Nitrites (NO2-) and nitrates (NO3-) act as plant nutrients but excessive amounts cause eutrophication. Sources of excess nitrates in the water are: sewage spills or leaks from sewer lines and septic tanks, excessive use of fertilizers on nearby lands, animal waste entering the water from runoff, and green waste.

Exceedance of the state criteria for nitrate + nitrite (as N) was observed in all study sites in He'eia Stream during 2002-2005, with the exception of site 3. It is noticeable that among all the sites, site 1 & site 2 consistently have the highest nitrate + nitrite levels. The higher levels at these two sites are most likely due to their proximity to residential areas causing fertilizer runoffs.

The Total Nitrogen and the Combined Nitrogen levels in Sites 1 and 2 are relatively close in values across the board. This indicates that the inorganic nitrate and nitrite found in fertilizers are the dominant sources of nitrogen and that organic nitrogen nutrients are minimal.

In reviewing the data, it is observed that the nitrate-nitrite content decreased substantially in site 3 (the Marsh). A majority of the nitrates + nitrite (as N) levels in this site ar below the state criteria. The slow movement of the water in the Marsh fosters settling of particulate matter and increases the contact of microorganisms with the chemical nutrients. Microorganisms remove nitrates from the water and metabolize them to nitrogen gas.

As the water moves down to the Restoration area (Site 4), the combined nitrate-nitrite levels begin to rise again. This area was densely covered with mangroves and is now being rehabilitated with native plants. Soil erosion and stream water mixing with the seawater in Kaneohe Bay are possible sources of the chemical nutrients. The water in the reef areas contains higher nitrate and nitrite levels than the estuaries. Elevated levels of nitrates in the reef areas can cause proliferation of seaweeds and damage to corals.

In order to ensure Kaneohe Bay's delicate eco-system for future generations, a public service campaign educating local residents how their property and habits impact the water quality would be desirable. Along with this, He'eia Stream needs to be continually monitored to detect any progress or decline in the combined nitrogen levels.

 

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