Whole-Pond Seepage Testing Using The Overnight Water Balance Test Method
Updated: Dec 5, 2018
KLA Environmental Services routinely performs whole-pond seepage tests using technology developed by Dr. Jay Ham. Dr. Ham pioneered this technology during his tenure at Kansas State University. This test method determines the seepage rate of the entire pond using research-grade instruments. It is more accurate than sampling methods, such as the standpipe and soil core test procedures, and does not require the pond to be drained or have sediment removed. Testing is typically completed during two consecutive nighttime periods, so wastewater can continue to be discharged into the pond during working hours. We have completed more than 100 seepage tests on wastewater storage ponds and lagoons serving livestock and industrial facilities.
Seepage tests are typically required to:
Fulfill facility permit requirements
Document the seepage rate of modified or newly constructed wastewater impoundments
Verify protection of sensitive groundwater area
The seepage rate is determined using the methods described by Ham (1999), Ham and DeSutter (1999) and Ham and Baum (2009). Data from test periods when no precipitation, waste inflow or waste outflow occurred are used to calculate the seepage rate. The following equation summarizes this calculation:
S = [(Dt2 – Dt1) - SE]/(t2 – t1)
where S is the average seepage rate (mm/d); Dt2 and Dt1 are the relative liquid levels of the pond at the ending and beginning of the test, respectively (mm); SE is the cumulative evaporation (mm); and t2 and t1 are the ending and beginning times of the test, respectively (d).
The instruments and equipment used in the test are identical to that described in Ham and Baum (2009) and/or complied with the criteria presented therein. The instruments and equipment are also deployed and operated in accordance with Ham and Baum (2009). This technique has been designated as the overnight water balance test.
A datalogger and pressure transducer are employed to measure the liquid level changes in the pond. Two sensors are deployed. The purpose of this redundancy is to improve data quality and detect bias errors or malfunctions. Each sensor is placed in a stilling well that is attached to a steel T-post and positioned such that the sensor is recording at a liquid depth that approximates the middle of the depth range. The datalogger and pressure transducer have an accuracy and sampling frequency that complies with the guidelines for pressure-based liquid level recorders detailed in Ham and DeSutter (1999 and 2003).
Meteorological conditions are measured with instruments contained in a portable weather station positioned on the berm or adjacent to the liquid surface of the waste storage pond. Prevailing wind direction, site conditions and air flow characteristics are considered when selecting a location for deployment of these instruments. Ultrasonic measurements of wind speed and direction, and shielded measurements of air temperature and relative humidity, are obtained with a Gill MetPak weather sensor. A tipping bucket rain gauge is used to measure precipitation.
The pond liquid surface (pond free-water surface) temperature is measured with a precision infrared thermometer (IRT). The IRT is mounted on a steel T-post installed in the pond approximately 2 m from the edge of the liquid surface. A PVC pipe extension is secured to the top of the post to position the IRT approximately 1.5 m above the liquid surface with a view angle of 45 degrees. IRT data are corrected for surface emissivity and downwelling long-wave radiation.
All sensors used at the site are sampled at 0.2 Hz and the data are stored as 30‑minute averages using computerized data acquisition equipment. Remote communications are provided by cellular telephone and modem, allowing data to be monitored and uploaded to computers.
Evaporation is determined using the bulk-transfer meteorological equation (Ham, 1999). This technique uses short-time-interval meteorological data and measurements of pond surface temperature to calculate evaporation every 30 to 60 minutes. Ham (1999) compared five methods for measuring lagoon (pond) evaporation and found that the bulk‑transfer method was the most accurate. Ham and Baum (2009) confirmed that this method of determining evaporation was also applicable to the overnight water balance test by comparing it with direct measurement of evaporation using eddy covariance.
Refer to Ham and Baum (2009) for more detailed information pertaining to the equipment and methods used in this seepage measurement technique.
The test will be conducted in accordance with guidelines provided by Ham and DeSutter (2003) and Ham and Baum (2009). The following criteria are used to determine if the test data are acceptable:
Average nighttime evaporation rates are less than 0.25 mm/hr (6 mm/d) during the test periods.
Duration of each test period is equal to or greater than eight hours.
Depth measurements at start and end of each test period are taken when wind speed is less than 3 m/s at 1 m above the liquid surface.
At least two tests are conducted on consecutive or near-consecutive nights.
Scum, crust or debris that would inhibit evaporation covers less than 5% of the liquid surface of the pond.