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
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
Illinois State Water Survey (ISWS). 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.