Effluent pond seepage testing

About the Effluent Pond Seepage Testing COP

DairyNZ has commissioned the development of the Effluent Pond Seepage Testing Code of Practice (COP) to provide a standardised test methodology for undertaking pond leakage assessment on effluent containment structures, such as an inground ponds. Councils have discretion in how they assess compliance, but this Code of Practice provides a NZ effluent industry accepted benchmark that councils can use as a reference.

This Code is intended to provide good practice guidelines for professional engineers and other technical specialists undertaking seepage testing and measurement for effluent ponds. A standardised approach ensures farmers receive a consistent level of service across the sector.

Pond drop testing for effluent storage ponds

The Pond Drop Test (PDT) is a precision test method developed to check whether an effluent storage pond is leaking beyond acceptable limits. The Pond Drop Test is the industry standard for confirming a pond’s sealing performance and is especially useful for demonstrating compliance.

A well designed and well constructed pond is built to minimise the loss of liquids to within the generally accepted maximum seepage rate or specific regional council requirements. A Pond Drop Test is used to continually verify that the pond is performing within acceptable seepage limits over time.

How a Pond Drop Test works

• The test measures the change in pond water level over time, using high precision equipment that can detect changes as small as a fraction of a millimetre.
• All inflows and outflows to the pond are isolated, and the pond must be at least 75% full before the test begins.
• The test typically runs for a minimum of 50 hours, with at least 36 hours of ‘good’ data required for analysis. A longer test duration to increase accuracy is reccomended.
• Environmental factors such as rainfall and evaporation are carefully measured and corrected for, often using a floating evaporation pan and an on site rain gauge.
• Wind speed is monitored, and data sections experiencing periods of high wind or rainfall are excluded to reduce error.
• Tests are carried out by specialist providers using purpose built equipment, and reported results are reviewed and signed off by a suitably qualified engineer.

Key steps involved in the process

Passing criteria

The generally accepted maximum seepage rate is no more than 1.0 mm per day, and the test result must have a total error of less than ±1 mm. Always check your regional council’s specific requirements.

How often does my clay pond need a retest?

Pond Drop Testing was originally developed for existing clay lined ponds to assess seepage where there were no other generally accepted criteria. While the test can be used for other storage pond types, clay ponds remain the most common application.

As good practice, DairyNZ recommends undertaking a pond drop test every five years, or sooner if there have been significant changes to the pond. This is an indicative guideline only. The actual frequency of retesting should be risk based and farm specific.

A test may be required before five years have elapsed to confirm sealing performance, including where there are:

• Large cracks, erosion or visible liner damage.
• Trees or shrubs on the embankment that may compromise the liners integrity.
• Solids have been removed from the base of the pond using a digger, and where the liner may have been damaged.
• The pond has been left empty for an extended period.
• There is excessive vegetation growth on the base or sides of the pond.
• Unexpected soft or wet ground around the banks of the pond.

Deciding whether to invest in a test

DairyNZ encourages farmers to assess the condition of their pond before investing in a pond drop test. The Code of Practice outlines methods that can be used to support this assessment.

Consider the current condition of your existing pond and weigh up whether a PDT is the best option compared with relining or building a new pond. The PDT may help determine how quickly you prioritise upgrading your pond compared to other investments in your farm plan.

As part of good practice, regular farm assessments can help track pond performance over time. These assessments should be recorded in your farm plan as part of routine maintenance. The Code of Practice also provides guidance on the types of checks that can be carried out between full Pond Drop Tests.