Mj. Goss et al., Transport of nitrogen, phosphorus and microorganisms from manure into surface- and groundwater, BIOLOGICAL RESOURCE MANAGEMENT: CONNECTING SCIENCE AND POLICY, 2000, pp. 31-55
The quality of water resources is an important factor that must be consider
ed in any analysis of sustainable agriculture or resource management. Most
contaminants affecting water quality in rural areas can be grouped into one
of three classes: simple inorganic ions, more complex organic molecules, a
nd particulates. Contaminants of all three classes can result from animal m
anure. In particular, it potentially provides the source of nitrate, phosph
ate, toxic metals, and bacteria as well as materials that generate high bio
logical oxygen demand.
The concentration of contaminants entering water can be different for each
class of contaminant, as they are affected by different processes. Organic
molecules are affected by vapour pressure, and solubility in water and soil
organic matter, simple inorganic compounds are influenced by exchange and
adsorption, whereas the transport of microorganisms are affected by surface
charge and population dynamics.
The organic matter in manure also alters the physicochemical properties of
the soil, thereby increasing the mobility of contaminants and resulting in
losses from soil under conditions that might suggest that the materials wou
ld be less available to leaching and runoff.
Preferential flow paths can develop due to structures present in the surfac
e soil or in the subsurface due to soil horizon boundaries. Preferential fl
ow can carry water and contaminants to depth very rapidly. This reduces pot
ential interactions occurring between the soil and contaminants in solution
, and, again, can result in greater impact on water quality than might be e
xpected.