Rain Changed Your Leachate: Re-Testing the Dose After Every 50 mm

Sixty millimetres of rain in thirty-six hours does something specific to a leachate sump: the level climbs 600-800 mm, the colour goes from black to weak tea, and the scoop of polymer that produced a firm, stackable mass in February now leaves a wet, glossy surface after ten minutes of mixing. Nothing about the polymer changed. The liquid did.

What Rain Actually Does to the Liquid

Infiltration through the cap dilutes leachate before it reaches the collection system. At one closed cell we sampled through a single autumn, conductivity fell from 17.8 mS/cm during the dry spell to 8.9 mS/cm after three weeks of rain, ammonia-nitrogen dropped from 2,300 mg/L to 780 mg/L, and suspended solids roughly halved. Those numbers matter because absorption is an ionic-strength story as much as a polymer story. A weaker brine lets the resin chain uncoil further, so a gram of polymer typically holds more water in September than in July — often 20-35% more.

The consequence is counter-intuitive: a fixed dose over-treats in the wet season. The gel that forms is softer and bulkier than the acceptance test allows, transport weight per tonne of waste climbs, and the receiving facility starts asking why the loads look wetter than the paperwork claims.

Three Measurements, Once a Week

You do not need a laboratory to follow this. Three field numbers, taken at the same point in the sump each time, catch almost every shift. Conductivity from a handheld meter, in mS/cm, recorded against rainfall for the preceding seven days. pH, because it moves the same way and changes how fast the granule surface wets out. And a cup test: 1 g of the production polymer into 100 mL of the actual liquid at the actual temperature, inverted ten times, read at ten minutes and again at one hour.

The decision rules follow from the same three numbers. If the cup test shows at least 25% more absorption than your established baseline on two consecutive samplings, cut the dose 10-15% and re-test the next batch. If it shows less absorption — which happens in a long dry spell, when the liquid concentrates and ionic strength climbs — raise the dose in 10% steps and stop at the point where free liquid disappears within fifteen minutes of the last turn. Two readings in the same direction trigger a change; a single reading is weather, not a trend.

A Sampling Calendar That Survives a Wet Year

Fortnightly is enough from June to August on a covered cell. Move to weekly once the wet season starts, and add an unscheduled sample within 24 hours of any single event above 50 mm. Take the sample 300 mm below the surface, not from the floating scum layer — the scum is a different material with a different dose and will mislead you in one direction only. Keep bottles in the shade; a sample left on the dashboard for two hours gives a cheerful, wrong answer.

The limit of this approach: conductivity is a proxy, not a measurement of absorption, and it will not warn you about a supplier change or a spike of oil and grease from a workshop drain. The cup test remains the decision-maker. Nor does a lower dose per cubic metre mean less polymer overall — dilution raises volume, and total consumption in a wet month often rises while the dose rate falls. Budget on kilos, not on grams per litre.

If you are re-baselining after a season like this one, the grades and typical absorption ranges we supply for variable-strength liquid are listed under SAP grades for variable-strength leachate, and we will send a 1 kg sample matched to your current conductivity band so the cup test starts from a realistic place.