Efforts to reduce pollution from agriculture paying off slowly

Monday, September 25, 2017

Efforts by farmers to reduce the amount of fertilizer that reaches drinking water sources can take years to have a positive impact, according to a recent study from the University of Waterloo.

Soil runoff from a local field.The study found that that, depending on the type of terrain, efforts to reduce algae-causing nutrients such as nitrogen from reaching water sources such as the Great Lakes and can take decades to bear fruit.

“In recent years, people involved in agriculture have gone to great lengths and expense to try to reduce the impacts of fertilizer on drinking water,” says Nandita Basu, an associate professor of Earth and Environmental Sciences at Waterloo. “What this study tells us, is that it can take a very long time to see the effects of pollution-reduction efforts, and that we must be careful to not rush to judgement. The fact is, it can take up to 30-40 years for our efforts to have the desired impact.”

The study, which was recently published in Environmental Research Letters, involved reviewing more than 50 years-worth of environmental data from 16 subwatersheds within Southern Ontario’s Grand River Watershed. The data included not only water quality data, but also agricultural census records of fertilizer application and livestock production going back nearly 120 years. 

“It was a monumental effort. Each data point represents a huge amount of work. Much of the agricultural census data we included isn’t even digitized,” says Kim Van Meter, a postdoctoral fellow and lead author on the study. “But this data was crucial to having the long-term trajectories we needed to show time lags between implementation of conservation measures and real improvements in water quality, which are often on the order of decades.”

After crunching the numbers, the researchers found that, after taking increasing crop yields into account, net nutrient inputs within the Grand River Basin have actually been decreasing since the late 1980s.  Despite these decreases, however, water quality has been slow to respond in many areas of the watershed, with time lags on the order of 20–30 years.

Interestingly, one exception to this trend is the Canagagigue, a tributary to the Grand and an area with high densities of tile drainage.  The Canagagigue has seen an approximate 30 per cent decrease in nitrogen loading since the early 1990s, with a time lag of only approximately 5–10 years behind decreases in inputs.

“Quantifying these time lags is crucial to setting water quality goals,” says Van Meter.  “When we set a policy goal to reduce nutrient loads by 40 per cent, it is important to understand that it may take decades to achieve this target, even if watershed stakeholders are doing everything right. ”

The researchers say that these nutrient-related time lags are primarily related to how quickly runoff reaches the river. In slower-responding areas of the watershed, where agricultural fields are not drained, rainfall and irrigation water percolate through the soil, down to the groundwater, reaching the river only decades later. In contrast, tile-drained areas, such as those around the Canagagigue, divert water straight to the river, with little delay.

“Tile drains can be a double-edged sword.  Subsurface drains are often linked to poorer water quality since the filtering capacity of the soil is lost," says Van Meter. "On the other hand, more efficient fertilizer use in tile-drained areas may also lead to faster reductions in stream concentrations than might be seen in non-drained areas."

Conversely, in areas where time lags are longer, the landscape has a greater ability to filter out nutrients, either trapping them in soils or sediments, or allowing them to escape into the atmosphere.  In addition, when nutrients are trapped within the upper soil layers, crops in subsequent years can potentially use those excess nutrients.

“Anything that slows the flow of water through the landscape increases the likelihood that excess nutrients will be kept out of our rivers and lakes,” said Basu. 

In related work, Basu and PhD student Frederick Cheng have explored how small wetlands, which can retain water and nutrients in upland areas, can reduce the potential for algal blooms in the Great Lakes themselves.

Kim Van Meter is a postdoctoral fellow in the Department of Earth and Environmental Sciences at Waterloo. Nandita Basu is an associate professor in both the Department of Civil and Environmental Engineering and the Department of Earth and Environmental Sciences and is also a member of the Water Institute.

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