Site-specific risk assessment and management of point and diffuse sources - Environmental Deterioration and Contamination: Problems and their Management - page 325

- The most important chemical pollutants from point sources subject to manage-

ment and control are oxygen depleting nutrients, toxic chemicals and heavy
metals.

2.3.2 Characteristics of diffuse pollution sources

Diffuse pollution is highly dispersed throughout a large area. It is often difficult to trace
the exact origin of these pollutants because they result from a wide variety of human
activities on the land and they cannot be traced back to properly controlled or derelict
point sources. Diffuse pollution is largely dependent on the natural characteristics of
soil, climate and topography. According to Novotny & Olem (1994) and Novotny
(2003), diffuse sources can be characterized as follows:

- Pollutant generation occurs over an extensive area of land and is in transit

overland before it reaches soil surfaces, surface waters or infiltrates shallow
aquifers;

- Diffuse sources are often individually minor but collectively significant in a larger

area or in a watershed;

- Diffuse secondary sources may originate from primary diffusion or not properly

identified point sources;

- Diffuse discharges enter the receiving surface or subsurface waters in a diffuse

manner at intermittent intervals that are related mostly to the occurrence of
meteorological events, i.e., rainfall or snowmelt;

- Diffuse pollution is highly episodic, associated with dynamic weather conditions

(moisture content of soil, storm events, etc.), responsible for the transport of pol-
lution from multiple or diffuse sources. Other episodic phenomena are connected
with activities such as slurry application on soil, aggregations of livestock in rela-
tion to shelter or watering needs, spills and incidents, plowing, harrowing, planting
or harvesting;

- The extent of diffuse pollution is related to certain uncontrollable climatic events,

as well as geographic and geologic conditions and may differ greatly from place
to place and from year to year;

- Pollution peaks are variable and difficult to predict;

- Impacts are often slow to develop and become evident years later (e.g., contami-

nation of groundwater, changes in trophic status of lakes);

- Diffuse sources are difficult or impossible to be monitored at the point of

origin, even the transport routes are changing and dependent on weather con-
ditions (temporary runoffs, floods, etc.), meaning that the evidence of pollution
impact is limited to the target environment, the air, surface water, groundwater
or soil;

- Abatement of diffuse load is focused on land and runoff management practices;

- Compliance monitoring is carried out on land rather than in water;

- Water quality impacts are assessed on a watershed scale;

- Pollutant emissions and discharges cannot be measured in terms of effluent

limitations;