Water quality criteria can include general narrative statements that describe good water quality and specific numerical concentrations that are known to protect aquatic life and human health. The criteria are adjusted as needed to reflect changes in law and science. Numerical criteria are for specific physical, chemical (toxics), and radiological characteristics of the waters (e.g. minimum of 4.0 mg/L dissolved oxygen, 2.5 ug/L ammonia, 9.0 ug/L copper). Narrative criteria include general protective statements known as the "free froms." This narrative criteria says that all state waters shall be free from substances attributable to sewage, industrial waste, or other waste in concentrations, amounts, or combinations which contravene established standards or interfere directly or indirectly with designated uses of such water or which are inimical or harmful to human, animal, plant, or aquatic life.
Numeric chlorophyll 'a' criteria exist for man-made reservoirs and natural lakes (special standard "dd"). The Chesapeake Bay and its tidal tributaries have dissolved oxygen, submerged aquatic vegetation, and water clarity criteria. Site specific criteria exist for the tidal Pamunkey and Mattaponi Rivers (special standard "aa") and for the tidal James River (special standard "bb"). The criteria were developed to protect these waters from the harmful effects of nutrient over-enrichment. Significant increases in algae due to nutrient over-enrichment can harm water quality, food resources and habitats, and decrease the oxygen that fish and other aquatic life need to survive. Additional information regarding nutrient criteria and ongoing nutrient criteria development efforts is located on the Nutrient Criteria Development web page.
These numerical and narrative criteria describe water quality necessary to protect designated uses such as swimming, drinking and the propagation and growth of aquatic life.