Chemical and Physical Characteristics

Chemical and physical properties, like other non-living ecosystem attributes, help shape the environment of living things. Many of EPA’s specific environmental protection responsibilities include measuring and addressing chemical changes. Chemical measurements are often based on water sampling for, among other substances, nitrogen and phosphorus compounds, dissolved oxygen, pesticides, and heavy metals.

Some data on chemical characteristics in U.S. waters have been collected by the U.S. Geological Survey National Water Quality Assessment (NAWQA)program. In an analysis done for the Heinz report, NAWQA reported on contaminants in stream waters from 109 sites and in sediments from 558 stream sites in 36 watersheds across the U.S. At least half of monitored streams had contaminant concentrations that exceed water quality criteria for wildlife.²⁵ However, no analyses yet relate these concentrations to the status of fish or invertebrate communities in the streams. Nitrate levels were highest in farmland streams, with 10 percent of the samples exceeding drinking water standards (Exhibit 5-6).²⁶

The NAWQA program provides consistent and comparable information on nutrient and pesticide concentrations in streams in agricultural areas, although the network design and number of sites do not allow estimates to be made for agricultural streams nationally. Nitrate loss from most forests does not appear to be resulting in high-nitrate concentrations in forest streams, but few streams are sampled in parts of the country where nitrate deposition tends to be high (e.g., eastern states).

A number of physical and chemical indicators are being monitored in Atlantic and Gulf Coast estuaries to help diagnose and interpret information on biotic condition. Eighteen percent of mid-Atlantic estuaries show high nitrogen concentrations, and 12 percent show high phosphorus concentrations. Twenty percent of Atlantic and Gulf Coast estuaries have low dissolved oxygen concentrations (i.e., less than 5 milligrams per liter). On average, 75 percent of the sediments contain elevated pesticide concentrations, and 40 percent show elevated concentrations of heavy metals.²⁷

Exhibit 5-6: Nitrate levels in streams by ecosystem, 1992-1998

Source: USGS, National Water Quality Assessment. The Quality of Our Nation's Waters-Nutrients and Pesticides. 1999.