Over the years, I have heard from a number of sources including professionals that when a compost meets Class A compost requirements, it is benign and there is no risk of causing pollution of either ground or surface water. A few years ago, someone insisted during a public meeting that their Class A compost could not be a source of nitrate contamination of groundwater because there was no nitrate in a sample of leachate that they collected from the base of their storage piles. Last year I heard a compost equipment vendor insist that after their two week windrow process, all of the product was perfectly safe and incapable of causing any harm to the environment!
Is this true? Is Class A compost really benign? From a potential pathogen perspective, if the compost meets Class A standards and has been stabilized long enough to prevent regrowth of potential pathogens, there is little risk. From the perspective of potential harm from E. coli or other potential pathogens, it may be benign.
Class A compost may contain significant concentrations of soluble nutrients, such as ammonium, potassium, calcium, sulphate, sodium etc. Most compost does not contain nitrate unless it is very mature. These soluble nutrients leach out of the compost if the compost is uncovered in a high rainfall climate. Of particular concern is ammonium because ammonia is toxic to fish at low concentrations, which is a particular concern for leachate entering ditches and waterways. Ammonium, when leached from the Class A compost into the soil profile, may convert to nitrate and increase the potential for leaching to groundwater.
In response to a question in 2010, I had prepared a summary and literature review that suggested that a pile of Class A compost may cause nitrate contamination of groundwater. Should Compost Piles be Covered in High Rainfall Climates
The first question is whether compost piles produce leachate? The answer depends on the timing and amount of rainfall, the moisture content of the compost, and the percentage of the composting pad covered with compost. Research concluded that 6 to 95% of the rainfall became leachate.
The second question is whether this leachate may contain ammonium? Research reported ammonium concentrations ranging from 1.4 to 260 mg N/L in leachate coming from finished compost piles. The highest concentrations of ammonium in leachate were found from food waste compost. Ammonium concentrations in leachate from animal manure compost depended on the source of the manure, where poultry manure compost produced highest ammonium concentrations. Maturity of the compost was also a factor, where ammonium concentrations in compost decreased with maturity (usually significantly greater than 100 days).
The third question is what happens to ammonium in surface water? Its actually ammonia, not ammonium that is toxic to fish at low concentrations, but the toxicity is dependent on the pH of the water. The ratio of ammonium (NH4+) to ammonia (NH3)is pH dependent, where increasing pH increases the proportion of ammonia relative to ammonium. Ammonia is lethal to fish at concentrations of 0.2 to 2 mg/L (http://www.water-research.net/Watershed/ammonia.htm).
The next question is what happens to ammonium contained in leachate when it enters the soil? The amount of movement of ammonium in soil depends on the permeability of the soil and the adsorptive capacity of the soil. Normally, we assume that positive cations such as ammonia do not move very far in soil. Ammonium may convert to nitrate in the soil profile, and nitrate moves easier in the soil profile because it is an anion. This nitrate is at risk of polluting groundwater if compost is stored on a permeable soil over a supply of water.
The solution to reducing surface and groundwater pollution from Class A compost simply means covering the piles. What is the best material to cover the piles with? Covering with an impervious tarp increases the risk of the pile becoming anaerobic and producing odor because Class A compost can still be very biologically active. It is recommended to keep piles covered with a breathable fabric that resists precipitation. We have used a product called Compostex very successfully in the last number of years.