Whether you’re dealing with a dry, semi-dry, or just a mildly leaky pond, take the time to see what’s there, a few inches or several inches into the dirt. You can learn a lot that will help your rehabilitation program.
Clay liners are generally considered favorably by pond owners looking for sustainable and low-impact solutions. Yet, clay has some significant contrary qualities. When you’re talking about an artificially installed clay liner, you’re usually referring to bentonite. Bentonite is a unique type of clay that absorbs many times its volume of water. When it’s compacted, it becomes both pliable and impermeable. When bentonite clay is installed over every surface of the pond bed, to within no more than a foot of the lowest expected water level, it can be a reasonably durable liner that may be able to self-heal when damaged.
If you have a pond or feature, especially a large one that appears to have a soil bottom, poke your finger a few inches below the bottom of the pond to see if there’s a liner buried under there. If there’s nothing but clay, you may have a bentonite liner. The bad news is that clay liners are highly vulnerable to diffuse damage, so identifying the source is sometimes impossible. The good news is that it’s pretty simple to install a more durable flexible liner and move on with your day.
PVC (Polyvinyl Chrloride)
PVC liners were once favored for small backyard ponds because of their low cost and straightforward installation. PVC is available everywhere, but the flexible sheets of PVC liner used in fishponds generally look like a thick version of your black, backyard, trash bag. PVC is lightweight and cheap, but it usually only lasts about ten years, and then only if you’re able to shield it from sunlight under about 12” of soil. When exposed to the sun (or even polluted air), PVC becomes brittle and will tear and crumble easily. If you have a moderate-to-small pond that’s been around for a few years, and if you see random scraps of black plastic sticking out of the ground around it, you probably have a PVC liner. The bad news is that PVC liner is very vulnerable to tears even when brand new and emptying a pond to identify the source of the problem might be more than the liner can handle. Once a PVC liner degrades enough to develop cracks, it’s generally in your best interest to replace it altogether. The good news is that you can use virtually anything and walk away with a more durable, reliable, and long-lasting liner.
EPDM (Ethylene Propylene Diene Monomer)
EPDM is a type of synthetic rubber that has been very popular for small ponds in recent years. Firestone is the most well-known manufacturer of EPDM pond liners, and you’ll see it promoted on many pond websites, both hobby and professional. Synthetic rubber is thicker than PVC, black, and substantially heavier. It’s very flexible and folds around corners with ease, but it’s so thick it generally needs to be trimmed and glued in tight spaces. It is less vulnerable to UV exposure than PVC but cannot be left exposed to the sun over time. If you have a pond with a thick black rubbery-feeling liner, this is your likely candidate. The good news is that you might be able to repair the liner, providing you can identify the exact cause of the leak. The bad news is that EPDM is highly vulnerable to punctures from sharp rocks and tree roots, even claws or beaks of wild animals. If you’re not able to ward off such random attacks, your EPDM liner will continue to spring mysterious, troublesome leaks.
HDPE (High-Density Polyethylene)
HDPE is the most commonly used geomembrane for its low price and abundant availability, making it a popular option for pond liners. HDPE can be white or black and is moderately heavy, thicker, and stiffer than PVC or EPDM. It’s more resistant to UV degradation, more resistant to tears and punctures, and lighter than EPDM. If your pond is in a northern climet, with extreme winter temperatures, is white, or has been around for 25 or more years with little trouble, it’s probably HDPE.
HDPE is an all-around solid liner, so the bad news is that you’re even having a problem. The good news is that most leaks are generally confined to a small area or a single hole that might be patchable. If you decide to replace it, it can make a highly protective, convenient underlayment for a new high-quality, durable liner.
RPE (Reinforced Polyethylene)
RPE was developed specifically for its exceptional durability against UV degradation, punctures, and tears. Any pond lined with RPE was probably built within the last 20 years, but RPE is not as well known, and the only quick way to distinguish it from HDPE is to compare weight and thickness. Unless you have other evidence, it’s safer to assume your old liner is HDPE.