A study of waterways flowing into Lake Erie has Ontario's environment ministry warning it "cannot support further greenhouse development" in the province's southwest unless wastewater treatment rules are tightened.
For now, the province said Wednesday, it plans to work with greenhouse growers in the region to "inspect and assess their operations, and to ensure our waterways are protected by undertaking measures to reduce phosphorus in their discharges."
The environment ministry added that it "will continue to test affected streams to monitor water quality progress."
When greenhouses' recycled irrigation water can no longer be used for growing plants, it must be treated before being discharged, the province said.
In particular, phosphorus from untreated discharges is a "major contributor" to algae blooms which in turn can cause eutrophication -- that is, choking out animal life in the water by depriving it of oxygen.
Southwestern Ontario, the province said Wednesday, already has the highest concentration of vegetable greenhouses in Canada.
The environment ministry has released the results of a two-year (2010-11) greenhouse wastewater monitoring project showing a "decrease in water quality" between upstream and downstream points in Ontario's Sturgeon Creek, Lane Drain and Lebo Drain watersheds -- all of which flow into Erie, the southernmost of the five Great Lakes.
One waterway receiving greenhouse wastewater discharges showed average phosphorus levels 38 times as high as nearby creeks, the province said.
The province said its water quality test results suggested 65 per cent of all greenhouses sampled had discharged high to "extremely high" levels of nitrate, phosphorous, potassium and copper -- findings that typically indicate "nutrient" water and/or "process water" -- either directly into watercourses or "indirectly" via control ponds.
Monitoring of greenhouse outfalls has shown greenhouse discharge is responsible for "direct degradation of water quality" in Sturgeon Creek and Lebo Drain, the province said.
Those two watercourses are "the most polluted waterways in Ontario with respect to phosphorous and nitrate" and in the top five most polluted with respect to potassium and copper, the ministry said in its study.
The province can legally enforce appropriate treatment to make sure a watercourse keeps the "fundamental water chemistry" needed to protect indigenous aquatic life and as many water uses as possible -- that is, to protect the waterway's "assimilative capacity."
Sturgeon Creek and Lebo Drain, the study said, "are sufficiently impacted by nutrient inputs such that no further assimilative capacity is available. The (environment ministry) cannot support further greenhouse development within either watercourse without appropriate treatment technology in place."
Some other small watersheds within the Kingsville and Leamington areas are also "heavily impacted by greenhouse inputs and are contributing to loadings of nutrients into Lake Erie," the study said.
The discharges from greenhouse operations are compromising, "to a significant extent," Ontario's commitments to make point-source reductions of phosphorous into Lake Erie, the study concludes.
Those commitments, the study said, were made under the Binational Great Lakes Water Quality Agreement, as well as the federal-provincial Lake Erie Lakewide Management Plan.
"Protecting the Great Lakes and their tributaries is vital to ensuring that everyone has access to safe drinking water, clean beaches and a healthy environment," Environment Minister Jim Bradley said in the province's release Wednesday.
"We are determined to work with greenhouse operators to improve their environmental performance."