Stormwater runoff is both a water quantity and water quality problem. Urbanization and construction increase the impervious surface areas and change the infiltration rates of soil through compaction and other engineering modifications. As a result:

• The volume of runoff increases
• Time-of-concentration (time-to-peak) is faster
• Peak flow rates are higher
• Frequency and duration of bank-full flow increases

This results in increased flooding as well as increased erosion and sedimentation.

To address these problems, municipalities have conducted stormwater master drainage plans to assess the:

• Adequacy of culverts and storm drain sizes
• Stormwater detention
• Drainage design and erosion control

Detention structures can reduce flow peaks and remove some of the sediment, but they do not adequately treat dissolved chemicals, nutrients, bacteria and the finer suspended sediment. Left untreated, these pollutants can cause aquatic habitat loss, fish kills, eutrophication (increase in nutrients and depletion of dissolved oxygen) of lakes and streams, sedimentation and scouring, and public health hazards from full and partial-body contact with the water as well as fish consumption.

The finer portion of stream sediment loads (silts and clays) have a residual negative charge on their surfaces and trace element and heavy metal ions in the stream water that are positively charged will adsorb onto the fine sediment particles. This adsorbed ionic layer will not dissolve back into the streamflow as long as the pH of the stream remains alkaline. Should the pH become acidic owing to increases in organic matter and pollutants in the runoff to the stream, the metals adsorbed onto the sediment can become solutes in the water again posing a hazard to both the aquatic life and the public. Even while still in the adsorbed state in alkaline conditions, mercury can enter the food chain when aquatic organisms convert it to a more soluble form and fish concentrate it in their tissues.

In 1966, the EPA in conjunction with other federal, state and local agencies monitored 19 percent of the 3.6 million miles of streams in the U.S. They found that 37 percent (7 percent of the stream miles) of the reaches studied showed some level of contamination and that a majority of the contaminants were from urban runoff conveyed by storm sewers and discharged into the streams.

The Arizona Department of Environmental Quality (ADEQ) participated in this national study and continues to monitor Arizona streams and lakes on an annual basis. In 2004, ADEQ monitored 3,450 miles of streams, washes and canals which included 77 percent of the perennial stream miles in the state. The top four categories of contaminants identified were:

• Metals
• Sediment
• Bacteria
• Pesticides

One or more of these contaminants were found at concentrations that either impaired or prevented the attainment of designated water uses along approximately 800 river miles.

Forty-five percent of Arizona’s lake and reservoir acreage was also monitored. The top four categories of contaminants and degradation indicators identified were:

• Metals (mercury)
• pH
• Nutrients (ammonia)
• Low dissolved oxygen

One or more of these contaminants or indicators were found at concentrations that indicated designated water uses were impaired or not attained in approximately 4600 acres of lakes and reservoirs.

The principal sources of metals in Arizona’s streams, lakes and reservoirs are from:

• Mining
• Road building
• Land development
• Atmospheric deposition.

Principal sources of sediment are from:

• Agricultural areas
• Construction sites
• Logging areas
• Mined areas
• Areas being hydrologically modified
• Urban runoff
• Heavy recreation use areas

The principal sources of bacteria are from:

• Untreated sewage
• Animal waste
• Heavy recreation use area

The identified principal sources of nutrients (nitrogen, phosphorus and ammonia) impacting Arizona’s water resources are:

• Fertilizers
• Livestock waste
• Sewage treatment plant effluent
• Failing septic systems
• Pet wastes