Basins, Ponds and Landfill Caps

With difficulty, and that is exactly why the choice of system should be made with this moment in mind. The first step is not to look for the hole but to establish whether the leak is real: measure the drop in level over several days and discount evaporation, which in summer on an exposed basin can exceed a genuine leak, and rainfall, which can mask it entirely. If the residual drop confirms the leak, the second step is to narrow the zone: watch whether the level stabilises at a certain height, because in that case the hole is right there, on the bank, and that is the lucky situation. If instead the level keeps dropping to the bottom, the damage is low down and the search becomes serious: geoelectrical methods exist that locate where current passes through the liner, and they are the only alternative to emptying. It is worth saying plainly: on a naturalistic basin buried under soil and planted, the cost of finding and repairing can approach that of the lining itself, and that is the concrete reason why bentonite is chosen in those situations.

Yes, and it is one of the commonest ways a naturalistic basin stops working years later, when nobody any longer connects the problem to the planting. Roots do not pierce the liner by sudden mechanical force: they work into points that are already weak, typically along laps and at folds, and widen them as they grow. The process is slow and therefore insidious, because the loss appears gradually and gets blamed on evaporation for a season or two. There are three countermeasures and they must be decided at the outset. The first is to choose species with a contained root system and keep them away from the edge, always the most vulnerable zone. The second is to interpose a dedicated root barrier, which is not the same thing as a protective geotextile: the geotextile stops stones, not roots. The third, and the most effective where vegetation is part of the design, is to use a system that tolerates damage rather than one that merely suffers it, which brings you back to the choice made in phase 02.

Constructionally yes, in purpose no, and the difference is worth understanding because it explains why bentonite is almost always chosen there. The sequence is identical: shaped and regulated surface, protective layer, impermeable barrier, upper protection, soil cover and seeding, with perimeter anchorage. What changes is the direction of the problem. In a pond you keep water in; in a landfill cap you keep rainwater out, because water entering the waste body becomes leachate, which then has to be collected and treated, at a cost that lasts decades. The mechanical context changes too: a landfill body settles greatly and unevenly for years, so the lining must tolerate deformations well beyond those of a basin on natural ground. It is precisely those two conditions, likely damage and a lining buried forever under soil and vegetation, that make a self-sealing material preferable to a barrier which, once damaged, stays damaged.

It is needed, for two distinct reasons worth keeping separate because they have different remedies. The first is protection from ultraviolet light. Exposed synthetic materials age many times faster than the same materials covered: they lose elasticity, stiffen and eventually craze. That is why, in a basin a few years old, the degradation always shows in the band above the water line and never below, and it is also why the edge fails first. The second reason is mechanical and gets underrated: beneath the liner, over time, gas produced by the ground and groundwater accumulate and push the lining upwards. With no weight to counter them, blisters form that lift the liner off the bottom, and the phenomenon worsens on its own because a blister, once formed, collects more gas. Ballast solves both at once, and it should be checked above all on the slopes, where it tends to be reduced for ease of laying and is in fact more necessary than on the bottom.