

Ohio EPA Technical Guidance Manual (TGM) Chapter 10: Ground Water Sampling. ASTM D4750: Standard Test Method for Determining Subsurface Liquid Levels in a Borehole or Monitoring Well. USGS Techniques of Water-Resources Investigations, Book 3, Chapter A1: General Introduction and Hydrologic Definitions.