Department of Health Seal

TGM for the Implementation of the Hawai'i State Contingency Plan
Section 6.3
GROUNDWATER GAUGING

6.3 GROUNDWATER GAUGING

The purpose of groundwater gauging is to construct a groundwater table map or a potentiometric surface map for the site under investigation. The data are used to calculate the hydraulic gradient(s) and the horizontal groundwater flow direction(s) across the site. Therefore, all measurements must be taken within a 24-hour period or less. Certain conditions require that measurements are taken within a shorter time period. These conditions include (USEPA, 1999a):

  • The magnitude of the observed changes between wells appears too large.
  • Atmospheric pressure changes occur.
  • Aquifer is tidally influenced.
  • Aquifer is affected by river impoundments and/or unlined ditches.
  • Aquifer is stressed by intermittent pumping of production wells.
  • Aquifer is recharged due to precipitation events or irrigation.

If time restrictions require the use of more than one measuring device, calibrate the devices against each other at the start or end of the work day. Calibrate the devices by comparing measurements in a single well. If free phase product is present at the site, the calibration well must contain free phase product to ensure proper calibration. In this case, calibrate the devices at the end of the day. To calibrate instruments at the start of the day, select a clean or the least contaminated well.

Gauge wells starting with the cleanest well and proceeding in order of increasing contaminant levels. Decontaminate the measuring device at the start and end of the day and between wells. Ensure that the decontamination procedures are adequate for the contaminants present.

The HEER Office requires a minimum of two rounds of groundwater gauging to verify the groundwater gradient(s) and flow direction(s). The two gauging events must be separated by a minimum of 30 days. Ensure that wells are continually connected to the aquifer, if gauging events are repeated over prolonged periods of time. Purge/redevelop wells as needed to reestablish hydraulic connectivity and equilibration with the aquifer.

On sites that are tidally influenced, perform a tidal study to determine the net groundwater flow direction. If site investigation, monitoring and remedial efforts continue over a year, include seasonal groundwater gauging into the investigation to determine the influence of seasonal variations in the aquifer.

Correct the measured groundwater elevations for tidal influences, barometric influences, and overlying free product thickness.

6.3.1 Monitoring Well Equilibration

During well development, a large amount of groundwater is pulled through the surrounding formation as well as the filter pack and the well casing. This may disturb the chemical equilibrium of the surrounding formation with the groundwater. In addition, the filter pack may not reach chemical equilibrium with the aquifer.

Time must be allowed for chemical equilibration of the filter pack and the surrounding formation with the aquifer before the well may be sampled. At the same time, hydraulic equilibrium with the aquifer should be attained. This is especially important in low permeability aquifers, where water extraction during well development may draw down the aquifer adjacent to the well.

The HEER Office recommends that groundwater gauging and sampling be conducted no sooner than 14 days after well development. However, equilibration time may be based on site-specific factors. If a different time frame from 14 days is proposed based upon known site conditions (e.g. evidence of high hydraulic conductivity, either for the site itself or immediately surrounding area), overall project considerations, or other pertinent information, the interval, rationale, and evidence supporting the proposal should be noted in detail in the SAP or documented through consultation with the HEER Office. Additional discussion on equilibration is provided in Subsection 6.2.1.9.

6.3.2 Depth-to-Water Measurement

Reference all water level measurements to the survey mark at the top of the casing. The reference point must be surveyed and marked as detailed in Subsection 6.2.1.6, Well Survey. Use either a weighted steel tape with chalk or an electronic water level indicator to measure the depth to water (USEPA, 2002b and 1999a). Select the measuring device carefully for wells deeper than 200 feet to ensure that the tape does not stretch (USEPA, 2002b). The measurement must be taken with an accuracy of ±0.01 feet.

Wells with submerged screens should have the well cap removed at least 15 minutes prior to gauging to eliminate the effects of rising or falling water levels prior to the gauging event. Rising or falling water levels prior to removing the well cap will create either pressure or a vacuum on the well that will impact the water level in the well unless sufficient time is allowed for the well to equilibrate. In addition, wells with submerged screens should be gauged continuously for several minutes to document that the water level has equilibrated.

If groundwater sampling is to be completed on the same day, measure the depth to water prior to sampling.

Calculate the water table/potentiometric surface elevation by subtracting the depth to water from the reference point elevation. Correct elevations for tidal, barometric and free phase density influences.

6.3.3 Total Well Depth

Measure the total well depth by lowering the probe to the bottom of the well and referencing the depth to the surveyor's mark. Note whether silt is encountered at the bottom of the well. Repeat the total well depth measurement at least once to confirm the measurement (USEPA, 1999a).

If groundwater sampling is to be completed on the same day, measure the total well depth after sampling has been completed to prevent suspension of silt into the water column (USEPA, 1999a).

Figure 6-10

Figure 6-10. Oil-water Interface Meter
[Source: Solinst, 2008]

6.3.4 Free Product Measurement

Measure wells with free phase product last. Use an oil-water interface probe manufactured for use in free-phase product (See example in Figure 6-10). Measure the depth to the top and bottom of the free phase in reference to the surveyor's mark at the top of the casing. In LNAPL plumes, this would be on top of the water column; in DNAPL plumes at the bottom of the well.

Correct the water table elevation for the thickness of the free product floating on top of the water column. The corrections have to be based on the actual density of the LNAPL present at the site.

6.3.5 Well Gauging Log

At a minimum record the following information on groundwater monitoring logs:

  • Date
  • Project name and location
  • Field personnel
  • Measuring devices used
  • Calibration of measuring devices against each other
  • For each well record:
    • Time of measurement
    • Depth to free product
    • Depth to groundwater
    • Depth to bottom of well
    • Observations (casing condition, well head condition, odor, color, sheen, silting, etc.)
  • Activities that may influence water level (groundwater pumping, irrigation, etc.)
  • Decontamination procedures

6.3.6 Tidal Effects

Aquifers at sites in proximity of the ocean with water table or potentiometric surface elevations close to sea level often show tidal influences. Tidal sea level changes result in changes of hydraulic pressure at the shoreline, where groundwater flows into the ocean. As the tide rises, hydraulic pressure increases and causes backpressure into the aquifer.

The larger the permeability or hydraulic conductivity of the aquifer formation, the farther the backpressure is felt inshore. The tidal efficiency and tidal lag of each well is dependent on its distance from the ocean and the hydraulic conductivity of the formation between the well and the shoreline.

6.3.6.1 Tidal Study

Completion of a tidal study may be necessary at sites that are tidally influenced. Continually gauge and record groundwater elevations at the site for a minimum period of 72 hours. Gauge at least three monitoring wells on small sites and more wells on larger sites. In addition, measure the water elevation in the ocean in a place protected from wave action. Enclose the pressure gauge in a stilling well to filter out most of the surface ripples. If gauging in the ocean is not possible or no protected area can be found nearby, use tidal charts corrected for the site. Record gauging data at a frequency of six minutes or less. Synchronize data logging between all pressure gauges.

Calculate the groundwater gradient and flow direction for each sampling time. Calculate the net groundwater gradient and net flow direction.

Compare the times of tidal fluctuations between the ocean and each monitoring well to determine the tidal lag of each well. Compare the groundwater elevation changes within each well to the sea level changes to calculate the tidal efficiency (percent of groundwater elevation change compared to sea level change).

On a map, present the groundwater flow directions as they change throughout one tidal cycle. Present the groundwater gradient for each flow direction. In addition, present the net groundwater flow direction and gradient on the same map. If variations are seen between tidal cycles, present each tidal cycle on a map.

6.3.6.2 Gauging at Tidally Influenced Sites

Groundwater gauging at tidally influenced sites requires careful planning. Choose the date and time of gauging according to tide charts. Choose the date when tidal fluctuations are minimal, such as on quarter moons. Choose the time of gauging when fluctuations in the wells are minimal. For example, bracket the time of high tide or low tide. Correct the times for tidal lag. Follow the same criteria for each gauging event.

Involve sufficient field personnel to accomplish gauging within a period of no more than 2 to 3 hours. Choose a reference well. Use the reference well to calibrate the field instruments against each other. Gauge the well at a minimum every half an hour between gauging other wells.

Assume that the change in groundwater elevation within the reference well is linear throughout the half hour between measurements. Use the linear regression of the groundwater elevation change in the reference well to correct the groundwater elevations for all wells for tidal influence. On sites with large variations in tidal efficiency choose more than one reference well.

6.3.7 Seasonal Effects

Groundwater flow may exhibit significant seasonal variations between the dry season and the wet season in Hawai`i. Characterizing seasonal and temporal variations in groundwater flow is important at sites where the groundwater flow direction may change due to seasonal variations. It is also important for site investigations involving aquifer tests.

Initially, weekly or monthly water level measurements may be needed to characterize seasonal fluctuations, followed by quarterly or semi-annual monitoring after the water level variations have been described (CalEPA, 1995a).

6.3.8 Temporal Variations

In addition to tidal and seasonal variations, the following processes can introduce temporal variations in the groundwater table/potentiometric surface and possibly in the groundwater flow direction (USEPA, 1992d; CalEPA, 1995a):

  • Barometric effects
  • Variations in precipitation and runoff/recharge rates
  • On-site or off-site well pumping, recharge, and discharge
  • Changes in land use by altering recharge or discharge patterns (e.g. paving, damming of rivers)
  • Changes in lagoons, ponds, or stream stage
  • Deep well injection

Identify and evaluate factors that result in short- or long-term variations in groundwater elevations and flow patterns. Measure the water levels frequently enough to detect and characterize temporal variations in groundwater flow.

6.3.9 Determination of Vertical Hydraulic Gradient and Flow Direction

Determine the vertical hydraulic gradient and flow direction on sites where vertical groundwater flow is significant. A deep vertical extent of a dissolved contaminant plume is an indication that vertical groundwater flow is significant.

To determine the vertical component of groundwater flow, install multiple piezometers or wells in clusters or nests, or multi-level wells or sampling devices. A piezometer or well nest is a closely spaced group of piezometers or wells screened at different depths, whereas a multi-level well is a single device providing access to more than one depth in a single well.

Both piezometer/well nests and multi-level wells allow for the measurement of vertical variations in hydraulic head. To obtain reliable measurements, consider the following criteria in the evaluation of data from piezometer/well nests and multi-level wells (USEPA, 1992d):

  • Data obtained from multiple piezometers or wells placed in a single borehole may be erroneous. Sealant from one piezometer or well may migrate into the screened interval of another.
  • Drilling and installing a piezometer close to others in a cluster may cause disturbance in the formation around neighboring piezometers.
  • Water levels measured in piezometers that are closely-spaced, but separated horizontally, may produce imprecise information regarding the vertical component of groundwater flow

Installation of multiple piezometers closely spaced or within the same borehole is difficult, and evaluation of the data has large uncertainties. The limitations can be overcome by installing a single multi-level monitoring well or sampling device in a single borehole. Refer to the USEPA document entitled "Handbook of Suggested Practices for the Design and Installation of Groundwater-Monitoring Wells" (USEPA, 1991a) for a discussion of the advantages and disadvantages of these types of devices.

Install a minimum of two piezometer nests or multilevel monitoring wells on site. The two or more measurement points must be aligned parallel to the horizontal groundwater flow direction. During groundwater gauging in the piezometers, follow the procedures for groundwater gauging detailed in Section 6.3.

Calculate the vertical groundwater flow directions and hydraulic gradient using the water level measurements. Generate a vertical cross section across the measuring points depicting the flow net, piezometer screen depths and length of screen interval, geological units, and water bearing units.

Refer for guidance (USEPA, 1989a; Cedergren, 1977; and Freeze et. al., 1979) on construction and evaluation of flow nets in cross sections (vertical flow nets). The design of the piezometer arrangement must be discussed in the SAP.