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National Trends Network

The NTN is the only network providing a long-term record of precipitation chemistry across the United States.

Sites predominantly are located away from urban areas and point sources of pollution. Each site has a precipitation chemistry collector and gage. The automated collector ensures that the sample is exposed only during precipitation (wet-only-sampling).

Site operators follow standard operational procedures to help ensure NTN data comparability and representativeness. They collect samples weekly on Tuesday morning, using only containers cleaned at the Central Analytical Laboratory (CAL) at the Wisconsin State State Laboratory of Hygiene (WSLH). They weigh the collection bucket to determine sample volume and transfer the sample from the collection bucket to a shipping bottle. All samples are sent to the CAL for analysis, and data entry, verification, and screening.

The CAL measures free acidity (H+ as pH), conductance, calcium (Ca2+), magnesium (Mg2+ ), sodium (Na+ ), potassium (K+ ), sulfate (SO42-), nitrate (NO3- ), chloride (Cl- ), and ammonium (NH4+). The CAL also measures orthophosphate, but only for quality assurance as an indicator of sample contamination.

The CAL reviews field and laboratory data for completeness and accuracy, and flags samples that were mishandled, compromised by precipitation collector failures, or grossly contaminated. The CAL delivers all data and information to the NADP Program Office, which applies a final set of checks and resolves remaining discrepancies. Data then are made available on the NADP Web site.