The Integrated Water Resources Conference was held on January 26th at the Wyndham Resort in Orlando. This is the fourth of several posts that will provide further discussion and comments prompted by the information each speaker provided. Readers are encouraged to continue the conference dialogue via comments (below) that will be moderated and posted. This entry is a special guest post by speaker, Jeff Herr of Brown & Caldwell.
Nutrient Management in Watersheds
There are almost 5,000 nutrient TMDLs in the US with many additional TMDLs still to be completed. Now that state surface water quality numeric nutrient criteria have been finalized for Florida, the number of water segments impaired for nutrients is expected to grow substantially. Nutrient TMDLs will need to be developed for these impaired water segments. US EPA is in a national stormwater rulemaking process with new criteria expected in late 2012 or early 2013. This new rule may expand NPDES MS4 jurisdiction and may require the retrofit of existing development to reduce pollutant loads to impaired surface waters.
There are many different potential point and non-point sources of nutrients in a watershed. It is very important to accurately quantify all sources and magnitudes of nutrients so that an effective solution can be developed and implemented. For example, if a lake is phosphorus limited, and a vast majority of the phosphorus load to the lake is from internal recycling from lake bottom sediment, treating all of the stormwater entering the lake would provide minimal water quality improvement. Millions of dollars could be spent with negligible water quality improvement. In this case sediment removal or chemical inactivation would be required to improve lake water quality.
Monitoring of water flow rates and chemical characteristics is essential to calibrate hydrologic budget volumes and nutrient budget loads developed by modeling or calculation. The larger the number of measurements the greater the confidence in the results. Many watershed nutrient loading studies are completed solely using literature values. This can lead to incorrect results and the expenditure of funds with no water quality improvement.
Most of our rain events (~90%) are one-inch or less in an average rainfall year. To reduce nutrient loads in stormwater runoff and improve surface water quality, we should focus treatment on runoff from common rain events, up to 2 or 3-inches. There is no need to treat the runoff from a 5-, 10- or 25-year storm for water quality improvement. Most of our runoff is produced by directly connected impervious areas, an impervious area connected directly to a closed drainage system. Even in hydrologic “D” soils, runoff from non-directly connected impervious and pervious areas is minimal for common rain events. Disconnecting impervious areas is an excellent and low cost method to reduce the runoff volume from a development.
At present, much of our stormwater runoff is not beneficially used and ultimately is lost to tide. Rainwater and stormwater are resources which should be captured and reused for irrigation, gray water, cooling, etc. Up to 60% of our water use does not require potable quality water. Capturing and reusing stormwater runoff reduces nutrient loads to receiving waters and reduces the demand for potable water.
An integrated watershed approach to nutrient management involves a holistic analysis considering all sources and nutrient loads in a watershed. If only point source reductions are mandated, evaluate alternative non-point source projects or in-water projects (i.e. dredging). Determine which solutions have the lowest life cycle cost per mass of nutrient removed and which are the most environmentally beneficial (reduced GHG emissions). The best option may involve treating point sources to a cost effective level and then adding non-point source projects to achieve the remaining required nutrient reduction. The triple bottom line approach considers economic, environmental and social issues.
A copy of Jeff’s presentation can be viewed by clicking the following link: Jeff Herr’s Presentation
Many thanks to Jeff Herr and Brown & Caldwell for their continuing support of FWEA and the IWRC.