Perhaps your soil has 30% clay and the dugout you’ve planned doesn’t need a liner at all. Ponds don’t get easier or less expensive than that, get digging! For the rest of us with “great drainage”, let’s look at the costs and benefits of plastic liners, imported clay, and the little known but promising method of fermented gley.
Elk Root Conservation Farm (ERC Farm) near Winlaw considered all these options and have just finished building a 3200 sqft pond in soil that’s 80% sand and fairly gravelly. They tilled and packed bentonite clay into the subsoil and topped it with a protective layer of compacted road crush. ERC Farm’s Director, Kate, and volunteer pond builder, Ryan, have their fingers crossed for long-term success of this innovative biodiverse catchment pond system which has filled up and is holding 350 cubic meters of November rain.
Synthetic Liners
Kate explained that EPDM rubber and plastic polyethylene liners were more expensive and not environmentally-friendly. First, liners are sized about 5% bigger than the area covered to allow for overlap. Next, all plastic liners do best in a “geotextile sandwich” with protective fabric above and below to keep sharp rocks at bay. Even then, a puncture could spell doom. How long will those synthetics last?
By contrast, clay is “self-healing”. Bentonite can even be added “wet” to seal up a leaking pond that’s full, albeit in double or triple the quantities required when clay is added to the dry base like ERC Farm has done.
To line ERC Farm’s pond with rubber, 3600 sqft of EPDM would come to $10,000, plus $5000 for 7200 sqft of geotextile, plus delivery. For plastic, a good quality 30-mil polyethylene comes in at about $5000, plus geotextile. Delivered, materials for a synthetic liner would have come to $4 to $6 per square foot.
Bentonite Clay
Ryan — an engineer before he was a farmer — went with 5 lbs/sqft of clay, exceeding industry guidelines of 4 lbs/sqft for sandy soil. More is better, of course, to avoid thin spots that might leak. The delivered clay (8 tons) and crush (30 yards) cost about $10,000, coming in at $3 per square foot in delivered materials before installation.
Best practice requires you to till or track-roll the clay layer, and generally it’s a good idea to backfill a layer of compacted subsoil. Ryan chose to add 6″ of road crush on the slopes and about 2″ in flat areas to protect the clay layer. Will it work? Well, Ryan did conduct multiple tests to determine a best practice for the conditions at ERC Farm. “We’ll see,” he says!
You might also consider whether your dugout is going to run dry, which can crack the clay layer and lead to leaks. In this case, plastic might be the better option, or cover the clay layer with a lots more subsoil to keep it moist all year. ERC Farm’s pond will stay relatively full with seasonal fluctuations to mimic a wetland system, because their first goal is to grow biodiversity on-farm, so clay was the obvious choice.
Clay-infused geotextiles known as “geosynthetics” are an alternative to powdered clay. ERC Farm did investigate this option and found it to be costly and still required similar amounts of unnatural fabrics and layers as other liner options. That said, these fabrics are used on some enormous reservoirs and presumably share some of the benefits of both clay and roll-out liners, such as steeper slopes (more on that below.)
Gley
ERC Farm’s next pond will be more experimental. Gley, an old Russian technique popularized by Bill Mollison in his Permaculture Designer’s Manual but rarely seen in practice, promises to be simple and, especially for farms, inexpensive.
A compacted layer of wet organics ferments under compacted backfill to create an impermeable biological slime. Green, sappy plants and cow or horse manure are classically recommended, but any wet, biological material that would normally compost should work. You could use old mouldy hay that’s no good for feed, leaves raked by suburban neighbours, fresh-cut hay straight off the field, or algae slime skimmed off another dugout.
For ranchers, the best option could be to bale-graze your herd on good hay in the excavated pond, adding bales until the leftover mix of trampled hay and manure is about 6 inches deep (and the cows are well-fed!) While we’re on livestock, small ponds have traditionally been sealed by mud-wallowing pigs.
Bulky materials should be shredded to make sure the layer can compact. Woodier, high-carbon materials like chopped straw and wood shavings should be avoided as the ferment likely needs more nitrogen to work, but perhaps this is worth some more experiments?
Opinions vary on how much material you need. Compacted, some sources say 3″ is enough: perhaps an inch of manure and a couple inches of crushed wet greens. Bill recommends 6″ as the minimum.
Whatever you use, get it moist, pack it down, and cover it up. You could use silage tarp or cardboard weighed down for a couple weeks to ferment away, then fill up the pond and peel off the tarps. Or it may be simpler just to backfill subsoil on top of the greens and leave it like that for good.
What will it cost? Let’s say you bought rained-on hay at $100 per large square bale ($125/ton). Each bale can cover about 180 square feet with 6 inches of highly compacted material. Using a bale on every 160 square feet (7” deep), ERC Farm’s pond would have required 20 bales for a cost of $2000, or about $0.65/sqft in materials. Maybe you can only find good hay at $300/ton? Then we’re closer to $1.50/sqft in materials, or $5000 for ERC Farm’s pond, but still half the price of clay-with-crush.
Slopes, Volumes and Evaporation
But wait, I’ve skipped one more piece of vital information: Gley needs gentle slopes. Typically 3:1 is considered the maximum for gley, and Bill recommends 4:1. Clay-lined ponds can get away with being steeper, up to 2:1, while synthetic liners allow for 1.5:1 slopes or sometimes steeper.
Gentle slopes are fantastic for many reasons, starting with safety (steep slopes are a drowning hazard) and biodiversity, allowing for a wide fringe of water-cleaning and habitat-creating wetland plants appreciated by dragonflies, turtles, and frogs.
But shallow slopes limit the pond’s volume and depth, particularly for small ponds. Depth is important because a pond is likely to evaporate up to 3 feet of water through a hot growing season, depending on the pond’s shade and wind protection.
For example, while ERC Farm’s small 0.07-acre pond with 2:1 slopes can hold 350 m3 of water and is an average of 4-feet deep (6-feet in the middle), had it been built with 4:1 slopes it could only have been an average of 2-feet deep and held half the water.
To store 20,000 m3 with 2:1 slopes, you’d need a one-acre pond, 18-feet deep on average (30-feet in the middle). To get the same volume with 4:1 sloped sides, the pond must be 1.6 acres and can only be about 12-feet deep on average (25-feet in the middle).
Considering this, if you need some slopes to be steep, you might get really experimental and consider a hybrid approach with synthetic liners for steep walls and clay or gley on the flat bottom, but be careful how you seal the overlap.
Costs for Stored Water
It’s very helpful to frame costs in terms of the volume of water stored. Clearly, bigger ponds are much more cost-effective.
At ERC Farm, the clay and crush for a 0.07-acre pond that holds 350 m3 came to $10,000, or about $30/m3 for materials. Synthetic liners would have come in around $40 to $60/m3. With gley, ERC Farm’s small pond could only hold 200 m3 at 3:1 slopes, so would likely have been $10/m3 for half-price hay, or $25/m3 for good hay.
$30/m3 is really inexpensive compared to a plastic or concrete tank that typically costs around $1 per US gallon, or $265/m3! It would take nine massive 10,000-gallon tanks to equal ERC Farm’s pond, and plastic tanks can only breed mosquitos, while a pond can be paradise for mosquito-eating turtles, frogs, salamanders, dragonflies, and so much more.
But $30/m3 is also much more expensive than we’d expect for materials to line a larger pond. For a 20,000 m3 pond, a one-acre clay liner might cost $120,000 ($6/m3), and a plastic liner around $180,000 ($9/m3). Gley on a 1.6-acre pond holding the same volume of water might cost $40,000 to $120,000 in hay (roughly $2 to $6/m3).
Add to these your estimates for labour and excavation costs. As a rough range, a small pond might be $15/m3 or more to excavate, while a multi-acre “lake” could cost less than $2/m3, but this is best quoted by an earthworking company. Cost is impacted by site conditions, scale, and what you plan to do with the fill — make terraces, fill a hole, or transport it off-site?
A word of regulatory caution while considering what you do with that extra soil: Don’t build up the sides higher than grade. This “turkey nest” will turn your dugout into a dam and will trigger the Dam Safety Regulation.
Dugout Benefits
Is the cost of storage worth it to you? To keep it all in perspective, a highly productive acre in the Kootenays requires about 2500 m3 of water to irrigate by sprinkler, depending on the crop, microclimate, and soils.
Perhaps you have plenty of dryland and a creek, but no water license? Your best bet for a license (though still a long shot, unfortunately) may be to go for freshet water, drawing off from March into May to use all summer long. Such spring storages can make a lot of sense for both farms and ecosystems.
After evaporative losses, and leaving plenty of water in the bottom and the wetland edges you designed in to keep the frogs happy, a 20,000 m3 pond might irrigate 6 productive acres all summer long.
If the pond cost you $200,000 to build, and irrigation on 6 acres allows you to net $20,000/year from, say, expanding an orchard, the pond will have paid itself off in 10 years. Better yet, if you get some “Beneficial Management Practice” funding for 75% of that, you’ll be golden in fewer than 3 seasons.
And the dragonflies, turtles, and frogs will thank you forever, so fill those pond edges with trees, shrubs, and wetland plants!
