December 2010, Vol. 22, No.12

Are 'green' farming practices driving algal blooms?

News art

Hazardous blue-green algae blooms and dead zones with no oxygen have returned to Lake Erie since the late 1990s. They were especially severe this summer, along with surging loads of dissolved phosphorus. New research hints that the main culprit could be an environmentally beneficial agricultural practice known as “no-till” farming, which was instituted to improve lake water quality.

“In the late 1970s, governments on both sides of Lake Erie agreed that phosphorus was the cause of nuisance algal growth and that annual loads should be limited to 11,000 metric tons,” said Dave Culver, a professor of aquatic ecology at Ohio State University (OSU; Columbus). Thanks to advanced nutrient removal techniques at municipal and industrial wastewater treatment plants in the watershed, the total phosphorus target has been met nearly every year since 1981. Algal growth fell to a minimum in 1995 but since then has rebounded, despite no change in the total phosphorus loading.

Now, a new study based on long-term stream monitoring implies that the return of water quality problems is due to changes in the kind of phosphorus reaching the lake, according to David Baker, director emeritus of the National Center for Water Quality Research at Heidelberg University (Tiffin, Ohio) and lead researcher on the study. He explained that total phosphorus consists of dissolved forms that are 100% available to feed plant and algal growth, and particulate forms that are only 30% bioavailable. Baker has found that since the 1980s, dissolved phosphorus levels have doubled and now make up 30% of total phosphorus reaching the lake.

The rise in dissolved phosphorus has coincided with profound changes in agricultural practices in the watershed, Baker said. After point sources cut their nutrient releases in the early 1980s, agricultural runoff became the major contributor of phosphorus to Lake Erie. “The most efficient way to cut sediment runoff was through no-till farming,” Baker said. By the 1990s, no-till practices dominated the landscape, reducing by 50% the loss of sediment and particulate forms of phosphorus to streams feeding Lake Erie.

“But since 1995, dissolved phosphorus levels in streams in farming areas have skyrocketed,” Baker said. “Because dissolved phosphorus has increased so much, the amount of bioavailable phosphorus loading into Lake Erie from agriculture is higher now than before the no-till programs were started. We’re going to have to rethink strategies for cutting farm runoff.”

Many causes

Farmers who avoid tillage can plant and fertilize crops without disturbing soil structure, improving moisture retention and allowing crop residues to slowly decompose on the soil surface. “But no-till agriculture stratifies the soil and concentrates phosphorus at the soil surface, where it dissolves in runoff water,” Baker said. Agricultural researchers have often documented increases in dissolved phosphorus runoff with no-till crop production, he added.

The problem is exacerbated by a growing trend to broadcast fertilizer on the soil surface instead of injecting it 130 to 150 mm deep (5 to 6 in. deep) in fields, Baker said. In a no-till system, the broadcast fertilizer granules lie on top of the crop debris and are vulnerable to rainfall runoff for a longer time than in tilled fields. In addition, large pores drilled by worms and roots in no-till fields help transport dissolved phosphorus from the soil surface straight down to drainage tiles under fields and out to streams, Baker said.

But not everyone agrees that no-till farming should take all the blame. “Increases in dissolved reactive phosphorus in streams are primarily due to soil compaction and poor nutrient management by farmers,” said Mark Scarpitti, state agronomist at the Ohio Natural Resources Conservation Service.

Soil compaction has increased since the 1970s, along with the ballooning size of farms. “When farms were smaller, farmers made one pass across their fields with a planter that injected seeds and fertilizer simultaneously,” Scarpitti explained. Now, larger farms and bigger pieces of equipment are the norm, and farmers find it more efficient to broadcast the fertilizer in the fall and make a second pass over the field in the spring to seed their crop. Up to 85% of the field can become compacted from heavy machinery. Because a compacted soil is more impervious to rainfall, the fertilizer lying on the surface doesn’t get a chance to soak in, and a hard rain will wash it off into streams, Scarpitti said. In addition, nutrients applied to frozen or snow-covered ground contribute to surface water contamination when the snow melts, he said.

Solutions

Because there are multiple causes of nutrient runoff, a whole system of best management practices (BMPs) will be needed to correct the problem, Scarpitti said. “We promote no-till farming as part of the solution,” he added. If soil is tilled, it oxygenates the soil and elevates microbial metabolism, leading to the loss of organic matter and carbon that absorb nutrients. Tillage also mineralizes inorganic nutrients, releasing them into the environment. “To compensate for this disruption, we have become overly dependent on commercial fertilizer,” he said.

In contrast, soil that is not tilled has a rich assemblage of microorganisms that produce vitamins, amino acids, and polysaccharide glues that improve soil structure, absorb nutrients, and prevent soil particles from escaping during rainstorms. “The use of no tillage, a diverse mix of cover crops, and good nutrient-management practices are all tools to help improve the health of the soil,” Scarpitti said. “If we improve the health of our soils, we will improve the health of our streams and lakes.”

Precision farming is a new high-tech method that can ameliorate compaction and fertilizer runoff, Scarpitti said. A geographical positioning system (GPS) controls farm equipment and can steer a tractor in the same wheel tracks year after year within an accuracy of 25 mm (1 in.). Fertilizer and seed go into the bands of untracked soil, which is now able to heal, Scarpitti explained. Variable-rate technology can be linked to the GPS, enabling fertilizer to be applied at different rates across the field, based on soil nutrient levels. Optimum yields can be achieved using exactly the right amount of fertilizer in the precise place it’s needed.

Partnerships between small treatment plants and farmers, spurred on by the U.S. Environmental Protection Agency’s looming nutrient criteria for streams, are another way to cut phosphorus runoff, said Richard Moore, an anthropologist at OSU. For example, a cheese factory in Winesburg, Ohio, is paying farmers to institute BMPs, such as storing and treating animal waste, and has met its nutrient reduction goals at a lower cost than upgrading its treatment plant.

Remaining challenge

Experts agree that farming practices are behind the rise in dissolved phosphorus flowing into Lake Erie, but determining which practices are at fault and how best to change them is a tall order. “Funding is needed to provide field-scale edge-of-field research to quantify the risk of phosphorus loss from different BMPs so that farmers can choose the BMP that will give them the biggest bang for the buck,” said Libby Dayton, a research scientist at OSU.

Baker agrees that there has to be a greater emphasis on nutrient management tailored to the specific soils and hydrology in an area. Meanwhile, he and his colleagues are moving to more-detailed studies of soil stratification.  

— Janet Pelley, WE&T


Chloride in winter runoff has toxic effects on urban streams, study finds

Research implicates road salt as source of high chloride levels in northern U.S. streams  

In northern climates, road salt is a ubiquitous feature of winter life, enabling safe passage for automobiles and pedestrians alike on roads and sidewalks that would otherwise pose an icy danger. But what happens when road salt washes into area streams as runoff? A recent study of the effects of road salt on water quality found “dramatic impacts” in terms of aquatic toxicity associated with periods of winter runoff. High chloride levels, in particular, pose a significant threat to aquatic life in urban streams located in areas in which road salt is applied.

In northern climates, road salt is a ubiquitous feature of winter life, enabling safe passage for automobiles and pedestrians alike on roads and sidewalks that would otherwise pose an icy danger. But what happens when road salt washes into area streams as runoff? A recent study of the effects of road salt on water quality found “dramatic impacts” in terms of aquatic toxicity associated with periods of winter runoff. High chloride levels, in particular, pose a significant threat to aquatic life in urban streams located in areas in which road salt is applied.

IIn northern climates, road salt is a ubiquitous feature of winter life, enabling safe passage for automobiles and pedestrians alike on roads and sidewalks that would otherwise pose an icy danger. But what happens when road salt washes into area streams as runoff? A recent study of the effects of road salt on water quality found “dramatic impacts” in terms of aquatic toxicity associated with periods of winter runoff. High chloride levels, in particular, pose a significant threat to aquatic life in urban streams located in areas in which road salt is applied.

In northern climates, road salt is a ubiquitous feature of winter life, enabling safe passage for automobiles and pedestrians alike on roads and sidewalks that would otherwise pose an icy danger. But what happens when road salt washes into area streams as runoff? A recent study of the effects of road salt on water quality found “dramatic impacts” in terms of aquatic toxicity associated with periods of winter runoff. High chloride levels, in particular, pose a significant threat to aquatic life in urban streams located in areas in which road salt is applied.

For the study, researchers from the U.S. Geological Survey (USGS) and the Wisconsin State Laboratory of Hygiene assessed the aquatic toxicity associated with chloride levels at a local level, evaluating several streams in the Milwaukee area. To examine the issue at a regional level, the researchers monitored specific conductance as a surrogate for chloride in nearly a dozen watersheds in southeast Wisconsin. To approach the issue on a national scale, the researchers examined historical data compiled by USGS for 13 northern and four southern metropolitan areas. “Dramatic impacts were observed on local, regional, and national scales,” the researchers conclude in their article, “A Fresh Look at Road Salt: Aquatic Toxicity and Water-Quality Impacts on Local, Regional, and National Scales,” which was published Sept. 1 in the online version of Environmental Science & Technology.

More development, more chloride

Concerns about the deleterious effects of road salt on water quality are not new, said Steven Corsi, the lead author of the article and research hydrologist for the USGS Wisconsin Water Science Center in Middleton, Wis. However, usage of road salt has increased each decade since the 1940s, as development results in more roads, parking lots, and sidewalks that are treated with salt for deicing purposes. Moreover, municipalities and other local governments face a “greater expectation” from the public to provide clean roads than in the past, Corsi said. If left unchecked, the increased use of road salt will pose an even greater threat to aquatic life, especially in urban streams, according to the research of Corsi and his colleagues.

At the local level, the researchers sampled 13 streams in the Milwaukee area — 12 of them urban, one rural — in February and March 2007. The samples were evaluated for chloride concentrations and specific conductance. In addition, bioassays were conducted using water fleas (Ceriodaphnia dubia) and fathead minnows (Pimephales promelas). Among the urban streams, eight were found to contain chloride concentrations exceeding the acute water quality criterion of 860 mg/L specified by the U.S. Environmental Protection Agency (EPA). Furthermore, 11 of the 12 urban streams had chloride concentrations in excess of EPA’s chronic water quality criterion of 230 mg/L. As for the bioassay tests, samples from seven of the 12 urban waterways exhibited toxicity. By comparison, the rural stream was found to have a chloride concentration of 20.4 mg/L and did not exhibit toxicity. Meanwhile, long-term data collected as part of the monitoring of another Milwaukee stream between 1997 and 2007 generated similar results.

At the regional level, the researchers monitored 11 streams in southeast Wisconsin using continuous specific conductance sensors during periods of cold and warm weather. Depending on the site, between 1 and 10 years of data were collected. The selected watersheds differed in terms of size, as well as degree of urbanization, and samples from areas with greater urban land use were found to have increased levels of specific conductance. In two of the more urbanized watersheds, for example, water samples frequently were found to have specific conductance exceeding 10,000 µS/cm. During winter, 55% of samples were found to have chloride levels exceeding EPA’s acute water quality criterion, while all of the streams exceeded the chronic water quality criterion.

To examine the effects of road salt at a national level, the researchers evaluated the chloride concentrations in water quality samples collected by USGS from 17 large metropolitan areas around the country between 1969 and 2008. To test for the possibility that chloride could be originating from sources other than road salt, the researchers selected four southern cities, where road salt is not used, to serve as a control.

The historical data revealed that, between November and April, 55% of the 168 monitoring locations in northern metropolitan areas exceeded EPA’s chronic water quality criterion for chloride. During this same timeframe, 25% of locations exceeded the acute water quality criterion for chloride. Between May and October, only 16% of monitoring locations exceeded EPA’s chronic water quality criterion for chloride, while only 1% exceeded the acute water quality criterion. For the southern sites, few samples exceeded the chronic criterion, and none exceeded the acute criterion.

Aquatic toxicity surprises

Overall, the elevated chloride levels found during the study were not unexpected, Corsi said. However, the extensive aquatic toxicity documented in the results was “somewhat surprising,” he said. “It was more than we expected,” he noted.

The findings at the national scale indicate that runoff containing high levels of road salt is “not just a Milwaukee problem,” Corsi said. By contrast, any urban stream located in an area where road salt is used is at risk of experiencing high chloride concentrations. “That’s a serious issue for urban streams in areas where road salt is applied,” Corsi said. In these waterways, aquatic species sensitive to salt are likely to be stressed or eliminated altogether, he noted.

No easy solutions

However, Corsi acknowledged that “no easy solutions” exist for local governments attempting to reduce chloride levels in local streams. For starters, the highly soluble nature of road salt complicates efforts to remove the compound by settling, the most common approach used to remove pollutants from stormwater. “There aren’t any current stormwater management practices that are going to effectively address this issue,” Corsi said. Instead, decreasing the amount of road salt that is applied seems to be the only effective means of reducing chloride contamination in runoff.

Of course, local governments do not wish to decrease public safety in the name of improving water quality. Therefore, efforts to reduce the amount of road salt applied should focus on four strategies, Corsi said. First, employees responsible for applying road salt must calibrate their equipment properly and ensure that they are using the correct amount of salt. Second, the use of modified application techniques, such as applying salt brine to roads before a snowfall, can lessen the extent to which snow adheres to the road surface, increasing the effectiveness of snowplows. Third, innovative plow designs capable of plowing more thoroughly are available, Corsi said. Finally, alternative chemicals can be used, but they may present their own water quality problems. For example, some alternative chemicals are organic in nature and could increase levels of biochemical oxygen demand if they enter a waterway in sufficient concentrations. Overall, the best solution involves “vigilance and a lot of attention paid to what’s being applied and how it’s being applied,” Corsi said.

The study was supported by the Milwaukee Metropolitan Sewerage District, Milwaukee’s General Mitchell International Airport, and USGS. The report may be downloaded free at http://pubs.acs.org/doi/abs/10.1021/es101333u 

Jay Landers, WE&T

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