Assimilative capacity is the relationship between water quality and quantity, land use (not only urban development, but also activities such as agriculture, aggregate extraction, and recreational facilities such as golf courses), and the ability of a watercourse or lake to resist the effects of landscape disturbance without the impairment of water quality below levels set by provincial standards. An assimilative capacity study (ACS) uses scientific modelling to support municipalities and other legislative authorities in predicting the impacts of land use.
The assimilative capacity of a waterbody is related to flow conditions and nutrient loads during a specified period or over a range of expected variations in these conditions. Understanding assimilative capacity means identifying the maximum contaminant loadings from the watershed, to assess whether a waterbody will meet pre-determined criteria for its ecological function and designated use.
MOECC has expressed concern about the assimilative capacity of groundwater sources in relation to large subsurface disposal systems, which may not be an appropriate way to handle sewage in certain areas, depending on the subsurface geology and hydrogeology. Mapping the geology and water table of the region may be useful in this regard.
These calculations will help identify "hot spots" of servicing and/or environmental constraints, which can be mapped and compared with growth allocations for the affected areas. Information on the costs of upgrading or expanding servicing can also be collected to connect growth management with the costs of infrastructure.
 Lake Simcoe Region Conservation Authority, Assimilative Capacity Study, 2006. Retrieved from http://www.lsrca.on.ca/watershed-health/reports/assimilative-capacity-study
 Either empirical or numerical modelling approaches may be used. The assimilative capacity of systems may be estimated by using a mass balance approach combined with empirical relationships where sufficient gauge and water quality measurement data exist. Alternatively, a desktop analytical approach may be used where regional data are available (either to estimate flows or loadings). Finally, numerical watershed and waterbody models may be used to predict watershed loading, point source loads, and to ultimately estimate the assimilative capacity downstream waterbody.