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As water quality challenges intensify globally, scientists face mounting pressure to develop more precise and efficient monitoring methods. The South Florida Water Management District (SFWMD) laboratory has announced a comprehensive overhaul of its total nitrogen (TN) measurement protocol—a breakthrough that promises to transform water quality assessment.

For decades, SFWMD relied on an indirect approach to measure total nitrogen—first determining Total Kjeldahl Nitrogen (TKN) and Nitrite+Nitrate Nitrogen (NOX) separately, then summing the values (TN = TKN + NOX). While functional, this method proved increasingly problematic. The TKN procedure required labor-intensive sample evaporation at high temperatures, followed by volume restoration after digestion—a process demanding specialized equipment including expensive digestion tubes and ventilated hoods to neutralize acid fumes.

The new direct measurement method eliminates these complications. By digesting samples in sealed containers under high temperature and pressure, technicians can now determine total nitrogen content without volume restoration or acid fume management. This streamlined process delivers lower detection limits and reduced measurement uncertainty while cutting operational costs.

Concurrent with the TN methodology upgrade, SFWMD has introduced Total Dissolved Nitrogen (TDN) as a new parameter—replacing the previous TDKN metric for filtered water samples. This addition strengthens the district's capacity to comprehensively assess nitrogen pollution across aquatic ecosystems.

Before implementation, SFWMD subjected the new protocol to extensive development and optimization, minimizing impacts on existing sampling and workflow. The method successfully passed multiple proficiency tests and demonstrated strong correlation with traditional techniques across 773 typical SFWMD surface water samples. After submitting certification applications in 2013, Florida's Department of Health Laboratory Services conducted onsite inspections and granted formal approval in April 2014.

The transition affects existing monitoring permits and parameter lists. When only TN measurement satisfies permit requirements, laboratories can now request TN alone rather than both TKN and NOX. For permits requiring both TN and NOX, TKN requests should be replaced with TN. The new TN reference method follows Standard Method SM4500NC from the 21st edition of Standard Methods for the Examination of Water and Wastewater, with SFWMD's standard operating procedure coded as SFWMD-SOP-3090-003.

Notably, the direct TN method's detection limit (0.02 mg/L) outperforms the calculated TN method's effective limit (0.055 mg/L). Sample collection procedures remain unchanged, though the new technique allows TN analysis from the same autosampler bottles used for total phosphorus—a previously impossible convenience.

Comparative analysis confirmed strong agreement between traditional and innovative TN measurement techniques. Testing across 773 routine water samples revealed consistent results between the NOX + TKN calculation and direct SM4500NC determination.

Further validation came through blind sample testing with Environment Canada and the Quality Assurance of Information for Marine Environmental Monitoring in Europe (QUASIMEME) program. The method demonstrated reliable performance across freshwater, brackish, and marine environments without detectable bias.

In Environment Canada's March 2014 proficiency test involving natural waters from lakes, rivers and rainfall, SFWMD's TN measurements achieved "excellent, unbiased" ratings across all 10 samples:

QUASIMEME's marine water testing (Exercise 1023) and estuarine/low-salinity testing (Exercise 1024) similarly confirmed the method's reliability, with all results receiving "Satisfactory" ratings under international standards.