Is chloride a concern?
Assessing the real impact of manure-leached chloride on soil and water quality
August 1, 2022 by James Careless
What impact does chloride leaching from livestock and poultry manure have on the rural landscape? That’s a question that researchers at the University of Minnesota hope to answer.
A three-year study, entitled “Assessing the implications of chloride from land application of manure for Minnesota waterways,” is led by principal investigator Melissa Wilson (department of soil, water and climate), along with co-investigators Erin Cortus (department of bioproducts and biosystems engineering) and Pedro Urriola (department of animal science) with support from graduate students including Matthew Belanger. Together, the team is working to measure the levels of chloride leachate in the state. The study began in 2021.
“The main purpose of this study is to see to what extent manure-based chloride moves through the soil profile and is leached out of the soil profile, and then comparing that to commonly used synthetic KCL fertilizers that are used in Minnesota,” says Belanger. “We want to know how much is leached out into the groundwater and surface waters, and to what extent does the storage in the soil profile change as well?”
In completing this study, the researchers will categorize their data based on species, geographical region, and correlating manure characteristics, quantify the movement of manure-based chloride in Minnesota fine clay and sandy soils, and evaluate the leaching risk potential based on manure types, chloride concentrations and soil characteristics. This study’s data will aid in the development of statewide standards for acceptable chloride leachate levels, plus strategies to manage this substance.
The need for these standards is very real, because “chloride accumulates over time in water resources, including surface water and groundwater, which makes it somewhat different compared to nitrogen and phosphorus management,” says Alycia Overbo, PhD candidate at the University of Minnesota. Overbo, who has previously undertaken research on the subject, adds, “There are no feasible or affordable ways to remove chloride from water bodies in the environment. Yet elevated chloride levels in surface waters can have detrimental effects on aquatic ecosystems and vegetation, while rising chloride levels in groundwater resources can degrade the quality of some drinking water sources. Additionally, research has suggested that elevated soil chloride can lead to nutrient release from sediment and soil, induce manganese and iron reduction, and increase the dissolution of phosphorus and toxic trace metals.”
The chloride study team is extracting 48 intact soil core samples from across Minnesota to gather their data. These samples are 12 feet deep and 12 inches in diameter. They are being taken to a laboratory and treated with liquid livestock manures, solid turkey manure or synthetic KCL fertilizer.
“Over the course of three weeks, we are simulating three two-inch rain events and letting the water percolate through these treated soil samples,” says Belanger. “And then we are collecting the leachate and evaluating how much chloride and other nutrients are in it. This provides us with an affordable way to do representative field tests in the lab. And then we can scale that up into the field if that makes sense.”
“With a lab-based study, we just have a little bit more control over the factors that we want to study,” adds Cortus. “Using intact soil cores provides a nice model of what would happen in a field.”
Results so far
Although the University of Minnesota study has two more years to run, previous research by Overbo has already been able to draw some conclusions about chloride leachate’s impact on soil and water sources.
A case in point: “Through our previous research, we learned that livestock manure is an important source of chloride in Minnesota,” says Overbo. “We estimated that livestock in Minnesota contribute 62,600 tons of chloride to the environment per year, compared to 209,900 tons from wastewater treatment plants, 221,300 tons from fertilizer, and 403,600 tons from road salt. Since livestock manure contributes significant amounts of chloride to the environment on an annual basis, this tells us that manure is an important source of chloride to consider in local settings.
“Areas with sensitive groundwater resources or chloride impairments may want to examine local chloride loading from manure application and encourage practices that can reduce chloride leaching.”
“Our research has been focussed on Minnesota, but our findings are relevant to areas outside of Minnesota with similar agricultural activity,” she adds. “Researchers in Illinois also conducted a chloride budget for their state and found similar results, indicating the importance of livestock and manure application as chloride sources.”
Manure managers need to monitor chloride leaching
Based on the data she’s seen to date, says Overbo, “Chloride may not be the number-one concern for the manure management industry, but it should definitely be on the radar. In Minnesota, the number of chloride impairments is increasing, and they are not limited to urban areas. We are seeing elevated chloride levels in some surface waters and groundwater resources in more agricultural, suburban, and mixed land-use areas of the state. Solid manure [in particular] has high chloride concentrations, and liquid manure still has concentrations one to two orders of magnitude above what is naturally found in the groundwater, so manure application definitely has the potential to impact groundwater and receiving surface waters.”
To address chloride leaching caused by manure sources, “Implementing best practices to prevent contamination of groundwater and surface water is key, since chloride can’t readily be removed from these water resources,” she says. “Testing manure and sampling groundwater downgradient can be very helpful in understanding the local impacts of manure application on groundwater. Research in Minnesota has shown that chloride levels can be particularly high in groundwater downgradient from earthen-lined basins, but that manure storage with synthetic liners helps limit chloride leaching into downstream and downgradient resources.”
The bottom line, says Overbo, is that even though chloride leaching is not as talked about as nitrogen or phosphorus, it is a concerning environmental problem, one the industry (and others) should begin tackling the issue sooner than later. Even halfway through the current study, the team agrees there are adverse impacts to chloride leaching.•