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GRI-GM13 requires 2.0 to 3.0 percent carbon black in an HDPE sheet, dispersed to Category 1 or 2 in at least 9 of 10 views. That loading is a UV screen, and together with the antioxidant package measured as a 100 minute minimum standard OIT under ASTM D3895, it is most of what keeps an exposed liner from going brittle. A buried liner barely draws on that reserve. A lagoon liner lives on it.
That is the real difference between lining a lagoon and lining almost anything else. A landfill cell gets covered by waste. A canal or a covered reservoir gets soil or a cover over the sheet. A lagoon liner sits open to the sky for its entire service life, and the band above the waterline takes the worst of it: full sun, daily thermal cycling, wetting and drying as the level swings, and wind. Choosing a lagoon liner material is mostly an argument about that freeboard band. For most municipal work, the wastewater chemistry is a secondary input.
HDPE, and why it is the default
HDPE holds the default position on lagoons for the same reasons it holds it almost everywhere: broad chemical resistance, good UV stability once the carbon black and antioxidants are in the resin, and commodity economics across acres of open floor. Typical wastewater lagoon specifications land between 40 and 80 mil, with textured sheet on the slopes where interface friction governs veneer stability. At 60 mil, GRI-GM13 sets the testable floor at 126 lb/in width tensile yield, 42 lb tear, and 108 lb puncture, so a submittal either meets published numbers or it does not.
The cost of that chemistry shows up in the field. HDPE is stiff and it moves with temperature, so sheet placed on a hot afternoon pulls tight overnight, and wrinkles are managed during placement rather than corrected later. The crystallinity that buys the chemical resistance also brings stress cracking as a long term failure mode, which is why GM13 carries a 500 hour requirement under ASTM D5397. On a lagoon, the stiffness is felt at the slope breaks and around every penetration.
LLDPE, for lagoons that will move
Relining an earthen lagoon that has been in service for decades is a different problem from lining a new cell on engineered subgrade. The floor has settled unevenly, the slopes have crept, and the liner has to conform to ground that is not going to be made flat. LLDPE is built for that case. GRI-GM17 requires 800 percent break elongation and a 30 percent minimum axi-symmetric break resistance strain per ASTM D5617, a property HDPE carries no requirement for at all, and it caps the 2 percent modulus at 2,400 lb/in width to screen out resin that is too stiff to qualify.
Flexibility has a price at equal thickness. At 60 mil, the GM17 puncture minimum is 66 lb against HDPE's 108 lb, and tear is 33 lb against 42 lb. The softer sheet conforms to a rough subgrade and is also easier to hurt on one. If the floor has rock or old debris in it, subgrade preparation or a cushion geotextile is part of the LLDPE decision rather than an upgrade to be argued about later.
Reinforced sheet for small lagoons and complicated geometry
Reinforced polypropylene is a different kind of product: polypropylene plies laminated over a woven polyester scrim that carries the load, with 36 and 45 mil the common reinforced grades. Because it is a coated fabric, it is tested through the ASTM D751 family rather than the sheet plastic methods, and the scrim arrests tear propagation, which is worth a great deal on exposed sheet under wind load. RPP folds instead of rolling, so panels are welded in the shop and shipped ready to deploy. ECA fabricates custom panels up to 25,000 SF or 4,000 lbs each.
On a small lagoon with a lot of pipe penetrations, that changes the schedule. Most of the seaming happens under a roof with factory quality control, and field welding is reduced to the joints that genuinely have to be made in the field. The reason not to default to reinforced sheet is area. Across acres of open floor, the field weld economics of polyethylene are hard to argue with, and reinforced membrane earns its place where geometry, exposure, or schedule is the binding constraint.
When the effluent actually governs
Industrial lagoons holding hydrocarbons, solvents, or aggressive process water invert the usual order, and that is where a reinforced ethylene interpolymer alloy such as XR-5 belongs. The 30 oz/yd2 grade publishes 550 lb minimum grab tensile in each direction, 800 psi minimum hydrostatic resistance, and a low temperature bend that passes at minus 30 F. It is specified against the manufacturer's data sheet rather than a consensus GRI standard, so the submittal review is a different exercise: the compatibility statement is checked against the actual effluent instead of against a published minimum property table.
PVC belongs in the conversation mainly to be ruled out. ASTM D7176 covers non-reinforced PVC for buried applications, and roughly a third of the formulation by weight is plasticizer, which is what makes an unreinforced sheet that flexible. Plasticizer also migrates out under sun and heat. At 40 mil, PVC is a legitimate lagoon material under a cover or soil. In the freeboard band it is the wrong specification.
What the material choice does not solve
No polymer solves gas. Polyethylene sits at 0.94 g/cc or lower and water sits at 1.0, so the only thing holding a lagoon liner down is the liquid on top of it, and biological gas generated in the subgrade will lift any of these materials into a whale. That is a design problem answered by a venting layer, usually a drainage geocomposite specified on transmissivity per ASTM D4716, and by where the vents terminate. Choosing a tougher sheet does not substitute for it.
The same is true of the details around the edge. An anchor trench sized for wind uplift, ballast where the design calls for it, and textured sheet where slope stability governs will do more for a lagoon liner's service life than moving one grade up the material list.
How the decision usually goes
For a municipal storage lagoon on competent subgrade, 60 mil HDPE with textured sheet on the slopes is the answer more often than not, and the engineering hours are better spent on subgrade preparation, the venting layer, and the anchor detail than on relitigating the polymer. LLDPE earns the change when the geotechnical report predicts settlement. Reinforced sheet earns it when the lagoon is small, penetration heavy, or on a schedule that will not absorb weeks of field welding, and reinforced EIA earns it when the effluent rather than the sun is the governing exposure. Facilities get into trouble by running that order backwards, buying chemical resistance the wastewater never needed and then losing the liner in the freeboard band.