What Happens to Precipitation Samples Once They Leave the Field?

Every Tuesday morning at 9:00 a.m., operators at approximately 250 sites across the U.S. carefully collect precipitation from rain collectors and data from gages and ship samples to the National Atmospheric Deposition Program at the Wisconsin State Laboratory of Hygiene for testing. These weekly samples enable scientists to track the types of pollutants that are distributed from the atmosphere to the earth in rain and snow (wet deposition).

Careful handling of precipitation samples from the field to the final review in the laboratory is critical to obtain precise data. Mishandled or compromised samples can easily become contaminated. Our laboratory, the Central Analytical Laboratory (CAL) uses standard operating procedures with checks and balances to ensure data accuracy.

The following four steps track precipitation samples through the CAL.

1. Shipping and Receiving

The CAL provides buckets and shipping containers, typically 1-liter bottles, to the network sites. Shipping and Receiving logs in approximately 250 samples each week into a database, assigns each sample a unique ID. At this point, a sample is uniquely identified, tracked through the lab, and is associated with a digital version of a sites sample information (date on and off, operator field notes, etc.)

Next, the sample is transported into the laboratory for measurements of pH, conductivity and filtration laboratory within the CAL. Meanwhile, bottles and white water buckets used in the field are sanitized in dishwashers, and then returned to site personnel at the U.S. network sites.

2. Analysis


The pH/conductivity analyst checks each sample in its 1-liter bottle for contamination and compares this information with initial data from the field operator. Water samples are poured into two 4-milliliter vials, and pH (a measurement of “acidity”) and conductivity (a measure of ionic strength) are completed. Within four days of receiving a sample, all samples are measured for pH and conductivity.

The remainder of the precipitation sample volume is filtered, removing solids from the sample in the 1-liter bottles into 60-milliliter bottles. These 60-ml subsamples are used for the remainder of the analysis. If there is a sufficient amount of liquid, a portion of the sample is refrigerated and included in a water sample archive at the CAL. These samples are kept for five years.

Major Ions

Within two weeks, the samples are analyzed for sulfate, nitrate, and chloride by ion chromatography; calcium, magnesium, sodium, and potassium by inductively coupled plasma-optical emission spectroscopy; and ammonium and orthophosphate by flow injection analysis.

These individual instruments are run independently by chemists in the laboratory.

3. Quality Assurance, Review, and Site Support

A preliminary report combining information from the sample entry form and chemistry results is generated. The preliminary report will be reviewed and is used to alert staff members to any potential errors.

The longest part of the process is the two stages of data review, during which CAL staff members look for data incompleteness and errors, such as equipment problems and collector inefficiencies, and then review data, searching for inconsistencies with past information.

The quality assurance process involves checking bottles, lids, and buckets at random for cleanliness. If there is a problem, the equipment is rewashed and rechecked. While handling samples and equipment, CAL staff must ensure that they don’t contaminate any samples. Even the smallest drop of water added to a bottle or bucket can alter the results.

Control samples are also analyzed to ensure the quality of sample collection analysis.

4. Data Dissemination

Finally, the data are disseminated through the NADP Web site at no charge to users. This site offers online retrieval of individual data points, seasonal and annual averages, trend plots, concentration and deposition maps, and reports.

In 2008, nearly 38,000 registered users logged in, and the site received more than 1.65 million hits. About 33 percent of users are from the federal government, 33 percent are from universities, 20 percent from k-to-12 schools, and the remainder is from other organizations. Data can be retrieved at http://nadp.slh.wisc.edu.