California Water Science Center (CAWSC) - San Diego Hydrogeology Project (SDH)

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Project Chief: Wes Danskin
Phone: 619-225-6132
Email: wdanskin@usgs.gov


Welcome to the United States Geological Survey (USGS) San Diego Hydrogeology project website, which provides geologic and hydrologic information for the transboundary San Diego–Tijuana area, USA and Mexico. This website provides background information about the project; a variety of news items; a large amount of data, in particular for USGS multiple-depth, monitoring-well sites; geologic and hydrologic models; and additional resources including photos, illustrations, and references. Last updated: April 2016

Some additional data may be available from the USGS database National Water Information System (NWIS).

Data Accuracy and Limitations as Defined in Groundwater Technical Procedures of the U.S. Geological Survey

Link to report: https://ca.water.usgs.gov/projects/sandiego/pdfs/tm1-a1.pdf

Data accuracy and limitations measuring water levels
by use of a graduated steel tape

1. A graduated steel tape is commonly accurate to 0.01 foot.
2. Most accurate for water levels less than 200 feet below land surface.
3. The steel tape should be calibrated against another acceptable steel tape. An acceptable steel tape is one that is maintained in the office for use only for calibrating steel tapes, and his calibration tape never is used in the field.
4. Oil, ice, or debris may interfere with a water-level measurement.
5. Corrections are necessary for measurements made through angled well casings.
6. When measuring deep water levels (greater than 500 feet), tape expansion and stretch is an additional consideration (Garber and Koopman, 1968).

Data accuracy and limitations for a minimum set of
data elements

1. Altitudes determined from topographic maps are accurate to within one-half the map contour interval; latitudes and longitudes are accurate to about 0.5 second.
2. Accuracy of latitude, longitude, and altitudes determined by use of GPS are dependent on each instrument's capabilities.
3. The accuracy of the measuring point, land-surface datum, measuring point correction, and reference marks depends on the measurement method used.
4. A graduated steel or electric tape commonly is accurate to 0.01 foot.

Data accuracy and limitations for measuring points
and reference marks

The "stickup" of a well is the length of well casing above the plane of the land-surface datum (LSD).

Altitude Accuracy: Vertical Stickup

The accuracy of the measuring point (MP) or LSD altitude depends on the measurement method used. When topographic maps are used, the accuracy typically is about one-half the contour interval of the topographic map. When geodetic differential GPS methods are used, the accuracy can be on the order of a couple of centimeters. When spirit leveling is used the accuracy is dependent on the order (1st, 2nd, 3rd) of surveying and the length of the survey line and typically can vary from tens of centimeters to a millimeter or less. Limitations: A high level of altitude accuracy is not critical when measurements obtained from a single well are compared to one another. Measurement accuracy is important, but altitude accuracy is not. If water-levels are to be compared among wells, however, a higher altitude accuracy (such as from spirit leveling) may be needed.

MP Correction Length Accuracy: Vertical Stickup

The MP correction length is the distance the measuring tape travels from the MP to the plane of the LSD. The accuracy of the MP correction length depends on the configuration of the MP with respect to the LSD. In the simplest example of a well with a vertical stickup and the LSD as a monument in the well pad or a file mark on the casing, the MP correction length can be measured directly with a measuring tape. In that instance, the accuracy of the measurement is 0.01 foot. In the case when the vertical distance between LSD and the MP cannot be directly measured with a tape, such as when a protective casing prevents direct measurement, the accuracy is a function of the measurement method used. A visual estimate using a measuring tape likely will have an accuracy slightly greater than 0.01 foot. When spirit leveling is used, the accuracy can vary from tens of centimeters to a millimeter or less. MP correction length accuracy is critical because a well may have more than one MP, all of which should be referenced to a single LSD. Limitations: Special considerations must be made for a well with a non-vertical stickup, when the configuration of the MP at the well does not allow the measuring tape to hang vertically directly from the MP through the plane of the LSD.

Altitude Accuracy: Non-Vertical Stickup

The altitude of the MP of a non-vertical stickup is not used directly, but may be measured for use in combination with the LSD altitude and the MP correction length. In the case of a non-vertical stickup, the accuracy of the LSD altitude is identical to that described in the vertical case. The accuracy of a water-level altitude calculated from the MP altitude and the MP correction length is equivalent to the least accurate measurement.

MP Correction Length Accuracy: Non-vertical Stickup

When the measurement tape does not hang vertically from the MP to the plane of the LSD, the MP correction length must be computed on the basis of the measurement path length and angles of deviation from vertical. The accuracy of this MP correction length is a function of the configuration of the well and the ability of the hydrographer to determine the tape path, but likely is greater than 0.01 foot.

Reference Mark Accuracy

A reference mark (RM) is used to determine whether the MP has moved with reference to LSD and, in extreme cases, to re-establish the LSD or MP at a well, thus the accuracy of the RM should be at least equivalent to that of the water-level measurement. In most instances, this is 0.01 foot. Limitation: comparability of water-level measurements made before and after re-establishment of the LSD or MP is limited by the accuracy of the RM.

Data accuracy and limitations for measuring water levels
by use of an electric tape

1. A modern graduated electric tape commonly is accurate to +/– 0.01 foot.
2. Most accurate for water levels less than 200 feet below land surface.
3. The electric tape should be calibrated against an acceptable steel tape. An acceptable steel tape is one that is maintained in the office for use only for calibrating tapes, and this calibration tape never is used in the field.
4. If the water in the well has very low specific conductance, an electric tape may not give an accurate reading.
5. Material on the water surface, such as oil, ice, or debris, may interfere with obtaining consistent readings.
6. Corrections are necessary for measurements made from angled well casings.
7. When measuring deep water levels, tape expansion and stretch is an additional consideration (Garber and Koopman, 1968).

Data accuracy and limitations for documenting the
location of a well

1. GPS instrument accuracy varies. Handheld, Wide Area Augmentation System (WAAS)-enabled GPS instruments typically are accurate within a few meters horizontally. Instrument manuals and field tests should be used to confirm instrument accuracy.
2. USGS 7.5-minute latitude-longitude scale should be accurate to 0.5 second or about 50 feet.

Data accuracy and limitations for recognizing and
removing debris from a well

1. Debris that is present in a well can affect the plumbness of the tape and cause errors in water-level measurements.
2. The quality of water-level data from a well is directly related to well maintenance. 3. Success rate for this procedure increases with increasing well diameter and decreasing well depth.

Data accuracy and limitations for estimating discharge
from a naturally flowing well

1. Under ordinary field conditions, with reasonable care, measurements may be made in which the error seldom exceeds 10 percent.
2. Not accurate for small flows of 30 gallons per minute or less, or when the crest of the flow is less than 1.5 inches. For small flows, connect a pipe tee to the top of the well casing and measure the well discharge with a bucket and stopwatch.
3. The most accurate estimated discharge will be obtained when the pipe is truly vertical.

Data accuracy and limitations for measuring water levels
in a flowing well

1. Low-pressure head measurements are most feasible with heads less than 6 feet above land surface.
2. With care and experience, low-pressure head measurements can be measured to an accuracy of 0.1 foot.
3. Accuracy is a function of calibration, maintenance, and the quality and range of the pressure gauge. Highpressure head measurements using a pressure gauge can be as accurate as 0.1 foot, but may only be accurate to 1 foot or more, depending on the gauge accuracy and range.
4. A pressure gauge is the most accurate in the middle third of the gauge's range. Never let the well pressure exceed the altitude/pressure gauge limits.
5. Never connect a gauge to a well that uses a booster pump in the system, because the pump could start automatically and the resulting pressure surge may ruin the gauge.
6. Closing or opening a valve or test plug in a flowing well should be done gradually. If pressure is applied or released suddenly, the well could be permanently damaged by the "water-hammer effect" by caving of the aquifer material, breakage of the well casing, or damage to the distribution lines or gauges. To reduce the possibility of water-hammer effect, a pressure-snubber should be installed ahead of the altitude/pressure gauge.
7. Ideally, all flow from the well should be shut down so that a static water-level measurement can be made. However, because of well owner objections or system leaks, this is not always possible. If the well does not have a shut-down valve, it can be shut-in by temporarily installing a soil-pipe test plug on the well or discharge line.
8. If a well has to be shut down, the time required to reach static pressure after shut-in may range from hours to days. Since it may be impractical or impossible to reach true static conditions, record the shut-in time for each gauge reading. During return visits to a particular well, it is desirable to duplicate the previously used shut-in time before making an altitude/pressure-gauge reading.

Data accuracy and limitations for measuring water levels in wells and piezometers by use of a submersible pressure transducer

1. Water-level measurements for the in-place calibration of pressure transducers should be made to the nearest 0.01 foot.
2. The accuracy of a pressure transducer differs with the manufacturer, measurement range, and depth to water. The measurement error and accuracy standard for most situations are 0.01 foot, 0.1 percent of range in waterlevel fluctuation, or 0.01 percent of depth to water above or below a measuring point (MP), whichever is least restrictive.
3. Pressure transducers are subject to drift, offset and slippage of the suspension system. For this reason, the transducer readings should be checked against the water level in the well on every visit, and the transducer should be recalibrated periodically and at the completion of monitoring.

Data accuracy and limitations for conducting an
instantaneous change in head (slug) test with a mechanical
slug and submersible pressure transducer

1. The accuracy of a slug test is a function of many factors, including well construction, field procedures, and analysis method. Rapidly changing the water level in a well can be done by submerging an object (slug) in the water, causing the water level to rise instantaneously. Displaced water will move from the well to the geologic formation until the hydraulic head falls to the original static or equilibrium level. This is called a falling head test or "slug in test." After the water level reaches equilibrium, quickly removing the slug causes the water level to fall instantaneously. Water will move from the formation into the well until the hydraulic head returns to the equilibrium level. This is called a rising head test, "slug-out test," or bailer test. Because the early-time data for these tests are most important for the subsequent analysis, the data logger should begin collecting data just before the slug is submerged or removed from the well. The initial time can be adjusted during analysis, but the logger must be collecting data at a frequency of at least several samples per second when the water level begins to change. After the first minute or two of data collection, the sampling interval can be increased. Data loggers designed for aquifer tests and slug tests frequently have internal programs that allow for rapid data collection at early time and gradual increase of the sampling interval over time (a logarithmic time scale).
2. Some transducers have more rapid recording rates than others. If the slug test is being done in a formation of high hydraulic conductivity, select a transducer that can transmit at very small time increments (tenths of a second).
3. Due to the accuracy limitations of slug tests, results should be reported to one significant figure.