Excess nutrients, especially nitrate-N, become diluted contaminants in the saturated zone but persist all the way to the sea. Cycling of manure. Contaminants in waste solutions are already mobile, and those in solid manure stockpiles, etc., are leached when added to the soil. Decay, as used here, destroys, inactivates, or dissipates the toxicity of a soil's foreign materials. Toxic substances also decay by contact with oxygen in surface water or the aerated zone.
Animal wastes degrade in an oxygen-rich environment that favors biochemical decomposition.
Sorption is how soil particles hold or chemically react with contaminants, so a fraction of the chemicals bond to the soil and do not move with the soil water. Clays and soils with much organic matter tend to retain many contaminants, while sands do not. Dense rocks have little surface area for sorption, allowing contaminants to move freely with percolating water. Dilution in water lowers contaminant strength, potency, and effect. Because concentrated toxic spills may need more dilution than soil water can offer, contain and remove the contaminants before they enter the ground.