DRAINAGE AND THE ENVIRONMENT

Making denitrification work

Putting specifications and recommendations on the design elements of denitrification systems to reduce nitrate outflow from agricultural lands has been the target of several research initiatives.

A dispersion sandwich trench installation near London, Ontario.	The finished beneath-stream bioreactor.

Now, under the auspices of Greencover Canada, which is part of the Agricultural Policy Framework of Agriculture and Agri-Food Canada, there has been some progress in this direction in Ontario using three different layouts.

Installing a dispersion sandwich trench beneath a stream.

In 1994, two colleagues at the University of Waterloo Earth Sciences department began work investigating which media worked best in reducing nitrate levels in tile outlet and groundwater flow. “Dave Blowes and Dr. Will Robertson installed 45 gallon bioreactor barrels and diverted tile water through them,” says Craig Merkley, project manager at the Upper Thames River Conservation Authority. “They found that woodchips were the most effective medium.”

Since then, things have progressed. Robertson now has design criteria (see Figure 1) for a woodchip filled reactor trench, called a dispersion sandwich. It can be used to treat tile water in either lateral flow or upflow designs. As well, two other alternative designs are being developed. Two examples of this type have recently been installed in Ontario, one in an agricultural application, the other in a municipal golf course application.

How the groundwater interceptor system works.	A cross-section shows how the trench is constructed.A cross-section shows how the trench is constructed.

The second concept is to use a woodchip filled trench to intercept shallow groundwater as it moves towards a flood plain. This design allows a groundwater plume to flow up through the bioreactor before entering an open watercourse.

Schematics of the lateral flow and upflow trench systems indicate flow rate (hours).

The latest concept is to construct a trench below stream-base grade in an open watercourse in such a way that stream flow is diverted beneath the stream bed, through the woodchip medium, then reintroduced to the main stream flow downslope.

In all cases, says Merkley, the idea is not to provide capacity to handle maximum or peak flows, but to process most of the water at other times. “Our studies have shown we can reduce nitrate levels in some of these systems from as high as 98mg/L to zero during the warm season,” he says. He believes, on average, the systems could reduce nitrate levels at discharge by 50 percent.

The working group, which now includes University of Waterloo masters candidate Peter Van Driel, has discovered that as well as removing nitrates from groundwater flow, the systems have shown they can remove other potential pollutants to surface waters, such as perchlorate.

Figure 1. Robertson has projected the volume of bioreactor media required to reduce nitrates.

An undesirable outcome of the bioreactor process is that in removing the nitrates, outflow water is depleted of oxygen and high concentrations of this water entering a stream is detrimental to the habitat of fish and other creatures. To counter this, stone weirs and a switchback aeration channel for outflow water has been developed. This channel provides a riffle over stones to effectively oxygenate the water before it discharges into a stream.

Consultations throughout the investigations and proving phases included numerous drainage contractors, municipal drainage superintendents, engineers and officials from various government departments, universities and agricultural producer groups. Now that the concepts are proven, Merkley anticipates more interest in installations on a wider scale. A series of four units are being installed near Kincardine, Ontario, in the Pine River watershed. The effectiveness of these and installations already complete will be monitored closely and if they are as successful as Merkley, Robertson and their colleagues expect, it is very likely other regions will soon begin promoting their use. -30-

Header installation for a beneath-stream installation.	A switchback re-aeration channel.