Nutrient trends in the Bay watershed are monitored at a network of 32 nontidal river sites—basically areas west and north of Interstate 95—by the U.S. Geological Survey and the states. Sampling is done during “normal” flow conditions every month. Sampling protocols also call for capturing at least eight high-flow storm events each year, including at least one per season.

Technicians find themselves standing on windy bridges in midwinter storms collecting samples with hands frozen in thin nitrile gloves—they can’t wear insulated gloves because the fibers can cause contamination.

That’s important because severe storms send disproportionately large amounts of sediment and phosphorus (much of which attaches to sediment) down the rivers. But even concentrations of nitrogen, which more readily dissolves in water, change dramatically during high flows. In rivers, where nutrient sources are largely from farms and other runoff sources, nitrogen concentrations can jump fourfold.

Further, the concentrations can change based on when a storm hits. A spring storm, when farmers are fertilizing their fields, may generate higher nutrient concentrations than one in late summer, when dry soils soak up much of the rain and nutrients.

The story can change from river to river. In a waterway where most nutrients come from wastewater treatment plants and other “point sources,” high river flows actually dilute discharged nitrogen and phosphorus, resulting in reduced concentrations.

When results come back from the lab, they go through an extensive quality assurance check to determine whether they fit within the expected range of results. If not, the samples may be retested. If scientists conclude the samples were in some way contaminated, they will be discarded to keep the record as clean as possible.

The network is gradually being expanded to provide better information about nutrient and sediment trends in the watershed. It’s not inexpensive, though: It costs about $45,000 a year to monitor each site.