Wastewater Lagoon Seepage Testing
Wastewater lagoons are designed to hold wastewater. Lagoons are required to be seepage tested periodically.
To understand how much liquid is seeping from a lagoon, a seepage test can be performed. Idaho's Wastewater Rules (IDAPA 58.01.16) have operational standards for allowable seepage and require municipal wastewater lagoons to meet facility-specific seepage rates corresponding to the date of construction or major modification. For certain industrial facilities, seepage testing may also be required by permit. Lagoons should be seepage tested (1) after construction, (2) as of April 15, 2012, by rule, and (3) every 10 years after the initial testing.
Testing Procedures and Data Analysis
Site conditions need to be understood, and testing procedures that demonstrate compliance with the rule and/or permit must be submitted to DEQ for review and approval before conducting the required seepage testing. Data of sufficient quality and quantity are required for a valid seepage test. Additionally, seepage testers should evaluate whether a sufficient number of calculated seepage rates exist to validate a compliance determination and to show data completeness through a statistical means that creates a defensible and objective number of seepage testing days needed for a complete dataset.
Recommendations for statistical seepage test data analysis were published in the Wastewater Lagoon Seepage Test Statistical Review, by Jeff Myers, a nationally and internationally recognized expert in characterization, sampling, statistics, uncertainty management, and data quality.
DEQ will employ all of the methods outlined in the Myers technical paper to evaluate seepage test data submitted for compliance determinations. DEQ recommends that facilities become familiar with the methods that will be used in DEQ seepage test report reviews.
The Myers technical paper recommends using free public domain software.
VSP was developed by the Pacific Northwest National Laboratory under the auspices of the US Department of Energy. VSP is designed for the nonstatistician to support multiple sampling objectives and ensure the right type, quality and quantity of data are gathered to support decisions.
DEQ strongly recommends using the VSP tool along with the other assessment metrics outlined in the Myers technical paper, to determine when sufficient data have been collected to validate calculated seepage rates for each lagoon. The VSP software generates a discomfort curve (i.e., a type of statistical power curve) to incorporate (1) a fixed regulatory limit, (2) a calculated average seepage rate, and (3) the variability around the seepage rate to create a defensible and objective number of seepage testing days needed to generate a complete dataset.
Calculations and Assessment Metrics
Beyond the VSP, DEQ held a Statistical Review Workshop for Evaluating Lagoon Seepage Rates where revised seepage testing spreadsheets and assessment metrics were discussed, and updated seepage spreadsheets that better accommodate the statistical recommendations from the Myers technical paper were reviewed. DEQ no longer requires data input at 4-hour intervals and will accept data input/calculations at 24-hour intervals as outlined in the revised calculations spreadsheet. The revised calculation spreadsheets are found below:
- Seepage Test Spreadsheet for Electronic Method
- Seepage Test Spreadsheet for Manual Method or Hook Gauge
Although DEQ only requires the calculations be conducted on a 24-hour interval, DEQ emphasizes that intermittent data should still be collected and submitted for evaluation to explain the behavior of the lagoon and evaporation pan during each test. The Meyers technical paper recommends that the interval around the datum used to calculate the lagoon and evaporation pan elevations be limited to 5 minutes (before and after the reading time) or less. DEQ is rewriting the April 2009 Guidance for Evaluating Wastewater Lagoon Seepage Rates to accommodate the Myers technical paper. The current guidance used in conjunction with the Myers technical paper recommendations for assessment metrics should assist facilities in reviewing and validating seepage data for compliance determinations.