Concrete is a sturdy material with which to build a garden pond, but it's also porous, which means that water has a tendency to work its way through concrete pond walls instead of staying where it's supposed to be. Of even greater concern is the tendency of concrete to slowly dissolve due to pH differences in water and concrete. A poorly sealed pond may release lime and other chemicals into the water. Waterproofing products counter the limitations of concrete, but vary considerably in their effectiveness, cost and ease of installation.
Integral waterproofing is the addition of latex or acrylic compounds in the concrete mixture when the pond is first poured. When combined with the water in the concrete mixture, these compounds fill the pores in the concrete after it's cured. Waterproofing additives may be more costly than other waterproofing solutions, and they're less effective in cold climates where frost may cause cracks in the concrete.
Rubber and Plastic Coatings
You can use synthetic coatings to waterproof a concrete pond that's already in place by painting them onto the surface of the pond shell. Synthetic rubber, polyurethane, epoxy resin or water-resistant latex paints provide an impermeable barrier between the pond water and the surface of the concrete. Some of the more robust and flexible coatings are capable of sealing small existing cracks, and they may also withstand some minor movement of the concrete caused by frost. In very cold climates, however, frost damage can limit the effectiveness of this solution.
Like a painted-on coating, a flexible pond liner creates a waterproof barrier over the surface of the pond's concrete shell, but unlike a surface coating, a flexible shell won't be compromised if frost heaving causes cracks in the concrete. Flexible liners are made from synthetic rubber or plastic, and in the simplest installation method, they simply lie on top of the concrete shell. Liner installation is more complicated in ponds with drains, pump inlets or returns, because the liner will have to be cut to accommodate them. Any breach in the liner must be sealed around its edges, and the site of the breach will always be vulnerable to leakage.
A more labor-intensive but potentially more effective way to use a liner to waterproof a pond is to integrate the liner directly into the pond's walls. Some contractors will install flexible synthetic liners between two layers of concrete, creating a sandwich that prevents water from seeping all the way through the wall but also protects the waterproof liner from damage. This approach will not prevent chemical leaching between the water and concrete.